Standard Article

You have free access to this content

Basic Elements of Toxicology

Basic Science

  1. Bryan Ballantyne MD, DSc, PhD, FRCPath, FFOM, FACOEM, FAACT, FATS, FIBiol, CBiol1,
  2. Timothy C. Marrs OBE, MD, DSc, FRCP, FRCPath, FBTS, FATS2,3,4,
  3. Tore Syversen MSc, Dr Philos5

Published Online: 15 DEC 2009

DOI: 10.1002/9780470744307.gat001

General, Applied and Systems Toxicology

General, Applied and Systems Toxicology

How to Cite

Ballantyne, B., Marrs, T. C. and Syversen, T. 2009. Basic Elements of Toxicology. General, Applied and Systems Toxicology. .

Author Information

  1. 1

    Independent Consultant in Occupational and Clinical Toxicology, Charleston, West Virginia, USA

  2. 2

    Edentox Associates, Pinehurst, Edenbridge, Kent, UK

  3. 3

    National Poisons Information Service (Birmingham Centre) and West Midlands Poisons Unit, Birmingham, UK

  4. 4

    University of Central Lancashire, Lancashire School of Health and Postgraduate Medicine, Preston, UK

  5. 5

    Norwegian University of Science and Technology, Department of Neuromedicine, Faculty of Medicine, Trondheim, Norway

Publication History

  1. Published Online: 15 DEC 2009

1 Introduction

  1. Top of page
  2. Introduction
  3. Historical Development of Toxicology
  4. Definition and Scope of Toxicology
  5. Descriptive Terminology of Toxic Effects
  6. Morphological and Functional Nature of Toxic Effects
  7. Dosage–Response Relationships
  8. Factors Influencing Toxicity
  9. Biohandling as a Determinant of Systemic Toxicity
  10. Routes of Exposure
  11. Exposure to Mixtures of Chemicals
  12. Toxicology of Drugs
  13. Nature, Design and Conduct of Toxicology Studies
  14. Review of Toxicology Studies
  15. Hazard Evaluation and Risk Assessment
  16. Special Considerations in Human Hazard Evaluation
  17. Professional and Ethical Issues
  18. References
  19. Further Reading

Toxicology, essentially addressing the potentially harmful effects of chemicals to living organisms, is now a universally recognized scientific and medical discipline devoted to a large and widespread number of basic and applied issues. Although only generally accepted as a specific and defined area of knowledge and investigation since the early part of the twentieth century, its principles and implication have been appreciated for aeons. Thus, the harmful and lethal effects of certain substances, plants, fruits, insect bites, animal venoms and minerals, have been known since prehistoric times. Indeed, the Greek, Roman and subsequent civilizations knowingly used certain substances and extracts for lethality in hunting, protection, warfare, suicide and homicide. Currently, activities in toxicology are mainly centred around, though not exclusively, determining the potential for chemicals, both naturally occurring and synthetic, to produce adverse effects, and as a consequence to assess hazard and risk from such chemicals to humans and lower animal forms, thus allowing the development of appropriate precautionary, protective, restrictive and therapeutic measures. For example, substances used, or of potential use, in commerce, the home, the environment and medical practice may present variable types of harmful effects, whose nature is determined by many factors, including particularly the physicochemical characteristics of the material, its potential to interact with biological materials and the pattern of exposure.

Toxicology investigations can have far-reaching implications for health-related issues in the workplace, commerce, home and general environment. For man-made and man-used materials, a balanced critical approach may be necessary in order to assess the risk–benefit ratio for their employment in specific circumstances, and to determine what precautionary and protective measures are needed for safe use. Indeed with drugs, pesticides, industrial chemicals, food additive and cosmetic/personal care preparations, mandatory toxicology testing and regulations exist. In the UK, safety evaluation toxicology has been closely associated with pathology and experimental pathology, but in many other countries, including the USA, toxicology has been regarded as a component or branch of pharmacology. However, with the growth of toxicology it has become a major multidisciplinary science with significant overlap into other health-related science.

2 Historical Development of Toxicology

  1. Top of page
  2. Introduction
  3. Historical Development of Toxicology
  4. Definition and Scope of Toxicology
  5. Descriptive Terminology of Toxic Effects
  6. Morphological and Functional Nature of Toxic Effects
  7. Dosage–Response Relationships
  8. Factors Influencing Toxicity
  9. Biohandling as a Determinant of Systemic Toxicity
  10. Routes of Exposure
  11. Exposure to Mixtures of Chemicals
  12. Toxicology of Drugs
  13. Nature, Design and Conduct of Toxicology Studies
  14. Review of Toxicology Studies
  15. Hazard Evaluation and Risk Assessment
  16. Special Considerations in Human Hazard Evaluation
  17. Professional and Ethical Issues
  18. References
  19. Further Reading

As noted in Section 1 above, toxicology in formal terms has been regarded as a relatively young science. However, the origins of toxicology are ancient, and it is likely that man, unknowingly, undertook the first experiments in toxicology in searches for an acceptable diet on moving out of the habitat in which he evolved. It is likely that many of these early excursions had an unfortunate outcome. In Greek and Roman periods, poisons, generally of plant origin, were used for murder and suicide, whilst the potential danger of medicinal products and their adulterants has been recognized since Babylonian times. Poisoning for nefarious purposes has remained a problem ever since, and much of the earlier impetus to the development of toxicology had been primarily forensic. Another motivation for the development of toxicology was the careful description of adverse reactions to medicinal products that began to appear in the eighteenth century. Thus William Withering described digitalis toxicity in 1785, and around 1790 Hahnemann, the founder of homoeopathy, carried out toxicological experiments on himself and his healthy friends with therapeutic agents of his time, including cinchona, aconite, belladonna, ipecacuanha and mercury. The introduction of anaesthesia was followed by formal enquiries into sudden deaths during chloroform anaesthesia in the closing years of the nineteenth century.

During World War I, a variety of poisonous chemicals were used in the battlefields of northern France and Belgium. This was the stimulus for much work on mechanisms of toxicity, as well as medical countermeasures to poisoning, In fact war, or the prospect of war, played as great a part in the development of toxicology as of many other sciences. Much of the basic work on organophosphates (OPs) was stimulated by the discovery of these compounds by the Germans in the 1930s. Although defence considerations stimulated this work, much of it, particularly related to treatment, is applicable to the use in industry and agriculture of OP pesticides. Similarly, chelation therapy, initially studied in relation to organic arsenicals, is now used in the treatment of poisoning by many metals.

Occupational toxicology (see Occupational Toxicology and Occupational Toxicology and Occupational Hygiene within the European Union (EU) Chemicals Regulation) originated in the late 1600s and early 1700s with the writings of Bernardino Ramazzini, a professor of medicine in the Universities of Modena and Padua, and referred to as the father of occupational medicine. It gained impetus in the nineteenth century as a product of the industrial revolution, with early descriptions of occupational diseases induced by chemicals, such as cancer of the scrotum in chimney sweeps. Although in theory, but much less in practice, affected workers in the past had some remedies at law, major advances in the control of occupationally related diseases of chemical origin came in the period after 1960 with the setting of threshed limit values (TLVs) and occupational exposure limits (OELs). Additionally, in Western countries the increasing wealth of workers and the activities of their Unions have enabled them to make use of existing legal remedies and make representation in respect of needed improvements in occupational litigation. This has resulted in many companies having to take care of their workers and devote greater resources to industrial hygiene and occupational medicine. However, there are still clear needs for improvement in underdeveloped countries and with a few employers in Western countries.

Regulatory toxicology (see Regulatory Toxicology) has its origins in the development of the chemical and pharmaceutical industries in the nineteenth and twentieth centuries. The first toxicology regulation is often said to be the Alkali Act of 1863 (UK, 1863), which was intended to control pollution by heavy industry. Regulatory toxicology now accounts for a very large majority of expenditure on toxicology testing and administration. Toxicology has only come of age as a defined scientific discipline over the past few decades, as concern for worker and consumer health, and for the adverse environmental impact of xenobiotics, increased. Additionally, the growth of toxicology has been fuelled by a series of disasters such as Seveso, Bhopal, tri-ortho-cresyl phosphate (TOCP) poisoning incidents, methyl mercury incidents and the thalidomide tragedy, which threw up lacunae in knowledge concerning the toxic effects of substances, as well as the inadequacy of testing procedures. One of the earliest of such disasters occurred in the USA in 1937 and resulted in the deaths of 105 individuals from poisoning by an elixir of sulfanilamide containing the solvent diethylene glycol (DEG) (Calvery and Klumpp, 1939). This led to the passing of legislation in the form of the US Federal Food, Drug and Cosmetic Act, forbidding the marketing of new drugs until cleared for safety by the US Food and Drug Administration (FDA). Regulations have been elaborated at national, continental (European Union (EU) and North Atlantic Free Trade Area (NAFTA) and international levels. It is therefore disappointing and frustrating to note that contamination of medicinal formulations with DEG and resultant large-scale outbreaks of poisoning with acute renal failure, often with mortalities, has occurred on several occasions up to the present time, since the original poisoning outbreak in 1937. Most of these have been recorded in children from countries having generally lower standards of human care, and have included the following: Cape Town, 1969 (seven deaths from DEG-contaminated sedative formulations; Bowie and McKenzie, 1972); Bombay, India, 1986 (14 deaths from DEG-contaminated glycerine: Pandya, 1988); Nigeria, 1990 (47 deaths from DEG-adulterated paracetamol (acetaminophen) syrup: Okuonghae et al., 1992); Dhaka, Bangladesh, 1990–1992 (236 deaths of 339 children having acute renal failure from DEG-containing paracetamol (acetaminophen) elixir: Hanif et al., 1995); Haiti, 1996 (85 deaths in a follow-up group of 87 cases of acute renal failure from DEG-contaminated paracetamol (acetaminophen) syrup: O'Brien et al., 1998); Gurgaon, India 1998 (acute renal failure with 33 deaths from contaminated cough expectorant: Singh et al., 2001); New Delhi, India (encephalopathy and renal failure in 11 children from DEG-contaminated paracetamol elixir; Hari et al., 2006). In Argentina, during 1992, there were 15 deaths of 29 victims of poisoning from the use of DEG-containing propolis syrup (for upper respiratory infection) (Ferrari and Giannuzzi, 2005). A series of 64 Chinese adult patients (Guangdong Province) with severe liver disease received intravenous (iv) Armillarisin-A (3-acetyl-5-hydroxylmethyl-7-hydroxycoumarin) that contained DEG as solvent. Of these, 15 had confirmed DEG poisoning, of whom 12 died; findings in the poisoned patients included metabolic acidosis, acute renal failure and acute renal tubular necrosis with interstitial nephritis on renal biopsy (Lin et al., 2008). Subsequently five employees of the Qiqihar No. 2 Pharmaceutical Company of Heilongjiang Province, who were responsible for allowing DEG to be used in the production of Armillarisin A, were jailed for between four and seven years (EAASM, 2008). With respect to all these cases of lethal human DEG poisoning the lessons are clearly there for all, but unfortunately unlearned by many. In some cases the DEG contamination was accidental, but in others was financially driven. DEG, an inexpensive solvent, was more profitable to use than the more expensive propylene glycol or glycerine. Stricter pharmaceutical manufacturing oversight and enforcement is required on a worldwide basis (Wax, 1996).

The main international organizations currently regulating chemicals are the Codex Alimentarious Commission (CAC), and its committees, for food standards, and the Organization for Economic Cooperation and Development (OECD) for the standardization of test methods. The International Conferences on Harmonization (ICH) and the Veterinary International Conferences on Harmonization (VICH) have attempted to harmonize test requirements for pharmaceuticals, both human and veterinary. Because of the tendency for new test methods to be introduced, often without eliminating older tests, regulations have been inclined to become ever more complex, with the result that the cost of laboratory toxicology testing has become a significantly large administrative and economic segment of product development. However, more recently introduced test methods do not always imply added costs; thus the introduction of in vitro genotoxicity studies may permit the avoidance of costly and time-consuming long-term in vivo carcinogenicity bioassays. However, the complexity of toxicological regulations may imply, not only an effect on the profits of companies developing the chemical or drug, but also loss of potentially useful substances. In some cases this has led to sufficient disquiet for legislative action to be taken. Examples of this are the ‘orphan drug’ procedure in the USA, and the clinical trials exemption in the UK. Industrial effects of the increasing complexity of regulatory activity have been the need to establish regulatory departments within chemical and pharmaceutical organizations for the interpretation of, adherence to, and oversight of regulations, and to establish discussion channels with appropriate government departments on safety issues related to company products. Also, for reasons of size and costs, many companies have chosen not to develop their own toxicology testing facilities, or abandon or significantly reduce their own ‘in-house’ facilities. This has resulted in the significant growth of the contract toxicology testing industry.

Organochlorine insecticides probably averted an epidemic of typhus at the end of World War II, but it was the persistence of these compounds in the environment that was probably the greatest stimulus to the evolution of environmental toxicology. A major landmark in the evolution of this branch of toxicology was the publication by Rachel Carson of Silent Spring (Carson, 1962).

The past decades have seen a gradual shift in the emphasis of toxicology from its origins in acute toxicity, particularly human, to long-term and nontarget species toxicity. More or less in parallel, stress has changed from studies of natural, usually plant, compounds to products of chemical synthesis. Additionally, in recent years considerable resources have gone into testing for carcinogenic potential, whilst there have been extensive and intensive investigations into in vitro alternatives to animal toxicology testing.

Clinical toxicology, concerned with the causation, diagnosis and management of poisoning was originally under the control of general physicians in general hospitals. Much of the impetus for the subsequent development of clinical toxicology came from the activities of government defence research establishments. For example, chelation therapy for heavy-metal poisoning was discovered during searches for a method to treat organic arsenical poisoning during World War II, while oximes for OP poisoning were developed during the ‘cold war’ in the 1950s and 1960s (see above). As a distinct specialization, clinical toxicology is relatively new domain, having developed as a consequence of the fact that general physicians may not necessarily have access to the information required to treat their patients. Currently clinical toxicology has its own certification requirements, specialized societies and journals specifically devoted to the specialization. Poison information services developed during the 1950s in the USA and UK, and the concept has since spread throughout the world. Thus poisons information centres, which have access to information on thousands of drugs and chemicals that people may become accidentally or deliberately poisoned with, are to be found in major cities in most developed countries. In many cases, units exist, not only to back up clinicians with information and to act as a general advisory service to the general population, but also to carry out hands-on management of poisoning (see also Susceptibility of Children to Environmental Xenobiotics; Ethical, Legal, Social and Professional Issues in Toxicology).

A recent development was the recognition that differing toxicology requirements may be a barrier to free trade. Within major trading blocks such as NAFTA and the EU it has been necessary to elaborate common toxicological requirements for clearance of materials, while the agreement on the application of sanitary and phytosanitary methods (‘SPS agreement’, WTO, 1994), achieved after tortuous negotiations in 1994, requires that, in most circumstances, CAC maximum residue levels (MRLs) for food additives, contaminants and pesticides, be accepted for world trade purposes. This inevitably raised the profile of the international expert committees, such as the Joint Expert Committee on Food Additives (JECFA) and the Joint Meeting on Pesticide Residues (JMPR).

Major, extensive and rapid developments in the scientific basis of toxicology and its practical applications have been obvious since the early 1950s. These developments have been a consequence of a variety of reasons, the major elements of which are listed in Table 1. Reflecting these developments has been a major proliferation in the number of textbooks and journals devoted to general and special aspects of toxicology; a proliferation of abstracting and information services; the provision of undergraduate and graduate courses and of certification requirements in general and applied toxicology; and the establishment of an industry devoted to toxicology testing and consultation. Along with these has been an increase in the number of professional organizations and certification boards specifically devoted to toxicology. As a consequence of the markedly expanded scope of toxicology, a number of circumscribed differing subdisciplines have emerged, and several major areas of subspecialization have been defined (Table 2).

Table 1. Major Driving Forces for the Development and Increasing Specialization of the Scientific Basis and Applications of Toxicology
  • Exponential increase in the number of synthetically produced industrial and domestic chemicals

  • Major increase in the numbers and nature of new drugs, pharmaceutical preparations, tissue-implantable materials and medical devices

  • Increase in the number and nature of pesticides and related products

  • Growing concern about the number of food-additive materials

  • Increasing concern that environmental agents, including pesticides, chemical pollutants and naturally occurring toxins, are contributing to the causation and pathogenesis of diseases, in particular cardiovascular, neurological, pulmonary and neoplastic.

  • Mandatory testing and regulation of chemicals and drugs used commercially, domestically and medicinally

  • Enhanced public awareness of the potential for adverse effects from xenobiotics and naturally occurring chemicals to man, animals and the general environment

  • The potential for interactions between occupational, domestic and environmental exposures and its influence on adverse effects

  • Litigation, principally as a consequence of occupationally related illness, unrecognized or poorly documented product safety concerns and environmental harm. Recently, certain lawyers (of the ambulance-chasing type) are inserting advertisements on television to draw attention of the general public to adverse drug effects and offering their services for litigation against companies and prescribers (a practice that will have detrimental implications for future drug development)

  • Activities of public awareness and ‘watchdog’ groups

Table 2. Major Subspecialties of Toxicology
SpecialtyMajor functional components
LaboratoryDesign and conduct of in vivo and in vitro toxicology testing programmes
RegulatoryAdministrative function concerned with the development and interpretation of mandatory toxicology testing programmes, and with particular reference to ensuring the safe use, handling and transportation of substances used commercially, domestically and therapeutically, and with the development of product-safety literature and labels
ClinicalThe causation, diagnosis and management of poisoning in humans
VeterinaryThe causation, diagnosis and management of poisoning in domestic and wild animals
ForensicEstablishing the cause for death or intoxication in humans, by analytical procedures, and with particular reference to legal processes
OccupationalDetermining the potential for adverse effects from chemicals and other agents in the occupational environment and the development of appropriate screening procedures and precautionary and protective measures
ProductAssessing the potential for adverse effects from commercially produced chemicals and formulations, and development of recommendations on safe patterns, protective and precautionary measures, and development of relevant literature for users
PharmacologicalAssessing the toxicity of therapeutic agents
AquaticAssessing the toxicity to aquatic organisms of chemicals discharged into marine and fresh waters
EnvironmentalDetermining the adverse effects of toxic pollutants, usually at low concentrations, released from commercial, industrial, domestic and natural sources into the immediate environment and subsequently widely distributed by air and water current and by diffusion through soil. It differs from ecotoxicology (see below) in focussing on the effects on individuals
EcotoxicologyDetermining adverse effects and impact from synthetic or natural pollutants on populations, communities, and terrestrial, freshwater and marine animal, vegetable and microbial ecosystems. If differs from environmental toxicology (see above) in that the aim is to integrate the effects at all levels of biological organization from molecular to whole communities, and therefore is a broader discipline.
ToxicologyDetermining the toxicity of substances of biological origin, including plants, animals and pathogenic microorganisms

An excellent informative demonstration on milestones in the history of toxicology has been prepared by Dr. Steven W. Gilbert and Dr. Antoinette Hayes, and is reproduced with their kind permission in Figure 1.

thumbnail image

Figure 1. Milestones of Toxicology. Prepared by Dr. Steven W. Gilbert (Institute of Neurotoxicology and Neurological Disorders, Seattle and Dr. Antoinette Hayes (Northeastern University). Originally published in Toxipedia (www.asmalldoseof.org). (Reproduced with permission from Gilbert, Institute of Neurotoxicology and Neurological Disorders.)

3 Definition and Scope of Toxicology

  1. Top of page
  2. Introduction
  3. Historical Development of Toxicology
  4. Definition and Scope of Toxicology
  5. Descriptive Terminology of Toxic Effects
  6. Morphological and Functional Nature of Toxic Effects
  7. Dosage–Response Relationships
  8. Factors Influencing Toxicity
  9. Biohandling as a Determinant of Systemic Toxicity
  10. Routes of Exposure
  11. Exposure to Mixtures of Chemicals
  12. Toxicology of Drugs
  13. Nature, Design and Conduct of Toxicology Studies
  14. Review of Toxicology Studies
  15. Hazard Evaluation and Risk Assessment
  16. Special Considerations in Human Hazard Evaluation
  17. Professional and Ethical Issues
  18. References
  19. Further Reading

Definitions of toxicology vary according to the relative importance that the defining author or authority ascribe to the component elements. Most toxicology considerations, and hence definitions, are concerned with the effects from natural or synthetic (man-made) substances, including biological toxins and a wide range of commercial and domestic chemicals. Thus, many definitions hinge around chemical toxicology. The central consideration in toxicology is the potential for the agent(s) under investigation to cause adverse (harmful) effects. Thus, in definitions of toxicology, adverse effects imply those that are detrimental to the survival or normal functioning of living organisms (Ballantyne, 1989). In view of this, some authorities and organizations have suggested that an overall definition should include ‘catch all’ causative agents that may produce adverse effects in biological materials. This has led to the inclusion of biological and physical agents in some overview definitions. The core of chemical toxicology is that of a discipline concerned with investigating the potential of chemicals, or mixtures of them, to produce harmful effects in living organisms and determining the implications of these effects; in this chemical-limited concept, chemicals cover naturally occurring substances (from plants, animals and microorganisms) and synthetic (man-made) materials, the latter often being referred to as xenobiotics. One overview definition covering the various elements of toxicology is as follows: ‘Toxicology is a study of the interaction between chemical, biological and physical agents in biological organisms in order to quantitatively determine the potential for these agents to produce morphological and/or functional injury that results in adverse effects in living organisms, and to investigate the nature, incidence, mechanism of production, factors influencing their development, and reversibility of such adverse effects’.

Within the scope of this definition, adverse effects are those that are detrimental to either the survival or the normal functioning of a living organism. Inherent in this definition are the following key elements in toxicology, particularly with respect to chemical toxicology:

  1. Chemicals, or their conversion products, are required to come into close structural and/or functional contact with tissue(s) or organ(s) for which they have a potential to cause injury.

  2. When possible, the observed toxicity (or an end point reflecting it) should be quantitatively related to the degree of exposure to the chemical (the exposure dose). Ideally, the influence of differing exposure doses on the magnitude and/or incidence of the toxic effect(s) should be investigated. Such dose–response relationships are of prime importance in confirming a causal relationship between chemical exposure and toxic effect, in assessing relevance of the observed toxicity to practical (in-use) exposure conditions, and to allow hazard evaluations and risk assessment.

  3. The primary aim of most toxicology studies is to determine the potential for harmful effects in the intact living organism, in many cases (and often by extrapolation) to man.

  4. Toxicological investigations should ideally permit the following characterization of toxicity to be evaluated:

    1. The basic structural, functional or biochemical injury produced

    2. The dose–response relationships

    3. The mechanism(s) of toxicity, that is, the fundamental chemical and biological interactions and resultant aberrations that are responsible for the genesis and maintenance of the toxic response

    4. The factors that may influence the toxic response, for example, route of exposure, species, gender, age, formulation of test chemical and environmental conditions

    5. The development of approaches and methods for the recognition of specific toxic responses

    6. The reversibility of toxic effects, either spontaneously or with treatment procedures.

The word ‘toxicity’ is used to imply the induction of adverse effects and to describe the nature of adverse effects produced and the conditions necessary for their induction, that is, toxicity is the potential for a material to produce injury in biological systems. In some related or subdisciples of toxicology, specific terminology may be used to describe adverse effects; for example, with pharmacologically active and therapeutic agents (‘drugs’) description of adverse or nondesired effects is most appropriately undertaken using certain specific terms, as discussed in Section 11.

Toxicity (i.e. the potential to injure) investigations require clear differentiation from the process of hazard evaluation, which determines the likelihood that a given material will exhibit its known toxicity under particular conditions of use.

4 Descriptive Terminology of Toxic Effects

  1. Top of page
  2. Introduction
  3. Historical Development of Toxicology
  4. Definition and Scope of Toxicology
  5. Descriptive Terminology of Toxic Effects
  6. Morphological and Functional Nature of Toxic Effects
  7. Dosage–Response Relationships
  8. Factors Influencing Toxicity
  9. Biohandling as a Determinant of Systemic Toxicity
  10. Routes of Exposure
  11. Exposure to Mixtures of Chemicals
  12. Toxicology of Drugs
  13. Nature, Design and Conduct of Toxicology Studies
  14. Review of Toxicology Studies
  15. Hazard Evaluation and Risk Assessment
  16. Special Considerations in Human Hazard Evaluation
  17. Professional and Ethical Issues
  18. References
  19. Further Reading

Precision in communication depends on a clear understanding of the definitions of technical and scientific terms in the context of their intended use. This section discusses the derivation and meanings of frequently used expressions in toxicology. A schematic representation of the basis for the general classification of toxic effects is given in Figure 2. Before toxicity can develop, a substance must come into contact with a body surface, such as skin, eye or mucosa of the alimentary or respiratory tract; these are, respectively, the cutaneous, ocular, peroral (po) and inhalation routes of exposure. Other routes of exposure, notably in experimental or therapeutic situations, are subcutaneous (sc), iv, intramuscular (im) and intraperitoneal (ip). Harmful effects that occur at the sites where a substance comes into initial contact with the body are referred to as local effects. If substances are absorbed from the sites of contact, they, or products of their bioconversion, may produce toxic effects in cells, tissues or organs remote from the site of exposure; these remote responses are referred to as systemic effects. Many substances may produce both local and systemic toxicity. Also, since the nature and probability of toxicity developing depends on the number of exposures, this forms an additional general means for classifying toxic effects into those developing after a single (acute) exposure or multiple (repeated) exposures. Repeated exposure toxicity can cover a wide timespan; however, it is descriptively convenient to refer to short-term repeated (not more than 5% of lifespan), subchronic (5–20% of lifespan) and chronic (entire lifespan or the major portion of it). Examples of toxic effects classified according to location and to number of exposures are given in Table 3.

thumbnail image

Figure 2. Schematic basis for the general classification of toxic effects.

Table 3. Examples of Toxic Effects Classified According to Timescale and Location
Exposure timeLocationEffectSubstance
AcuteLocalSkin corrosionMethylamine
  Lung injuryHydrogen chloride
 SystemicKidney injuryPhenacetin
  HaemolysisArsine
 MixedLung injury and methaemoglobinaemiaOxides of nitrogen
Short-termLocalSkin sensitizationEthylenediamine
RepeatedLocalLung sensitizationToluene di-isocyanate
  Nasal septum ulcerationChromates
 SystemicNeurotoxicityAcrylamide
  Liver injuryArsenic
 MixedRespiratory irritation and neurobehaviouralPyridine
ChronicLocalBronchitisSulphur dioxide
  Laryngeal carcinomaNitrogen mustard
 SystemicLeukaemiaBenzene
  Angiosarcoma (liver)Vinyl chloride
 MixedEmphysema and kidney injuryCadmium
  Pneumonitis and neurotoxicityManganese

Additional descriptions of toxicity are by the time to development and the duration of induced effects. Thus they may be described as temporary (reversible or transient) or permanent (persistent). Latent (delayed-onset) toxicity exists when there is a period free from signs following (usually) an acute exposure. Latent toxicity is of particular importance in clinical toxicology since individuals exposed to chemicals of known latency in toxicity should be kept under review in order that any delayed adverse effects may be both promptly recognized and treated. Cumulative toxicity involves progressive injury produced by summation of incremental injury resulting from successive exposures. Examples of toxicity according to the timescale for development and duration of effect are given in Table 4. Effects may also be classified, and described, according to the primary tissue or organ forming the target for toxicity, for example, hepatotoxic, nephrotoxic, neurotoxic, genotoxic, ototoxic, immunologic. A description of toxicity from a material requires inclusion of the following: if effects are local, systemic or mixed; their nature and (if known) mechanism of toxicity; organs and tissues affected and condition of exposure resulting in toxicity (including species, route and number or magnitude of exposure).

Table 4. Examples of Toxic Effects Classified According to the Time for Development or Duration of the Lesion
TimescaleEffectSubstance
PersistentTesticular injuryDibromochloropropane
 Scarring (skin/eye)Corrosives
 Pleural mesotheliomaAsbestos
TransientNarcosisOrganic solvents
 Sensory irritationAcetaldehyde
CumulativeSquamous metaplasiaFormaldehyde
 Liver fibrosisEthanol
LatentPulmonary oedemaPhosgene
 Peripheral neuropathyAnticholinesterase organophosphates
 Pulmonary fibrosisParaquat

5 Morphological and Functional Nature of Toxic Effects

  1. Top of page
  2. Introduction
  3. Historical Development of Toxicology
  4. Definition and Scope of Toxicology
  5. Descriptive Terminology of Toxic Effects
  6. Morphological and Functional Nature of Toxic Effects
  7. Dosage–Response Relationships
  8. Factors Influencing Toxicity
  9. Biohandling as a Determinant of Systemic Toxicity
  10. Routes of Exposure
  11. Exposure to Mixtures of Chemicals
  12. Toxicology of Drugs
  13. Nature, Design and Conduct of Toxicology Studies
  14. Review of Toxicology Studies
  15. Hazard Evaluation and Risk Assessment
  16. Special Considerations in Human Hazard Evaluation
  17. Professional and Ethical Issues
  18. References
  19. Further Reading

The nature and magnitude of a toxic effect depend on many factors, amongst which are the physicochemical properties of the substance, its bioconversion, the conditions of exposure, and the presence of bioprotective mechanisms. The last factor includes physiological mechanisms, such as adaptive enzyme induction, DNA repair mechanisms and phagocytosis. Some of the frequently encountered types of morphological and biochemical injury constituting a toxic response are listed below. They may take the form of tissue pathology, aberrant growth processes, altered or aberrant biochemical pathways, or extreme physiological responses.

  • Inflammation

    This is a frequent local response to irritant chemicals or may be a component of systemic tissue injury. The inflammatory response may be acute with irritant or tissue-damaging materials, or chronic with repetitive exposure to irritants or the presence of insoluble particulate material.

