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Keywords:

  • chemical analysis;
  • hair growth;
  • hair treatment;
  • safety testing;
  • straightening
  • analyses chimiques;
  • croissance capillaire;
  • traitement capillaire;
  • tests de sécurité;
  • lissage

Synopsis

  1. Top of page
  2. SynopsisRésumé
  3. Introduction
  4. Structure of the hair fibre
  5. Keratins
  6. Intermolecular bonds, stability and strength of the hair fibre
  7. What determines the curl of the hair fibre?
  8. Products and techniques for hair straightening
  9. Chemical straighteners and their potential damage to the hair fibre
  10. Health risks caused by chemical straightening
  11. Legal aspects of the use of chemical hair straightening
  12. Conclusions
  13. Acknowledgements
  14. References

Personal image, as it relates to external beauty, has attracted much attention from the cosmetic industry, and capillary aesthetics is a leader in consumption in this area. There is a great diversity of products targeting both the treatment and beautification of hair. Among them, hair straighteners stand out with a high demand by costumers aiming at beauty, social acceptance and ease of daily hair maintenance. However, this kind of treatment affects the chemical structure of keratin and of the hair fibre, bringing up some safety concerns. Moreover, the development of hair is a dynamic and cyclic process, where the duration of growth cycles depends not only on where hair grows, but also on issues such as the individual's age, dietary habits and hormonal factors. Thus, although hair fibres are composed of dead epidermal cells, when they emerge from the scalp, there is a huge variation in natural wave and the response to hair cosmetics. Although it is possible to give the hair a cosmetically favourable appearance through the use of cosmetic products, for good results in any hair treatment, it is essential to understand the mechanisms of the process. Important information, such as the composition and structure of the hair fibres, and the composition of products and techniques available for hair straightening, must be taken into account so that the straightening process can be designed appropriately, avoiding undesirable side effects for hair fibre and for health. This review aims to address the morphology, chemical composition and molecular structure of hair fibres, as well as the products and techniques used for chemical hair relaxing, their potential risk to hair fibre and to health and the legal aspects of their use.

Résumé

L'image personnelle, en ce qui concerne la beauté extérieure, a beaucoup attiré l'attention de l'industrie cosmétique, et l'esthétique capillaire est un leader de la consommation dans ce domaine. Il existe une grande variété de produits ciblant à la fois le traitement et la beauté du cheveu. Parmi eux, les défrisants se détachent par une forte demande de la part des consommateurs visant la beauté, l'acceptation sociale et la facilité de l'entretien quotidien des cheveux. Cependant, ce genre de traitement affecte la structure chimique de la kératine et de la fibre capillaire, suscitant des inquiétudes en ce qui concerne la santé. En outre, la croissance des cheveux est un processus dynamique et cyclique, où la durée des cycles de croissance dépend non seulement de l'endroit où poussent les cheveux mais également de questions liées à l'âge de l'individu, à ses habitudes alimentaires de même qu’à des facteurs hormonaux. Ainsi, bien que la fibre capillaire soit composée de cellules épidermiques mortes, quand celles-ci émergent du cuir chevelu, il existe une très grande variation de l'ondulation naturelle et de la réponse aux cosmétiques capillaires. Bien qu'il soit possible, grâce à l'utilisation de produits cosmétiques, de donner à la chevelure une belle apparence cosmétique, pour l'obtention de bons résultats pour tous les traitements capillaires, il est essentiel de bien comprendre les mécanismes du processus. Des informations aussi importantes que la composition et la structure des fibres capillaires et la composition des produits et des techniques disponibles pour le lissage des cheveux doivent être prises en compte afin que le processus de lissage soit défini de façon appropriée, en évitant les effets secondaires indésirables pour la fibre capillaire et la santé. Cette analyse vise à traiter la morphologie, la composition chimique et la structure moléculaire de la fibre capillaire ainsi que les produits et les techniques utilisés pour les relaxants capillaires chimiques, leur potentiel de risque pour la fibre capillaire et pour la santé ainsi que les aspects légaux de leur utilisation.


