Pharmaceuticals and personal care products (PPCPs) include numerous classes of chemicals with unique physiochemical properties and biological activities. Over the past decade research on the occurrence, fate, effects, risk assessment, and management of PPCPs in the environment has peaked. It is important to appreciate the utility of traditional approaches for examining contaminant hazard and risk while understanding relevant limitations and important research needs to advance environmental risk assessment (ERA) and management efforts for PPCPs. Spurred initially by the critical reviews of Halling-Sorensen et al.  (755 citations as of 6 July 2009) and Daughton and Ternes  (778 citations as of 6 July 2009), this special issue of Environmental Toxicology and Chemistry includes a timely collection of manuscripts examining the environmental chemistry, toxicology, risk assessment, and management of PPCPs.
Occurrence, fate, and exposure
A vast number of PPCPs have now been detected in surface waters across the world. For human PPCPs, effluent-dominated ecosystems appear to represent worst-case scenarios for waterborne exposure and potential adverse effects . For veterinary medicines, inputs from manure application to soils and the use of aquaculture are probably the most important exposure routes . The nature of exposure to human PPCPs and veterinary medicines are also very different. Human PPCPs are typically released continuously from wastewater treatment plants and are therefore considered to be pseudo-persistent  with increased effective exposure duration in effluent-dominated systems , whereas veterinary drugs are either transported to surface waters in pulses during rainfall (livestock treatments) or are applied directly to aquatic systems in a single dose. The risk assessments of human use substances and veterinary medicines should therefore be addressed very differently. While most monitoring studies have focused on wastewater treatment systems and surface waters, researchers are beginning to recognize that other environmental compartments are also important. Hence, recent studies have explored the occurrence of PPCPs in biosolids, soils, sediments, biota, drinking water, and even food crops . For example, in this issue Ramirez et al. present findings from a National Pilot Study of PPCPs in Fish Tissue from the United States, which expanded previous observations of fish bioaccumulation in an urban effluent-dominated river .
In addition to environmental monitoring studies, a number of studies have investigated the fate and transport of PPCPs. Most transport studies have been on terrestrial systems and have investigated the movement of PPCPs from soils to surface waters in runoff and drain flow  and to groundwaters via leaching. The focus has been on veterinary medicines although researchers are now recognizing that these transport processes are also important for PPCPs applied to land associated with biosolids or during irrigation with wastewater . It is clear from these studies that the application matrix (e.g., manure, biosolids) has a significant impact on the transport behavior of many compounds although the underlying reasons for this have yet to be established.
For pharmaceuticals in particular, traditional approaches to characterize partitioning to soils and sediments (i.e., relating sorption to a compound's hydrophobicity and to soil or sediment organic carbon content) are often inadequate. Because many pharmaceuticals are weak acids, weak bases, or zwitterions, sorption processes are due not only to hydrophobic interactions but also are driven by other binding processes such as cation exchange, cation bridging, surface complexation, and hydrogen bonding. In fact, site-specific water, soil and sediment conditions such as pH and cation exchange capacity can have a major influence on PPCP sorption behavior in terrestrial and aquatic environments [10,11] as can the presence of biosolids and manure . A few studies have attempted to model the interaction of pharmaceuticals with different environmental matrices. For example, Aristilde and Sposito  used molecular dynamic simulations to model metal complexation interactions of fluoroquinolone antibiotics. Ter Laak et al.  developed relationships between soil properties and the sorption behavior of antibiotics. However, far more modeling work of this type will be required before we have the predictive tools needed for risk assessment.
The degradation behavior of PPCPs can be complex, but recent studies are beginning to characterize the degradation rates and pathways of PPCPs in the environment. Persistence in solid matrices such as biosolids, manures, soils, and sediments indicate that for many PPCPs the observed dissipation is not due to degradation but rather to the formation of nonextractable residues . Questions are being raised over the implications of these bound residues. The importance of PPCP metabolites and transformation products is also being increasingly recognized. In this issue Monteiro and Boxall, Williams et al., and Buth et al. specifically examine relevant questions of importance to defining environmental fate and transport processes of PPCPs in aquatic and terrestrial ecosystems.
Effects and risk
In developed countries most human PPCPs lack acute toxicity or fail to elicit mortality responses under typical environmental postconsumer exposure conditions . Exceptions may be observed, however, in developing countries when environmental management and regulatory practices are not as robust. Carlsson et al. report in this issue ambient aquatic toxicity resulting from exposure to effluents from bulk manufacturing facilities in India. Exceptions are also apparent for select veterinary medicines with intended use as insecticides; for example, ivermectin, a parasiticide, is acutely toxic to crustaceans at low or even part per trillion levels . Chronic exposures to PPCPs occur particularly for organisms residing in effluent-dominated systems , and potentially adverse ecological responses to select substances may occur at environmentally relevant levels at much lower concentrations than mortality thresholds .
Thus, it is critical to consider a priori the mechanisms of action (MOA) for PPCPs in the environment. Leveraging pharmacological safety information, for example, appears useful for supporting such efforts, but for a response to be meaningful as a measure of effect in ecological risk assessment, MOA-related responses should be linked to the population level of biological organization, with endangered or threatened species being an important exception. Ankley et al.  presented a framework for performing such examinations. In this issue, Zellinger et al. describe how synthetic estrogens adversely affect fish reproduction at low or even sub-part per trillion concentrations, which could be anticipated based on the MOA of these compounds. Although the approach of Ankley et al.  appears promising, it is important to note that a therapeutic MOA may be more predictive for nontarget vertebrates than invertebrates and algae. Brooks et al.  highlighted this observation because lower adverse effect thresholds were determined for algae growth following exposure to the antidepressant fluoxetine than endpoints employed in standardized crustacean and fish testing methods. Such an observation was interesting because green algae do not possess the therapeutic target of fluoxetine, though this therapeutic is known to have antimicrobial properties .