  • Fibrosis

    This production and accumulation of fibrous connective tissue may occur as a consequence of the inflammatory process.

  • Necrosis

    Pathologically used to describe circumscribed death of tissues or cells, necrosis may result from a variety of pathological processes induced by chemical injury, for example, corrosion, severe hypoxia, membrane damage, reactive metabolite binding, inhibition of protein synthesis and chromosome injury. With certain substances, differing patterns of zonal necrosis may be seen. In the liver, for example, galactosamine produces diffuse necrosis of the lobules (Mehendale, 1987), acetaminophen (paracetamol) mainly centrilobular necrosis (Goldfrank et al., 1990) and certain organic arsenicals peripheral lobular necrosis (Ballantyne, 1978).

  • Enzyme Inhibition

    This may decrease biological activity in biologically vital pathways, producing impairment of normal function. The induction of toxicity from enzyme inhibition may be due to accumulation of substrate or to deficiency of product or function. For example, OP anticholinesterases produce toxicity by accumulation of acetylcholine at cholinergic synapses and neuromuscular junctions (Ellenhorn and Barceloux, 1988). Cyanide inhibits cytochrome oxidase and interferes with mitochondrial oxygen transport, producing cytotoxic hypoxia (Ballantyne, 1987).

  • Biochemical Uncoupling

    Agents capable of biochemical uncoupling interfere with the synthesis of high-energy phosphate molecules, but electron transport continues resulting in excess liberation of energy as heat. Thus, uncoupling produces increased oxygen consumption and hyperthermia. Examples of uncoupling agents are dinitrophenol and pentachlorophenol (Williams, 1982; Kurt et al., 1988).

  • Lethal Synthesis

    This occurs when foreign substances of close structural similarity to normal biological substrates become incorporated into biochemical pathways and are then metabolized to a toxic product. A classical example is that of fluoroacetate, which becomes incorporated in the Krebs cycle as fluoroacetyl coenzyme A, which combines with oxaloacetate to form fluorocitrate. The latter inhibits aconitase, blocking the tricarboxylic acid cycle and results, particularly, in cardiac and nervous system toxicity (Albert, 1979).

  • Lipid Peroxidation

    In biological membranes free radicals start a chain of events causing cellular dysfunction and death. The complex series of events includes oxidation of fatty acids to lipid hydroperoxides which undergo degradation to various products, including toxic aldehydes. The generation of organic radicals during peroxidation results in a self-propagating reaction (Horton and Fairhurst, 1987). Carbon tetrachloride, for example, is activated by a hepatic cytochrome P450-dependent mono-oxygenase system to the trichloromethyl and trichloromethyl peroxy radicals; the former radical probably covalently binds with macromolecules and the latter initiates the process of lipid peroxidation leading to hepatic centrilobular necrosis. The zonal necrosis is possibly related to high cytochrome P450 activity in centrilobular hepatocytes (Albano et al., 1982).

  • Covalent Binding

    Electrophilic reactive metabolites covalently binding to nucleophilic macromolecules may have a role in certain genotoxic, carcinogenic, teratogenic and immunosuppressive events. Important cellular defence mechanisms exist to moderate these reactions, and toxicity may not be initiated until these mechanisms are saturated.

  • Oxidative Stress

    Oxidative stress induces injury to cells by excessive production of reactive oxygen species (ROS) having high reactivity against DNA, lipids and proteins (Pérez et al., 2006).

  • Receptor Interaction

    Occurs at a cellular or macromolecular level and involves specific chemical structures modifying the normal biological effect mediated by the receptor; these may be excitatory or inhibitory. An important example is effects on Ca2+ channels (Braunwald, 1982).

  • Endocrine Disruption

    A number of xenobiotics found in both human community and wildlife environments have been shown to have a potential to disrupt endocrine functions. Endocrine disrupting effects of chemicals have a potential to cause reproductive problems and increase the risk for development of endocrine-dependent cancers. Interference with steroid biosynthesis may have adverse effects on sexual differentiation, growth and development (Sanderson, 2006). Additionally, there have been studies on the interactions between xenobiotics and hormone receptors, particularly thyroid hormone, oestrogen and androgen receptors.

  • Immune-mediated Hypersensitivity

    Such reactions stimulated by antigenic materials are particularly important considerations for skin and lung resulting in allergic contact dermatitis, respiratory sensitization and asthma (Cronin, 1980; Brooks, 1983; Bardana et al., 1992; Isola et al., 2008; see also Cytogenetics; Allergic Asthma and Rhinitis: Toxicological Considerations; Assessing Impacts of Environmental Contaminants on Wildlife). Skin-sensitizing potential, a Type VI hypersensitivity reaction, can be investigated by standard in vivo studies in laboratory animals, such as occluded patch tests (Buehler, 1965) and the guinea-pig maximization procedure (Magnusson and Kligman, 1969; Magnusson and Kligman, 1970), and by induction of the response by the local lymph node proliferation assay (Gerberick et al., 2000; Hilton et al., 1998; Kimber et al., 1994; Kimber et al., 2001; Ryan et al., 2008), and where considered necessary confirmed by the human repeat insult patch test. Recently, various changes in epidermal Langerhans cells have been suggested as possibly forming the basis for the development of in vitro assays for assessing skin sensitization potential (Ryan et al., 2005). Respiratory sensitization, mediated by IgE- and Th2-cell responses, cannot be predicted by conventional repeated inhalation exposure studies, but can addressed by studies such as the mouse IgE test and cytokine fingerprinting (Dearman and Kimber, 2001; Dearman et al., 2003; Kimber et al., 2002; Holsapple et al., 2006).

  • Immunosuppression

    Suppression of the immune system by xenobiotics may have important repercussions with respect to increased susceptibility to infective agents and certain aspects of tumorigenesis.

  • Neoplasia

    Agents stimulating neoplasia cause an aberration of tissue growth and control mechanisms of cell division, and result in abnormal proliferation and growth. This is a major consideration in repeated exposure to xenobiotics. The terms tumorigenesis and oncogenesis are general expressions used to describe overall the development of neoplasms; the word carcinogenesis should be restricted specifically to malignant neoplasms. In experimental and epidemiological situations, oncogenesis may be exhibited as an increase in the total number of neoplasms, an increase in specific types of neoplasm, the occurrence of ‘rare’ or ‘unique’ neoplasms or a decreased latency to detection of neoplasms.

Chemical carcinogenesis is a multistage process. Simplistically, the first, and critical, stage is a genotoxic event followed by other processes leading to the pathological, functional and clinical expression of neoplasia. One multistep model that has received much attention is the initiator–promoter scheme (Figure 3). The first stage, that of initiation, requires a brief exposure to a genotoxically active material which results in binding of the initiator or reactive metabolite to cellular DNA; there is a low, or no, threshold for this initiation stage. The second stage, that of promotion, permits the expression of the carcinogenic potential of the initiated cell. Promoting agents have the following characteristics:

  • They need not be genotoxic

  • Pepeated exposure is required after initiation

  • They show some evidence for reversibility

  • They may have a threshold for promoting activity.

thumbnail image

Figure 3. Schematic representation of inter-relationships between initiator and promoter in the two-stage mechanism of carcinogenesis: (A) an initiating dose of a genotoxic carcinogen is not by itself oncogenic; (B) if the initiating dose is followed by multiple applications of an epigenetic promoter, neoplasia results (the classical initiator–promoter relationship); (C) if promotion is delayed following initiation, a neoplastic response occurs indicating a persisting initiating effect; (D) if promoter dosing is infrequent or doses are small, there may not be a neoplastic response or a low tumour incidence, indicating a threshold for the the promoting effect; (E) multiple applications of an epigenetic promoter alone due not result in neoplasia; (F) initiation must precede promotion; (G) a genotoxic carcinogen may act as both initiator and promoter.

Genotoxic initiators may also act in a promotional manner following initiation. Substances causing or enhancing a carcinogenic process may be conveniently described as genotoxic and epigenetic carcinogens; the former are capable of causing DNA injury and the latter exert onocogenic effects by mechanisms (epigenetic) other than direct genotoxicity. Genotoxic materials acting directly with DNA are referred to as primary carcinogens; those requiring metabolic activation are procarcinogens, with the metabolically active electrophile being the ultimate carcinogen. Examples of primary carcinogens include alkylene epoxides, sulfate esters and nitrosoureas; procarcinogens include polycyclic aromatic hydrocarbons, aromatic amines, azo dyes and nitrosamines.

Epigenetic carcinogens include the following differing functional classes: promoters, cocarcinogens, hormones, immunosuppressives and solid-state materials. Cocarcinogens, when applied just before or with genotoxic carcinogens, enhance the oncogenic effect. Various mechanisms may cause enhancement, including increased absorption, increased metabolic activation of procarcinogen, decreased detoxification or inhibition of DNA repair. One group of epigenetic carcinogens are the peroxisome proliferators, which induce liver tumours in experimental rodents. These materials produce hepatomegaly and hepatocyte peroxisome proliferation and induce several liver enzymes, including those of the peroxisomal fatty acid β-oxidation system. Phthalate esters are one class of compound producing peroxisomal proliferation and experimental hepatocarcinogenesis (Rao and Reddy, 1987). A threshold may exist with epigenetic carcinogens, but there is disagreement about the way in which data from studies with epigenetic carcinogens should be analysed for risk-assessment purposes.

5.1 Genotoxicity

Genotoxic chemicals, which interact with DNA and possibly lead to heritable changes, may be conveniently classified as clastogenic or mutagenic.

Clastogenic effects occur at the chromosomal level and are usually visible by light microscopy. They may involve simple breaks, rearrangement of segments, or gross destruction of chromosomes. Severe clastogenic effects may be incompatible with normal function, and cell death occurs. The relevance of chemically induced sublethal cytogenetic effects is not totally understood, but could lead to dysfunction of the reproductive system and other tissues with rapid cell turnover rates (see Genetic Toxicology Testing and its Relevance to Human Risk and Safety Evaluation; Carcinogenesis and Carcinogens that are also Genotoxic).

Mutagenic effects are focal molecular events in the DNA molecule, which involve either substitution of a base pair or deletion or addition of a base. Base-pair transformations (‘point mutations’) may occur by direct chemical transformation, incorporation of abnormal base analogues or alkylation. Addition or deletion of a base will result in a disturbance of the triplet code and hence alteration of the codon sequence distal to the addition or deletion (‘frameshift mutation’). Intracellular DNA repair enzymes are present, but if the capacity of repair mechanisms is exceeded then abnormal coding will be transcribed into RNA and expressed as altered protein structure and possibly function, depending on the molecular segment affected. As noted above, it is considered that in genotoxic carcinogenesis the molecular DNA event of initiation is fundamental to multistage oncogenesis. There is now considerable evidence showing a good correlation (with some test systems) between carcinogenic and mutagenic potential. Thus, the use of certain mutagenicity tests procedures has become widely accepted as a means of screening chemicals for their carcinogenic potential (Krisch-Volders, 1984; Brusick, 1988; see Genetic Toxicology Testing and its Relevance to Human Risk and Safety Evaluation).

Over the past decade there has been considerable interest and major developments in the field of toxicogenomics (Borlak, 2005; Hamadeh and Afshari, 2004; see Reactive Oxygen Species in the Induction of Toxicity). Toxicogenomics is concerned with a study of the complex interactions between the cellular genome, toxic agents, organ dysfunction and the disease state (Gatzidou et al., 2007). Following exposure of an organism to a toxic agent there is a cellular response exhibited as an alteration of the gene expression, which through transcription into messenger RNA (mRNA) results in altered protein synthesis and function. A major technical advance relating to defining molecular events in toxicogenomics is that of microarray technology, which permits the simultaneous analysis of the transcriptional expression level for thousands of individual genes under various physiological conditions (de Longueville et al., 2004). As a molecular complement, post-transcriptional and post-translational events can be investigated using high-throughput proteomics or wider metabolic aspects by metabolomics (Robertson, 2005). These approaches permit a detailed molecular perspective on how an organism responds to stress or xenobiotics. The applications of toxicogenomics have been broadly classified into two overlapping areas: mechanistic research and predictive toxicology (Cunningham, 2006; Cunningham et al., 2003; Gatzidou et al., 2007; Pennie et al., 2000). Transcript profiling is a major factor for understanding a mode of action and the detection of the induction of patterns of gene expression changes, and is the basis for predictive toxicology. Studies indicate that short-term gene expression profiles are likely to have significant potential for predicting carcinogenicity (Ellinger-Ziegelbauer et al., 2004; Fielden et al., 2008). Microarray technology has become a powerful tool to explore the expression levels of thousands of genes or even complete genomes after exposure to xenobiotics, but a major current challenge in toxicogenomics is functional interpretation, linking potentially inter-related alterations in gene expression to conventional toxicological end points (Moggs, 2005; Yu et al., 2006).

5.2 Reproductive and Developmental Toxic Effects

These aspects of toxicology are concerned with, respectively, adverse effects on the ability to conceive and adverse effects on the structural and functional integrity of the conceptus up to and around parturition.

Adverse effects on reproduction may result from a variety of differing mechanisms on reproductive organs and their neural and endocrine control mechanisms (Barlow and Sullivan, 1982). Developmental toxicity deals with adverse effects on the conceptus from the stage of zygote formation, through the stages of implantation, germ layer differentiation, organ formation and growth processes during intrauterine development and the neonatal period. The most extreme toxicity, death, may occur as preimplantation loss, embryo resorption, foetal death or abortion. Nonlethal foetotoxicity may be expressed as delayed maturation, including decreased body weight and retarded ossification. Structural malformations (morphological teratogenic effects) may be external, skeletal or visceral. The preferential susceptibility of the conceptus to chemical (and other environmental) insults in comparison with the adult state is related to: (i) the small numbers of cells and their rapid proliferation rates; (ii) a large number of nondifferentiated cells lacking defence capabilities; (iii) requirements for precise spatial and temporal interactions of cells; (iv) limited metabolic capacity and (v) immaturity of the immunosurveillance system (Tyl, 1988). There is now considerable awareness that functional, in addition to structural, malformations of development may occur, and this is reflected in the increased monitoring in developmental toxicity studies. Malformation from chemical exposure may result from, amongst other mechanisms: (i) genotoxic injury; (ii) interference with nucleic acid replication, transcription or translation; (iii) essential nutrient deficiency and (iv) enzyme inhibition. The most sensitive period for induction of structural malformations is during organogenesis. Functional teratogenic effects may be induced at later stages, particularly neurobehavioural disturbances (Rodier, 1980).

5.3 Pharmacological Effects

These may be induced by drugs and chemicals generally and usually involve interaction between causative agent and cell receptors resulting in suppression or exaggeration of physiological functions expressed normally by the involved receptor(s). This differentiates pharmacological effects from toxic effects, with the latter being, by definition, adverse in nature. The receptor–agent complex is reversible and thus the induced effects are transient. However, marked (potent) pharmacological effects may cause significant physiological hyper- or hypofunction, and in these cases there are borderline or overlapping considerations with toxicity. Pharmacological effects can be a cause of temporary incapacitation or inconvenience in the occupational environment, as well as side-effects of medication. For example, narcosis from acute overexposure to an organic solvent may clearly be of relevance in safe workplace considerations; such a reversible narcosis needs to be differentiated from central nervous system injury resulting from long-term, low-concentration solvent exposure (World Health Organization, 1985). Another important pharmacological effort, particularly from airborne materials in the workplace, is peripheral sensory irritation. Materials having such effects interact with sensory nerve receptors in skin or mucosae, producing local discomfort and related reflex effects. For example, with the eye there is pain or discomfort, excess lacrimation and blepharospasm. Although such effects are warning and protective in nature, they are also distracting and thus likely to predispose to accidents. For this reason, peripheral sensory irritant effects are widely used in defining, along with other considerations, exposure guidelines for workplace environments (Ballantyne, 1984; see The Influence of Temperature on Toxicity).

6 Dosage–Response Relationships

  1. Top of page
  2. Introduction
  3. Historical Development of Toxicology
  4. Definition and Scope of Toxicology
  5. Descriptive Terminology of Toxic Effects
  6. Morphological and Functional Nature of Toxic Effects
  7. Dosage–Response Relationships
  8. Factors Influencing Toxicity
  9. Biohandling as a Determinant of Systemic Toxicity
  10. Routes of Exposure
  11. Exposure to Mixtures of Chemicals
  12. Toxicology of Drugs
  13. Nature, Design and Conduct of Toxicology Studies
  14. Review of Toxicology Studies
  15. Hazard Evaluation and Risk Assessment
  16. Special Considerations in Human Hazard Evaluation
  17. Professional and Ethical Issues
  18. References
  19. Further Reading

6.1 General Considerations

A fundamental concept in biology is that of variability. Individual members of the same grouped species and strain differ to variable degrees with respect to their biochemical, cellular, tissue, organ and overall characteristics. Additionally, within a given individual there is a spectrum of variability in certain features, for example, cell size and biochemical function within a particular cell series. The differences between individuals are, at least in part, a consequence of genetic factors and age. Since toxicity is the result of adverse effects on biological systems, or modifications of defence mechanisms, it is not unexpected that the majority of toxic responses will also show variability between individuals of a given strain. Also, because of genetic and biochemical variability, even larger discrepancies in response will be observed between species. It is axiomatic to the toxicologist that, within certain limits and under controlled conditions, there is a positive relationship between the amount of material to which given groups of animals are exposed and the toxic response, and that the response of a given animal may differ quantitatively from that of other animals in the same dosage group. As the amount of material given to a group of animals is increased, so does the magnitude of the effect and/or the number who are affected. For example, a specific amount of a potentially lethal material given to a group of animals may not kill all of them; however, as the amount of material is increased, so the proportion dying increases. This reflects the variability in the susceptibility of the population studied to the lethal toxicity of the test substance. Likewise, if an irritant material is applied to the skin, as the amount is increased over a given area this is associated with: (i) an increase in the number of the population affected and (ii) an increase in the severity of the inflammation. For the two examples given above, death is an ‘all-or-none’ response (a quantal response), whereas inflammation may be considered from a dose–response viewpoint as having two elements, that is, its presence or otherwise, and the degree of inflammation which represents a continuous (or graded) response. The above considerations, which reflect variability in biological systems, form the basis for the fundamental concept of dose–response relationships in both pharmacology and toxicology, there usually being a positive relationship between dose and response in vivo and in many in vitro test systems.

It follows from the above discussion that the amount of material to which an organism is exposed is one prime determinant of toxicity. The dose–response relationships for differing toxic effects produced by a given material in a particular species may vary. Thus, as discussed later, dose–response relationships have to be carefully interpreted in the context of the effect studied and the particular conditions under which the information was collected.

The word ‘dose’ is most frequently used to denote the total amount of material to which an organism or test system is exposed, and ‘dosage’ defines the amount of material given in relation to a recipient characteristic (e.g. weight). Dosage allows a more meaningful and comparative indicator of exposure. For example, 500 mg of a material given as a po dose to a 250 g rat or a 2000 g rabbit will result in dosages of 2 and 0.25 mg (kg body weight)−1, respectively. It follows that comparative dosing studies should be expressed in dosage units. Dose in most reports usually implies the exposure dose, that is, the total amount of material that is given to an organism by the particular route of exposure, or the amount incorporated into a test system. Another expression of dose is absorbed dose, which is the amount of material penetrating into the organism through the route of exposure. Absorbed dose may show a closer quantitative relationship with systemic toxicity than exposure dose, since it represents the amount of material directly available for metabolic interactions and systemic toxicity. A further expression of dose is target-organ dose, which is the amount of material (parent or metabolite) received at the organ or tissue exhibiting a specific toxic effect of interest. This should be expressed (if possible) in terms of the mechanistically causative molecule (parent chemical or reactive metabolite). Clearly, target-organ dose is a more precise quantitative indication of toxicity than exposure dose, since it is a measure of the amount of material at the site of toxicity, whereas exposure dose is total dose to the organism and only a proportion of this (or a metabolite) will ultimately gain access to the target site(s) for the toxic response. However the estimation of target or organ-tissue dose requires a detailed knowledge of the pharmacokinetics and metabolism of the material. For this reason, most information relates to the exposure dose.

The exposure dose is of practical importance since it reflects the amount of material to which the organism is actually exposed and its relationship to the likelihood of the development of a particular toxic end point, and therefore is of particular use for hazard-evaluation purposes. Absolute target-organ doses, on the other hand, allow a more detailed biological evaluation of toxicity in relation to bioavailable chemical, and when related to exposure dose may be used for rational risk-assessment procedures.

If a material is capable of inducing several differing types of toxicity, the dose (or dosage) of material required to cause the individual effects may differ, with the more sensitive toxic effect appearing at the lower dosages. The first distinct toxicity, at lower dosages, may not necessarily be the most logically significant effect. For example, with epicutaneously applied materials, local inflammation may appear before more sinister systemic toxicity. Conversely, if the most significant toxicity occurs at lower dosages, then other toxicity at higher dosages may be of lesser interpretive significance.

6.2 Expression of Dosage–Response Relationships

As discussed above, with a given population there is a quantitative variability in susceptibility to a chemical by individual members of that population. Thus, with a genetically homogeneous population of animals of the same species and strain, the proportion exhibiting a particular toxic effect will increase as the dosage increases. This is shown schematically in Figure 4 as a cumulative frequency distribution curve, where the number of animals responding (as a proportion of the total in the group) is plotted as a function of the dosage given (as a log10 function). In many instances there is a sigmoid curve, with a log-normal distribution and being symmetrical about the mid-point. This is a typical dosage–response relationship, often loosely referred to as a dose–response relationship. There are several important elements to this curve that require consideration when interpreting its toxicological significance:

  • The majority of individuals responding do so symmetrically around the mid-point (i.e. the 50% response value). The position of the major portion of the dosage–response curve around its mid-point is sometimes referred to as the potency.

  • The mid-point of the curve (50% response point) is a convenient description of the average response, and is referred to as the median effective dosage (ED50) for the effect being considered. If mortality is the end point, then this is specifically referred to as the median lethal dosage (LD50). The ED50 is used for the following reasons: (i) it is at the mid-point of a log-normally distributed curve and (ii) the 95% confidence limits are narrowest at this point.

  • A small proportion of the population, at the left-hand side of the dosage–response curve, respond to low dosages; they constitute a hypersusceptible or hyper-reactive group.

  • Another small proportion of the population, at the right-hand side of the curve, do not respond until higher dosages are given; they constitute a hyposusceptible or hyporeactive group.

  • The slope of the dosage–response curve, particularly around the median value, gives an indication of the range of doses producing an effect. It indicates how greatly the response will be changed when the dosage is altered. A steep slope indicates that a majority of the population will respond over a narrow dosage range, and a shallower slope indicates that a much wider range of dosages is required to affect the majority of the population.

thumbnail image

Figure 4. Sigmoid dosage–response curve for a toxic effect plotted as proportionate response against log10 dosage. The curve is typically symmetrical about the average (50% response) point. The major response (potency) occurs around the average response. The slope of the curve is is determined by the increase in response as a function of incremental increases in dosage. Hyper-reactive and hyporeactive individuals in the group are at the extreme left- and right-hand sides of the curve, respectively.

The shape of the dosage–response curve, and its extreme portions, depend on a variety of endogenous and exogenous factors; the former may include cellular defence mechanisms and reserves of biochemical function. Thus, toxicity may not be initiated until cellular defence mechanisms are exhausted, or a biochemical detoxification path is near saturation. Also, saturation of a biochemical process that produces toxic metabolites may result in a plateau for toxicity.

An important variant of the sigmoid dosage–response curve may be seen with genetically heterogeneous populations, where the presence of an usually high incidence in the hypersusceptible area could indicate the existence of a special subpopulation that have a genetically determined hypersusceptibility to the substance being tested (Figure 5).

thumbnail image

Figure 5. Variant of the sigmoid cumulative dosage–response curve resulting from an enhanced hyper-reactive response. This can represent a genetic variant in a proportion of the population causing an enhanced sensitivity to the toxic effect.

Data plotted on a dosage–response basis may be quantal or continuous. The quantal response is ‘all-or-none’, for example, death. The graded, or variable, response is one involving a continual change in effect with increasing dosage, for example, enzyme inhibition, degree of inflammation or physiological function, such as heart rate. The dosage–response curve is often linearly transformed into a log-probit plot (log10 dose vs. probit response) because it permits the examination of data over a wide range of dosages, and allows certain mathematical procedures (e.g. calculation of confidence limits and slope of response) (Figure 6). Quantal data can also be plotted as a frequency histogram or frequency distribution curve; this is done by plotting the percentage response at a given dose minus the percentage response at the immediately lower dose (i.e. response specific for the dosage). This procedure usually results in a Gaussian distribution (Figure 7), reflecting the differential biological susceptibility of the test organism to the treatment. In such a normal frequency distribution curve the mean ±1 standard deviation (SD) represents 68.3% of the population, the mean ±2 SD represents 95.5% and the mean ±3 SD is 99.7% of the population. It is important to stress that not only will the incidence of the effect of interest vary with dosage, and determine the dosage–response relationship, but also the severity or magnitude of the effect will change with varying dosage. Thus, for any given dosage producing a particular response incidence, those responding may show a difference in the magnitude of the effect.

thumbnail image

Figure 6. Linear transformation of dosage–response data by log-probit plot.

thumbnail image

Figure 7. Relationships between a cumulative frequency distribution curve and a normal frequency distribution curve for quantal data. The cumulative frequency distribution curve shows the proportionate response for each dosage, and the expected total response for any given dosage. The normal frequency distribution curve shows the response specific for a given dosage compared with lower dosages. For the normal frequency distribution curve, the response (c) at any given dosage (e.g. at B) is obtained by taking the total response at that dosage (b) and subtracting the response (a) at the immediately lower dosage.

The absence of a clear dosage–response relationship in a controlled experiment may indicate a nontoxic or nonpharmacological action of the material. For example, an aminoalkyltrialkoxydisilane given by gavage to rats resulted in the following mortalities (expressed as (number dying/number dosed)): 16 g kg−1 (4/5), 8 g kg−1 (0/5), 4 g kg−1 (3/5) and 2 g kg−1 (0/5). Clearly, there was no dosage–response relationship in this study. Necropsy of dying rats showed that polymerization of the material had occurred in the stomach, producing a hard, opalescent, solid mass completely occluding the stomach. Hence, the cause of death was a consequence of mechanical obstruction and nutritional deprivation, rather than intrinsic toxicity.

For drugs, one convenient indication of ‘safety’ often used is the ratio of the median effective dose causing death to that producing the desired therapeutic response (i.e. LD50/ED50); this is frequently referred to as the therapeutic index (TI50). In general, the higher this ratio, the greater is the degree of safety with respect to lethality. However, very considerable caution is needed in applying this information. For example, if the slopes of the dosage–response curves for drug effectiveness and lethality are parallel, then the assumption of an equal therapeutic ratio over a range of dosages and to a majority of the population is justified (Figure 8). If, however, the dosage–response curve for lethality is shallower than that for the therapeutic response (Figure 9), then there will be a decreasing TI at the lower dosages, and the hyperreactive groups may be at greater risk. One approach which can be used to take into account differences in slopes is to calculate the ratio between the dosage causing a 1% mortality (LD1) and that producing near maximum therapeutic efficacy (ED99). This ratio, LD1/ED99 is referred to as the ‘margin of safety’ (Figure 9). A complete appraisal of safety-in-use, of course, also requires considerations on sublethal and long-term toxicity, and at therapeutic dosages the likelihood of side effects and idiosyncratic reactions.

thumbnail image

Figure 8. One simplistic method to assess ‘safety ratios’ for drugs is by comparing the ratio of the therapeutically effective dosage (ED50) to that causing mortality (LD50); the ratio LD50/ED50 is referred to as the ‘therapeutic index’ (TI). For parallel pharmacological effect and lethality dosage–response lines, the TI will be similar over a wide range of dosages, as shown in the figure above. However, nonparallel lines may give misleading conclusions if the TI50 is calculated (see Figure 9).

thumbnail image

Figure 9. The TI50 may give misleading information if the dosage–response lines for pharmacological and lethal effects are not parallel. In the example shown here, there may be a reasonable safety margin based on the LD50 and ED50 considerations. However, due to the shallower slope of the mortality dosage–response line, the TI will be significantly lower at the 1 and 5% levels, and thus the hyper-reactive group may be at greater risk. In this case a better index of safety will be the ratio of LD1/ED99, which is referred to as the ‘margin of safety’.

The slope of the dosage–response relationship, particularly around the mid-point, can be of value for more precisely assessing hazard or potential for overdose situations. Thus, for example, in considering lethality, a steep slope indicates that a large proportion of the population will be at risk over a small range of doses. Likewise, with a material producing central nervous system depression, a steep slope implies that a small incremental increase of dosage may result in coma rather than sedation.

In most cases of acute lethal toxicity, the dosage–response curve is a log-normal cumulative frequency distribution or Gaussian frequency distribution. In a few cases, however, there may be two definite peaks in the frequency-distribution curve; this is distinct from an increase in hyper-reactive groups at the left-hand side of the dosage–response curve and is known as a bimodal distribution; it may reflect different modes of toxicity possibly with differing latencies. Such a bimodal distribution may reflect different modes of lethality toxicity possibly with differing latency. Earlier deaths at the lower dosages, producing the first phase of the bimodal distribution, represent a quantitatively more potent toxicity; those surviving the first phase toxicity may succumb to the higher dosage latent toxicity. For example, with op anticholinesterase, the first deaths are due to the cholinergic crisis resulting from acetylcholinesterase inhibition, and late toxicity may result from delayed-onset peripheral neuropathy. In some cases, log-probit plots will allow the determination of ED50 values for each subgroup in the bimodal distribution.