Introduction

  1. Top of page
  2. SynopsisRésumé
  3. Introduction
  4. Structure of the hair fibre
  5. Keratins
  6. Intermolecular bonds, stability and strength of the hair fibre
  7. What determines the curl of the hair fibre?
  8. Products and techniques for hair straightening
  9. Chemical straighteners and their potential damage to the hair fibre
  10. Health risks caused by chemical straightening
  11. Legal aspects of the use of chemical hair straightening
  12. Conclusions
  13. Acknowledgements
  14. References

Attempts at beautification, mainly in women, especially involve the skin and its annexes [1]. Personal image, as it relates to external beauty, has been the target of investment in the beauty industry, and in this context, the branch of capillary aesthetics has attracted much attention from the cosmetic industry because it is considered a leader in consumption in this area [2]. As hair is one of the few physical features that can be easily modified to create a totally different style, be it in length, colour, or shape [3], there is a great diversity of products targeted for both the treatment and the beautification of hair; among them, hair relaxers and straighteners stand out. Generally, the term ‘relaxer’ refers to products intended for the treatment of kinky hair, while ‘straightener’ refers to products used for the treatment of curly hair – in this work, the term ‘straightener’ is used when referring to both products. The reasons for the use of hair straighteners include beauty, social acceptance and ease of daily hair maintenance [1]. However, these cosmetics affect only the hair shaft. As the newly developing hair will not be affected by these alterations, the new emerging hair will grow with its natural, original shape, and therefore, hair straightening needs to be repeated every 4–6 weeks [3]. Thus, the emphasis in this cosmetic treatment should be only on new growth, as repeated treatments can lead to hair breakage [3], and scalp and hair disorders [4], among others [1, 4-6]. Moreover, although the hair fibres are composed of dead epidermal cells, when they emerge from the scalp, there is huge variation in natural wave and the response to hair cosmetics [5]. Consequently, for obtaining good results, it is essential to understand the mechanism of the process and other important information such as the composition of natural hair fibres, the composition of products and techniques available for hair straightening. Thus, this review aims to address a comprehensive summary of the morphology, chemical composition and molecular structure of hair fibres, as well as the products and techniques used for chemical hair straightening, their potential risk to hair fibre and to health and legal aspects of their use.

Structure of the hair fibre

  1. Top of page
  2. SynopsisRésumé
  3. Introduction
  4. Structure of the hair fibre
  5. Keratins
  6. Intermolecular bonds, stability and strength of the hair fibre
  7. What determines the curl of the hair fibre?
  8. Products and techniques for hair straightening
  9. Chemical straighteners and their potential damage to the hair fibre
  10. Health risks caused by chemical straightening
  11. Legal aspects of the use of chemical hair straightening
  12. Conclusions
  13. Acknowledgements
  14. References

The hair is an appendage derived from the epidermis; it is a keratinized structure formed from the invagination of the epidermis into the dermis. From this invagination, small saccular structures called hair follicles originate [3, 7, 8]. Thus, it can be divided into two major parts: the hair follicle and the hair shaft [3]. The hair shaft extends from its root or bulb (located within the follicle), passing through the various layers of the epidermis, surpassing the stratum corneum and then continuing with a stem. Despite its shine, body, and texture, it is a dead structure (Fig. 1) [3, 5, 8].

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Figure 1. Structure of the human skin showing hair as an epidermal annex.

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Hair follicles are essential growth structures of hair, being strongly invaginated into the scalp tissue. At the base of each hair follicle, cells proliferate in upflow. The complex and intertwined processes of protein synthesis, structural alignment and keratinization transform the cytoplasm of these cells into a fibrous material known as hair [8, 9]. Thus, the primary component of hair fibre is keratin (about 65–95%), the remaining constituents being represented by other proteins, water, lipids (structural or free), pigments and trace elements [10-12].

Hair fibres (about 50–100 μm in diameter) are not continuous in their entire length, but rather the result of the combination of compact groups of cells within the follicle, from which originate three basic morphological components: (i) the cuticle, which is the outermost region covering the core of the fibres; (ii) the cortex, which comprises most of the hair volume (75%) and is responsible for sustaining the hair shaft; and (iii) the medulla, which is the central area of the hair and is not always present [3, 10, 12-14].