For many toxic effects, except genotoxic carcinogenesis, there is a dose below which no effect or response can be elicited; this corresponds to the extreme left-hand side of the dosage–response curve. This dosage, below which no effect occurs, is referred to as the ‘threshold dosage’. The threshold concept, a corollary of the dosage–response relationship, is important in that it implies that it is possible to determine a ‘no-observable effect level’ (NOEL), which can be used as a basis for assigning ‘safe levels’ for exposure.

6.3 Applications of Dosage–Response Information

It is important to reiterate that conclusions drawn from dosage–response studies are valid only for the specific conditions under which the information was collected. Within this constraint, dosage–response information allows at least the following:

  1. Confirmation that the effect under consideration is a toxic (or pharmacological) response to the chemical or therapeutic agent. Thus, a positive dosage–response relationship is good evidence for a causal relationship between exposure and the development of toxicity or pharmacological effects.

  2. Quantitative dose–response information allows the determination of an average (median) response, gives the range of susceptibility in the population studied, and indicates where the dosage for hypersusceptible groups is expected.

  3. The slope of the dosage–response curve gives information on the range of effective dosages and the differential proportion of the population affected for incremental increases in dosage. With a shallow slope the range of effective doses is widespread; the proportion of the population additionally affected by incremental increases in dosage is small. In contrast, a steep slope implies that the effective dose for the majority of the population is over a narrow range, and there will be a significant increase in the proportion of the population affected for small incremental increases in dosage.

  4. The shape of the left-hand side of the dosage–response curve may indicate the existence of an unusually high hypersusceptible proportion of the population. This may, for example, indicate a genetically determined increased susceptibility to the chemical or pharmacologically active substance studied.

  5. Quantitative comparison for a specific end point may be made between different materials with respect to average and range of response, particularly if the information has been collected under similar conditions.

  6. The data may allow conclusions on ‘threshold’ or ‘no-effect’ dosages for the response or the determination of those concentrations or doses that do not result in the appearance of any effects due to exposure to the substance tested (NOEL) or do not cause the development of effects regarded as potentially biologically harmful (no observed adverse effect level (NOAEL)). Recently there has been the development of a concept of benchmark dose (BMD) as an alternative to the NOAEL, or compromise between the NOAEL and the lowest observable adverse effect level (LOAEL). The BMD concept, introduced by Crump 1984, is the dose of a material that is required to achieve a predetermined response of a toxicological effect. For quantal data, the BMD involves fitting a dose–response curve to bioassay data, and the dose (or lower bound on the dose) corresponding to a given low-level response on the fitted curve (e.g. a 5 or 10% elevation in incidence over background: BMD05, BMD10) selected as a characterization of the dose level at which a detectable increase in the measured effect of interest occurs (Sand et al., 2008; Rhomberg, 2005). Also cited are the lower 95% confidence limits on the BMD; for example, BMDL05.

For continuous data, it has been standard to define the BMD as corresponding to a percentage change in response relative to background (Sand et al., 2008). Also, for continuous dose–response data, Sand et al. 2006 proposed an approach that defines the BMD as the dose at which the slope of an S-shaped dose–response relationship changes the most in the low dose region. The dose is in a transition region where the sensitivity to chemical exposure may start to change noticeably. After the application of appropriate uncertainty factors (UFs), the BMD is used for the determinations of certain safe dose values, such as acceptable intakes or reference doses (RfDs). The fact that the BMD approach involves uncertainty analysis has been cited as a major improvement over the NOAEL approach (Sand et al., 2008). Another frequently cited dose is the reference dose (RfD), which is an estimate of a daily exposure to the human population (including sensitive subgroups) that is likely to be without appreciable risk of deleterious effects during a lifetime (Barnes, 2000). In general, the RfD is calculated from the NOAEL, derived from animal studies, with the application of UFs and a modifying factor (MF):

  • mathml alt image(1)

In the simplest case, the NOAEL is divided by two Ufs of 10 each, in order to address extrapolation from animal to human and the sensitivity within the human population respectively; in such cases an MF of 1 is used:

  • mathml alt image(2)

As noted above, the derivation has generally involved the application of a 100-fold safety factor to a measure of the threshold for toxicity, such as the NOAEL or BMD. This 100-fold factor is composed of two 10-fold factors allowing for human variability and species differences. It has been proposed that the 10-fold factors can be refined with a chemical-specific adjustment factor (CSAF) when suitable data are available (WHO, 2001). Such refinements would emphasize pathway-related factors associated with variability in kinetics, and derived from databases that quantify interspecies differences and human variability in Phase I metabolism, Phase II metabolism and renal excretion (Dorne and Renwick, 2005).

The above considerations are briefly illustrated in the following section for acute lethal toxicity.

6.4 Dosage–Response Considerations for Acute Lethal Toxicity

Death, a quantal response, is an end point incorporated in many acute toxicity studies, and often used for the calculation of LD50 values. Acute lethal toxicity studies involve giving differing dosages of the test material to groups of laboratory animals of the same strain by a specific route of exposure and under controlled experimental conditions, e.g. with respect to diet, caging, temperature, relative humidity and time of dosing. Mortalities at each dosage are recorded over a specified period of time, usually 14 days. By epicutaneous or respiratory exposure, the exposure time should be stated, since the degree of local injury and the potential for systemic toxicity are a function of this time, as well as the exposure dosage. For routes other than inhalation, the exposure dosage is usually expressed as mass (or volume) of test material given per unit of body weight, for example, ml (kg body weight)−1 or mg (kg body weight)−1. For inhalation, the exposure dose is expressed as the amount of test material present per unit volume of exposure atmosphere: mg m−3 or ppm. Dose–response information collected for differing concentrations of an atmospherically disposed material should be over similar periods of time in order to allow the most meaningful comparisons to be made. Alternatively, the effect of differing inhalation exposure doses can be made by exposing different groups to the same concentration of test substance for various exposure periods; this may allow the calculation of a median time to death (50% response rate) for the population exposed to a specific atmospheric concentration of test material (LT50). By using both of these approaches it is possible to reach conclusions on the differential sensitivity of a population to varying concentrations for a specified period of time, or to differing exposure periods for a given concentration.

Dosage–mortality data usually conforms to the sigmoid cumulative frequency distribution curve (Figure 10A), which may be converted to a linear form using a log-probit plot (Figure 10B). Lethal toxicity is usually initially calculated and compared at a specific mortality level; most frequently used is that causing 50% mortality in the population studied, since this represents the midpoint of the dosage range about which the majority of deaths occur and usually with a symmetrical distribution. This is the median lethal dose for 50% of the population studied (LD50), that is, that dose, calculated from the dosage–mortality data, which causes death of half of the population dosed under the specific conditions of the test. This concept of the LD50 was introduced by Trevan 1927. By inhalation, the reference is the lethal concentration50 (LC50) for a specified period of time (i.e. x h LC50). Other values calculated include the LD5 and LD95, which give statistical indications of near-threshold and near-maximum lethal toxicity, respectively, and the range of doses over which a lethal response may occur.

thumbnail image

Figure 10. Dosage–mortality data plotted (A) as a cumulative frequency distribution curve (% response versus log10 dosage) and (B) linearly transformed by log-probit plot.

Since the LD50, for economical and ethical reasons, is usually conducted with only small numbers of animals, there is a UF associated with the calculation of the LD50 (or LC50 or LT50). This is estimated from the 95% confidence limits; that is, the dosage range for which there is only a 5% chance that the LD50 (or other LD value) lies outside. The 95% confidence limits are narrowest at the LD50 (Figure 11), which is a further reason why this is an appropriate point for the comparison of acute lethal toxicity.

thumbnail image

Figure 11. Dosage–mortality curve fitted with 95% confidence limits. The limits are closest at the ED50 and diverge at the extremes of the dosage response.

The LD50, by itself, is an insufficient index of lethal toxicity, particularly if comparisons are to be made between different materials. The whole of the dosage–response information should be examined, including the slope of the dosage–response line and 95% confidence limits. For example, two materials with differing LD50 values, but overlapping 95% confidence limits are not regarded as being of significantly different lethal toxicity, since there is a statistical probability that the LD50 of one material will be within the 95% confidence limits of the other. However, when there is no overlap of 95% confidence limits, then the materials are considered to have significantly different lethal toxicity at the LD50 level (Figure 12). A particularly important consideration is that of the slope of the dosage–response curve (Figure 13). For example, if two materials have similar LD50 values with overlapping 95% confidence limits and identical slopes on the dosage–response lines (and therefore statistically similar LD10 and LD90 values), they are lethally equitoxic over a wide dosage range (A and B, Figure 12). However, materials having similar LD50 values but differing slopes (and hence significantly different LD10 and LD90 values) may not be considered to be lethally equitoxic over a wide dosage range (A or B versus C, Figure 13). Thus, materials having a steep slope (A or B, Figure 13) may affect a much larger proportion of the population by incremental increases in dosages than is the case with materials having a shallow slope; thus, acute overdose may be a more serious problem affecting the majority population for materials with steeper slopes. In contrast, materials having a shallower slope (C, Figure 13) may present problems for the hyper-reactive groups at the left-hand side of the dosage–response curve, and may occur at significantly lower dosages than for hyper-reactive individuals associated with the steep slope group. It follows from the above that a proper interpretation of acute lethal toxicity information should include examination of LD50, 95% confidence limits, slope and extremes of the dosage–response curve.

thumbnail image

Figure 12. Comparison of the acute lethal toxicity for three compounds based on LD50 data alone. Compounds A and B have overlapping 95% confidence limits, and therefore have comparable acute lethal toxicities. Compound C has 95% confidence limits that are numerically separate from those of A and B, and is thus significantly less lethally toxic than either A or B, based on LD50 considerations.

thumbnail image

Figure 13. The influence of the slopes of dosage-mortality plots on the interpretation of LD50 data. All three illustrative materials (A, B and C) have overlapping 95% confidence limits at the 50% response level, and are therefore of comparable LD50. Materials A and B have parallel dosage–response lines and overlapping 95% confidence limits at the 5 and 95% levels; therefore, these two materials are of comparable acute lethal toxicity over a wide range of dosages. Material C, in contrast, has a shallower slope and significantly different LD5 and LD95 values, and therefore over a wide range of dosages C has a differing lethal toxicity to A and B. With materials A and B, because of the steep slope of the dosage–mortality line, a much larger proportion of the population will be affected by small incremental increases in dosage. With material C, there may be a greater risk for the hyper-reactive group since the LD5 lies at a much lower dosage than for A and B.

It needs to be stressed that dosage–response information requires to be interpreted in terms of the conditions by which it was obtained; the following few examples are used to illustrate the care necessary:

  1. The numerical precision of the LD50 lies only in the statistical procedures by which it is calculated. If an experiment to determine LD50 is repeated at a later time, slightly different dosage–response data may be obtained because of biological and environmental variability, resulting in a different numerical value for the LD50. Therefore, LD50 values should be regarded as representing an order of lethal toxicity under the specific circumstances by which the information was collected.

  2. An important consideration in interpreting the acute hazard from a chemical is the time to toxic effect. Thus, materials of similar LD50, but differing times to death may present different hazards. For example, with substances having similar LD50 and slope values, those having more rapid times to death can be considered as presenting a greater acute hazard. However, those substances with longer latency to effect may have a potential to produce cumulative toxicity by repeated exposure. For example, the acute po LD50 of 2,4-pentanedione in the rat is 0.58 g kg−1 and that of 2,2-bis(4-aminophenoxyphenyl)propane (BAPP) is 0.31 g kg−1, with respective times to death of 2–5 hours and 13–14 days; on this basis, 2,4-pentanedione would be regarded as presenting a greater acute potential hazard than BAPP (Tyler and Ballantyne, 1988).

  3. A more complete interpretation of the significance of LD50 data may require consideration of the cause of death. If differing potentially lethal toxic effects are produced, it is important to know if this can lead to a multimodal dosage–response curve, and thus to differing hazards by immediate or latent mortality or morbidity. Clearly, latency is of importance in clinical toxicology for decisions on immediate medical management and observations and treatment for latent toxicity. For example, tert-butyl nitrite given by acute ip injection to mice has a 30 minute LD50 of 613 mg kg−1 and a seven day LD50 of 187 mg kg−1. The earlier deaths were probably related to cardiovascular collapse and methaemoglobin formation, whereas later deaths were due to liver injury (Maickel and McFadden, 1979).

  4. Acute LD50 data may not be a direct guide to defining lethal toxicity by multiple exposures. Thus, with a material producing significant cumulative toxicity, the acute lethal dose (and dosage) may be significantly higher than that producing death by multiple smaller exposures. For example, the four hour LC50 for trimethoxysilane is 47 ppm (rat); however, for rats given 20 exposures, each of seven hours over four weeks, the LC50 was 5.5 ppm for that time period (Ballantyne et al., 1988). Thus, the potentially lethal vapour concentration of trimethoxysilane for repeated exposure is significantly less than that by acute exposure.

Any investigation into lethal toxicity should attempt to allow the maximum amount of usable information to be obtained. For this reason, acute toxicity studies should be designed not only to determine lethal toxicity, but also to monitor for sublethal and target organ toxicity; this is possible by incorporating into the protocol observations for signs, body weight, haematology, clinical chemistry, urinalysis, gross and microscopic pathology and other specialized procedures as considered appropriate for the material under test. In this way a significantly greater amount of relevant information can be obtained, and the most useful and meaningful information collected to allow a comparative evaluation of acute toxicity and potential hazards, and the potential for cumulative toxicity (Zbinden and Flury-Roversi, 1981).

Detailed discussions on dosage–response relationships and their toxicological and pharmacological relevance have been written by Sperling 1984, Tallarida and Jacob 1979 and Timbrell 1982.

7 Factors Influencing Toxicity

  1. Top of page
  2. Introduction
  3. Historical Development of Toxicology
  4. Definition and Scope of Toxicology
  5. Descriptive Terminology of Toxic Effects
  6. Morphological and Functional Nature of Toxic Effects
  7. Dosage–Response Relationships
  8. Factors Influencing Toxicity
  9. Biohandling as a Determinant of Systemic Toxicity
  10. Routes of Exposure
  11. Exposure to Mixtures of Chemicals
  12. Toxicology of Drugs
  13. Nature, Design and Conduct of Toxicology Studies
  14. Review of Toxicology Studies
  15. Hazard Evaluation and Risk Assessment
  16. Special Considerations in Human Hazard Evaluation
  17. Professional and Ethical Issues
  18. References
  19. Further Reading

With animal studies and human poisoning, the nature, severity, incidence and probable induction of toxicity depend on a large number of exogenous and endogenous factors. Some of the more important are as summarized as follows.

7.1 Species and Strain

Species and strain differences in susceptibility to chemical-induced toxicity may be due, to variable extents, to differences in rates of absorption, metabolic conversions, detoxification mechanisms and excretion. In some cases animal studies may give underestimates, and in other instances overestimates, for acute po toxicity to humans. For example, the acute po LD50 of ethylene glycol has been determined in several laboratory mammals to range from 4.7 to 7.5 g kg−1 (Sweet, 1985–1986a) and that for methanol to range from 5.63 to 7.50 g kg−1 (Sweet, 1985–1986b); however, these chemicals are significantly more lethally toxic to humans, with both having a minimal lethal dosage in humans around 0.5–1.0 g (kg body weight)−1.

7.2 Age

With some substances age may significantly affect toxicity, probably mainly due to relative differences in metabolizing and excretory capacities. In one extensive compilation of LD50 values for drugs to neonatal and adult mammals (Goldenthal, 1971), the ratio (LD50 adult) /(LD50 neonate) varied from <0.02 (for amidephrine) to 750 (for digitoxin).

7.3 Nutritional Status

Nutritional status may significantly influence the level of cofactors and biotransformation mechanisms important for the expression of toxicity, and in this way diet can affect toxicity (Rao and Knapka, 1998). Moreover, diet may markedly influence the natural tumour incidence in animals and modulate carcinogen-induced tumour incidence (Grasso, 1988). Khanna et al. 1988 studied the effect of protein deficiency on the neurobehavioural effects of acrylamide in rat pups exposed during the intrauterine and early postnatal stages. They found acrylamide to be more toxic in protein-deficient hosts, owing to a significant decrease in dopamine and benzodiazepine receptor binding. Based on a two-year study, Hubert et al. 2000 found that a dietary restriction of around 25% is appropriate for Sprague–Dawley rats in toxicity and carcinogenicity assays to improve survival without an impairment of growth and routine clinical chemistry monitors. Feeding is an important factor in the design and interpretation of acute po toxicity studies. For example, Kast and Nishikawa 1981 compared the acute po toxicity of several anti-gastric-ulcer drugs and β-adrenoceptor agonists and blockers; the ratio (LD50 fed)/(LD50 fasted) for rats and mice ranged from 1.3 to 1.47, indicating a higher toxicity in the starved animals. The authors concluded that the greater acute toxicity in the starved animals was due to accelerated gastric emptying and intestinal absorption. The importance of dietary factors in toxicity has been reviewed by Angeli-Greaves and McLean 1981 and Grasso 1988.

7.4 Time of Dosing

Diurnal and seasonal variations in toxicity may relate to similar variations in biochemical, physiological and hormonal profiles. Examples of temporal variations in biological activity include circadian dependence of metabolic adverse effects of cyclosporin (Malmary et al., 1988), toxicity of methotrexate (Marks et al., 1985) and seasonal variations in gentamicin nephrotoxicity (Pariat et al., 1988). Circadian toxicology is discussed in Chronotoxicology.

7.5 Environmental Factors

A variety of environmental factors are known to influence the development of toxicity, including temperature, relative humidity and photoperiod. The influence of temperature may vary between differing chemicals and the effects investigated. For example, colchicine and digitalis are more toxic the higher the temperature (Lu, 1985); in contrast, studies on the behavioural toxicity of the anticholinesterase soman suggest that the lower the temperature, the greater the susceptibility (Wheeler, 1987). The influence of temperature on toxicity is clearly an important consideration for materials used in arctic and tropical areas. The influence of temperature on toxicity is discussed in The Influence of Temperature on Toxicity.

7.6 Dosing Characteristics

The nature, severity and likelihood of inducing toxicity are influenced by the magnitude, number, frequency and profiling of dosing. Thus, local or systemic toxicity produced by acute exposure may also occur by a cumulative process with repeated lower-dosage exposures; also, additional toxicity may be seen with the repeated exposure situations. For example, acute exposure to formaldehyde vapour causes peripheral sensory irritant effects and (with sufficiently high concentrations) injury and inflammatory change in the respiratory tract; short-term repeated vapour exposure can result in the development of respiratory sensitization; longer-term vapour exposure may cause squamous metaplasia and nasal tumours (Wartew, 1983). The relationships for cumulative toxicity by repetitive exposure compared with acute exposure toxicity may be complex, and the potential for repeated exposure cumulative toxicity from acutely subthreshold doses may not be quantitatively predictable. For example, the LC50 for a four hour exposure to trimethoxysilane vapour is 47 ppm; by repeated exposure over a four week period (seven hours a day, five days a week) the LC50 is 5.5 ppm (Ballantyne et al., 1988). In contrast, acute exposure to benzene vapour for 26 hours (95 ppm) or 96 hours (21 ppm) produced severe bone marrow cytotoxicity, whilst a similar exposure dose given over a longer period of time (95 ppm for 2 hours a day for two weeks) produced little toxicity (Toft et al., 1982).

For repeated exposure toxicity, the precise profiling of doses may significantly influence toxicity. For example, with formaldehyde in a four week vapour inhalation study, it was determined that exposure of rats to 10 or 20 ppm by interrupted exposure over eight exposure periods produced more nasal mucosal cytotoxicity than did continual exposures (Wilmer et al., 1987). In a four-week inhalation study with carbon tetrachloride, it was found that interruption of a daily six hour exposure by 1–5 hour periods of nonexposure caused more severe hepatotoxicity than with continuous exposures, but five minute peak loads superimposed on a steady background only slightly aggravated the hepatotoxic effect of carbon tetrachloride vapour (Bogers et al., 1977).

7.7 Formulation and Presentation

For chemicals given perorally or applied to the skin, toxicity may be modified by the presence of materials in formulations that facilitate or retard the absorption of the chemicals. With respiratory exposure to aerosols, particle size significantly determines the depth of penetration and deposition in the respiratory tract (see Neurotoxicology).

7.8 Miscellaneous

A variety of factors in addition to the above may influence the nature and exhibition of toxicity, depending on the conditions of the study; for example, housing conditions, handling and dosing volume. Variability in test conditions and procedures may result in significant interlaboratory variability in results of otherwise standard procedures; for example, LD50 determination (Griffith, 1964; Hunter et al., 1979).

All the above factors need to be taken into consideration during risk assessments for xenobiotic exposures in order to more accurately predict human responses (Aldridge et al., 2003).

8 Biohandling as a Determinant of Systemic Toxicity

  1. Top of page
  2. Introduction
  3. Historical Development of Toxicology
  4. Definition and Scope of Toxicology
  5. Descriptive Terminology of Toxic Effects
  6. Morphological and Functional Nature of Toxic Effects
  7. Dosage–Response Relationships
  8. Factors Influencing Toxicity
  9. Biohandling as a Determinant of Systemic Toxicity
  10. Routes of Exposure
  11. Exposure to Mixtures of Chemicals
  12. Toxicology of Drugs
  13. Nature, Design and Conduct of Toxicology Studies
  14. Review of Toxicology Studies
  15. Hazard Evaluation and Risk Assessment
  16. Special Considerations in Human Hazard Evaluation
  17. Professional and Ethical Issues
  18. References
  19. Further Reading

The induction of systemic toxicity results from a complex inter-relationship between absorbed parent material and conversion products formed in tissues, their distribution in body fluids and tissues, binding and storage characteristics and their excretion. Some of these factors are considered below (see also Figure 14).

thumbnail image

Figure 14. Possible pathways for the fate of a chemical absorbed from the route of exposure.

8.1 Absorption

The absorption of a substance from the site of exposure may result from passive diffusion, facilitated diffusion, active transport or the formation of transport vesicles (pinocytosis and phagocytosis). The process of absorption may be facilitated or retarded by a variety of factors, which include elevated temperature, that increases percutaneous (pc) absorption by cutaneous vasodilation, and surface-active materials, that facilitate penetration. The integrity of the absorbing surface is important; for example, the acute PC LD50 for HCN (solution) is 6.89 mg kg−1 for rabbits with intact skin, and 2.34 mg kg−1 if the skin is abraded (Ballantyne, 1987).

8.2 Biodistribution

Following absorption, materials circulate either free or bound to plasma protein or blood cells; the degree of binding, and factors influencing the equilibrium with the free form, may influence availability for metabolism, storage or excretion. Within tissues there may be binding, storage, metabolic activation or detoxification; binding may produce a high tissue/plasma partition and be a source for slow titration into the circulation following the cessation of environmental exposure. Examples of storage sites include fat for lipophilic materials (e.g. chlorinated pesticides) and bone for fluoride, lead and strontium. The relationship between exposure dose and release rate may be complex; for example, volatile lipophilic materials are generally more rapidly desorbed than nonvolatile lipophilic substances. Permeability of tissues may be modified by tissue-specific barriers; for example, the blood–brain barrier and placenta. This may affect differential toxicity within classes of compounds, for example, neurobehavioural effects produced by organomercurials and, to a lesser degree, with inorganic mercury compounds (Lu, 1985).

8.3 Biotransformation

Metabolism of substances is conveniently classified under the following two major headings (Williams, 1959):

  • Phase I Reactions

    A functional group is introduced into the molecule by oxidation, reduction or hydrolysis.

  • Phase II Reactions

    There is conjugation of an absorbed material or its metabolite with an endogenous substrate.

For many materials there is an initial Phase I reaction to produce materials which are conjugated by Phase II processes. In other instances, only a Phase II process may be utilized. Reactions of a Phase I type include oxidation, reduction and enzymatic hydrolysis; Phase II reactions include conjugation with glucuronic acid, sulfate, glycine and glutathione, and acetylation and methylation. Phase I reactions, particularly, may result in the formation of toxic metabolites from relatively innocuous precursors, that is, metabolic activation. Phase II conjugates are generally more water-soluble than the parent compound or Phase I metabolites and hence usually more readily excreted. With toxic parent compounds, or toxic metabolites, there may be conversion to less toxic products; that is, detoxification has occurred. Examples of metabolic activation and detoxification are given in Table 5. Many activation reactions are catalysed by a cytochrome-P450-dependent mono-oxygenase system, which is particularly active in the liver. Clearly, a major determinant of the likelihood of toxicity developing, and its severity, is the overall balance between the absorption rate of a chemical, its metabolic activation and detoxification, and the excretion of toxic species (see Toxicokinetics).

Table 5. Examples of the Metabolic Transformation of Chemicals
BiotransformationChemicalConversion
DetoxificationCyanideEnzymatic conversion to less toxic thiocyanate
 Benzoic acidConjugation with glycine to produce hippuric acid
 Bromobenzene3,4-Epoxide reactive metabolite is enzymatically hydrated to the 3,4-dihydrodiol or conjugated with glutathione
ActivationCarbon tetrachlorideMicrosomal enzyme metabolic activation to hepatotoxic trichloromethylperoxy radicals
 2-AcetylaminofluoreneN-Hydroxylation to more potent carcinogen
  N-Hydroxyacetylaminofluorene
 ParathionOxidative desulfuration to the potent cholinesterase inhibitor paraoxon

8.4 Excretion

Substances may be excreted as parent compound, metabolites and/or Phase II conjugates. A major route of excretion is by the kidney, and in some cases the urinary elimination of parent compound, metabolite or conjugate, may be used as a means for assessing absorbed dose. Some materials may be excreted in bile and thence in faeces; in such cases there may also be enterohepatic cycling. Certain volatile materials and metabolites may be eliminated in expired air. The excretion of materials in sweat, hair, nails and saliva is usually quantitatively insignificant, but these routes may be of importance for a forensic or industrial diagnosis or confirmation of intoxication (Paschal et al., 1989; Randall and Gibson, 1989). Materials excreted in milk may be transferred to the neonate.

8.5 Biohandling Interactions

The probability of adverse effects developing in response to chemical exposure depends particularly on the magnitude, duration, frequency and route of exposure. These will determine the amount of material to which an organism is exposed (the exposure dose), and hence to the amount of material which can be absorbed (the absorbed dose). The latter determines the amount of material available for distribution and toxic metabolite formation, and hence the likelihood of inducing a toxic effect. Opposing absorption and metabolite accumulation is elimination. Hence, for a given environmental exposure situation, the probability of inducing toxicity, and its magnitude, depend on the relationship between rate of absorption, metabolism (activation and detoxification) and elimination of parent material and metabolites.

The amount of a material in contact with the absorbing surface is one of the principal determinants of absorbed dose. In general, the higher the concentration, the greater is the absorbed dose. However, if mechanisms other than simple diffusion across a concentration gradient are operating, a simple proportionate relationship between concentration and absorbed dose may not be present. In such instances, a rate-limiting factor could result in proportionately smaller increases in absorbed dose for incremental increases in concentration at the absorption site. Also, and in particular when there is absorption by active transport, there may be saturation of the absorption process and a ceiling value.

When there is repeated exposure, the relative amounts of biotransformation products, and the distribution and elimination of metabolites and parent compound, may be different from that following an acute exposure. For example, repeated exposure may induce and enhance mechanisms responsible for the biotransformation of the absorbed material, and thus alter the relative proportions of parent molecules and metabolites (activation and detoxification), and hence the probability for target-organ toxicity. Also, if there is slow detoxification, storage and/or slow excretion, repeated exposures may lead to the accumulation of toxic species and hence a potential for cumulative toxicity.

9 Routes of Exposure

  1. Top of page
  2. Introduction
  3. Historical Development of Toxicology
  4. Definition and Scope of Toxicology
  5. Descriptive Terminology of Toxic Effects
  6. Morphological and Functional Nature of Toxic Effects
  7. Dosage–Response Relationships
  8. Factors Influencing Toxicity
  9. Biohandling as a Determinant of Systemic Toxicity
  10. Routes of Exposure
  11. Exposure to Mixtures of Chemicals
  12. Toxicology of Drugs
  13. Nature, Design and Conduct of Toxicology Studies
  14. Review of Toxicology Studies
  15. Hazard Evaluation and Risk Assessment
  16. Special Considerations in Human Hazard Evaluation
  17. Professional and Ethical Issues
  18. References
  19. Further Reading

The primary tissue or system by which a material comes into contact with the body, and from where it may be absorbed in order to exert systemic toxicity, is the route of exposure. The usual circumstances of environmental exposure are by ingestion (po), inhalation and skin (epicutaneous/pc) or eye contact. Also, for investigational, therapeutic and certain forensic purposes, im, iv and sc injections may be routes of exposure.

The relationship between route of exposure, biotransformation and potential for toxicity may be complex and also influenced by the magnitude and duration of dosing. Materials that undergo hepatic activation are likely to exhibit greater toxicity when given po than if absorbed across the lung or skin, owing to the high proportion of material passing directly via the portal vein following po dosing. In contrast, materials that undergo hepatic detoxification are likely to be less toxic perorally than when absorbed pc or across the respiratory tract. However, in determining the relevance of route to biotransformation and toxicity, both the magnitude and timescale for dosing should be considered. Thus, when a single, large dose (bolus) of a metabolically activated material is given po, its rapid metabolism may result in the immediate development of a severe acute toxicity. However, if the same material is given po at much lower rates (e.g. by dietary inclusion), then there will be slow and sustained absorption, and in such circumstances the rate of generation of the toxic species may approach that resulting from continuous exposure by other routes. With materials that are detoxified by the liver, a slow continuous alimentary absorption will result in an anticipated low toxicity, compared with other routes of exposure. However, a po bolus may result in the detoxifying capacity of the liver being overwhelmed, and unmetabolized material may enter the circulation to initiate. A few comments on specific routes of exposure follow.

9.1 Peroral

If a material is sufficiently irritant or corrosive, it will cause local inflammatory or corrosive effects on the upper alimentary tract. This may lead to, for example, fibrosis, dysphagia and perforation with mediastinitis and/or peritonitis and the complications thereof. Additionally, carcinogenic materials may induce tumour formation in the alimentary tract. The gastrointestinal tract is an important route by which systemically toxic materials may be absorbed (see Peroral Toxicity).

9.2 Cutaneous Contact and Percutaneous Absorption

Skin contact is an important route of exposure in the occupational and domestic environments, and can result in local and/or systemic toxicity. Local effects may include acute inflammation and corrosion, chronic inflammatory responses, immune-mediated reactions and neoplasia. The PC absorption of materials can be a significant route for the absorption of systemically toxic materials (Billingham, 1977; Bronough and Maibach, 1985; see Cutaneous Toxicology), and indeed is now a means for the systemic titration of pharmacologically active materials (Woodford and Barry, 1986). Factors influencing the PC absorption of substances include skin site, integrity of skin, temperature, formulation and physicochemical characteristics, including charge, molecular weight and hydrophilic and lipophilic characteristics (Billingham, 1977; Dugard, 1977; Stuttgen et al., 1982; Kemppainen and Reifenrath, 1990). In addition to enhancing pc absorption by topical epicutaneous contact, some materials may also do so following systemic absorption. For example, ethanol can act as a topical penetration enhancer through solubility and skin-disrupting actions (Cornwell and Barry, 1995; Walters et al., 1997). Also, Brand et al. 2006 demonstrated that ethanol dosed acutely to rats by gavage increased the pc absorption of epicutaneously applied paraquat, N,N-dimethylformamide, and N,N-diethyl-m-tolbutamide.

Although it is well appreciated that pc absorption of materials may occur when they contaminate the skin as liquid or solid, it has been shown that pc absorption can also result from exposure of the skin to vapour. In the majority of situations, absorption by the inhalation route is generally regarded as being significantly greater than by pc absorption from the vapour phase (McDougal et al., 1986; Jacobs and Phanprasik, 1993). When controlled studies are conducted in humans, difference of conduct and interpretive opinion may exist, particularly with the need for artefact-free techniques. For example, Johanson and Boman 1991 conducted a comparative study on human volunteers who were exposed for two hours mouth-only to 50 ppm 2-butoxyethanol, followed by one hour of no exposure, followed by a further two hours of skin-only exposure to 50 ppm 2-butoxyethanol. Using areas under the curve for concentrations of 2-butoxyethanol in finger-prick blood samples, they calculated that approximately 75% of the total uptake of 2-butoxyethanol vapour was absorbed through the skin. Subsequently, Corley et al. 1997 criticized this approach, mainly on the basis that the finger-prick sampling was compounded by locally high concentrations of 2-butoxyethanol at the site of absorption. They conducted a study involving exposure of one arm of human volunteers subjected to 50 ppm [13C2]-2-butoxyethanol vapour (50 ppm) for two hours. Blood samples were taken by finger-prick from the exposed arm and by catheter from the unexposed arm, and analysed for 2-butoxyethanol and its major haemolytic metabolite, butoxyacetic acid. They found the concentration of 2-butoxyethanol in the finger-prick blood samples to be almost 1500 times those taken by catheter from the unexposed contralateral arm. Blood butoxyacetic acid concentrations were found to be within a factor of four of each other for the two sampling techniques, and it was considered that the metabolite was a better indicator of absorption into the systemic circulation. In a physiologically based pharmacokinetic model of a ‘worst case’ scenario (100% body exposure) of an eight hour exposure to 25 ppm vapour, they calculated that only 15–27% of the total uptake of 2-butoxyethanol would be by pc absorption. In contrast, with styrene, measurement of urinary metabolites in workers showed no significant pc absorption of styrene (Limasset et al., 1999).

9.3 Inhalation

The likelihood of toxicity from atmospherically dispersed materials depends on a number of factors, the most important of which include physical state, physicochemical properties, and concentration, time and frequency of exposure. The water solubility of a gas or vapour influences the depth of penetration of a material into the respiratory tract. As water solubility decreases, and lipid solubility increases, there is more effective penetration towards the alveoli. Water-soluble molecules, such as formaldehyde, are more effectively scavenged by the upper respiratory tract.

The penetration and distribution of fibres and particulates in the respiratory tract are determined principally by their size. Thus, in general, particles having a mass medium aerodynamic diameter (MMAD) greater than 50 µm do not enter the respiratory tract; those of diameter >10 µm are deposited in the upper respiratory tract; those having a range of 2–10 µm are deposited in the trachea, bronchi and bronchioles; and only particles whose diameter is <1 µm reach the alveoli. Thus, larger insoluble particles are more likely to cause local reactions in the upper respiratory tract, and the potential for alveolar injury is greater with smaller diameter particles. Fibres have aerodynamic characteristics such that those having diameters >3 µm are unlikely to penetrate the lung. In general, fibres having a diameter <3 µm and length <200 µm will enter the lung. Fibres of diameter >10 µm may not be removed by normal clearance mechanisms. Several studies have indicated that fibres of diameter >1.5 µm and length <8 µm have maximum biological activity (Asher and McGrath, 1976; Stanton et al., 1981). Dust may be a significant cause of lung disease (Conference, 1990).

The likelihood that inhaled substances will produce local effects in the respiratory tract depends on their physical and chemical characteristics (particularly volatility), reactivity with lining fluids, reactivity with tissue components and site of deposition. Depending on the nature of the material, conditions of exposure and biological reactivity, the types of response produced include acute inflammation and injury, chronic inflammation, immune-mediated hypersensitivity reactions and neoplasia. The degree to which inhaled gases, vapours and particulates are absorbed and, hence, their potential to produce systemic toxicity, depend mainly on molecular weight, solubility in tissue fluids, metabolism by lung tissue, diffusion rate and equilibrium state.

A relatively recent, but rapidly expanding area relating mainly, though not exclusively, to the inhalation route of exposure is nanotoxicology. This subject area is concerned with assessing toxicity related to particles in the 10−9 m size. More specific definitions indicate that the size is ≤100 nm in one dimension, or a range of 1–100 nm (Borm and Kreyling, 2004). Nanoparticles currently in use have been made from transition metals, silicon, carbon and metal oxides (zinc dioxide and titanium dioxide) (Dreher, 2004), and in many cases engineered nanoparticles exist as nanocrystals (Murray et al., 2000). Nanoparticles are of technological interest because, for a given particle-type, as the particle size is decreased within the nanoscale range, certain fundamental physicochemical characteristics change, resulting in the development of new and different properties; for example, change from electrically insulating to conducting or insoluble materials becoming soluble (Warheit et al., 2000). Manufactured nanoparticles have physicochemical properties that result in their having unique electrical, mechanical, thermal and imaging properties that are highly desirable in the commercial, medical and environmental areas (Dreher, 2004; EPA, 2003). There are indications from pulmonary toxicology studies that nanoparticles cause enhanced toxicity in comparison with larger-sized particles of similar chemical composition (Donaldson et al., 2001; Lam et al., 2004; Oberdörster, 2000). A substantial proportion of inhaled nanoparticles are likely to deposit in the respiratory tract; ∼30% to >90%, depending on breathing rate and particle size. They readily reach the deep lung, and the alveolar region is the primary site of deposition for nanoparticles in the range 10–100 nm. In addition to compartmentalization within the respiratory tract, dissolution is probably another relevant factor in determining the fate and effects of nanoparticles (Borm et al., 2006). There are indications that these materials may pose a hazard locally to the lung through oxidative stress, inflammation, and tumorigenesis, and also cause systemic effects by redistribution to other organs following pulmonary deposition (Donaldson et al., 2005; 2006). Translocation from lungs has been shown for brain, liver, kidney and spleen (Elder et al., 2006; Geiser et al., 2005), and the rate depends on the physicochemical properties of the nanoparticles, including size, chemical nature, shape and charge (Oberdörster et al., 2002; Kreyling et al., 2006). Studies with nanoparticles require that the material tested has detailed characterization (Murdock et al., 2008), and a suggested prioritized list for physicochemical characteristics is as follows (Warheit, 2008): particle size and distribution (wet state) and surface area (dry state); crystal structure; aggregation status; composition/surface coatings; surface reactivity; method of nanomaterial synthesis; sample purity. Currently many resources, particularly in the USA, Europe and Japan, are being devoted to the development of risk and safety evaluations of nanomaterials, environmental fate, human health effects, workplace and environmental monitoring, and precautionary measures (Stern and McNeil, 2008; Thomas and Sayre, 2005; Thomas et al., 2006a; 2006b). A detailed review of nanotoxicology is given in Nanotoxicology—The Toxicology of Nanomaterials.

9.4 Eye

Local and systemic adverse effects may be produced by ocular contamination with liquids, solids and atmospherically dispersed materials. Local effects include transient inflammation, permanent injury and hypersensitivity reactions. Penetration may lead to iritis, glaucoma and cataract. Also, with pharmacologically or toxicologically potent materials, systemically active amounts of material may be absorbed from periocular blood vessels and/or nasal mucosa following passage down the nasolachrymal duct (Shell, 1982; Ballantyne, 1983; see Ophthalmic Toxicology).

10 Exposure to Mixtures of Chemicals

  1. Top of page
  2. Introduction
  3. Historical Development of Toxicology
  4. Definition and Scope of Toxicology
  5. Descriptive Terminology of Toxic Effects
  6. Morphological and Functional Nature of Toxic Effects
  7. Dosage–Response Relationships
  8. Factors Influencing Toxicity
  9. Biohandling as a Determinant of Systemic Toxicity
  10. Routes of Exposure
  11. Exposure to Mixtures of Chemicals
  12. Toxicology of Drugs
  13. Nature, Design and Conduct of Toxicology Studies
  14. Review of Toxicology Studies
  15. Hazard Evaluation and Risk Assessment
  16. Special Considerations in Human Hazard Evaluation
  17. Professional and Ethical Issues
  18. References
  19. Further Reading

Circumstances involving exposure to several xenobiotics can result in prior, coincidental or successive exposure to these chemicals, and the nature of the toxicity may vary considerably depending on the conditions of exposure. Thus, an evaluation of the hazards from exposure to multiple chemicals can be much more demanding than is the case for a single chemical. In assessing toxicity from mixtures it is important to consider: (i) chemical and/or physical interactions of the individual materials, (ii) the effect that one chemical may have on the absorption, metabolism and pharmacokinetic characteristics of another and (iii) the possibility for interaction between parent compound and metabolites (Ballantyne, 1985). One longstanding descriptive classification for effects produced by binary mixtures of chemicals is as follows:

  • Independent Effects

    Substances qualitatively and quantitatively exert their own toxicity independently of each other.

  • Additive Effects

    Materials with similar qualitative toxicity produce a response that is quantitatively equal to the sum of the effects produced by the individual constituents.

  • Antagonistic Effects

    Materials oppose the toxicity of each other, or one interferes with the toxicity of another; a particular example is that of antidotal action.

  • Potentiating Effects

    One material, usually of low toxicity, enhances the expression of toxicity by another; the result is more severe injury than that produced by the toxic species alone.

  • Synergistic Effects

    Two materials, given simultaneously, produce toxicity significantly greater than anticipated from that of either material alone; the effect differs from potentiation in that each substance contributes to toxicity, and the net effect is always greater than additive.

In assessing the toxicity of mixtures, the following need to be taken into consideration:

  • Possible physical and chemical interaction, which may result in the formation of new substances or groupings, or influence bioavailability

  • Time relationships of the exposure for the various components

  • Route and conditions of exposure

  • Physical and physiological factors affecting absorption

  • Mutual influence of materials and metabolites on biotransformation, pharmacokinetic characteristics and target organ doses of toxic species

  • Relative affinities of the target sites

  • Potential for independent, additive, antagonistic and interactive processes between the various chemical species.

Mixtures may be complex and contain unreacted parent materials, major reaction and degradation products, contaminants and trace additives. It is important to be aware that small quantities of high-toxicity materials may have equal, or greater, significance with respect to adverse health effects than major components. For example, serious consideration needs to be given to repeated exposure toxicity from small quantities of monomer residuals in polymeric materials; for example, ethylene oxide, propylene oxide, vinyl chloride and formaldehyde (Ballantyne, 1989). The contribution to toxicity by trace materials is well illustrated by, for example, the presence of trialkyl phosphorothioate or phosphorothionate impurities in op anticholinesterases (Hollingshaus et al., 1981) such as malathion (JMPR, 1998), and the presence of 2,3,7,8-tetrachlorodibenzodioxin in chlorophenols (Kimbrough et al., 1984).

Many instances of enhancement of toxicity by specific routes are known. Thus, by skin contact, the systemic toxicity of a material may be enhanced by other materials that facilitate pc absorption. For example, the presence of a surface-active material may result in a carrier function, and the presence of a primary irritant may produce local erythema resulting in increased skin blood flow. If the viscosity of a material is increased, this may enhance local or systemic toxicity due to persistence on the skin.

The inhalation exposure dosage of chemicals may be modified by, for example, the presence of sensory irritants or HCN, which can alter the rate and depth of breathing. Some substances may cause anosmia and hence remove an olfactory warning for other inhaled materials. Particulates may absorb other materials that, if inhaled, cause an increased local burden. When trace quantities of highly volatile and toxic materials are present in a substance, they may, depending on the condition of air movement, have a significant influence on toxicity and hazard. For example, if materials containing trace amounts of acrolein are handled in stagnant air conditions, then potentially toxic vapour concentrations of acrolein may develop; in contrast, when there is free airflow, the acrolein vapour concentration may remain low (Ballantyne et al., 1989a). Thus, the degree of ventilation of a space may significantly influence the toxicity of the atmosphere resulting from vapourization of the individual constituents of a liquid mixture.

The endogenous determinants of overall toxicity resulting from exposure to a mixture of chemicals can be very complex. For example, toxicity may be modulated by prior or simultaneous exposure, resulting in enhancement or suppression of metabolic activation or detoxification pathways. The potential for toxicity may depend on the equilibrium state, although this may be continually fluctuating. Modification of toxicity can also result from modulation of pharmacokinetic characteristics, variation in the biodistribution of absorbed materials and metabolites, modifying elimination of the toxic species, and competition for binding sites or receptors. All the above factors will influence the relative and absolute concentration of toxic species at target sites for toxicity. For complex mixture situations, such as environmental considerations, the extremely large number of potential chemical combinations (constituents and concentrations) limit the usefulness of standard toxicity testing for establishing hazard, and modelling approaches have been employed. Most of these models have been constructed on no-interaction assumptions, and rely on concentration-addition or response-addition approaches. Concentration addition is based on an assumption that mixture components contribute to toxicity through a common mechanism of action and an additive effect is obtained from the individual components of the mixture, and thus is regarded as a ‘toxic equivalency’ approach (Altenburger et al., 2000; Safe, 1990). Response addition models are used when mixture constituents have different mechanisms of action, and the combined effects of the constituents are based on the probability that individual constituents will affect the exposed organism (Backhaus et al., 2000; Walter et al., 2002). Combined concentration addition and response addition models can be built into a comprehensive model for noninteracting mixtures (Altenburger et al., 2005; Olmstead and LeBlanc, 2005; Teuschler et al., 2004). However, the concentration-addition and response-addition models have limited application to complex mixtures, since they do not address the problem of component interactions. Toxicokinetic interactions between chemicals result in one chemical altering the effective concentration of another, and toxicodynamic interactions can occur in which one chemical influences the response of the organism to another chemical (Anderson and Dennison, 2004). In order to integrate interactions into models for mixture toxicity, Rider and LeBlanc 2005 developed a mathematical model that combines concentration addition, response addition and toxicokinetic interaction into the toxicity assessment of chemical mixtures. Haddad et al. 2001 developed a physiologically based pharmacokinetic (PBPK) model for the assessment of health risks from chemical mixtures based on an approach to take into account the influence of multichemical pharmacokinetic interactions at a quantitative level.

Detailed discussions on the toxicity and hazard evaluation of mixtures of substances have been presented by the World Health Organization 1981, Murphy 1983, Ballantyne 1985, Bhat and Ahangar 2007 and the National Research Council 1988 (see also Evaluation of Toxicological Interactions for the Dose-Response Assessment of Chemical Mixtures; Toxicology of Chemical Mixtures).

11 Toxicology of Drugs

  1. Top of page
  2. Introduction
  3. Historical Development of Toxicology
  4. Definition and Scope of Toxicology
  5. Descriptive Terminology of Toxic Effects
  6. Morphological and Functional Nature of Toxic Effects
  7. Dosage–Response Relationships
  8. Factors Influencing Toxicity
  9. Biohandling as a Determinant of Systemic Toxicity
  10. Routes of Exposure
  11. Exposure to Mixtures of Chemicals
  12. Toxicology of Drugs
  13. Nature, Design and Conduct of Toxicology Studies
  14. Review of Toxicology Studies
  15. Hazard Evaluation and Risk Assessment
  16. Special Considerations in Human Hazard Evaluation
  17. Professional and Ethical Issues
  18. References
  19. Further Reading

11.1 Undesirable Effects

It has been estimated that in the USA adverse dug reactions affect somewhere in the region of two million patients, and are lethal to about 100 000 individuals annually (Nakamura, 2008). Drugs and medicinal products can produce adverse effects in the usually defined manner of toxicity and, additionally, they can cause prolongation or exaggeration of the known pharmacological effect(s) of the substance and in this respect cause the adverse inconvenience of pharmacological activity. Undesirable and unwanted effects can be classified and described according to the following schemes:

  1. One system divides them into two types: Type A, which are the results of an exaggerated, but otherwise normal, pharmacological action of a drug, such as bleeding induced by anticoagulant drug or hypoglycaemia induced by diabetic drugs; Type B, which are totally aberrant effects not expected from the known pharmacological mode of actions of a drug, such as deafness from streptomycin.

  2. A second system divides them into three groups: dose dependent, as in Type A above, dose independent, which are of an allergic nature involving antigen–antibody reactions, and pseudoallergic reactions, where allergic reactions are mimicked by mediator release due to direct action of the drug or its metabolite on mast cells.

  3. In this approach, specific terms are used to describe grouped effects of an apparently common aetiological factor. They are usually characterized as follows:

    • Side effects: undesirable effects which result from the normal pharmacological actions of the drug.

    • Overdosage: implies that toxicity will occur by dosing in excess of that recommended for the desired pharmacological effect.

    • Intolerance: implies that the threshold dose to produce a pharmacological effect is lowered; this may be a consequence of a genetic abnormality.

    • Idiosyncrasy: an abnormal reaction to a drug due to an inherent, frequently genetic, anomaly.

    • Secondary effects: those arising as an indirect consequence of the pharmacological action of a drug.

    • Adverse drug interactions: adverse effects produced by a combination of drugs, but not seen when the drugs are given separately at the same dose.

Some of the terms used in this third scheme are difficult to define, and particular reactions may difficult to classify into a single one of them. Also, there is clearly bridging with some descriptive classes, notably intolerance and iodiosyncrasy with respect to genetic influences. Amongst the potential causes for adverse drug reactions, genetic variants are a likely large reservoir, and this has given rise to the study of pharmacogenomics (Huang et al., 2006; Nakamura, 2008). This specialization has as its main objectives investigation of the influence of genetic variability on bioresponse to drugs, identification of the gene loci responsible for adverse drug reactions and the design of screening procedures for identifying susceptible individuals. Genes currently known to be associated with adverse drug reactions have been classified into three major categories: drug-metabolizing enzymes, drug transporters and human leucocyte antigens (HLAs); those in the first two categories influence pharmacokinetics and those in the latter may possibly influence cellular immune response (Wilke et al., 2007). Illustrative examples are as follows. Antituberculosis drugs, including isoniazid, are known to be hepatotoxic, and it has been proposed that the production and elimination of toxic metabolites is dependant on the activities of several enzymes, including N-acetyl transferase 2, cytochrome P450 oxidase (CYP2E1) and glutathione S-transferase. There is evidence that polymorphisms at the loci for these enzymes could modify the activities, and hence the risk for hepatotoxicity. The prevalence of polymorphisms varies geographically, and correspondingly the risk for hepatotoxicity differs in populations; thus, knowledge of the polymorphisms at these loci might be useful in evaluations for, and in controlling, hepatotoxicity (Roy et al., 2008). Decreased drug metabolism and clearance can increase the body load of a drug or metabolite to a toxic level, and thus result in an adverse reaction. For example, genetic variants in the genes encoding thiopurine-S-methyl transferase and uridine diphosphate glucuronosyl transferase 1A1 are known to reduce the enzymatic conversion of azothioprine and irinotecan, respectively, and increase the risk for myelotoxicity associated with therapy (Nakamura, 2008). HLA molecules may involve interaction between a certain HLA molecule and a drug, or metabolite, and trigger a cellular immune repose. For example, there is a strong association in Han Chinese between HLA-B 1502 and the Stevens–Johnson syndrome induced by carbamazepine (Chung et al., 2004).

11.2 Factors Influencing Drug Toxicity

Factors influencing dose-dependent drug toxicity include formulation, route of administration, pregnancy, age, genetic polymorphism of metabolism, environmental influences on metabolism, renal and hepatic excretion, disease, drug interactions and patient compliance. Genetic factors have been considered above in Section 11.1 Nutritional status can affect both dosing efficacy and toxicity (Thomas et al., 1998). Additionally, certain common foods or drinks may interact with drugs or metabolites or modify their metabolism. One well-known example is the interaction of monoamine oxidase inhibitors with food rich in tyramine, causing systemic accumulation of amines and hypertensive crises (Lloyd, 1991). Another, but as yet not as widely appreciated, finding is drug interactions with grapefruit juice (Fuhr, 1998). A major factor in this interaction is suppression of the cytochrome P450 enzyme CYP3A4 in the wall of the small intestine, resulting in diminished first-pass metabolism with higher bioavailability and increased maximum plasma concentrations of the drug substrates. The P-glycoprotein pump, found in the brush border of the intestinal wall, transports many of the cytochrome P450 3A4 substrates, has also been noted to be inhibited by grapefruit juice. A consequence of inhibiting these enzyme systems is an alteration in the pharmacokinetics of several drugs leading to an elevation of their plasma concentrations (Kiani and Imam, 2007). The inhibition of intestinal CYP3A4 was, apparently, was first noted in an interaction study of felodipine with ethanol, where grapefruit juice was used to ‘blind’ for the administration of ethanol (Bailey et al., 1989). It was noted that felodipine concentrations were considerably higher than those previously reported for the dose used. In addition to increasing the bioavailability of drugs, grapefruit juice may prolong the metabolic elimination of some drugs (Fuhr et al., 1993; Bailey et al., 1994). The interaction between drugs and grapefruit juice is marked for felodipine, nimodipine and saquinavir, for which there are marked increases in the area under the curve (AUC) and/or maximum plasma concentrations greater than 70% of controls (Fuhr, 1998). Major drugs implicated in grapefruit interactions include dihydropyridine calcium antagonists, verapamil, terfenadine, cyclosporin, ethinyloestradiol, 17β-oestradiol, prednisone, midazolam, triazolam, quinidine and saguinavir; all these materials share the property of having significant first-pass metabolism by cytochrome P450 AL/5, predominantly in the gut wall, resulting in Phase I metabolites (Fuhr, 1998). It has been suggested that psoralens, principally 6′,7′-dihydrobergamottin, are major inhibitors, with a possible contribution from naringenin (Edwards et al., 1996; Runkell et al., 1997). It is proposed that patients be advised to refrain from drinking grapefruit juice when taking a drug that is extensively metabolized, unless a lack of interaction has been demonstrated (Fuhr, 1998).

In many countries, evidence of adverse drug reactions is sought in normal volunteers and patients in all phases of clinical trial leading up to licensing and marketing of a new product. Postmarketing surveillance (PMS) is then carried out to assess long-term safety in thousands of patients in order to detect low-frequency reactions that were not recognized in the relatively small number of patients studied in premarketing trials. There are a variety of PMS schemes, including voluntary reporting of possible cases of drug reaction, while standard epidemiological techniques, such as retrospective studies, prospective cohort studies and case–control studies may be useful.

12 Nature, Design and Conduct of Toxicology Studies

  1. Top of page
  2. Introduction
  3. Historical Development of Toxicology
  4. Definition and Scope of Toxicology
  5. Descriptive Terminology of Toxic Effects
  6. Morphological and Functional Nature of Toxic Effects
  7. Dosage–Response Relationships
  8. Factors Influencing Toxicity
  9. Biohandling as a Determinant of Systemic Toxicity
  10. Routes of Exposure
  11. Exposure to Mixtures of Chemicals
  12. Toxicology of Drugs
  13. Nature, Design and Conduct of Toxicology Studies
  14. Review of Toxicology Studies
  15. Hazard Evaluation and Risk Assessment
  16. Special Considerations in Human Hazard Evaluation
  17. Professional and Ethical Issues
  18. References
  19. Further Reading

12.1 General Considerations

Toxicology studies should permit, within the constraints of the time period studied, a quantitative determination of the potential for a chemical, or mixture of chemicals, to produce local and systemic adverse effects and allow a determination of factors that may influence the nature, severity and possible reversibility of effects. Specific features that any toxicology testing programme should allow are as follows:

  • The nature of any adverse effects

  • Relationship of the adverse effects to in-use and practical situations

  • Dose–response relationships (average, range, hyper-reactive groups, no-effects and minimum-effects concentrations or dosages)

  • Modifying factors

  • Effects of gross acute overexposure

  • Effects of repeated exposure (short and long term)

  • Definition of allowable and nonallowable exposures

  • Definition of monitoring procedures

  • Guidance on protective and restrictive procedures

  • Guidance on first aid and medical management

  • Definition of ‘at-risk’ populations (e.g. by sex, pre-existing disease, or genetic susceptibility).

Information necessary for the above purposes can be obtained only from carefully designed and conducted studies. In some cases, it may not be economically possible to undertake a complete range of toxicology studies, and in such circumstances it is necessary to carefully consider the most appropriate investigational approaches based on known physiochemical properties, existing and suspected toxicology, and anticipated conditions of use. The relevance and credibility of a toxicology study can be no better than its design and conduct permit. For the purposes of hazard evaluation, there is a need to emphasize exposure conditions that may exist under practical conditions of use.

Toxicology testing programmes generally start with single-exposure in vivo or in vitro studies and progress to investigations evaluating the effects of long-term repeated exposures. Studies having specific end points, such as teratology and reproductive effects, are conducted as the emerging toxicology profile and end-use exposure patterns dictate. Toxicology testing procedures can be conveniently subdivided into general and specific. General toxicology studies are those in which animals are exposed to a test material under appropriate conditions, and are monitored for all types of toxicity that the monitoring procedures permit. Specific toxicology studies are those in which exposed animals, or in vitro test systems, are monitored for defined end point(s).

12.2 General Toxicology Studies

These are usually conducted as a programme in the sequence of acute, short-term repeated, subchronic and chronic. Ideally, the protocol for general studies should include provision for some animals to be kept for a period after the end of dosing in order to determine latency and reversibility, or otherwise, of toxic effects. Acute studies give information on toxicity produced by a single exposure, including the effects of massive overexposure; they also give information of use for setting exposure conditions for short-term repeated-exposure studies. The type of monitoring employed in general toxicology studies will be determined by several considerations, including the chemistry of the test material, its known or suspect toxicology, degree of exposure, and the rationale for conducting the investigation. In general, since multiple-exposure studies are most likely to produce the widest range of toxicity, it is usual to employ the most extensive monitoring in these studies. The monitoring employed to detect functional toxicity and toxicological pathology includes the following:

  • Inspection, on a regular basis, for signs of toxic and/or pharmacological effects

  • Body weight before dosing and at appropriate intervals during the dosing phase

  • Food and water consumption

  • Haematology for assessment of peripheral blood and haematopoietic tissue responses

  • Clinical (blood) chemistry of various substances and of specific enzyme activities, and appropriate urinalysis

  • Gross and microscopic pathology with organ weight measurement

  • Special pathological or functional tests may be required on a case-by-case basis.

12.3 Specific Toxicology Studies

Many of these procedures are directed at determining a specific toxic or pathological effect for hazard-evaluation and risk-assessment purposes, but others are employed as ‘screening’ or ‘short-term’ tests to assess the potential of a substance to induce chronic effects or toxicity with a long latency. Some of the most frequently employed special toxicology methods are listed below.

12.3.1 Primary Irritation

These studies are designed to determine the potential of substances to cause local inflammatory effects, notably in skin and eye (see Ophthalmic Toxicology; Cutaneous Toxicology). In order to reduce the use of animals for eye irritancy testing, a variety of alternative procedures have been proposed, which include the use of enucleated eyes, various in vitro cell or tissue cultures, and the noninvasive measurement of corneal thickness (Nardone and Bradlaw, 1983; Shopsis and Sathe, 1984; Ballantyne, 1986; Borenfreund and Borrero, 1984).

12.3.2 Peripheral Sensory Irritation

Methods to assess the potential to cause eye or respiratory tract discomfort with associated reflexes are particularly useful to perform, notably with respect to occupational toxicology, since such effects may be distracting (Owens and Punte, 1963; Ballantyne et al., 1977; Ballantyne, 1984; see Peripheral Chemosensory Irritation: Fundamentals, Investigation and Applied Considerations).

12.3.3 Immune-Mediated Hypersensitivity

Allergenic materials may produce hypersensitivity reactions by skin contact or inhalation, and several methods are available to determine the potential for chemicals to produce allergic contact dermatitis or respiratory sensitization (Goodwin et al., 1981; Maurer et al., 1984; Karol et al., 1985).