The cuticle is composed primarily of keratin and displays a stepped structure with five to ten superimposed flat overlapping cells (scales) of 0.3–0.5 μm thick, which are stacked like shingles on a roof and are oriented towards the distal (tip) end of the fibre. The outer surface of the cuticle's scale cells is coated by a thin membrane, the epicuticle, and each cuticle cell consists of three layers of protein: the A-layer, a resistant layer with high cystine content (>30%); the exocuticle, also rich in cystine (~15%); and the endocuticle, low in cystine content (~3%) (Fig. 2) [3, 8, 10, 14].

image

Figure 2. Schematic cross-section of a hair fibre showing medulla, cortex and cuticle cell layers.

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The cuticle encircles the cortex, which forms the most voluminous part of the hair fibre and is comprised of macrofibrils, long filaments oriented parallel to the axis of the fibre. Each macrofibril consists of keratin intermediate filaments (IF), also known as microfibrils, and the matrix, constituted by keratin-associated proteins (KAP) (Fig. 3). The cortical cell is spindle-shaped, about 100 μm long and generally 1-6 μm thick [8, 10, 11, 13, 14]. The medulla of human hair, if present, generally makes up only a small percentage of the mass of the whole hair and is believed to contribute negligibly to the mechanical properties of human hair fibres [11].

image

Figure 3. Schematic representation of the human hair fibre structure and its insertion into the scalp.

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The hair appearance importantly depends on the health of the cuticle. When the cuticle is strong and healthy, hair appears to be strong and healthy. Intact and closed cuticle act as a protective shield against harmful environmental elements; when cuticle scales are open (raised), substances can be deposited in their structure (Fig. 4). Physical-chemical manipulation of the scales of the cuticle causes the appearance of hair to be changed, creating all kinds of different effects which can vary in softness, colour and even texture. Thus, from a cosmetic point of view, the cuticle is a very important component of the hair fibre [8]. The cortex also has a great cosmetic importance, as its optical properties strongly affect the colour and shine of hair fibre [15].

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Figure 4. Cuticles scales closed (A) and open (B).

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Keratins

  1. Top of page
  2. SynopsisRésumé
  3. Introduction
  4. Structure of the hair fibre
  5. Keratins
  6. Intermolecular bonds, stability and strength of the hair fibre
  7. What determines the curl of the hair fibre?
  8. Products and techniques for hair straightening
  9. Chemical straighteners and their potential damage to the hair fibre
  10. Health risks caused by chemical straightening
  11. Legal aspects of the use of chemical hair straightening
  12. Conclusions
  13. Acknowledgements
  14. References

Keratins are a group of over 30 cytoskeletal proteins that, by possessing a diameter between 7 and 11 nm, are called intermediate filaments, that is, between microtubules (20–25 nm) and actin microfilaments (5–6 nm) [16]. They are high molecular weight polymers formed by long chains of amino acids linked together by different types of interactions. The keratin fibres consist of long molecular chains intertwined and firmly attached in different ways through covalent bonds (including disulphide bonds) and also weaker interactions such as hydrogen bonding, Coulomb electrostatic interactions and Van Der Waals, and when water is present, hydrophobic bonds [8-10].

Electrophoretic studies have divided hair keratins into two subfamilies: type I and type II. The type I keratins are acidic and have molecular weights varying from 40 to 48 kD, while those of type II are basic to neutral and have molecular weights varying from 58 to 65 kD [13]. Heid et al. (1986) [17] identified eight major hair keratins, four of each type. Other additional keratins were subsequent findings [18]. They are arranged in pairs of heterotypic chains of type I and type II and are distinguished from their epidermal counterparts by having a relatively high content of cysteine (7.6% vs. 2.9%, respectively) and a disulphide bond between two adjacent cysteines, forming a cystine that produces a stronger and more durable structure [19]. Thus, among numerous amino acids that comprise the keratin of human hair, cystine is one of the most important. Each unit of cystine contains two amino acids of cysteine from different portions of the peptide chains which are interconnected by two sulphur atoms, forming a very strong bond known as a disulphide bond (Fig. 5) [11].