12.3.4 Neurological and Behavioural Toxicity

To confirm the existence, nature, site and mechanism of toxic injury to the central and/or peripheral nervous system, a variety of approaches with varying degrees of sophistication are available (see Neurotoxicology; The Role of Behavioural Toxicity in Risk Assessment). These include observational test batteries (Gad, 1982), light and electron microscopy (Spencer et al., 1980), selective biochemical procedures (Abou-Donia et al., 1987), electrophysiological, pharmacological, tissue culture and metabolism techniques (Dewar, 1981; Mitchell, 1982). The potential to produce delayed polyneuropathy, carried out in hens and sometimes in rodents, and used in the toxicological assessment of OPs and some other neurotoxic substances, has been increasingly refined (Veronesi, 1992; OECD, 1995).

12.3.5 Developmental Toxicity

Most studies are directed at assessing the potential for chemicals to induce structural defects of development, and essentially involve administering the test material to the pregnant animal during the period of maximum organogenesis (Tuchmann-Duplessis, 1980; Beckman and Brent, 1984; Tyl, 1988). However, there has been increasing interest in the development of test methods to assess possible adverse functional effects resulting from exposure of the foetus both during gestation and in the early neonatal period; for example, developmental neurobehavioural toxicology and immunotoxicology (Zbinden, 1981; Vorhees, 1983; see Developmental Toxicology; Developmental Neurotoxicity).

12.3.6 Reproductive Toxicity

Reproductive studies are conducted to assess the potential for adverse structural and functional effects on gonads, fertility, gestation, foetuses, lactation and general reproductive performance. Exposure to the chemical may be over one or several generations. In view of the necessarily comparatively low doses used during these long-term studies, they may not be sufficiently sensitive to detect most potentially teratogenic materials. The basis for these reproductive studies has been reviewed (Mattison, 1983; Baeder et al., 1985; Rao et al., 1987; see Reproductive Toxicology).

12.3.7 Metabolism and Pharmacokinetics

These studies may be of very considerable importance in the interpretation of conventional toxicology studies, in helping determine the mechanism of toxicity, in assessing the relationship between environmental exposure concentration and target organ toxicity, and in the design of additional studies to elucidate mechanisms of toxicity. Metabolic studies should yield information on the biotransformation of a material and the nature of the products, the sites at which this occurs and the mechanism of biotransformation. Pharmacokinetic studies should allow a quantitative determination of the rate of uptake, the absorbed dose, the biodistribution, tissue binding and storage, and the routes and rates of excretion of test material and metabolites (Oehme, 1980; Gibaldi and Perrier, 1982).

12.3.8 Genotoxicity

A number of tests, both in vitro and in vivo, are available to assess the mutagenic or clastogenic potential of chemicals (see Mutagenesis; Cytogenetics; Genetic Toxicology Testing and its Relevance to Human Risk and Safety Evaluation; Short-term Tests for the Determination of Genotoxic and Carcinogenic Potential of Xenobiotics). A positive genotoxic result is not necessarily a directly usable end point per se, but may assist in defining a potential for adverse health effects or be used in screening for potential longer-term toxicity. Thus, materials with clear mutagenic activity may be suspected of being genotoxic carcinogens, and appropriate further studies may be required; clastogenic materials may be suspect of reproductive or haematological toxicity.

The most widely used in vitro mutagenicity test has probably been that described by Ames 1982 and which utilizes histidine-dependent mutants of Salmonella typhimurium. The bacteria are incubated in a medium deficient in histidine; if the added test chemical is genotoxic it causes a reverse mutation to the histidine-independent state, which permits bacterial growth. Since the introduction of this bacterial assay, there have been a host of other mutagenic tests developed, including the use of other strains of bacteria and a variety of in vitro cell-line cultures. For example, a commonly used test system is a forward gene mutation assay in Chinese hamster ovary (CHO) cells with a strain which is deficient in the enzyme hypoxanthine-guanine phosphoribosyl transferase (HGPRT), which confers resistance to toxic purine analogues such as 6-thioguanine and permits growth of the cells in a medium containing such substrates. The presence of a mutant chemical will restore sensitivity to the presence of purine analogues, and this may be used to assess mutagenic potential quantitatively. Clastogenic potential can be assessed in vitro by exposing cultured cells and subsequently examining them by light microscopy for chromosome damage. In vivo tests for clastogenicity involve direct approaches by examining cell preparations from dosed animals for chromosomal damage (cytogenetic studies) or indirect approaches of detecting products from damaged cells such as micronuclei in erythrocytes.

It is usual to conduct in vitro genotoxicity studies in the presence and absence of a metabolic activation system in order to assess the possible influence of metabolism on the genetoxic potential of the test chemical. Frequently employed is a homogenate of liver from animals given the polychlorinated biphenyl, arochlor, which induces a broad range of hepatic P450-metabolizing enzymes.

In vivo genotoxicity studies can be conducted in a variety of ways. For example, the specific locus test in mice involves exposure of nonmutant mice to the test substance and subsequently mating them with multiple-recessive stock. Mutant offspring have altered phenotypes such as hair or eye colour, ear length or hair structure. As noted above, clastogenic potential can be assessed in vivo by exposure to the test chemical and subsequently examining mitotically active tissue, such as bone marrow, for chromosome injury.

12.3.9 Combustion Toxicology

It has been estimated that 50–75% of deaths occurring within a few hours of being exposed to a fire are the result of inhalation injuries and systemic toxicity (Ballantyne, 1981). The primary aim of combustion toxicology is to determine the adverse effects produced as a result of exposure to heated or burning materials. Although considerable emphasis has been placed on acute effects, there is increasing concern about the long-term consequences of repeated exposure to the products of combustion in occupationally exposed individuals, such as fire fighters. The design and interpretation of appropriate studies may be difficult because of the large number of variables that may affect the nature, concentration and temporal variations of combustion products. The major, though not exclusive, factors that influence the toxicity and hazard from a fire atmosphere include the nature of the material available for heating or burning, the phase of the combustion process, temperature, air flow and oxygen availability, and potential for interaction between the combustion materials generated. All of these factors may be required to be investigated and considered in evaluating the continually changing hazard from a fire. Principal lines of investigation and sources of information about toxicity and hazards from fire atmospheres are as follows:

  1. Physicochemical studies to determine the nature of the products of combustion generated under differing conditions of temperature and oxygen availability

  2. Animal exposure studies

  3. Clinical and forensic observations on fire casualties to determine the nature and cause of morbidity and mortality from exposure to a fire atmosphere.

Although investigations designed to investigate the nature and determinants for materials producing local respiratory or systemic toxicity are of clear importance, it is also necessary to be aware of the presence of materials that may produce sensory irritant or central nervous system depressant effects. Clearly, irritant effects on the eye or narcosis may impede escape from a potentially hazardous situation. Polymers, which constitute a major component of commercial and domestic buildings, provide good examples of the generation of toxic, irritant and neurobehavioural chemical species on combustion (Ballantyne, 1989; see Combustion Toxicology and Implications for Adverse Human Health Effects).

12.3.10 Antidotal Studies

In addition to being aware of the likelihood of spontaneous reversibility of toxic injury (i.e. biochemical, physiological and morphological healing), it is of clear practical importance to investigate the induction of reversibility of toxicity by antidotal procedures (Marrs, 1988; see Antidotal Studies). Indications for such studies include high acute toxicity (including dose and time to onset of effects); serious (but potentially reversible) repeated exposure toxicity; where there are indications that early treatment may reduce or abolish latent toxicity; suspicions of a potential for antidotal effectiveness based on considerations of mechanism of toxicity and confirmation that antidotal treatment is effective for a new member of a chemical series for which a generic antidote has been established. Examples of chemicals for which specific antidotal treatment has been investigated include cyanides (Marrs, 1987; Meredith et al., 1993; Ballantyne et al., 2007), ethylene glycol and DEG (Baud et al., 1988; Brent et al., 1999; Buchanan et al., 2008; Velez et al., 2007) and OP anticholinesterases (Arena and Drew, 1986; Ellenhorn and Barceloux, 1988; Bismuth et al., 1992).

In addition to investigating specific antidotal therapy, it may also be necessary to confirm, or otherwise, if standard methods of management and support are appropriate for particular substances or groups of materials. This may include, for example, potential for aspiration hazards, influence of dilution (by giving fluid to drink) and potential for adverse interaction with drugs used to maintain cardiovascular or respiratory homeostasis.

12.3.11 Human Studies

Information resulting from human exposures may be available for certain chemicals, and can be divided into:

  1. That generated experimentally, including volunteer studies and clinical trials

  2. That resulting from and observing exposed populations using epidemiological techniques.

Investigational exposures of human subjects have been carried out to variable extents with a heterogeneous collection of substances, including medicinal products, cosmetics and body-care products, certain pesticides (JMPR, 1999) and industrial chemicals, food additives and chemical warfare agents (Marrs et al., 1996). The types of studies conducted include efficacy determinations with careful detailed monitoring for potential averse effects (clinical studies), pharmacokinetic and metabolism studies, confirmatory (to laboratory animal findings) irritancy and sensitization studies, monitoring of epidemiological studies. Carefully controlled volunteer studies are conducted for the following basic reasons:

  1. To confirm the absence of potentially adverse health effects with substances that by the nature of their use patterns are deliberately brought into contact with specific routes of exposure. These may include, as examples, skin irritation or sensitization tests with cosmetic formulations, or phototoxicty/photoallergy studies with materials dosed orally or epicutaneously.

  2. To investigate the metabolism and/or pharmacokinetics of a material. This may be important where prior studies have shown species variability in the pharmacokinetic or metabolic behaviour, which can be correlated with the development, or otherwise, of a particular toxic end point. In such cases, human volunteer studies can aid in determining which animal model may best represent the human situation, and hence allow a comparative risk assessment for the use of the particular material for the human population (Wilks and Weston, 1990).

  3. To study quantifiable dose–exposure relationships, where a detailed hazard evaluation or risk assessment is required for the in-use application in the human population. One example is with OPs, where an assessment of dose–response and NOEL values are required with respect to cholinesterase inhibition.

  4. To meet national and international governmental regulations where specified.

The conduct of human studies has implications for ethical and potential litigation considerations. Although the objective of a human volunteer study is to obtain information relative to human safety evaluation, the prime concern should be for the volunteers and to ensure that they are not exposed to unacceptable risks (van Gelderen et al., 1990). Clearly, detailed forethought is required to avoid the production of adverse effects that could lead to harm and possible subsequent litigation. Occasionally, clinical drugs trials in human volunteer subjects have been associated with severe reactions; for example, in a small trial involving the testing of an anti-inflammatory agent, the adverse effects were critical and volunteers required treatment in an intensive care unit (BBC, 2006). In addition to study design in order to obtain the most relevant and meaningful information, detailed attention should be given to professional, ethical, regulatory and legal factors. Sass and Needleman 2004 have noted that human studies with low statistical power are not useful for determining the presence or magnitude of adverse effects from xenobiotics, and that industry-sponsored studies may (knowingly or unknowingly) lead to bias in study design, analysis and interpretation. In 2003, the US National Research Council noted that because of potential bias in industry-sponsored and conducted studies they should receive careful scrutiny. At an expert workshop held during 2002 in New York it was noted that even at that time there were no general ethical guidelines for studies of pesticide toxicity conducted in humans and no governmental oversight. The participants developed several ethical and public policy recommendations regarding human testing of pesticides (Oleskey et al., 2004).

In selecting volunteers for human studies it is necessary to ensure their good health, but constraints may be necessary on certain sectors of the population: for example, women of child-bearing age, atopics and those with genetically determined biochemical disorders, such as glucose-6-phosphate dehyrogenase deficiency. Proposals for human volunteer studies should receive the approval of an independent ‘ethics committee’ or ‘institutional review board’ (IRB). They hold responsibility for deciding that the proposals are justified, possible to conduct, that the procedures are acceptable and that the study carries the minimum of risk for the participants. The IRB should be subsequently informed of any changes in the project, if there are any early indications of adverse effects or any other significant untoward events. In general, IRB approval mainly centres on risk–benefit relationships and the adequacy of the informed consent processes (Goldman and Links, 2004). The ethical committee or IRB should be totally independent of the institution or organization conducting the study. It has been proposed that the traditional IRB should be supplemented with an environmental health and community review board that would have an extended ethical construct of dignity, veracity, sustainability, justice and community-based issues (Gilbert, 2006).

The major guidelines under which human volunteer studies are permissible are as follows:

  1. The reasons for conducting the study have been clearly defined, including an understanding of why the study will give information relevant to hazards evaluation and/or risk assessment in the human that may not be possible by nonhuman studies.

  2. The concept in 1 above should be clearly stated in the protocol, which should also contain all technical details, the rationale for the route and mode of exposure, number of volunteer subjects required and monitoring procedures. The protocol should be reviewed and approved by the IRB.

  3. A risk–benefit analysis should ideally be carried out in advance of the study; the smaller the direct benefit to the community, the lower should be the risk to individual volunteers.

  4. The investigators conducting the study should be appropriately qualified, and this is documented.

  5. Compensation to volunteers for participating in a study should be limited to inconveniences and not be an inducement.

  6. Informed consent should be obtained from volunteers who must sign an appropriate consent form that should embrace the following: (i) the purpose of the study, the procedures involved and any potential risks have been comprehensively explained to the volunteers (Melnick and Huff, 2004), (ii) there is a right to refuse to participate without penalty and (iii) it is clearly stated that the volunteer can withdraw at any stage of the study.

It is of importance that the publication policies of medical and scientific journals that accept papers dealing with toxicological research involving the participation of human volunteer subjects should stress compliance with ethical, legal and human rights protection aspects. This accords with the clear intent of Article 27 of the Helsinki Declaration: ‘Reports of experimentation not in accordance with the principles laid down in the declaration should not be accepted for publication’. This qualification on the publication of toxicological research involving human volunteer participation has been adopted by several professional societies who require documentation of compliance with ethical and regulatory guidelines on submission of manuscripts for publication; for example, the Society of Toxicology (Schwetz et al., 2005).

13 Review of Toxicology Studies

  1. Top of page
  2. Introduction
  3. Historical Development of Toxicology
  4. Definition and Scope of Toxicology
  5. Descriptive Terminology of Toxic Effects
  6. Morphological and Functional Nature of Toxic Effects
  7. Dosage–Response Relationships
  8. Factors Influencing Toxicity
  9. Biohandling as a Determinant of Systemic Toxicity
  10. Routes of Exposure
  11. Exposure to Mixtures of Chemicals
  12. Toxicology of Drugs
  13. Nature, Design and Conduct of Toxicology Studies
  14. Review of Toxicology Studies
  15. Hazard Evaluation and Risk Assessment
  16. Special Considerations in Human Hazard Evaluation
  17. Professional and Ethical Issues
  18. References
  19. Further Reading

A critical review of toxicology studies requires detailed case-by-case considerations, but attention should be generally directed to at least the following:

  • That the laboratory or institution reporting the study has the necessary scientific and/or medical credibility, capabilities, experience and expertise in the areas being investigated.

  • The objectives of the investigation should be precisely stated, and the study protocol should reflect this in detail.

  • The work should be reported in a clear and unambiguous manner, with all the necessary detail to allow the reader to undertake his/her own assessment and conclusions about the study.

  • There should have been adequate quality control procedures, and standards appropriate to good laboratory practices (GLPs) should have been followed.

  • The material tested should be precisely specified, including stability and the nature and amounts of any impurities, conversion products or additives.

  • It should be confirmed that the methodology that is used for exposure and to monitor the in vivo or in vitro studies is sufficiently specific and sensitive to allow the various objectives and end points to be determined.

  • Studies should be designed to allow a determination of the significance of the results and permit hazard and risk assessment procedures. For example, the number of test and control animals should be sufficient to allow for the detection of biological variability in response to exposure, to allow trends to be appreciated and to permit statistical analyses. There should be sufficient dose–response information to allow decisions on causal relationships and the magnitude of dosages which produce definite and threshold effects and those not producing toxicity.

  • Monitoring should allow a determination of whether any injury produced is a direct consequence of toxicity or an effect that is secondary to toxicity at another site. A primary effect is one produced as a result of a direct toxic effect of a chemical, or metabolite(s), on a target organ or tissue. Secondary effects are those occurring, often at another, but nontarget, site, as a consequence of toxicity in the primary tissue or organ. For example, primary pulmonary injury produced by inhaled potent irritant materials may result in significant hypoxaemia and secondary hypoxic injury to other organs, including liver, kidney or brain. Ideally the study should be carefully assessed to allow a conclusion as to whether the toxicity induced is a consequence of the action of parent material or metabolite, for example, comparison of routes involving and not involving first-pass effects.

  • Detailed assessment is required to determine if the numerical data have been appropriately and correctly evaluated. Thus, although there may be a statistically significant difference between a test group and the controls, this may not be of biological or toxicological significance. Conversely, changes or trends, not of statistical significance, may be of biological and toxicological relevance. Quantitative information should be viewed against the study as a whole, normal biological variability, quantitative changes which imply pathological processes and the magnitude of any changes as they may relate to an adverse effect.

The above considerations demand the careful design of toxicology studies, taking into account all factors that are inherent in the defined and inferred objectives of the investigation. To illustrate the care required in the interpretation of toxicology studies, a few examples are given below of different specific factors that need attention in particular studies:

  • The acute po LD50 of the undiluted diethylamine is <0.25 ml kg−1, whereas with a 10% aqueous solution the acute po LD50 is 1.41 ml kg−1, illustrating the influence of dilution of the test material on toxicity. A reciprocal relationship has been demonstrated with glutaraldehyde (Figure 15); in this case, acute po toxicity (as mg active material per kg body weight) increases with dilution within the confines of the study.

  • Materials with similar LD50 values may have differences in acute toxicity shown by other monitors of their toxicity. For example, 2,4-pentanedione has an acute po LD50 (rat) of 0.58 g kg−1, similar to that of BAPP at 0.31 g kg−1; however, times to death were 2–5 hours with 2,4-pentanedione and three to four days with BAPP, indicating a more serious potential hazard with the former.

  • With inhalation studies, the method of generation of the test material in the atmosphere may be a highly important consideration, as indicated by the following three illustrative examples:

    • For acute vapour inhalation studies, and in tests concerned with defining the effects of saturated vapour atmospheres, the vapour may be generated by static or dynamic methods. Static methods involve placing a sample of the test material in the exposure chamber and allowing the atmosphere to equilibrate for an appropriate period of time; thus, all volatile components accumulate to vapour saturation in the chamber. Dynamically generated atmospheres are produced by passing air through the test material and transferring the atmosphere so generated into and through the chamber in a continuous manner; this results in components of the test material being present in the atmosphere in proportion to both their concentration in the test material and volatility. Thus, trace contaminants of highly volatile toxic materials will be present in much higher concentrations with static, as opposed to dynamic, conditions. For example, methoxydihydropyran (MDP), containing 0.037% acrolein, when generated dynamically did not produce mortalities with rats for a four hour exposure period (MDP vapour concentration 7748 ppm; acrolein, trace). However, when the same material was used in a static exposure system there were mortalities due to the accumulation of acrolein vapour in the atmosphere (MDP vapour concentration 8044 ppm; acrolein, 240 ppm); acrolein has a four hour LC50 of 8.3 ppm (Ballantyne et al., 1989a).

    • The relative humidity of the chamber atmosphere may influence inhalation toxicity with hydrolysable materials. For example, when tris(dimethylamino) silane (TDMAS) was generated as a vapour with moistened air, a four hour LC50 of 734 ppm was determined (female rat), which accords stoichiometrically with toxicity due to dimethylamine formed by hydrolysis of TDMAS. However, when the vapour was generated under dry air conditions, a four hour LC50 of 38 ppm was calculated from the exposure–mortality data, indicating a highly significantly greater intrinsic toxicity for the TDMAS molecule (Ballantyne et al., 1989b).

    • A marked difference in toxicity may be obtained for the same material generated in different modes of exposure or different physical states. For example, short-term repeated exposure to vapour from 2-methacryloxypropyltrimethoxysilane does not produce any respiratory tract injury. However, when generated as an aqueous respirable aerosol it produces laryngeal granulomas (Klonne et al., 1987).

    • The use conditions of the test material may influence toxicity. Thus, the potential for cutting oil to induce cutaneous neoplasms is significantly enhanced after its industrial use, possibly owing to the generation of polycyclic aromatic hydrocarbons (Agarwal et al., 1986).

thumbnail image

Figure 15. Effect of aqueous dilution on the acute peroral lethal toxicity (as LD50) of glutaraldehyde to rats. As dilution increases the LD50 becomes smaller; that is, greater toxicity. (Data after Ballantyne and Jordan, 2001.)

14 Hazard Evaluation and Risk Assessment

  1. Top of page
  2. Introduction
  3. Historical Development of Toxicology
  4. Definition and Scope of Toxicology
  5. Descriptive Terminology of Toxic Effects
  6. Morphological and Functional Nature of Toxic Effects
  7. Dosage–Response Relationships
  8. Factors Influencing Toxicity
  9. Biohandling as a Determinant of Systemic Toxicity
  10. Routes of Exposure
  11. Exposure to Mixtures of Chemicals
  12. Toxicology of Drugs
  13. Nature, Design and Conduct of Toxicology Studies
  14. Review of Toxicology Studies
  15. Hazard Evaluation and Risk Assessment
  16. Special Considerations in Human Hazard Evaluation
  17. Professional and Ethical Issues
  18. References
  19. Further Reading

Toxicology is concerned with determining the potential for materials to produce adverse effects, whereas hazard evaluation is a process to determine if any of the known potential adverse effects will develop under specific conditions of use. Thus, toxicology is but one of the many considerations to be taken into account in the hazard evaluation process. The following are some of the other factors that need to be considered in defining whether a specific use of a material will be hazardous, and are discussed in detail by Tyler and Ballantyne 1988:

  • Physicochemical properties of the material

  • Use pattern

  • Characteristics of the handling procedure

  • Source of exposure and route of exposure, both normal and possible misuse

  • Control measures

  • Magnitude, duration and frequency of exposure

  • Physical nature of exposure conditions (e.g. solid, liquid, vapour, gas, aerosol)

  • Variability in exposure conditions

  • Population exposed (e.g. number, sex, age, health status)

  • Any experience and information derived from exposed human populations.

The general approach used to assess hazards is as follows:

  1. A search for all available health-related information on the substance and, if appropriate, substances of close chemical structure. This may include information on physicochemical properties, in vivo and in vitro toxicology, epidemiology, known occupational and domestic incidents, case reports, monitoring and use patterns. Mention should be made of the use of structure–activity relationships (SAR) for the prediction of toxicity. Basically this generally involves a computer-based modelling approach to relate chemical structure to (adverse) biological activity and, where sufficient information exists, to develop quantitative structure–activity relationships (QSAR) of biological potency. These approaches may yield valuable information that can be used in the planning of toxicology studies and, within the limits of the methodology and its assumptions, supply interim predictive statements on potential hazards. When used for hazard-evaluation purposes, it should clearly be stated that the information has been derived by SAR methods. Although a useful approach in the absence of ‘hard’ information, it has been noted that given the large range and variability of possible interactions of chemicals in biological systems, it is highly unlikely that SAR models will ever achieve absolute certainty in predicting toxicity, particularly with respect to the whole-animal system (McKinney et al., 2000).

  2. Detailed impartial review of all information obtained, emphasizing those studies conducted by credible scientific standards and by relevant routes of exposure.

  3. Interpretation of the credible and relevant literature in order to define toxicity and, if possible, mechanism, dose–response relationships and factors influencing toxicity (endogenous and exogenous).

  4. Conclusions regarding potential adverse effects from the substance under specific conditions of use.

  5. Determination of acceptable handling or in-use conditions and acceptable exposure to the substance with respect to immediate and long-term conditions of use.

  6. Determination of the management of overexposure situations.

The process and understanding of hazard evaluation, and its scientific basis, are now at a level where reliable interpretation and prediction can be made. A less reliable and scientifically limited evaluation process is that of risk assessment, which is an important developing component of regulatory and occupational toxicology. It is the objective of risk-assessment processes to assess the probability that adverse health effects will develop from known, or suspect, xenobiotics in the environment (e.g. drinking water or air) or workplace. Such quantitative risk assessments are most frequently conducted for work-time or lifetime exposure to low concentrations of xenobiotic. They are based on extrapolating dose–response relationships from animal studies, or occasionally human epidemiological data to: (i) determine risk at known or anticipated ranges of occupational or environmental exposure dosages or (ii) to assess ‘risk-free’ dosages. The approaches most frequently employed are to assess risk from carcinogens, teratogens, reproductively active substances and genotoxic materials.

With most materials there is usually insufficient information on mechanisms of toxicity for particular materials to allow scientifically valid, appropriate mathematical models to be developed for a specific toxic effect. The methods of extrapolation often make many, often biologically unreasonable, assumptions which include: (i) the existence (or otherwise) of thresholds for specific toxic end points; (ii) linearity of dose–response relationships; (iii) comparability of metabolism and pharmacokinetic parameters between species; (iv) the interaction between xenobiotics and biological systems at low concentrations and (v) the statistical reliability and biological variability resulting from the relatively small numbers of animals that may technically and ethically be incorporated into animal studies. Thus, with current mathematical approaches to data extrapolation, quantitative risk assessments should probably be regarded as ‘best guesses’ for environmentally safe exposure dosages. The findings from quantitative risk assessment may result in risk-management measures being undertaken (Hallenbeck and Cunningham, 1986). This involves the development and implementation of regulatory action, taking into account additional factors such as available control measures, cost–benefit analyses, ‘acceptable’ levels of risk, and taking note of various policy, social and political issues. More specific and mechanistically reliable estimates can be obtained from the process of ‘biological risk assessment’, which allows a more rational risk analysis based upon incorporation of metabolism and pharmacokinetic findings, including interspecies differences, mechanisms of toxicity and influence of physiological variables (Clayson, 1987; National Research Council, 1987).

15 Special Considerations in Human Hazard Evaluation

  1. Top of page
  2. Introduction
  3. Historical Development of Toxicology
  4. Definition and Scope of Toxicology
  5. Descriptive Terminology of Toxic Effects
  6. Morphological and Functional Nature of Toxic Effects
  7. Dosage–Response Relationships
  8. Factors Influencing Toxicity
  9. Biohandling as a Determinant of Systemic Toxicity
  10. Routes of Exposure
  11. Exposure to Mixtures of Chemicals
  12. Toxicology of Drugs
  13. Nature, Design and Conduct of Toxicology Studies
  14. Review of Toxicology Studies
  15. Hazard Evaluation and Risk Assessment
  16. Special Considerations in Human Hazard Evaluation
  17. Professional and Ethical Issues
  18. References
  19. Further Reading

By the very nature their intended design, laboratory toxicology studies are conducted under highly controlled conditions using healthy animals often of a particular weight range. The extrapolation of such information to a heterogeneous human population, with differing lifestyles and variable states of health, needs to be undertaken with considerable caution, taking into account all possible known and predictable variables. The possible interactions of multiple exposures to a variety of chemicals or drugs has been discussed earlier in this chapter. Other illustrative examples are presented below.

15.1 Personal Habits

Many personal habits, including diet and the taking of medicinal products, may influence the response to a toxic chemical. Two factors that have received special attention are cigarette smoking and excessive alcohol consumption. Cigarette smoking may lead to increased body burdens of many of the combustion products found in smoke, in particular carbon monoxide. Owing to the significantly increased carboxyhaemoglobin concentrations in smokers, they may be at greater risk in the occupational environment from carbon monoxide and carbon monoxide-generating materials, such as methylene chloride. Other materials in cigarette smoke that may increase the exposure burden include hydrogen cyanide, hydrogen sulfide, acrolein and polycyclic aromatic hydrocarbons. In some instances, there are clear indications of significantly enhanced toxicity, for example, synergism between cigarette smoking and asbestos (Hammond and Selikoff, 1973) or radon (Archer et al., 1972). Heavy alcohol consumption may lead to chronic progressive liver injury and fibrosis, and thus increase susceptibility to hepatotoxic substances and impair detoxification pathways.

15.2 Coexisting Disease

Individuals with certain illnesses may be at greater risk from particular drugs or industrial chemicals. For example, those with established cardiovascular disease may be at increased risk from exposure to carbon monoxide or methaemoglobin-generating substances, since both may compromise the available oxygen supply to the myocardium. Inhalation of irritant materials may aggravate chronic progressive pulmonary disease.

15.3 Genetically Susceptible Subpopulations

Individuals with genetically determined biochemical variants may be at greater risk from certain drugs and chemicals than those with normal biochemical features. Some examples are as follows:

  • Individuals with hereditary methaemoglobinaemia may generate significant amounts of methaemoglobin at exposure doses of nitrites or aromatic amines that cause only minor methaemoglobin concentrations in the normal population.

  • It is well known that slow acetylators are significantly more susceptible to the neurotoxic potential of isoniazid, whereas fast acetylators are more likely to develop liver injury, since hepatotoxicity is caused by the metabolite acetylhydrazine (Breckenridge and Orme, 1987). Another aspect of acetylator status relates to the potential of arylamines to induce bladder cancer. Slow acetylators may be more susceptible to arylamine-induced bladder cancer (Cartwright et al., 1982), possibly related to a higher urinary excretion of free arylamine (Derwan et al., 1986).

  • Individuals with glucose-6-phosphate dehydrogenase-deficient erythrocytes may be at increased risk from haemolytic effects of oxidants because of the inability of the erythrocyte to generate sufficient NADPH and maintain an adequate concentration of reduced glutathione, resulting in haemolysis (Calabrese et al., 1987). However, animal studies suggest that haemolytic effects occur only on exposure to otherwise toxic concentrations (Amoruso et al., 1986).

  • Exposure of persons with inherited uroporphyrinogen decarboxylase deficiency to dioxin can cause latent chronic hepatic porphyria to develop into porphyria cutanea tarda (Doss and Columbi, 1987).