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Figure 5. Disulphide bond (A), coulomb interactions (B) and hydrogen bond (C) that hold different keratin protein together, giving strength to the hair fibre.

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Intermolecular bonds, stability and strength of the hair fibre

  1. Top of page
  2. SynopsisRésumé
  3. Introduction
  4. Structure of the hair fibre
  5. Keratins
  6. Intermolecular bonds, stability and strength of the hair fibre
  7. What determines the curl of the hair fibre?
  8. Products and techniques for hair straightening
  9. Chemical straighteners and their potential damage to the hair fibre
  10. Health risks caused by chemical straightening
  11. Legal aspects of the use of chemical hair straightening
  12. Conclusions
  13. Acknowledgements
  14. References

Disulphide bonds

The high cysteine content of the protein comprising human hair results in a significant effect on the physical properties of the fibres [11]. Two adjacent cysteine residues are linked together, thus generating cystine, which forms a bridge between two proteins or between two portions of the same protein. In hair fibre, the high incidence of these cross-links (0.8 mM g−1), added to their susceptibility to oxidation or reduction, is the key for most chemical modifications of hair, which affect the physicochemical properties of the hair fibres [9].

Coulomb interactions

The high content of acidic and basic side chains (1.6 mM g−1) gives rise to Coulomb interactions that are relatively stable in aqueous environments but easily broken by acids and alkalis [9].

Hydrogen bonds

Although relatively weak and easily broken by water, hydrogen bonds are most numerous (about 4.6 mM g−1) in the hair fibres. Such interchain bonds between the amide groups along the polypeptide are essential for the stability of the α-helix structure of keratins [9].

Figure 5 illustrates these three main types of interchain interactions that occur between molecules of keratin of hair fibre.

What determines the curl of the hair fibre?

  1. Top of page
  2. SynopsisRésumé
  3. Introduction
  4. Structure of the hair fibre
  5. Keratins
  6. Intermolecular bonds, stability and strength of the hair fibre
  7. What determines the curl of the hair fibre?
  8. Products and techniques for hair straightening
  9. Chemical straighteners and their potential damage to the hair fibre
  10. Health risks caused by chemical straightening
  11. Legal aspects of the use of chemical hair straightening
  12. Conclusions
  13. Acknowledgements
  14. References

Evaluations of hair fail to demonstrate biochemical differences among ethnic groups, but some structural differences are seen [20]: all kinds of hair present common features of morphology, chemical composition (Table 1) and molecular structure [20-24], but the shape of the hair varies greatly between different ethnic groups [9, 11, 20, 24-28]. Thus, categorizing different types of hair into three large groups – African, Caucasian and Asian – makes it easier to recognize the specific characteristics of each type of hair including colour, curling and other parameters [9]. Methods for categorizing hair based on its curvature, regardless of the ethnical origin, have been described elsewhere [29] and are useful for the standardization of technical terms in hair science.

Table 1. Ranges of amino acid composition in the whole cosmetically unaltered hair fibre and of human hair from various ethnic origins. Data are expressed in micromoles per gram (μM g−1) of dry hair
Amino acidSymbolWhole fibre*AfricanBrown/CaucasianAsian
  1. *The ranges of composition of amino acids were assembled from the results of Robbins and Kelly (1970) [21], Ward and Lundgren (1955) [22] and Clay et al. (1940) [23], obtained from Robbins (2002) [24]. Whole fibre results approximated by cortex analysis.

  2. Data obtained from Wofram (2003) [9].

  3. Cysteines (Cys) are usually determined by quantification of their oxidation product, cysteic acid, generated by treatment with performic acid.

  4. §Cystine (Cs) is a dimeric amino acid formed by the oxidation of two Cys residues that are covalently linked through a disulphide bond. It only forms after the protein chain has been synthesized and the protein starts to fold.

  5. ‡,§Both Cys and Cs residues can be oxidized to cysteic acid.