Other genetically determined variants that have been implicated as causing increased susceptibility to chemicals include α1-antitrypsin deficiency (emphysema), aryl hydrocarbon hydroxylase deficiency (lung cancer), pseudocholinesterase variants (anticholinesterase toxicity) and thalassaemia (lead). An interaction between environmental factors and specific genotypes has been suggested as being involved in the aetiology of a number of diseases, including parkinsonism (Menegon et al., 1998; Cummings, 1999). Genetic screening of workers or prescreening for the identification of susceptible subpopulations to occupational exposure has raised several issues of ethical and social concerns (Christiani et al., 2008), and it has been proposed that workers and/or their representatives should be involved in deciding when and where genetic testing in the workplace is done (Holtzman, 2003).

16 Professional and Ethical Issues

  1. Top of page
  2. Introduction
  3. Historical Development of Toxicology
  4. Definition and Scope of Toxicology
  5. Descriptive Terminology of Toxic Effects
  6. Morphological and Functional Nature of Toxic Effects
  7. Dosage–Response Relationships
  8. Factors Influencing Toxicity
  9. Biohandling as a Determinant of Systemic Toxicity
  10. Routes of Exposure
  11. Exposure to Mixtures of Chemicals
  12. Toxicology of Drugs
  13. Nature, Design and Conduct of Toxicology Studies
  14. Review of Toxicology Studies
  15. Hazard Evaluation and Risk Assessment
  16. Special Considerations in Human Hazard Evaluation
  17. Professional and Ethical Issues
  18. References
  19. Further Reading

The right to search for truth implies also a duty. One must conceal any part of what one has recognised as true.

(Albert Einstein)

16.1 General Comments and Principles

There is no utopia with respect to the protection of the human population and the environment against xenobiotics, and it is thus of prime importance that the toxicologist and professional colleagues should be acutely and constantly aware and certain of the ethical and professional foundations of their work and its potential implications, and that they should give opinions in a credible, transparent and professional manner. All professional branches of science and technology have in common the objectives of high standards of learning, practice, personal and professional conduct, and visibility, in addition to more specific considerations relevant to the individual specialization. As a consequence many have developed codes of conduct and guidance note on ethical practices; for example, the Institute of Biology 2005 and the Royal Society of Chemistry 1995. However, codes of conduct and ethics should allow for constructive communication between scientists and the community. This sometimes falls short of expectations, as did the UK Council for Science and Technology ‘Universal Ethical Code for Scientists’ which was a series of awkwardly worded exhortations and platitudinous rules (Editorial, 2006). Members of the profession of toxicology should also follow the demands of ethical integrity as dictated by their individual conscience and the guidance of representative professional organizations. By their very nature and intended applications, toxicology investigations can have far reaching implications for health-related issues in the workplace, commerce, home and general environment. The same consideration applies to the application and expression of specialist knowledge by the toxicologist in his/her professional capacity. There are, as a consequence, many ethical issues associated with the practice of the science of toxicology and the practical applications of toxicology information. Whereas morality often involves a distinction between what is right and what is wrong, professional ethical considerations may not have such a clear distinction and individual experience, lifestyle, mental orientation and professional outlook are contributing factors. Against these considerations, various competing factors may have an opposing and possibly detrimental effect on the ethical values, and the value of ethical practices, by an individual; these include biases, motivations and potentially conflicting attitudes within a group organization. Thus, toxicologists working in commerce/industry and government service may find themselves in the midst of situations with the potential for group or organization self interest and expectations for bias in attitude and interpretation which could conflict with professional and personal ethical standards, and lead to difficult, strained and possibly inharmonious relationships. Such situations may lead to problems associated with community values, where the behaviours that govern moral and ethical conduct depend on a multiplicity of considerations, principal amongst which are majority viewpoints of the general population, the existing background of accepted general concepts and principles of moral and ethical values, and what are considered by the majority of a democratic society to the advantage of society relative to health, welfare and harmonious coexistence. Professionals often find that their ability to maintain and adhere to ethical standards is often undermined and enforceably questioned by others with self interests outside the professional field; these may include financial, commercial, economic and politically motivated considerations. All of these may result in significant influences to bias or attempt to manipulate the interpretation and implications of scientific observations, including iconoclasm of credible findings. Some professional society codes of conduct clearly recognize these potential employer–employee conflicts. For example, the Institute of Biology advises employees that—‘In serving the interests of their employer in good faith and to the best of their ability, always keep in mind the obligation to serve the public interest and maintain and enhance the reputation of the profession’ (Institute of Biology, 2005, p. 5).

Current society is experiencing an unfortunate, disturbing and widening gap between democratic societal moral and ethical values and what constitutes legal dictates. This is particularly noticeable in the adversarial society, notably in the USA; in dictatorial truth denials by commercial and political organizations; as a consequence of the decreasing separation of government and dictatorial religious dogma and in the distorted and smear tactics employed by corporations and government against those with ethical, reasoned arguments for causation.

From a practical standpoint and on the basis of the above considerations it is not unexpected to find in some situations that the design, conduct and reporting of toxicology studies as well as the interpretation of the findings and their implications with respect to human health and the environment, may be adversely influenced by considerations of a political, economic, personal, security or other self-interest nature. In many circumstances such factors may result in biased interpretation (in some cases deliberate) of otherwise credible experimental findings and thus a distortion or even suppression of findings and of the truth. Additionally, as noted by Golberg 1982, it is not unusual to see uncritical acceptance of adverse conclusions drawn from poor data. It follows that toxicologists should not only focus on the credibility and scientific significance of their work, but also within ethical and professional perspectives on the practical use(s) to which to which scientific discovery and knowledge can (or will) be used and for its general societal repercussions or misapplications. This is not, and should not be, the prerogative, as claimed, of those having hierarchical wider political and commercial interests. There is no universal scheme or consensus approach to ensure that all health-related information be made generally available to the scientific, medical and other interested communities, but this is not only a highly desirable, but indeed regarded by many of us as a necessary feature of all research and development activities having implications with respect to human health and the environment. However, self-interest by various individuals, groups, organizations, on a national or global basis, implies that no such utopia of knowledge is ever likely to exist.

Toxicologists can and do, at various times, become involved in legally orientated situations and their consequences. In cases involving litigation, the professional toxicologist should anticipate that his/her credibility will be questioned and should be appropriately prepared for critical evaluation (Furst, 1997; Furst and Reidy, 1999). On a more routine work basis, other major areas where the toxicologist may become involved in processes interfacing with national and international laws and regulations include the following: animal welfare; conduct and reporting of laboratory studies; regulatory test requirements; mandatory reporting of potential adverse health effects, as, for example, under Section 8(e) of the Toxic Substances Control Act (TSCA); human volunteer studies. Ethical aspects of human volunteer studies has been reviewed above (see Section 12.3.11), and several other aspects of ethical–legal interactions in toxicology are briefly discussed below.

16.2 Conduct of Laboratory Studies

Before there was formal regulatory control of the conduct of toxicology laboratory studies in the USA, some commercial and industrial in-house laboratories conducted studies without due regard to the development and adherence to specific protocols, kept poor records of findings and often only reported selected findings in a study. Although such badly conducted studies were rampant in only a few laboratories, instances of lesser unacceptable procedures and practices were not infrequent in many laboratories. Following enquiries by the FDA into deficiencies, inaccuracies and fraudulent data contained in reports, legislation was introduced in 1979 to ensure integrity and reproducibility of studies, in the form of GLP. Following introduction of the FDA GLP regulations, several other countries recognized the need for such legislation, and now most countries have programmes with a GLP core in operation that deal with laboratory facilities and personnel, protocols, reports and archives (summarized by Ballantyne, 2005).

16.3 Animal Welfare

The use of animals can be an emotive subject, and has been the subject of deep and extensive augments, public demonstrations, the use of violence by activists, legislation and the introduction of alternative methodologies (Ballantyne, 2005; Edmunds, 2005). Legal controls on the use of animals in biomedical research lie with government. The most extensive and demanding legislation has been in the UK, where a bill was passed through the British parliament in the nineteenth century (Animal Cruelty Act of 1876), and was updated in 1986 by the Animals (Scientific Procedures) Act. The legislation requires detailed licensing of research facilities and of research scientists, annual returns on animal usage and random audits by the Home Office inspectorate. In the USA, animal welfare became an integral component of conducting sound studies with the adoption of the EPA GLP regulations. As a consequence of the activities of credible animal rights groups and concerns by ethically motivated scientists, there have been considerable efforts to reduce animal discomfort and use in vitro toxicology research. These have included reduction in the numbers of animals used, refinement of in vivo procedures including improved and noninvasive monitoring procedures, and the development of in vitro alternatives. Partly because of the increased activities of animal welfare groups, and associated media reporting, several professional organizations have issued guidance documents for good animal welfare practices; for example, the (American College of Toxicology, 1988, Academy of Medical Sciences, 2007, Society of Toxicology, 2005). These, however, should only be regarded as itemized guidance notes, and not be used as a replacement for a carefully developed individual institutional programme. Continual individual and institutional vigilance is necessary in order to remain within the ethical and legal boundaries of animal care and handling. In-depth reviews on the legal and ethical aspects of animal usage in are to be found in Chemical Terrorism.

16.4 Regulatory Activity

The professional activities of many toxicologists will be influenced by the requirements of governmental regulatory agencies. Also, the toxicology community and the general population should be aware of the biased, and possibly misleading, influence that extensive lobbying of political organizations by industry for economic reasons (Loewenberg, 2006). Regulatory activities are to be found principally in relation to the production and in-use aspects of pesticides, industrial chemicals, pharmaceutical preparations, veterinary drugs, food additives, food contact materials, medical devices and cosmetics. The structure of regulations, their precise requirements, and whether statutory or voluntary, varies with the nature of the material regulated and the geographical location of the authority.

Toxicologists in commerce should have responsibility for advising on the scientific aspects of conforming to regulatory requirements, and on study design, conduct and the interpretation of findings. Regulatory agencies themselves have had impact on these aspects of toxicology studies, being obvious from the introduction of GLP principles. There have been phases when the attitude has been ‘studies conducted to meet regulatory requirements’, and during these phases this has resulted in a fragmentation of intellectual approaches to regulatory-based toxicology. Thus, some toxicology testing requirements that have been proposed by regulatory agencies are too specific for the intended purpose(s) and may contain fine detail that does not permit the flexibility necessary to undertake a vital hazard evaluation procedure. This has, at times, led to an attitude that some mandated regulatory toxicology testing is conducted merely to ‘satisfy regulations’ rather than to undertake a well-designed study to determine toxicity and permit objective evaluation of hazards. Clearly such attitudes may be conducive to inappropriate toxicology practices, excess use of in vivo procedures, and result in documentation that does not allow reasonable assessments of in-use hazards. Such a less than satisfactory and scientifically delinquent approach has resulted in reference to ‘cook book’ regulatory practices. A rationalized toxicology testing programme requires that for any particular material there is an in-depth consideration of a multitude of factors that may be unique to the material being tested and to its intended use. In no way can a generic protocol substitute for a scientifically reasoned approach to study design. In many cases, however, a scientifically relevant testing programme can be developed in collaborative discussions with regulatory agencies. The current complexities, tardiness and almost incomprehensibility of regulatory toxicology activities, including attempts at global harmonization, has been, to a not inconsiderable extent, a consequence of increasing bureaucracy with excesses of committee structures, permanent and advisory staffing, with personnel lacking the necessary educational and experience backgrounds, infiltration with legislative language and the uncontrolled misuse of meaningless synthetic management and legal language. Attempts at international harmonization, often marred by national interests, have on occasion resulted in flagrant disregard of credible scientific truths, concepts and criteria (Ballantyne, 2005). It is important that competent regulatory agencies who review data for classification and regulatory purposes should have access to credible independent and professionally qualified advisors and advisory committees that do not have vested interests in the economics of any submitting commercial organizations. Advisors should have documented national and international respect for their professional knowledge and integrity (Ballantyne and Marrs, 2004).

16.5 Publications

It is a core belief and value of many of us that the privilege of being allowed to conduct medical and scientific research, and to search for truth, is inseparable from the duty that no part of the truth is withheld. Implicit in this belief is that knowledge is the property of civilization and as such should not be subject to suppression by political agendas or commercial interests. In addition to the manipulation and suppression of the truth for political, commercial, economic and other limited interest reasons, it is clear in some cases that unethical and unprofessional practices in the conduct of basic and applied research are often associated with willful aberrations in the communication of scientific and medical information. Whilst the majority of full-length publications in respected scientific, medical and technical journals are of the highest credibility and significantly add to our knowledge and understanding, some are less desirable. The reasons for manipulating and/or fabricating information in publications are usually the same as those for deviant conduct in the research process. Malpractices have included the publication of fictitious data (Lock, 1995), manipulation of data to support a predetermined viewpoint, data modified to achieve a desired statistical significance and misquoting others (often cited as ‘unpublished data’). Editors and referees should be constantly vigilant to detect unprofessional activity in submitted manuscripts. A further unfortunate trend is for some workers to duplicate publication, in which the same, or a marginally altered, paper is published in different journals. It is very unlikely that this is a desire on the part of the author(s) to ensure that the widest audience has access to the information, but more likely the motivation is related to publication proliferation in research-restricted or competitive environments. Those who see such unethical duplication of information, often against the stated objective of the journals, should make this known to the appropriate editor(s). With the objective of preventing, or minimizing, publication of papers containing biased protocols and containing falsified or manipulated data, many journals and scientific societies have developed guidelines and codes of practice for the publication and presentation of scientific information. Editors should ensure that their referees act with professional scientific integrity and impartiality when reviewing manuscripts, and do not attempt to impose their opinions on the subjective interpretation of the findings. Indeed it is reprehensible for a referee to insist that the author's interpretation of findings be amended to accord with the viewpoints of the referee, a situation which some of us have encountered on occasion. In such circumstances the author(s) should formally complain to the journal editor and publisher. The major functions of journal editors and referees should be to ensure, as far as is possible, that experimental design is appropriate for the stated objective of the study, and the information submitted in the manuscript represents a true, complete and unbiased record of the methods and findings.

As a component in the process of ensuring credible reporting and interpretation of information, most toxicology-orientated journals now require that the author(s) include a disclosure statement, for publication, of any possible conflicts of interest that could result in misrepresentation of the facts and opinions expressed. Also, many journals also require that participants in the peer review process must disclose, to the editor, any conflict of interest(s) that could result in reviewer bias of the manuscript (Lehman-McKeeman and Peterson, 2003; Maurissen et al., 2005).

Peer review of manuscripts by credible scientific journals is an integral component of codes of practice to ensure quality and integrity of publications. It involves the scrutiny of an author's manuscripts by others who are recognized experts in the same field, and to render an opinion as to the suitability of the submitted paper for publication in a particular journal. In an international survey of 3040 academics with respect to their attitudes to the peer-review process, it was generally concluded (90% of those surveyed) that the process helps scientific communication and, in particular, improves the quality of published papers (Ware, 2008). The major findings were: peer review is widely supported by academics; it improves the quality of published papers; double-blind review is preferred and most effective and postpublication review may be a useful supplement to formal peer review. Although peer review of manuscripts submitted to scientific and medical journals is a well-established and accepted process, there have been attempts to question the value and integrity of this standard practice. The motivations of these moves against the standard practice of peer review are variable, but in many cases have a commercial interest. For example, in May 2007 the New England Journal of Medicine was served with a subpoena from Pfizer's lawyers demanding that the Journal produce peer review and other editorial documents concerning several of their products. Apparently, Pfizer required the documentation not only for defence in product-liability litigation, but also to discover ‘flaws in methodology’ in the research that might have been noted by the Journal's peer reviewers. The request was refused, and in January 2008 Pfizer's attorneys filed a motion to compel the production of the required documents. The journal again refused and filed with the court an Opposition and Motion for Protective Order, on the basis that reviewers had been promised confidentiality and that to provide the requested information would hamper the Journal's ability to serve the medical community. The motions were heard in federal court on 13 March 2008, where the judge concluded that ‘the New England Journal of Medicine's interest in maintaining the confidentiality of the peer-review process is a very significant one, especially in the light of its non-party status, and tips the scale in favour of the New England of Medicine’. He also noted that ‘the batch or wholesale disclosure by the New England Journal of Medicine of the peer review comments communicated to authors will be harmful to the New England Journal of Medicine's ability to fulfil both its journalistic and scholarly missions’ (Curfman et al., 2008b). Pfizer also issued similar subpoenas on the Journal of the American Medical Association and Archives of Internal Medicine, who both argued the sanctity of the peer-review process, and both had similar progression to that of the New England Journal of Medicine. In a 14 March 2008 ruling in federal court in Chicago, the judge concluded, ‘… given the strong policy behind preserving confidentiality in the peer review process, the Court finds any probative value would be outweighed by the burden imposed on the Journals in invading the sanctity of that process’ (DeAngelis and Thornton, 2008; Keys, 2008). Also relevant to the peer-review process is the possibility for breach of reviewer confidentiality by the reviewer communicating review-related information to parties other than the journal for whom a review was conducted (Editorial, 2008). There should be provision for declaration of any conflicts of interest by reviewers that might prohibit confidentiality of the process.

There have, from time-to-time, been proposals from some societies and journals that research sponsored by certain commercial organizations should not be accepted for publication because of possible self-interest bias and possible effect of this on credible reporting and interpretation of information. For example, the American Thoracic Society decided that no longer would it accept medical research sponsored by the tobacco industry in its peer-reviewed journals (Rutter, 1995). Decisions of this nature could be interpreted as journals or societies having a political agenda and exhibiting censorship (Roberts and Smith, 1996), a practice that, if uncontrolled, could extend into research sponsored by other organizations such as pharmaceutical and other industries, or even in-house research activities. One effective control approach is by drawing the attention of the reader to potential ethical and professional issues through the mechanism of disclosure of interests, with particular reference to sources of funding (Landrigan, 1995). Disclosure of interest would be by the inclusion in the published paper of a statement recording source(s) of funding and any vested interest(s) of the author(s) in the work conducted for the sponsoring organization (Lehman-McKeeman and Peterson, 2003). The need for disclosure has been stressed by the recently created nonprofit foundations housed in academic institutions, but organized for the benefits of individual investigators and funded by industry sponsors (Schwartz et al., 2008). Although such foundations may be helpful in providing needed research funds, the foundations may not be required to publically disclose details of their funding sources. Funding disclosure in published papers is one critical factor in transparency of biomedical research and its implications for the public health. Perhaps the extreme of industry funding and its potential implications with respect to totally credible and unbiased medical science is afforded by the fact that bioethicists and academic bioethics centres have sought and accepted funding, particularly from pharmaceutical and biotechnology industries (Elliott, 2005).

16.6 General Legal–Ethical–Professional–Employer Interfaces

16.6.1 Overview

By the very nature of their activities, professional toxicologists may find themselves in conflict situations resulting from pressures exerted by others (individuals and organizations) to engage in, or condone, decisions and/or actions that do not accord with personal and professional integrity and ethical principles. Whilst the majority of toxicologists have a high level of ethical and professional standards in the practice of their specialization, a small proportion have lower standards with respect to these qualities; the proportion is about the same as for any other profession (Ballantyne, 1988). In some instances the actions and consequences of lower standards may clearly be illegal and unethical, but in other cases whilst there may not be a direct conflict with the law there is clearly professional misconduct; there is a spectrum of malpractices between these two extremes. Although the majority of toxicological research is undertaken and published against a background of scientific impartiality and integrity, some investigations are conducted with preconceived considerations for the outcome, and in this respect the investigators may design, conduct and interpret the study towards obtaining the desired outcome. The reasons for such professional dishonesty may be multiple, and extend from an individual investigator to a whole organization. They include, but are not limited to, economic considerations, including profit margins, competitive activity within a given production line, product registration, attraction of funding, enforced direction from management, political motivation and career objectives. Malpractices include biased protocol development, nonstated deviations from the protocol, data falsification and fabrication, and omission of data that do not further a preconceived bias. On an organizational basis, instances of lack of scientific integrity, malpractices and professional dishonesty have been noted at times with industry, government and academia, and in some disgraceful distances there have been indications of collusion. For example, in the UK, the Institute of Professionals, Managers and Specialists obtained evidence for concern amongst scientists working for government quangos or for newly privatized laboratories who reported that they were asked to adjust their research conclusions and/or resulting advice. This included 17% who were asked to change their conclusions to suit the customer's preferred conclusions, in some cases this was to obtain further contract work (Ballantyne, 2005). Other examples are almost beyond understanding, since it is clear that the deceit will almost certainly become known. One reprehensible instance in the USA was that of the EPA giving misleading information about health effects to New York citizens from debris, dust and smoke in the air around the World Trade Center following the terrorist attacks on 11 September, 2001. Another example of possible government-department withholding of health-related information is by the Centers for Disease Control (CDC) delaying a report indicating greater health risks for people living in some areas around the US Great Lakes areas (Bristol, 2008). The adverse influences on public health issues by some dishonest business organizations, lobbyists and corrupt political administrations are transparently evident on a continuing basis. At the other end of the spectrum of professional dishonesty is misconduct by individuals or small groups, whose motivations include pressure to publish, the need for funding, financial gain and promotion. In order to limit biased or fraudulent activities in studies conducted for commercial organizations by contact, there should be a clear recorded formal agreement that should include the following: ownership and access to data, study design according to credible scientific principles, conduct, auditing, publication rights and indications for project termination (Paris, 1996). One reason for apparent professional and organizational misconduct may stem from the unfortunate trend, seen in both industry and government service, to appoint ‘managers’, not on the basis of qualifications, knowledge and relevant experience, but on their tenacious ‘team player’ attitude for the organization, often in the face of obvious truth and credibility. This has resulted in the direction of work being steered towards what is politically or economically desirable by senior organizational management. It has been ‘reasoned’ that such an organizational structure of lay management with side lining of experienced professionals permits ‘the various factors to be viewed in their widest perspective’, whilst in fact it is planned to obtain the ‘desired interests’ of the organization. The ramifications of lay control of professional activity and expression argue for independence of expression without repercussions (see Section 16.6.5). Under research activities, professional malpractice may include the undertaking of work in a clandestine manner for ‘reasons’ of national security or commercial advantages, and for similar reasons the suppression or manipulation of data. For example, studies may be undertaken confidentially and with no intentions to make the findings publicly known for commercial (monetary) and deceptive business practices, as identified with the tobacco industry, who have studied the relationship between smoking behaviour and product design (Hammond et al., 2006). Strategies resulting from these confidential internal studies relate to exploiting the limitations of toxicology testing and to concealing from consumers and regulatory agencies the potential health dangers of products. This underlines the greed and blatant disregard for pubic health by the tobacco industry, and their current advantage in the USA that there is no (at the time of writing) regulatory activity by the FDA with regard to tobacco products (Curfman et al., 2008a).

Toxicologists need to be continually vigilant in order to maintain a credible, ethical and professional approach to their work and its implications. This covers work practices and inter-relationships with others, including advisory functions and employers. Some major areas where caution is necessary are as follows.

16.6.2 Professional Conduct

To ensure that scientific investigations are conducted and communicated in publications in accordance with the highest scientific and ethical standards, several professional societies and journals have developed codes of practice for their membership and contributors. Such codes require thoughtful development against a myriad of challenges (Goodman, 1996). They should not be regarded as static documents for display, but as guidelines that require periodic revision against the changing background of investigational procedures and evolving knowledge. It has been proposed that there are three broad approaches to preventing scientific fraud and misconduct: education, standards and training (Evered and Lazar, 1995). These should be conducted by, demonstrated by and practised by the staff of research and development organizations, academic institutions, government organizations, professional organizations and their society members. When there are accusations of professional misconduct and/or behaviour, these should be dealt with promptly, impartially, by transparent methods and with subsequent access to arbitration and appeal if considered necessary.

16.6.3 Personal Integrity and Denigration

Toxicologists may encounter situations in which there are attempts to degrade their personal integrity and credibility, sometimes due to jealousy, on occasion for financial gain, or because of differing competitive scientific opinion. This may be intradepartmental, conflict between organizations or part of the armamentarium of the legal business. This may be seen at its most unpleasant in some court of law proceedings with so-called expert witnesses making attorney-stimulated derogatory remarks, often with financial incentive, at the opposing expert witnesses, and for which there is often little chance for open rebuttal (Hernberg, 1995).

16.6.4 Expert Witness

Appearing in court proceedings as an expert witness is, in most cases, a voluntary activity, sometimes carried out for sociopolitical reasons, but usually for financial gain. Acting as an ‘expert’ witness is now a career with some qualified toxicologists in contract organizations or private practice, and their specialist areas can be found in advertising services online. The toxicologist should be aware that as a voluntary expert witness he will be subject to extensive questioning by opposing lawyers concerning his/her qualifications, professional integrity and motivations, and there will be attempts made to destroy credibility as an expert witness. Before accepting an invitation to become an expert witness, the credible toxicologist should make it clear that he/she will act in an ethical and professional manner and not distort the truth for legal augment.

The toxicologist should be aware of, and wary of, the growing nauseating variant on the ‘ambulance chasing’ activity of the legal business, who seek to further their finances by taking advantage of adverse effects from drugs and medicinal products. In this practice, most noticeable in the USA, the legal business seeks clients, by the medium of television advertisements, who ‘might’ have developed averse health effects as a result of being exposed to certain industrial chemicals or having been legally prescribed certain specific and named medications. Clients are invited to have a free legal consultation to determine if they have a case against the manufacturer of the named chemical or drug. These advertisements are seen daily and cover, in the main, asbestos, analgesics, antidepressants, cholesterol-lowering drugs and anti-inflammatory agents. Presumably the legal business conduct regular searches of the medical and pharmacological literature for newly reported adverse reactions to drugs. This practice has recently extended into client litigation for general medical and surgical complications; for example, abdominal aortic surgery, haemodialysis, cardiac pacemakers. This practice will ultimately result in significant suppression of new drug developments.

16.6.5 Conflict and Hounding

Leading a credible and ethical professional lifestyle can, if the employee discovers malpractices by his management and/or fellow workers, result in hounding, unfair treatment, dismissal or antisocial activities against the employee, if he/she draws attention to unprofessional or unethical behaviour, including suppression or misrepresentation of health-related information. Discrimination against employees based on disclosure of unsafe and/or unethical practices has sometimes been classed as ‘whistleblower’ activity, based on the employee making public statements about the suppression or misrepresentation of health-related information. Such unethical employer activity has been recorded to occur in industry, government and academia. In the USA, employees in some states are protected by the so-called ‘whistleblower’ laws. In New Jersey, for example, this legislation is called the ‘Conscientious Employee Protection Act’ and is intended to protect employees who act in the public interest, possibly against business interests (Soskolne, 1999). In the UK, protection is provided by the Public Interest Disclosures Act, 1998, which became law on 1 January 1999. It provides that where employees disclose certain types of wrongdoing inside or outside the company or organization they are protected from any form of discrimination or dismissal by their employer (Faculty of Occupational Medicine, 1999). The Act covers situations where a worker reasonably believes that he/she has credible and supportable information and evidence that there has been, or continues to be: (i) the commission of a criminal offence; (ii) failure to comply with a legal obligation; (iii) a miscarriage of justice; (iv) a danger to the heath and safety of any individual; (v) damage to the environment and (vi) the deliberate concealment of information supporting a conclusion of the preceding. ‘Whistleblowers’ (a demeaning term for ethically and professionally motivated individuals), should be aware that the vicious Mafia-like tactics adopted by industrial and government organizations are likely to persist, and that they will continue to face continued career suicide, economic and emotional deprivation, victimization and personal abuse (Yancy, 2000). Thus, in a survey of 87 ‘whistleblowers’ from public service and industrial organizations in the USA, all but one experienced retaliation (Sockon and Sockon, 1987). There is doubt as to whether protection by legislation is supported by some political administrations; this includes the US Federal Executive Branch from 2000 to 2008 who had a clear industry/commerce financial-motivated approach. The public community should be grateful to those who risk their careers and well-earned reputations by drawing attention to employing organizations who covertly undertake malpractices and misinformation for political, commercial and financial motivations, particularly if the issues involved are health-related matters. The increasing recognition of the value of ‘whistleblower’ activities and the protection they should be afforded is emphasized by the formation of a National Whistleblowers Center based in Washington DC, which has an extensive informational web site (http://whistleblower.org) and several publications, such as Kohn et al. 2004.

There is a clear need for greater emphasis to be placed on the utmost importance of professionalism and ethical approaches in toxicology, particularly because of its wide implications for public health. These need to be infused into the early stages in the training of toxicologists at graduate and postgraduate levels. Educationalists need to draw attention to the social responsibility of the toxicologist and to audits of the practice and standards of the profession in academia, government service and industrial organizations.