AlanineAla (A)314–384370–509345–475370–415
ArginineArg (R)499–620482–540466–534492–510
Aspartic acidAsp (D)292–578436–452407–455456–500
Cysteic acidCya22–4010–3022–5835–41
1/2 Cystine½ Cs§1380–15341310–14201268–16081175–1357
Glutamic acidGlu (E)930–1036915–1017868–10531026–1082
GlycineGly (G)463–560467–542450–544454–498
HistidineHis (H)40–8660–8556–7057–63
IsoleucineIle (I)244–366224–282188–255205–244
LeucineLeu (L)489–529484–573442–558515–546
LysineLys (K)130–222198–236178–220182–196
MethionineMet (M)47–676–428–5421–37
PhenylalaninePhe (F)132–226139–181124–150129–143
ProlinePro (P)374–708642–697588–753615–683
SerineSer (S)705–1091672–1130851–1076986–1101
ThreonineThr (T)588–714580–618542–654568–593
TyrosineTyr (Y)121–195179–202126–194131–170
ValineVal (V)470–513442–573405–542421–493

As shown in Table 1, although there is a considerable variation within this set of data, the amino acid composition of hair fibres is always the same and the ranges of their concentration overlap and do not appear to vary greatly with ethnicity. Moreover, apart from their ethnicity, all hair fibres have a high content of cystine disulphide bonds, which contribute significantly to the stability of the fibre [9, 24]. What then does determine variations in the shape of hair? A definitive answer to this question has not yet been found, but appears to involve several factors, as explained below.

Shape of the hair follicle

The shape of the hair fibre is conferred during development, especially during keratinization [9], when hardening of the fibre occurs. Thus, it is logical to propose that the shape of the follicle in the zone of keratinization determines the shape of the hair fibre. This suggests that the growing fibre takes the shape of the mould, where hardening or keratinization occurs. Thus, if the follicle where the fibre is formed is curved in the area of keratinization, the emerging hair fibre will be highly wavy, but if the follicle is relatively straight, the emerging hair will be straight [24]. In this context, the shape of the cross-section of the hair and how it grows appear to be particularly related to the shape of the hair follicle and its position on the scalp (Table 2) [9, 11, 25-27]. The cross-section of the hair is an ellipse that may tend more or less to a circle. Similarly, in the same way that a thin strip is wound more easily than a cylindrical rope, a hair with a flat and thin cross-section, like the African type, tends to be curled or crimped, with rings of up to a few millimetres' diameter, while a hair with a thicker and cylindrical cross-section, like the Asian type, tends to be straight. The Caucasoid hair, on the other hand, has a high variation in the cross-sectional shape between different individuals, which makes the shape of the fibres vary from wavy to very curly [25, 26]. Figure 6 shows a model that schematizes the relation between the curvature of the hair fibre and its cross-sectional shape.

Table 2. Variation in growth and cross-section of the hair, according to ethnicity (11,26–28)
Hair typeGrowthShapeDiameter (μm)Characteristic
African0.9 cm per monthFlat oval44–89As hair grows almost parallel to the scalp, it grows curled
Caucasian1.2 cm per monthAlmost oval47–74Hair grows at an oblique angle to the scalp and is slightly curved
Asian1.3 cm per monthAlmost circular71–92The mode as the follicle is implanted causes the hair to grow straight and perpendicular to the scalp
image

Figure 6. Schematic diagram of the relationship between the cross-section shapes of the hair fibre and the angle of its curvature.

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Bilateral distribution of the cortical cells

Another possible factor contributing to the shape of hair considers the bilateral asymmetric structure of some keratinous fibres. Three types of cortical cells have been observed in the hair fibre: orthocortical (O), paracortical (P) and mesocortical (M) cells, and these are sometimes segregated into two distinct regions. Although it has been proposed in the late 1990s that, using phosphotungstic acid (PTA) staining methods described by Zhan (1980) [28], it should be revealing that the human orthocortex contains more non-keratin intermacrofibrillar matrix than the human paracortex [30, 31], it has been shown that orthocortical cells contain less matrix between intermediate filaments composed of keratin and a low sulphur content (~3%); paracortical cells are smaller in diameter, have smooth and rounded edges, and a sulphur content high (~5%); and mesocortical cells contain an intermediate level of cystine. If the opposite halves of the fibre grow at different rates or contract to different degrees during drying, a coiled fibre will emerge [24]. In curly hair, paracortical cells are present on the concave side of the capillary curve, while orthocortical cells are present on the opposite face, and mesocortical cells are absent. In straight hair, the cell distribution is concentric, with the orthocortical cells distributed along the whole perimeter of the fibre, around a layer of mesocortical cells surrounding paracortical cells (Fig. 7) [24].

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Figure 7. Representation of the distribution of orthocortical (O), mesocortical (M) and paracortical (P) cells in straight (left) and curly (right) hair fibres (Adapted from Robins, 2002 [24]).

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Scientific data support the fact that differences in hair curvature reflect the distribution profile of the different cell types constituting the fibres and that fibre types vary from straight to heavily crimped with different intermediates [24], as exemplified in Fig. 8.

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Figure 8. Classification of hair samples: (A) straight, (B) wavy, (C) curly.

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Thibaut et al. (2007) observed that the smaller the interior angle of the fibre curvature, the fewer the mesocortical cells therein, while the location of the paracortical cells was more restricted to the concave face of the curve. This is independent of ethnic origin of the hair and is closely related to the curvature of the hair fibre [29]. Thus, the conclusion that can be drawn from this evidence is that an asymmetry in the composition of the hair fibre along its transverse axis determines its curvature. In the concave part of the hair fibre, which concentrates paracortical cells, the concentration of a subtype of acid keratin known as hHa8 and of keratin-associated proteins (KAP) is high [32]. The latter are mainly those that form the matrix of the cortex and the proteins with a high content of cystine of the cuticle [24].

Products and techniques for hair straightening

  1. Top of page
  2. SynopsisRésumé
  3. Introduction
  4. Structure of the hair fibre
  5. Keratins
  6. Intermolecular bonds, stability and strength of the hair fibre
  7. What determines the curl of the hair fibre?
  8. Products and techniques for hair straightening
  9. Chemical straighteners and their potential damage to the hair fibre
  10. Health risks caused by chemical straightening
  11. Legal aspects of the use of chemical hair straightening
  12. Conclusions
  13. Acknowledgements
  14. References

Straightening consists of temporary or permanent breaks in the chemical bonds that maintain the three-dimensional structure of keratin protein in its original rigid form, followed by straightening and mechanical fixing of the new form [33, 34]. It is an effective treatment for hair, which alters almost all aspects of the hair fibre structure to accomplish its objective: to confer on the hairs a durable and different configuration from that which is present in its native form [3, 8, 9].

Temporary straightening, using physicochemical techniques such as dryer, flat iron and the old hot comb, lasts only until the next wash. Hair has to be pre-wetted, to break the hydrogen bonds of keratin, thus permitting temporary opening of its original structure. With this, the strand straightens. Rapid drying with the hair dryer maintains the flat shape of the strand. The application of a hot iron shapes the strand (scales), providing the desired end-look. The strand gets smooth and shiny, to reflect more light [3, 33].

More permanent straightening of hair is affected by altering the disulphide bonds of keratin [33]. It can be achieved with alkaline creams containing up to 3.5% sodium hydroxide (lye-based straighteners), or else guanidine hydroxide, potassium hydroxide, or lithium hydroxide in place of sodium hydroxide (non-lye straighteners). Guanidine hydroxide needs to be activated by adding 4–7% calcium hydroxide to produce calcium carbonate and guanidine hydroxide, the active agent [3, 35]. The aforementioned reagents cause lanthionization of hair and irreversible hair straightening. Chemical straightening can be also accomplished by ammonium or ethanolamine thioglycolate or bisulphite creams [33, 35, 36]. In the procedure with ammonium thioglycolate, the disulphide bonds are converted to sulphhydryl groups to allow the mechanical relaxation of the protein structure of hair fibres. After relaxation, free sulphhydryl groups are reoxidized (neutralized) to reform the disulphide bonds, thus looking in the desired conformation [37]. In principle, the treatment can be seen as a combination of reverse and gradual redirection of these disulphide bonds processes, with softening of the keratin, moulding into the desired shape and stabilizing the newly given geometry [9].