References

  1. Top of page
  2. Introduction
  3. Historical Development of Toxicology
  4. Definition and Scope of Toxicology
  5. Descriptive Terminology of Toxic Effects
  6. Morphological and Functional Nature of Toxic Effects
  7. Dosage–Response Relationships
  8. Factors Influencing Toxicity
  9. Biohandling as a Determinant of Systemic Toxicity
  10. Routes of Exposure
  11. Exposure to Mixtures of Chemicals
  12. Toxicology of Drugs
  13. Nature, Design and Conduct of Toxicology Studies
  14. Review of Toxicology Studies
  15. Hazard Evaluation and Risk Assessment
  16. Special Considerations in Human Hazard Evaluation
  17. Professional and Ethical Issues
  18. References
  19. Further Reading
  • Abou-Donia, M. B., Lapadula, D. M. and Carrington, C. D. (1987). Biochemical methods for the assessment of neurotoxicity. In Ballantyne, B. (Ed.), Perspectives in Basic and Applied Toxicology. Wright, London, pp. 130.
  • Academy of Medical Sciences (2007). The use of animals in medical research, http://www.acmedsci.ac.uk/images/publication/panimals.pdf.
  • Agarwal, R., Gupta, K. P., Sushil, K. P. and Mehrotra, N. K. (1986). Assessment of some tumorigenic risks associated with fresh and used cutting oil. Indian Journal of Experimental Biology, 24, 508510.
  • Albano, E., Lott, K. A. K., Slater, T. F., Stier, A., Symons, M. C. and Tomasi, A. (1982). Spin-trapping studies on the free-radical products formed by metabolic activation of carbon tetrachloride in rat liver microsomal fractions. The Biochemical Journal, 204, 593603.
  • Albert, A. L. (1979). Selective Toxicity. Chapman & Hall, London, p. 469.
  • Aldridge, J. E., Gibbons, J. A., Flahery, M. M., Kreider, M. L., Romano, J. A. and Levin, E. D. (2003). Heterogeneity of toxicant response: sources of human variability. Toxicological Sciences, 76, 320.
  • Altenburger, R., Baukhaus, T., Boedeker, W., Faust, M., Scholze, M. and Grimme, L. H. (2000). Predictability of the toxicity of multiple chemical mixtures to Vibrio fischeri: mixtures composed of similarly acting chemicals. Environmental Toxicology and Chemistry, 19, 23412347.
  • Altenburger, R., Schmitt, H. and Schüürmann, G. (2005). Algal toxicity of nitrobenzenes: combined effect analysis as a pharmacological probe for similar modes of interaction. Environmental Toxicology and Pharmacology, 16, 111.
  • American College of Toxicology (1988). Policy statement on the use of animals in toxicology, Bethesda MD.
  • Ames, B. N. (1982). The detection of environmental mutagens and potential carcinogens. Cancer, 53, 20342040.
  • Amoruso, M. A., Ryer, J., Easton, D., Witz, G. and Goldstein, B. D. (1986). Estimation of risk of glucose-6-phosphate dehydrogenase-deficient red cells to ozone and nitrogen dioxide. Journal of Occupational Medicine, 28, 473479.
  • Anderson, M. E. and Dennison, J. E. (2004). Mechanistic approaches for mixture risk assessments–present capabilities with simple mixtures and future directions. Environmental Toxicology and Pharmacology, 16, 111.
  • Angeli-Greaves, M. and McLean, A. E. M. (1981). Effect of diet on the toxicity of drugs. In Gorrod, A. W. (Ed.), Drug Toxicity. Taylor and Francis, London, pp. 91100.
  • Archer, V. E., Wagner, J. R. and Lurdin, F. E. Jr. (1972). Uranium mining and cigarette smoking effects in man. Journal of Occupational Medicine, 15, 204211.
  • Arena, J. M. and Drew, R. H. (1986). Poisoning. Charles C. Thomas, Springfield, pp. 146188.
  • Asher, I. M. and McGrath, P. V. (1976). Symposium on Electron Microscopy of Microfibers. Stock No. 017-012-002244-7, Superintendent of Documents, United States Government Printing Office, Washington, DC.
  • Backhaus, T., Altenburger, R., Boedeker, W., Faust, M., Scholze, M. and Grimme, L. H. (2000). Predictability of the toxicity of a multiple mixture of dissimilarly acting chemicals to Vibrio fischeri. Environmental Toxicology and Chemistry, 19, 23482365.
  • Baeder, C., Wickramarante, G. A. S. and Hummler, H. (1985). Identification and assessment of the effects of chemicals on reproduction and development (reproductive toxicology). Food and Chemical Toxicology, 23, 377388.
  • Bailey, D. G., Arnold, J. M. and Spence, J. D. (1994). Grapefruit juice and drugs. How significant is the intimation. Clinical Pharmacokinetics, 26, 9198.
  • Bailey, D. G., Spence, J. D., Edgar, B., Bayliff, C. D. and Arnold, J. M. (1989). Ethanol enhances the hemodynamic effects of felodipine. Clinical and Investigative Medicine, 12, 357362.
  • Ballantyne, B. (1978). The comparative short-term mammalian toxicology of phenarsazine oxide and phenoxarsine oxide. Toxicology, 10, 341361.
  • Ballantyne, B. (1981). Inhalation hazards of fire. In Ballantyne, B. and Schwabe, P. H. (Eds), Respiratory Protection. Chapman & Hall, London, pp. 351372.
  • Ballantyne, B. (1983). Acute systemic toxicity of cyanides by topical application to the eye. Journal of Toxicology: Cutaneous and Ocular Toxicology, 2, 119129.
  • Ballantyne, B. (1984). Peripheral sensory irritation as a factor in the establishment of workplace exposure guidelines. In Oxford, R. R., Cowell, J. W., Jamieson, G. G. and Love, E. J. (Eds), Occupational Health in the Chemical Industry. Medichem, Calgary, pp. 119149.
  • Ballantyne, B. (1985). Evaluation of hazards from mixtures of chemicals in the occupational environment. Journal of Occupational Medicine, 27, 8594.
  • Ballantyne, B. (1986). Applanation tonometry and corneal pachymetry for prediction of eye irritating potential. Pharmacologist, 28, 173.
  • Ballantyne, B. (1987). Toxicology of cyanides. In Ballantyne, B. and Marrs, T. C. (Eds), Clinical and Experimental Toxicology of Cyanides. Butterworths, London, pp. 41126.
  • Ballantyne, B. (1988). Editorial. In Ballantyne, B. (Ed.), Perspectives in Basic and Applied Toxicology. John Wright and Sons, Ltd, Bristol, pp. vviii.
  • Ballantyne, B. (1989). Toxicology. Encyclopedia of Polymer Science and Engineering, Vol. 16, John Wiley & Sons, Inc., New York, pp. 879930.
  • Ballantyne, B. (2005). The occupational toxicologist: professionalism, morality, and ethical standards in the context of legal and non-litigation issues. Journal of Applied Toxicology, 25, 496513.
  • Ballantyne, B., Bismuth, C. and Hall, A. H. (2007). Cyanides: chemical warfare agents and potential terrorist threats. In Marrs, T. C., Maynrad, R. L. and Sidell, F. R. (Eds), Chemical Warfare Agents. Toxicology and Treatment, 2nd edn. John Wiley & Sons, Ltd, Chichester.
  • Ballantyne, B., Dodd, D. E., Pritts, I. M., Nachreiner, D. S. and Fowler, E. M. (1989a). Acute vapor inhalation toxicity of acrolein and its influence as a trace contaminant of 2-methyoxydihydropyran. Human Toxicology, 8, 229235.
  • Ballantyne, B., Dodd, D. E. and Myers, R. C. (1989b). The acute toxicity of tris(dimethylamino)silane. Toxicology and Industrial Health, 5, 4556.
  • Ballantyne, B., Gazzard, M. F. and Swanston, D. W. (1977). Irritation testing by respiratory exposure. In Ballantyne, B. (Ed.), Current Approaches in Toxicology. Wright, Bristol, pp. 129138.
  • Ballantyne, B. and Jordan, S. L. (2001). Toxicological, medical and industrial hygiene aspects of glutaraldehyde with particular reference to its biocidal use in cold sterilization procedures. Journal of Applied Toxicology, 21, 131151.
  • Ballantyne, B. and Marrs, T. C. (2004). Pesticides: an overview of fundamentals. In Marrs, T. C. and Ballantyne, B. (Eds), Pesticide Toxicology and International Regulation. John Wiley & Sons, Ltd, Chichester, pp. 123.
  • Ballantyne, B., Myers, R. C., Dodd, D. E. and Fowler, E. H. (1988). The acute toxicity of trimethoxysilane (TMS). Veterinary and Human Toxicology, 30, 343344.
  • Bardana, E. J., Montanara, A. and O'Hollaren, M. T. (1992). Occupational Asthma. Hanley and Belfus, Philadelphia.
  • Barlow, S. M. and Sullivan, F. M. (1982). Reproductive Hazards of Industrial Chemicals. Academic Press, London.
  • Barnes, D. G. (2000). Reference dose (RfD): the possible impact of hormesis. Journal of Applied Toxicology, 20, 127130.
  • Baud, F. J., Galliot, M., Astier, A., Bien, D. V., Garnier, R., Likforman, J. and Bismuth, C. (1988). Treatment of ethylene glycol poisoning with intravenous 4-methylpyrole. The New England Journal of Medicine, 319, 97100.
  • BBC (2006). Six Taken Ill After Drug Trial. BBC News, UK. http://news.bbc.co.uk/go/pr/fr/-/2/hi/uk_news/england/london/4807042.stm
  • Beckman, D. A. and Brent, R. L. (1984). Mechanisms of teratogenesis. Annual Review of Pharmacology, 24, 483500.
  • Bhat, A. S. and Ahangar, A. A. (2007). Methods for detecting chemical-chemical interaction in toxicology. Toxicology Mechanisms and Methods, 17, 441450.
  • Billingham, D. J. (1977). Cutaneous absorption and systemic toxicity. In Drill, V. A. and Laza, P. (Eds), Cutaneous Toxicity. Academic Press, New York, pp. 5362.
  • Bismuth, C., Inns, R. H. and Marrs, T. C. (1992). Efficacy, toxicity and clinical use of oximes in anticholinesterase poisoning. In Ballantyne, B. and Marrs, T. C. (Eds), Clinical and Experimental Toxicology of Organophosphates and Carbamates. Butterworth-Heinemann, Oxford, pp. 555577.
  • Bogers, M., Appelman, L. M. and Feron, V. J. (1977). Effects of the exposures profile on the inhalation toxicity of carbon tetrachloride in male rats. Journal of Applied Toxicology, 7, 185119.
  • Borenfreund, E. and Borrero, O. (1984). In vitro cytotoxicity assays. Potential alternatives to the Draize ocular allergy test. Cell Biology and Toxicology, 1, 5565.
  • Borlak, J. (2005). Handbook of Toxicogenomics. John Wiley & Sons, Ltd, Chichester.
  • Borm, P., Klaessig, F. C., Landry, T. D., Moudgil, B., Pauluhn, J., Thomas, K., Trottier, R. and Wood, S. (2006). Research strategies for safety evaluation of nanomaterials. Part V: role of dissolution in biological fate and effects of nanoscale particles. Toxicological Sciences, 90, 23322.
  • Borm, P. A. and Kreyling, W. (2004). Toxicological hazards of inhaled nanoparticles–potential implications for drug delivery. Journal of Nanoscience and Nanotechnology, 4, 521531.
  • Bowie, M. D. and McKenzie, D. (1972). Diethylene glycol poisoning in children. South African Medical Journal, 46, 931934.
  • Brand, R. M., Jendrzejjewski, J. L., Henery, E. M. and Charron, A. R. (2006). A single oral dose of ethanol can alter transdermal absorption of topically applied chemicals in rats. Toxicological Sciences, 92, 349355.
  • Braunwald, E. (1982). Mechanism of action of calcium-channel-blocking agents. The New England Journal of Medicine, 301, 16181627.
  • Breckenridge, A. and Orme, M. L. E. (1987). Principles of clinical pharmacology and therapeutics. In Wetherall, D., Ledington, J. G. S. and Warrell, D. A. (Eds), Oxford Textbook of Medicine, 2nd edn, Vol. 1, Oxford University Press, Oxford, p. 77.
  • Brent, J., McMartin, K., Phillips, S., Burkhart, K., Donovan, J. W., Wells, M. and Kulig, K. (1999). Fomepizole for the treatment of ethylene glycol poisoning. The New England Journal of Medicine, 340, 832838.
  • Bristol, N. (2008). CDC criticised for delaying report on environmental health. Lancet, 371, 640.
  • Bronough, R. L. and Maibach, H. (1985). Percutaneous Absorption. Marcel Dekker, New York.
  • Brooks, S. M. (1983). Bronchial asthma of occupational origin. In Rom, W. M. (Ed.), Environmental and Occupational Medicine. Little, Brown, Boston, pp. 223250.
  • Brusick, D. (1988). Principles of Genetic Toxicology. Plenum Press, New York.
  • Buchanan, J. A., Alhelail, M. A., Cetaruk, E. W., Schaeffer, T. H., Palmer, R. and Brent, J. (2008). Massive ethylene glycol ingestion treated with formepizole alone. Clinical Toxicology, 46, 606.
  • Buehler, E. V. (1965). Delayed contact hypersensitivity in the guinea pig. Archives of Dermatology, 91, 171177.
  • Calabrese, E. J., Moore, G. S. and Williams, P. (1987). The effect of methyl oleate ozonide, a possible ozone intermediate, on normal and G-6-PD deficient erythrocytes. Bulletin of Environmental Contamination and Toxicology, 29, 498504.
  • Calvery, H. O. and Klumpp, T. G. (1939). The toxicity for human beings of diethylene glycol with sulfanilamide. Southern Medical Journal, 32, 11051109.
  • Carson, R. (1962). Silent Spring. Middleton Mifflin Company, Boston.
  • Cartwright, R. A., Glashan, R. W., Rodgers, M. J., Ahmad, R. A., Barham-Had, D., Higgins, E. and Kahn, M. A. (1982). Role of N-acetyltransferase phenotypes in bladder carcinogenesis: a pharmacogenetic epidemiological approach to bladder cancer. Lancet, 2, 842846.
  • Christiani, D. C., Mehta, A. J. and Yu, C.-L. (2008). Genetic susceptibility to occupational exposure. Occupational and Environmental Medicine, 65, 430436.
  • Chung, W. H., Hung, S. I., Hong, H. S., Hsih, M. S., Yang, L. C., Ho, H. C., Wu, J. Y. and Chen, Y. T. (2004). Medical genetics: a marker for Stevens-Johnson syndrome. Nature, 428, 486.
  • Clayson, D. B. (1987). The need for biological risk assessment in reaching decisions about carcinogens. Mutation Research, 185, 243269.
  • Conference (1990). Organic dust and lung disease. American Journal of Industrial Medicine, 17, 1148.
  • Corley, R. A., Markham, D. A., Banks, C., Delorme, P., Masterman, A. and Houle, J. M. (1997). Physiologically based pharmacokinetics and dermal absorption of 2-butoxyethanol vapor by humans. Fundamental and Applied Toxicology, 39, 120130.
  • Cornwell, P. A. and Barry, B. W. (1995). Effects of permeation enhancer treatment on the statistical distribution of human skin permeability. International Journal of Pharmaceutics, 117, 101112.
  • Cronin, E. (1980). Contact Dermatitis. Churchill Livingstone, Edinburgh.
  • Crump, K. S. (1984). A new method for determining allowable daily intakes. Fundamental and Applied Toxicology, 4, 854871.
  • Cummings, J. L. (1999). Understanding Parkinson disease. The Journal of the American Medical Association, 281, 376378.
  • Cunningham, M. L. (2006). Putting the fun into functional genomics. Toxicological Sciences, 92, 347348.
  • Cunningham, M. L., Bogdanffy, M. S., Zacharewski, T. R. and Hines, R. N. (2003). Workshop overview: use of genomic data in risk assessment. Toxicological Sciences, 73, 209215.
  • Curfman, G. D., Morrissey, S. and Drazen, J. M. (2008a). The FDA and tobacco regulation. The New England Journal of Medicine, 359, 10.
  • Curfman, G. D., Morrissey, S., Annas, G. J. and Drazen, J. M. (2008b). Peer review in the balance. The New England Journal of Medicine, 358, 22762277.
  • DeAngelis, C. D. and Thornton, J. P. (2008). Preserving confidentiality in the peer review process. The Journal of the American Medical Association, 299, 1956.
  • Dearman, R. J., Betts, C. J., Humphreys, N., Flanagan, B. F., Gilmour, N. J., Basketter, D. A. and Kimber, I. (2003). Chemical allergy. Considerations for the practical application of cytokine fingerprinting. Toxicological Sciences, 71, 137145.
  • Dearman, R. J. and Kimber, I. (2001). Cytokine fingerprinting and hazard assessment of chemical respiratory allergy. Journal of Applied Toxicology, 21, 153163.
  • Derwan, A., Jani, J. P., Shah, K. S. and Kashyap, S. K. (1986). Urinary excretion of benzidine in relation to acetylator status of occupationally exposed subjects. Human Toxicology, 5, 9597.
  • Dewar, A. J. (1981). Neurotoxicity testing. In Gorrod, J. W. (Ed.), Testing for Toxicity. Taylor and Francis, London, pp. 199218.
  • Donaldson, K., Aitken, R., Tran, L., Stone, V., Duffin, R., Forrest, G. and Alexander, A. (2006). Carbon nanotubes: a review of their properties in relation to pulmonary toxicology and workplace safety. Toxicological Sciences, 92, 522.
  • Donaldson, K., Stone, V., Clouter, A., Renwick, L. and MacNee, W. (2001). Ultrafine particles. Occupational and Environmental Medicine, 58, 211216.
  • Donaldson, K., Tran, L., Jimenez, L. A., Duffin, R., Newby, D. E., Mills, N., MacNee, W. and Stone, V. (2005). Combustion-derived nanoparticles: a review of their toxicology following inhalation exposure. Particle and Fibre Toxicology, 2, 10.
  • Dorne, J. L. C. M. and Renwick, A. G. (2005). The refinement of uncertainty/safety factors in risk assessment by the incorporation of data on toxicokinetic variability in humans. Toxicological Sciences, 86, 2026.
  • Doss, M. O. and Columbi, A. M. (1987). Chronic hepatic porphyria induced by chemicals: the example of dioxin. In Foa, V., Emmett, E. A., Maroni, M. and Columbus, A. (Eds), Occupational and Environmental Chemical Hazards. Ellis Horwood, Chichester, pp. 231240.
  • Dreher, K. L. (2004). Health and environmental impact of nanotechnology: toxicological assessment of manufactured nanoparticles. Toxicological Sciences, 77, 35.
  • Dugard, P. H. (1977). In Marzulli, F. N. and Maibach, H. I. (Eds), Dermatotoxicology and Pharmacology, Chapter 22. Hemisphere, Washington, DC, pp. 525550.
  • EAASM (2008). Five Chinese Jailed Over Lethal Fake Medicine Scandal. European Alliance for Access to Safe Medicine, Cardiff, UK. www.eaasm.eu/Media_centre/News/April_2008-23k
  • Editorial (2006). A universal code of ethics falls badly short. Lancet, 367, 86.
  • Editorial (2008). The pitfalls and rewards of peer review. Lancet, 371, 447.
  • Edmunds, M. (2005). Animals in research. Biologist, 52, 259260.
  • Edwards, D. J., Bellevue, F. H. and Woslee, P. M. (1996). Identification of 6′,7′-dihydroxybergamottin, a cytochrome P450 inhibitor, in grapefruit juice. Drug Metabolism and Disposition, 24, 12871290.
  • Elder, A., Gelein, R. and Silva, V. (2006). Translocation of inhaled ultrafine manganese oxide particles in the central nervous system. Environmental Health Perspectives, 114, 11721178.
  • Ellenhorn, M. J. and Barceloux, D. G. (1988). Medical Toxicology. Elsevier, New York, pp. 10701103.
  • Ellinger-Ziegelbauer, H., Stuart, B., Wahle, B., Bomann, W. and Ahr, H.-J. (2004). Characteristic expression profiles induced by genotoxic carcinogens in rat liver. Toxicological Sciences, 77, 1934.
  • Elliott, C. (2005). Should journals publish industry-funded bioethics articles. Lancet, 366, 422424.
  • EPA (2003). Proceedings: Environmental Protection Agency Nanotechnology and the Environment: Applications and Implications. STAR Progress Review Workshop. EPA Document Number EPA/600/R-02/080.
  • Evered, D. and Lazar, P. (1995). Misconduct in medical research. Lancet, 345, 11611162.
  • The Faculty of Occupational Medicine (1999). Guidance on Ethics for Occupational Physicians. Faculty of Occupational Medicine of the Royal College of Physicians of London, London, UK.
  • Ferrari, L. A. and Giannuzzi, I. (2005). Clinical parameters, post-mortem analysis and estimation of acute lethal dose in victims of a massive intoxication with diethylene glycol. Forensic Science International, 153, 4551.
  • Fielden, M. R., Nie, A., McMillian, M., Elangbam, C. S., Trela, B. A., Yang, Y., Dunn, R. T., II, Dragan, Y., Fransson-Stehen, R., Bogdanffy, M., Adams, S. P., Foster, W. R., Chen, S.-J., Rossi, P., Kasper, P., Jacobson-Kram, D., Tatsuoka, K. S., Wier, P. J., Gollub, J., Halbert, D. N., Roter, A., Young, J. K., Sina, J. F., Marlowe, J., Martus, H.-J., Aubrecht, J., Olaharski, A. J., Roome, N., Nioi, P., Pardo, I., Snyder, R., Perry, R., Lord, P., Mattes, W. and Car, B. D. for the Predictive Safety Testing Consortium and Carcinogenicity Working Group (2008). Interlaboratory evaluation of genomic signatures for predicting carcinogenicity in the rat. Toxicological Sciences, 103, 2834.
  • Fuhr, V. (1998). Drug Interactions with grapefruit juice. Drug Safety, 18, 251272.
  • Fuhr, V., Klittich, K. and Staib, A. H. (1993). Inhibiting effect of grapefruit juice and its bitter principal, naringemin, on CYP1 A2-dependent metabolism of caffeine in man. British Journal of Clinical Pharmacology, 35, 431436.
  • Furst, A. (1997). The Toxicologist as an Expert Witness. Taylor and Francis, Bristol, Pennsylvania.
  • Furst, A. and Reidy, D. F. (1999). The toxicologist and the law. In Ballantyne, B., Marrs, T. C. and Syversen, T. (Eds), General and Applied Toxicology, 2nd edn, Vol. 3, Macmillan Reference, London, pp. 14731488.
  • Gad, S. C. (1982). A neuromuscular screen for use in industrial toxicology. Journal of Toxicology and Environmental Health, 9, 691704.
  • Gatzidou, E. T., Zira, A. N. and Theocharis, S. E. (2007). Toxicogenomics: a pivotal piece in the puzzle of toxicological research. Journal of Applied Toxicology, 27, 302309.
  • Geiser, M., Rothen-Rutishauser, B. and Kapp, N. (2005). Ultrafine particles cross cellular membranes by nonphagocytic mechanisms in lungs and noncultured cells. Environmental Health Perspectives, 113, 15551600.
  • van Gelderen, C. E. M., Savelkourl, J. T. F. and Sangster, B. (1990). Safety studies in humans. II. Human volunteer studies. Food and Chemical Toxicology, 28, 775778.
  • Gerberick, G. F., Ryan, C. A., Kimber, I., Dearman, R. J., Lea, L. J. and Basketter, D. A. (2000). Local lymph node assay: validation assessment for regulatory purposes. American Journal of Contact Dermatitis, 11, 318.
  • Gibaldi, M. and Perrier, D. (1982). Pharmacokinetics. Marcel Dekker, New York.
  • Gilbert, S. G. (2006). Supplementing the traditional institutional review board with an environmental health and community review board. Environmental Health Perspectives, 114, 16261629.
  • Golberg, L. (1982). A code of ethics for scientists reporting and reviewing information on chemicals. Fundamental and Applied Toxicology, 2, 289292.
  • Goldenthal, E. I. (1971). A compilation of LD50 values in newborn and adult animals. Toxicology and Applied Pharmacology, 18, 185207.
  • Goldfrank, L. R., Flomenbaum, N. E., Lewin, N. A., Weisman, R. S. and Howland, M. A. (1990). Goldfrank's Toxicologic Emergencies, 4th edn. Appleton and Lange, Norwalk, p. 183.
  • Goodman, K. W. (1996). Code of ethics in occupational and environmental health. Journal of Occupational and Environmental Hygiene, 38, 882883.
  • Goldman, L. R. and Links, J. M. (2004). Testing toxic compounds in human subjects: ethical standards and good science. Environmental Health Perspectives, 112, A458A459.
  • Goodwin, R. F. J., Crevel, W. R. W. and Johnson, A. W. (1981). A comparison of three guinea-pig sensitization procedures for the detection of 19 reported human contact sensitizers. Contact Dermatitis, 7, 248258.
  • Grasso, P. (1988). Carcinogenicity tests in animals: some pitfalls that could be avoided. In Ballantyne, B. (Ed.), Perspectives in Basic and Applied Toxicology. Wright, London, pp. 268284.
  • Griffith, J. F. (1964). Interlaboratory variations in the determination of acute oral LD50. Toxicology and Applied Pharmacology, 6, 726730.
  • Haddad, S., Béliveau, M., Tardiff, R. and Krishnan, K. (2001). A PBPK modelling-based approach to account for interactions in the health risk assessment of chemical mixtures. Toxicological Sciences, 63, 125131.
  • Hallenbeck, W. H. and Cunningham, K. M. (1986). Quantitative Risk Assessment for Occupational and Environmental Health. Lewis Publishers, Chelsea.
  • Hamadeh, H. K. and Afshari, C. A. (Eds) (2004). Toxicogenomics. Principles and Applications. John Wiley & Sons, Ltd, Chichester.
  • Hammond, D., Collishaw, N. E. and Callard, C. (2006). Secret science: tobacco industry research on smoking behaviour and cigarette toxicity. Lancet, 367, 781787.
  • Hammond, E. L. and Selikoff, J. J. (1973). Relation of cigarette smoking to risk of death of asbestos associated disease among insulation workers in the United States. In Bogorski, D., Timbrell, J. C., Wagner, J. C. and Davis, W. (Eds), Biological Effects of Asbestos, IARC Scientific Publications No. 8. IARC, Lyon, pp. 312317.
  • Hanif, M., Mobarak, M. and Ronan, A. (1995). Fatal renal failure caused by diethylene glycol poisoning in paracetamol elixir. The Bangladesh epidemic. British Medical Journal, 311, 8891.
  • Hari, P., Jain, Y. and Kabra, S. K. (2006). Fatal encephalopathy and renal failure caused by diethylene glycol poisoning. Journal of Tropical Pediatrics, 52, 442444.
  • Hernberg, S. (1995). Ethics in research. Scandanavian Journal of Work, Environment and Health, 21, 241243.
  • Hilton, J., Dearmna, R. J., Harvey, P., Evans, P., Basketter, D. A. and Kimber, J. (1998). Evaluation of relative skin sensitizing potential using the local lymphnode assay: a comparison of formaldehyde with glutaraldehyde. American Journal of Contact Dermatitis, 9, 2933.
  • Hollingshaus, J. G., Armstrong, D. and Toia, R. F. (1981). Delayed toxicity and delayed neurotoxicity of phosphorothionate and phosphonothionate esters. Journal of Toxicology and Environmental Health, 8, 619627.
  • Holsapple, M. P., Jones, D., Kawabata, T. T., Kimber, I., Sarlo, K., Delgrade, M. J. K., Shah, J. and Woolhiser, M. R. (2006). Assessing the potential to induce respiratory hypersensitivity. Toxicological Sciences, 9, 413.
  • Holtzman, N. A. (2003). Ethical aspects of genetic testing in the workplace. Community Genetics, 6, 136138.
  • Horton, A. A. and Fairhurst, S. (1987). Lipid peroxidation and mechanism of toxicity. CRC Critical Reviews in Toxicology, 18, 2779.
  • Huang, S.-H., Goodsaid, F., Rahman, A., Fruch, F. and Lesko, L. J. (2006). Application of pharmacogenomics in clinical pharmacology. Toxicology Mechanisms and Methods, 16, 8999.
  • Hubert, M.-F., Laroque, P., Gillet, J.-P. and Keenan, P. (2000). The effects of diet, ad libitum feeding, and moderate and severe dietary restriction on body weight, survival, clinical pathology parameters, and cause of death in control Sprague-Dawley rats. Toxicological Sciences, 58, 1952007.
  • Hunter, W. J., Lingk, W. and Recht, P. (1979). Intercomparison study on the determination of single administration toxicity in rats. Journal of the Association of Official Analytical Chemists, 62, 864873.
  • Institute of Biology (2005). Code of Conduct and Guide on Ethical Practise. Institute of Biology, London.
  • Isola, D., Kimber, I., Sarlo, K., Lalko, J. and Sipes, G. (2008). Chemical respiratory allergy and occupational asthma. What are the key areas of uncertainty? Journal of Applied Toxicology, 28, 249253.
  • Jacobs, R. R. and Phanprasik, W. (1993). An in vitro comparison of the permeation of chemicals in vapor and liquid phase through pig skin. American Industrial Hygiene Association Journal, 54, 560575.
  • JMPR (1998). Pesticide Residues in Food-1997. Toxicological and Environmental Evaluations. Joint Meeting of the FAO Panel of Experts and the WHO Core Assessment Group, 22 September-1 October 1997, Lyon.
  • JMPR (1999). Report. Joint Meeting of the FAO Panel of Experts and the WHO Core Assessment Group, 23 September-3 October, 1998, Rome.
  • Johanson, G. and Boman, A. (1991). Percutaneous absorption of 2-butoxyethanol vapor in human subjects. British Journal of Industrial Medicine, 48, 788792.
  • Karol, M. H., Stadler, J. and Magreni, C. (1985). Immunotoxicologic evaluation of the respiratory system: animal models for immediate and delayed-onset pulmonary hypersensitivity. Fundamental and Applied Toxicology, 5, 459472.
  • Kast, A. and Nishikawa, J. (1981). The effect of fasting on oral acute toxicity of drugs in rats and mice. Laboratory Animals, 15, 359364.
  • Kemppainen, B. W. and Reifenrath, W. G. (1990). Methods for Skin Absorption. CRC Press, Boca Raton.
  • Keys, A. (2008). In RE: Bextra and Celebrex Marketing Sales Practises and Product Liability Litigation. Case No. 09 C 402, US District Court, North District of Illinois, Eastern Division, March 14, 2008.
  • Khanna, V. K., Husain, R. and Seth, P. R. (1988). Low protein diet modifies acrylamide neurotoxicity. Toxicology, 49, 395401.
  • Kiani, J. and Imam, S. Z. (2007). Medicinal importance of grapefruit juice and its interaction with various drugs. Nutrition Journal, 6, 33.
  • Kimber, I., Basketter, D. A., Berthold, K., Butler, M., Garrigue, J.-L., Lea, L., Newsome, R., Roggeband, R., Steiling, W., Stroppp, G., Waterman, S. and Weimann, C. (2001). Skin sensitisation testing in potency and risk assessment. Toxicological Sciences, 59, 198208.
  • Kimber, I., Dearman, R. J., Basketter, D. A., Ryan, C. A. and Gerberick, G. F. (2002). The local lymph node assay: past, present and future. Contact Dermatitis, 47, 315328.
  • Kimber, I., Dearman, R. J., Scholes, E. W. and Basketter, D. A. (1994). The local lymph node assay. Developments and applications. Toxicology, 93, 1331.
  • Kimbrough, R. D., Falk, H. and Stehr, P. (1984). Health implications of 2,3,7,8-tetrachlorodibenzodioxin (TCDD) contamination of industrial soil. Journal of Toxicology and Environmental Health, 14, 4793.
  • Klonne, D. R., Garman, R. H., Snellings, W. M. and Ballantyne, B. (1987). The Larynx as a Potential Target Organ in Aerosol Studies in Rats. Abstracts, 1987 International Symposium on Inhalation Toxicity, Karger, Basel, p. 86.
  • Kohn, S. M., Kohn, M. D. and Colapinto, D. K. (2004). Whistleblower Law. A Guide to Legal Protections for Corporate Employees. National Whistleblower Center, Washington, DC.
  • Kreyling, W. G., Semmler-Behnke, M. and Moller, W. (2006). Ultrafine particle-lung interactions: does size matter? Journal of Aerosol Medicine, 19, 7483.
  • Krisch-Volders, M. (1984). Mutagenicity, Carcinogenicity and Teratogenicity of Industrial Pollutants. Plenum Press, New York.
  • Kurt, T. L., Anderson, R., Petty, C., Bost, R., Reed, G. and Holland, J. (1988). Dinitrophenol in weight loss: the poison center and public health safely. Veterinary and Human Toxicology, 28, 574575.
  • Lam, C.-W., James, J. T., McCluskey, R. M. and Hunter, R. L. (2004). Pulmonary toxicity of single-wall carbon nanotubes in mice 7 and 90 days after intratracheal instillation. Toxicological Sciences, 77, 126134.
  • Landrigan, L. J. (1995). Disclosure of interest. American Journal of Industrial Medicine, 28, 581582.
  • Lehman-McKeeman, L. and Peterson, R. E. (2003). Guidelines governing conflict of interest (Editorial). Toxicological Sciences, 72, 183184.
  • Limasset, J. C., Simon, P., Poirot, P., Subre, I. and Grzeby, K. M. (1999). Estimation of the percutaneous absorption of styrene in an industrial situation. International Archives of Occupational and Environmental Health, 72, 4651.
  • Lin, B. I., Zhao, Z. X., Chong, J. T., Li, J. G., Zuo, X., Tao, Y., Lou, T. Q. and Gao, Z. L. (2008). Venous diethylene glycol poisoning inpatients with preexisting severe liver disease in China. World Journal of Gastroenterology, 14, 32363241.
  • Lloyd, G. (1991). Psychiatry. In Edwards, C. R. W. and Bouchier, I. A. D. (Eds), Davidson's Principles and Practise of Medicine. Churchill Livingstone, Edinburgh, p. 937.
  • Lock, S. (1995). Lessons from the Pearce affair: handling scientific fraud. British Medical Journal, 310, 15471546.
  • Loewenberg, S. (2006). US chemical companies leave their mark on EU law. Lancet, 367, 556557.
  • de Longueville, F., Bertholet, V. and Remacle, J. (2004). DNA microarrays as a tool in toxicogenomics. Combinatorial Chemistry and High Throughput Screening, 7, 207211.
  • Lu, F. C. (1985). Basic Toxicology. Hemisphere, Washington, DC.
  • Magnusson, B. and Kligman, A. M. (1969). The identification of contact allergens by animal assay. The guinea pig maximization test. The Journal of Investigative Dermatology, 52, 268276.
  • Magnusson, B. and Kligman, A. M. (1970). Allergic Contact Dermatitis in the Guinea Pig. Charles C, Thomas, Springfield.
  • Maickel, R. P. and McFadden, D. P. (1979). Acute toxicology of butyl nitriles and butyl alcohols. Research Communications in Chemical Pathology and Pharmacology, 26, 7583.
  • Malmary, M.-F., Kabbaj, K. and Oustrin, J. (1988). Circadian dosing stage dependence in metabolic effects of cicyclosporinein the rat. Annual Review of Chromopharmacology, 5, 3538.
  • Marks, V., English, J., Aherne, W. and Arendl, J. (1985). Chronopharmacology. Clinical Biochemistry, 18, 154157.
  • Marrs, T. C. (1987). The choice of cyanide antidotes. In Ballantyne, B. and Marrs, T. C. (Eds), Clinical and Experimental Toxicology of Cyanides. Butterworths-Heinemann, Oxford, pp. 383401.
  • Marrs, T. C. (1988). Experimental approaches to the design and assessment of antidotal procedures. In Ballantyne, B. (Ed.), Perspectives in Basic and Applied Toxicology. Wright, London, pp. 285308.
  • Marrs, T. C., Maynard, R. L. and Sidell, F. R. (1996). Chemical Warfare Agents. Toxicology and Treatment. John Wiley & Sons, Ltd, Chichester, pp. 115137.
  • Mattison, D. R. (1983). Reproductive Toxicology. Alan R. Liss, New York.
  • Maurer, T., Weirich, E. G. and Hess, R. (1984). Predictive contact allergenicity influence of the animal strain used. Toxicology, 31, 217222.
  • Maurissen, J. P., Gilbert, S. G., Sander, M., Beauchamp, T. L., Johnson, S., Schwetz, B. A., Goozner, M. and Barrow, C. S. (2005). Workshop proceedings: managing conflict of interest in science. A little consensus and a lot of controversy. Toxicological Sciences, 87, 1114.
  • McDougal, J. N., Jepson, G. W., Clewell, H. J., MacNaughton, M. G. and Anderson, M. E. (1986). A physiologically-based pharmacokinetic modal for dermal absorption of vapors in the rat. Toxicology and Applied Pharmacology, 83, 286294.
  • McKinney, J. D., Richard, A., Waller, C., Newman, M. C. and Gerberick, F. (2000). The practise of structure activity relationships (SAR) in toxicology. Toxicological Sciences, 56, 817.
  • Mehendale, H. M. (1987). Hepatotoxicity. In Haley, T. J. and Berndt, W. O. (Eds), Handbook of Toxicology. Hemisphere, Washington, DC, pp. 74111.
  • Melnick, R. L. and Huff, J. (2004). Testing toxic pesticides in humans: health risks with no health benefits. Environmental Health Perspectives, 112, A459A461.
  • Menegon, A., Board, P. G., Blackburn, A. C., Mellick, G. D. and LeCouteur, D. G. (1998). Parkinson's disease, pesticides, and glutathione transferase polymorphisms. Lancet, 352, 13441346.
  • Meredith, T. J., Jacobsen, D., Haines, J. A., Berger, J.-C. and van Heist, A. N. P. (1993). Antidotes for Poisoning by Cyanide, IPCS/CEC Evaluation of Antidote Series, Vol. 2, Cambridge University Press, Cambridge.
  • Mitchell, C. L. (1982). Nervous System Toxicology. Raven Press, New York.
  • Moggs, J. G. (2005). Molecular responses to xenoestrogens. Mechanistic insight from toxicogenomics. Toxicology, 213, 177193.
  • Murdock, R. C., Braydich-Stolle, L., Schrand, R. M., Schlanger, J. J. and Hussain, S. M. (2008). Characterization of nanometric dispersion in solution prior to in vitro exposure using dynamic light scattering technique. Toxicological Sciences, 101, 239253.
  • Murphy, S. D. (1983). General principles in the assessment of toxicity of chemical mixtures. Environmental Health Perspectives, 48, 141144.
  • Murray, C. B., Kagan, C. R. and Bawendi, M. G. (2000). Synthesis and characterization of monodisperse nanocrystals and close-packed nanocrystal assemblies. Annual Review of Materials Science, 30, 545610.
  • Nakamura, Y. (2008). Pharmacogenomics and drug toxicity. The New England Journal of Medicine, 359, 856858.
  • Nardone, R. M. and Bradlaw, J. A. (1983). Toxicity testing with in vitro systems: I. Ocular tissue culture. Journal of Toxicology: Cutaneous and Ocular Toxicology, 2, 8188.
  • National Research Council (1987). Pharmacokinetics in Risk Assessment. National Academy Press, Washington, DC.
  • National Research Council (1988). Principles of Toxicological Interactions Associated with Multiple Chemical Exposures. National Academy Press, Washington, DC.
  • Oberdörster, G. (2000). Toxicology of ultrafine particles. In vivo studies. Philosophical Transactions of the Royal Society of London, Series A, 358, 27192740.
  • Oberdörster, G., Sharp, Z. and Atudorei, V. (2002). Extrapulmonary translocation of ultrafine carbon particles following whole-body inhalation exposure of rats. Journal of Toxicology and Environmental Health, Part A, 65, 15311543.
  • O'Brien, K. L., Selanikio, J. D., Hecdivert, C., Placide, M.-F., Louis, M., Barr, D. B., Barr, J., Hospedales, C. J., Lewis, M. J., Schwartz, B., Philen, R. M., St. Victor, S., Espindola, J., Needham, L. L. and Denerville, K. (1998). Epidemic of pediatric deaths from acute renal failure caused by diethylene glycol poisoning. The Journal of the American Medical Association, 279, 11751180.
  • OECD (1995). OECD Guidelines for Testing of Chemicals. Delayed Neurotoxicity of Organophosphorus Substances Following Acute Exposure. OECD Guideline 418. Organization for Economic Cooperation and Development, Paris.
  • Oehme, F. W. (1980). Absorption, biotransformation, and excretion of environmental chemicals. Clinical Toxicology, 17, 147158.
  • Okuonghae, H. O., Ighogboja, I. S., Lawson, J. O. and Nwana, E. J. C. (1992). Diethylene glycol poisoning in Nigerian children. Annals of Tropical Paediatrics, 12, 235238.
  • Oleskey, C., Fleischman, A., Goldman, L., Hirschhorn, K., Landrigan, P. J., Lappé, M., Marshall, M. F., Needleman, H., Rhodes, R. and McCally, M. (2004). Pesticide testing in humans: ethics and public policy. Environmental Health Perspectives, 112, 914919.
  • Olmstead, A. W. and LeBlanc, G. A. (2005). Toxicity assessment of environmentally relevant pollutant mixtures using a heuristic model. Integrated Environmental Assessment and Management, 1, 114122.
  • Owens, E. J. and Punte, C. L. (1963). Human respiratory ocular irritation studies utilizing o-chlorobenzylidene malononitrile aerosols. American Industrial Hygiene Association Journal, 24, 262264.
  • Pandya, S. K. (1988). An unmitigated tragedy. British Medical Journal, 297, 117179.
  • Pariat, C., Courtois, P., Cambar, J., Piriou, A. and Bouquet, S. (1988). Circadian variations in the renal toxicity of gentamicin in rats. Toxicology Letters, 40, 175182.
  • Paris, P. M. (1996). Relationship between academia and industry: ethical considerations. Annals of Emergency Medicine, 27, 416417.
  • Paschal, D. C., DiPietro, E. S., Phillips, D. L. and Gunter, E. W. (1989). Age dependence of metals in hair in selected U. S. population. Environmental Research, 48, 1728.
  • Pennie, W. D., Tugwood, J. D., Oliver, G. J. A. and Kimber, I. (2000). The principles and practise of toxicogenomics: applications and opportunities. Toxicological Sciences, 54, 277283.
  • Pérez, L. M., Milkiewicz, P., Elias, E., Coleman, R., Pozzi, E. J. S. and Roma, M. G. (2006). Oxidative stress induces internalisation of the bile salt export pump, Bsep, and bile salt excretory failure in isolated rat hepatocyte couplets: a role for protein kinase C and prevention by protein kinase A. Toxicological Sciences, 91, 150158.
  • Randall, J. A. and Gibson, R. S. (1989). Hair chromium as an index of chromium exposure of tannery workers. British Journal of Industrial Medicine, 46, 171175.
  • Rao, G. N. and Knapka, J. J. (1998). Animal diets in safety evaluation studies. In Ioanides, C. (Ed.), Nutrition and Chemical Toxicity. John Wiley & Sons, Ltd, Chichester, pp. 345374.
  • Rao, M. S. and Reddy, J. K. (1987). Peroxisome proliferation and hepatocarcinogenesis. Carcinogenesis, 8, 631636.
  • Rao, K. S., Schwetz, B. A. and Park, C. N. (1987). Reproductive risk assessment of chemicals. Veterinary and Human Toxicology, 23, 167175.
  • Rhomberg, L. R. (2005). Seeking optimal design for animal bioassay studies. Toxicological Sciences, 84, 13.
  • Rider, C. V. and LeBlanc, G. (2005). An integrated addition and interaction model for assessing toxicity of chemical mixtures. Toxicological Sciences, 87, 520528.
  • Roberts, J. and Smith, R. (1996). Publishing research supported by the tobacco industry. British Medical Journal, 312, 133134.
  • Robertson, G. G. (2005). Metabonomics in toxicology: a review. Toxicological Sciences, 885, 809822.
  • Rodier, P. M. (1980). Chronology of neuron development: animal studies and their clinical implications. Developmental Medicine and Clinical Neurology, 22, 525545.
  • Roy, P. D., Majumder, M. and Roy, B. (2008). Pharmacogenomics of anti-TB drug-related hepatotoxicity. The Pharmacogenomics Journal, 9, 311321.
  • Royal Society of Chemistry (1995). Ethical guidelines to publication of chemical research. Chemical Reviews, 95, 11A13A.
  • Runkel, M., Bourian, M., Teglmeier, M. and Legrum, W. (1997). The role of naringin in the interaction of drugs with juices of grapefruit juice. Archives of Pharmacology, 355, R123.
  • Rutter, T. (1995). US journals veto tobacco funded research. British Medical Journal, 312, 11.
  • Ryan, C. A., Chaney, J. G., Kern, P. S., Patlewitcz, G. Y., Basketter, D. A., Betts, C. J., Dearman, R. J., Kimber, I. and Gerberick, G. F. (2008). The reduced local lymph node assay: the impact of group size. Journal of Applied Toxicology, 28, 518523.
  • Ryan, C. A., Gerberick, G. F., Gildea, L. A., Hulette, B. C., Betts, C. J., Cumberbatch, M., Dearman, R. J. and Kimber, I. (2005). Interactions of contact allergens with dendritic cells: opportunities and challenges for the development of novel approaches to hazard assessment. Toxicological Sciences, 88, 411.
  • Safe, S. (1990). Polychlorinated biphenyls (PCBs), dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs) and related compounds: environmental and mechanistic considerations which support the development of toxic equivalency factors (TUFs). CRC Critical Reviews in Toxicology, 21, 5188.
  • Sand, S., ovn Rosen, D., Victorin, K. and Filipsson, A. F. (2006). Identification of a critical dose level for risk assessment: developments in benchmark dose analysis of continuous endpoints. Toxicological Sciences, 90, 241251.
  • Sand, S., Victorin, K. and Filisson, A. F. (2008). The current state of knowledge on the use of the benchmark dose concept in risk assessment. Journal of Applied Toxicology, 28, 405421.
  • Sanderson, J. T. (2006). The steroid hormone biosynthesis pathway as a target for endocrine-disrupting chemicals. Toxicological Sciences, 94, 321.
  • Sass, J. B. and Needleman, H. L. (2004). Industry testing of toxic pesticides on human subjects concluded ‘no effect’ despite the evidence. Environmental Health Perspectives, 112, A154A155.
  • Schwartz, R. S., Curfman, G. D., Morrissey, S. and Drazen, J. M. (2008). Full disclosure and the funding of biomedical research. The New England Journal of Medicine, 358, 17.
  • Schwetz, B. A., Lehman-McKeeman, L. and Birnbaum, L. S. (2005). Toxicological research involving humans: ethical and regulatory considerations. Toxicological Sciences, 65, 419421.
  • Shell, J. W. (1982). Pharmacokinetics of topically applied ophthalmic drugs. Survey of Ophthalmology, 26, 207218.
  • Shopsis, C. and Sathe, S. (1984). Uridine uptake inhibition as a cytotoxicity test: correlations with the Draize test. Toxicology, 29, 195206.
  • Singh, J., Dutta, A. K., Khare, S., Dubey, N. K., Harit, A. K., Jain, N. K., Wadhwa, T. C., Gupta, S. R., Dhariwal, A. C., Jain, D. C., Bhatia, R. and Sokhey, J. (2001). Diethylene glycol poisoning in Gurgaon, India, 1998. Bulletin of the World Health Organization, 79, 8895.
  • Society of Toxicology (2005). Using Animals for Toxicological Research and Testing: Best Practices for Assuring Compliance with Animal Welfare Regulations. 45th Annual Meeting. March 5–9, 2005. San Diego, CA. http://iccvam.niehs.nih.gov/meetings/SOT06/sot06wsSpSess.pdf.
  • Sockon, K. and Sockon, D. (1987). A survey of whistle blowers: their stresses and coping strategies. Association of Mental Health Specialities. Laurel, Maryland.
  • Soskolne, C. L. (1999). Professional integrity and whistle blower protection putto the test: a success story. Epidemiology, 10, S173.
  • Spencer, P. S., Bischoff, M. C. and Schaumburg, H. H. (1980). Neuropathological methods for the detection of neurotoxic diseases. In Spencer, P. S. and Schaumburg, H. H. (Eds), Experimental and Clinical Neurotoxicology. Williams and Wilkins, Baltimore, pp. 743757.
  • Sperling, F. (1984). Quantitation of toxicology-the dose-response relationship. In Sperling, F. (Ed.), Toxicology: Principles and Practise, Vol. 2, John Wiley & Sons, Inc., New York, pp. 199218.
  • Stanton, M. F., Layard, M., Tegeris, A., Miller, E., May, M., Morgan, E. and Smith, A. (1981). Relation of particle dimension to carcinogenicity in amphobole asbestoses and other fibrous minerals. Journal of the National Cancer Institute, 67, 965975.
  • Stern, S. T. and McNeil, S. E. (2008). Nanotechnology safety concerns revisited. Toxicological Sciences, 101, 421.
  • Stüttgen, G., Siebel, T. and Aggerbeck, B. (1982). Absorption of boric acid through human skin depending on the type of vehicle. Archives of Dermatological Research, 272, 2129.
  • Sweet, D. V. (Ed.) (1985–1986a). Registry of Toxic Effects of Chemical Substances, Vol. 3. US Department of Health and Human Services, Centers for Disease Control, NIOSH, Washington, DC, p. 2360.
  • Sweet, D. V. (Ed.) (1985–1986b). Registry of Toxic Effects of Chemical Substances, Vol 3. US Department of Health and Human Services, Centers for Disease Control, NIOSH, Washington, DC, p. 3060.
  • Tallarida, R. J. and Jacob, L. S. (1979). The Dose-Response Relation in Pharmacology. Springer, New York.
  • Teuschler, L. K., Rice, G. E., Wilkes, R. C., Lipscomb, J. C. and Power, F. W. (2004). A feasibility study of cumulative risk assessment methods for drinking water disinfection by-product mixtures. Journal of Toxicology and Environmental Health, 67, 755779.
  • Thomas, K., Aguar, P., Kawasaki, H., Morris, J., Nakanishi, J. and Savage, N. (2006a). Research strategies for safety evaluation of nanomaterials: Part VIII: international efforts to develop risk-based safety evaluations for nanomaterials. Toxicological Sciences, 92, 2332.
  • Thomas, T., Thomas, K., Sadrich, N., Savage, N., Adair, P. and Bronaugh, R. (2006b). Research strategies for safety evaluation of nanomaterials, Part VII. Evaluating consumer exposure to nanoscale materials. Toxicological Sciences, 91, 1419.
  • Thomas, K. and Sayre, P. (2005). Research strategies for safety evaluation of nanomaterials. Part I: evaluating the human health implications of exposure to nanoscale materials. Toxicological Sciences, 87, 316321.
  • Thomas, J. A., Stargel, W. W. and Tschanz, C. (1998). Interactions between drugs and diet. In Ioannides, C. (Ed.), Nutrition and Chemical Toxicity. John Wiley & Sons, Ltd, Chichester, pp. 161182.
  • Timbrell, J. A. (1982). Principles of Biochemical Toxicology, Chapter 2. Taylor and Francis, London.
  • Trevan, J. W. (1927). The error of determination of toxicity. Proceedings of the Royal Society of London. Series B, 101, 483514.
  • Toft, R., Olofason, T., Tuneck, A. and Berlin, M. (1982). Toxic effects on mouse bonemarrow caused by inhalation of benzene. Archives of Toxicology, 51, 295302.
  • Tuchmann-Duplessis, M. (1980). The experimental approach to teratogenicity. Ecotoxicology and Environmental Safety, 4, 422433.
  • Tyl, R. W. (1988). Developmental toxicity in toxicologic research and testing. In Ballantyne, B. (Ed.), Perspectives in Basic and Applied Toxicology. Wright, London, pp. 206241.
  • Tyler, T. R. and Ballantyne, B. (1988). Practical assessment and communicationof hazards in the workplace. In Ballantyne, B. (Ed.), Perspectives in Basic and Applied Toxicology. Wright, London, pp. 330378.
  • UK (1863). Alkali Act London, UK. Her Majesty's Stationary Office, London.
  • Velez, L. I., Shepherd, G., Lee, Y. C. and Keyes, D. C. (2007). Ethylene glycol ingestion treated only with fomepizole. Journal of Medical Technology, 3, 125128.
  • Veronesi, B. (1992). Validation of a rodent model of organo-phosphorus induced delayed neuropathy. In Ballantyne, B. and Marrs, T. C. (Eds), Clinical and Experimental Toxicology of Organophosphates and Carbamates. Butterworth-Heinemann, Oxford.
  • Vorhees, C. V. (1983). Behavioural teratogenicity testing as a method of screening for hazards to human health: a methodological proposal. Neurobehavioral Toxicology and Teratology, 5, 469474.
  • Walter, H., Consolaro, F., Gramatica, P., Scholze, M. and Altenburger, R. (2002). Mixture toxicity of priority pollutants at No Observed Effect Concentrations (NOECs). Ecotoxicology, 11, 299310.
  • Walters, K. A., Brain, K. R., Dressler, W. E., Green, D. M., Howes, D., James, V. J., Kelling, C. K., Watkinson, A. C. and Gettings, S. D. (1997). Percutaneous penetration of N-nitroso- N-methyl dodecylamine through human skin in vitro: application from cosmetic vehicles. Food and Chemical Toxicology, 35, 705712.
  • Ware, M. (2008). Peer review: benefits, perceptions and alternatives. Publishing Review Consortium Summary Paper Number 4. Publishing Research Consortium, London.
  • Warheit, D. B. (2008). How meaningful are the results of nanotoxicity studies Ithe absence of adequate material characterization. Toxicological Sciences, 101, 183185.
  • Warheit, D. B., Hartsky, M. A. and Webb, T. R. (2000). Biodegradability of inhaled p-aramid respirable fibre-shaped particulates: representative of other synthetic organic fibre-types? International Archives of Occupational and Environmental Health, 73, S75S78.
  • Wartew, G. A. (1983). The health hazards of formaldehyde. Journal of Applied Toxicology, 3, 121126.
  • Wax, P. M. (1996). It's happening again–another diethylene glycol mass poisoning. Clinical Toxicology, 34, 517520.
  • Wheeler, T. G. (1987). The behavioural effects of anticholinesterase insult following exposure to different environmental temperatures. Aviation, Space, and Environmental Medicine, 58, 5459.
  • WHO (2001). Guidance Document for the Use of Chemical-Specific Adjustment Factors (CSAFs) for Interspecies Differences and Human Variability in Dose-Concentration Response Assessment. International Programme on Chemical Safety. World Health Organization, Geneva.
  • Wilke, R. A., Lin, D. W., Roden, D. M., Watkins, P. B., Flockhart, D., Zineh, I., Glacomini, K. M. and Krauss, R. M. (2007). Identifying genetic risk factors for serious adverse drug reactions: current progress and challenges. Nature Reviews: Drug Discovery, 6, 904916.
  • Wilks, M. F. and Weston, B. H. (1990). Human volunteer studies with non-phase pharmaceutical chemicals: metabolism and pharmacokinetic. Human and Experimental Toxicology, 13, 383392.
  • Williams, R. T. (1959). Detoxification Mechanisms, 2nd edn. Chapman & Hall, London.
  • Williams, P. L. (1982). Pentachlorophenol, an assessment of the occupational hazard. American Industrial Hygiene Association Journal, 43, 799810.
  • Wilmer, J. W. G. M., Wouterson, R. A. and Appleman, L. M. (1987). Subacute (4-week) inhalation toxicity study of formaldehyde in male rats: 8-hour intermittent versus 8-hour continuous exposures. Journal of Applied Toxicology, 71, 2526.
  • Woodford, R. and Barry, B. W. (1986). Penetration enhancers and the percutaneous absorption of drugs: an update. Journal of Toxicology: Cutaneous and Ocular Toxicology, 5, 167177.
  • World Health Organization (1981). Health Effects of Combined Exposures in the Workplace. Technical Report Series, No. 647. WHO, Geneva.
  • World Health Organization (1985). Chronic Effects of Solvents on the Central Nervous System and Diagnostic Criteria. Environmental Health Criteria Series, No. 5. WHO, Copenhagen.
  • WTO (1994). Agreement on the Application of Sanitary and Phytosanitar Measures. Uruguay Round of the General Agreement on Tariffs and Trade. World Trade Organization, Geneva, 15th April 1994. Also obtainable from the Stationery Office, London, CM2562.
  • Yancy, G. (2000). Protecting whistle blower. British Medical Journal, 320, 7071.
  • Yu, X., Griffith, W. C., Hanspers, K., Dillman, J. F. III, Ong, H., Vredevoogd, M. A. and Faustman, E. M. (2006). A system-based approach to interpret dose- and time-dependent microassy data: quantitative integration of gene ontology analysis for risk assessment. Toxicological Sciences, 92, 560577.
  • Zbinden, G. (1981). Experimental methods in behavioural teratology. Archives of Toxicology, 48, 6988.
  • Zbinden, G. and Flury-Roversi, M. (1981). Significance of LD50 test for the toxicological evaluation of chemical substances. Archives of Toxicology, 47, 7799.

Further Reading

  1. Top of page
  2. Introduction
  3. Historical Development of Toxicology
  4. Definition and Scope of Toxicology
  5. Descriptive Terminology of Toxic Effects
  6. Morphological and Functional Nature of Toxic Effects
  7. Dosage–Response Relationships
  8. Factors Influencing Toxicity
  9. Biohandling as a Determinant of Systemic Toxicity
  10. Routes of Exposure
  11. Exposure to Mixtures of Chemicals
  12. Toxicology of Drugs
  13. Nature, Design and Conduct of Toxicology Studies
  14. Review of Toxicology Studies
  15. Hazard Evaluation and Risk Assessment
  16. Special Considerations in Human Hazard Evaluation
  17. Professional and Ethical Issues
  18. References
  19. Further Reading
  • Bingham, E., Cohrssen, B. and Powell, C. H. (Eds) (2005). Patty's Industrial Hygiene and Toxicology, 5th edn. Wiley-Blackwell, Chichester.
  • Boelsterli, U. A. (2007). Mechanistic Toxicology. CRC Press, Boca Raton.
  • Decker, W. J. (1987). Introduction and history. In Maley, T. J. and Berndt, W. O. (Eds), Handbook of Toxicology. Hemisphere, Washington, DC, pp. 19.
  • Doull, J. and Bruce, M. C. (1980). Orgin and scope of toxicology. In Klaassen, C. D., Amdur, M. O. and Doull, J. (Eds), Casarett and Doull's Toxicology. The Basic Science of Poisons, 3rd edn. Macmillan, New York, pp. 310.
  • Gad, S. C. (2007). Animal Models in Toxicology. CRC Press, Boca Raton.
  • Greim, H. (2008). Toxicology and Risk Assessment. Wiley-Blackwell, Chichester.
  • Hamadeh, H. K. and Afshari, C. A. (2004). Toxicogenomics. Wiley-Blackwell, Chichester.
  • Hayes, A. W. (Ed.) (2007). Principles and Methods of Toxicology, 5th edn. CRC Press, Taylor and Francis Group, Boca Raton.
  • Hodgson, E. (2004). A. Textbook of Modern Toxicology, 3rd edn. Wiley-Blackwell, Chichester.
  • Josephy, P. D. and Mannervik, B. (2006). Molecular Toxicology. Oxford University Press, Oxford.
  • Kumar, C. S. S. R. (2006). Nanomaterials. Toxicity, Health and Environmental Issues. Wiley-Blackwell, Chichester.
  • Singh, J., Dutta, A. K., Khare, S., Dubey, N. K., Harit, A. K., Jain, N. K., Pariat, C., Courtois, P. and Cambar, J. (1988). Seasonal variations in gentamicin nephrotoxicity in rats. Annual Review of Chronopharmacology, 5, 461463.
  • Smart, R. C. and Hodgson, E. (Eds) (2008). Molecular and Biochemical Toxicology. Wiley-Blackwell, Chichester.
  • Tomalik-Scharte, D., Lazar, A., Fuhr, U. and Kirchheiner, J. (2008). The clinical role of genetic polymorphisms in drug-metabolizing enzymes. The Pharmacogenomics Journal, 8, 415.
  • Wadhwa, T. C., Gupta, S. R., Dhariwal, A. C., Jain, D. C., Bhatia, R. and Sokhey, J. (2001). Diethylene glycol poisoning in Gurgaon, India, 1998. Bulletin of the World Health Organization, 97, 8895.