If mechanical straightening is carried out at a high temperature (use of flat iron, for example), it can occur with a much lower conversion of sulphhydryl groups to disulphide. If at these high temperatures the density of the sulphhydryl groups formed is low, returning to room temperature will be sufficient to stabilize the curvature of the hair fibres. This eliminates the need to reoxidize the sulphhydryl groups back to disulphide and results saving time and lesser degradation of fibre [37].

As mentioned previously, all chemical straighteners (pH 13.2–13.5) use chemical reactions to change about 35% of cystine content of the hair to lanthionine, along with minor hydrolysis of peptide bonds [33, 35, 36]. When the hair is treated with hydroxide, disulphide bonds undergo rearrangement and cystine is ultimately converted to lanthionine. This monosulphide thio ether analogue of cystine (containing only one sulphur atom) helps stabilize the hair's straight configuration. The difference between a cystine and a lanthionine is the loss of one sulphur atom. The conversion of cystine to lanthionine weakens hair fibres, a loss of strength that can be measured readily through tensile measurements. As lanthionine is the main product of the reaction between alkali and cystine, these chemical straightening processes are known as lanthionization (Fig. 9) [34, 36, 38].

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Figure 9. Summarized overall reaction of lanthionine (Lan) formation through reactions between cystine with alkali (without intermediate details) (Adapted from Mack, 2009 [35]).

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Besides the mentioned chemical straighteners, one of the most popular and dangerous is formaldehyde (solution 37%) and, more recently, glutaraldehyde. Formaldehyde, despite being a banned substance at any concentration for hair straightening [39], became frequent in this procedure, because besides being cheaper, it is a quick process and leaves the strands shiny [33]. Since 2005, the Brazilian Sanitary Surveillance Agency (ANVISA) declared itself against the use of that substance as a straightener, but it was only in 2009 that Resolution RDC 36, 17 June 2009 was published, which provides for the prohibition of exposure, sale and delivery for consumption of formaldehyde in drugstores, pharmacies, supermarkets, warehouses, emporia and convenience stores [40]. This was a measure to counter the use of formaldehyde and its derivatives as hair straighteners. It can cause serious damage to the tissues of the upper respiratory tract for the user and for the professional who applies the product and has carcinogenic and teratogenic potential [40-42]. When absorbed in the body by inhalation, and mainly by prolonged exposure, it presents a risk of developing cancers of the mouth, nostrils, lungs and blood to the head [42]. Formaldehyde can irritate the eyes and nose, cause allergic reactions of the skin, eyes and lungs and is a cancer hazard [43].

Glutaraldehyde is a saturated dialdehyde, slightly acidic in its natural state, and it has been used as a straightener since the prohibition of formaldehyde. It is a relatively common preservative in cosmetics and can be used in concentrations up to 0.2%. Its disinfectant activity is due to its reactivity with sulphhydryl, hydroxyl, carboxyl and amino groups, altering DNA, RNA and protein synthesis. Glutaraldehyde mutagenicity is extremely similar to that of formaldehyde. However, glutaraldehyde is six to eight times stronger than formaldehyde in producing cross-links in the DNA and about ten times more intense than formaldehyde in the production of tissue damage inside the nose after inhalation [33].

Due to the potential health hazard presented by preservative-based straighteners, since the onset of the progressive brushing craze, large companies have been trying to develop a product with a new active ingredient, not based on formaldehyde or any other type of preservative. In 2011, carbocysteine, whose chemical name is S-(carboxymethyl)-1-cysteine, was developed. It is a dibasic amino acid, with molecular weight of 179.2 g mol−1 and molecular formula C5H9NO4S, which reduces hair volume by up to 90%, moisturizes and adds shine to hair [44]. The primary objective of this brushing technique is not to straighten the hair. This to seal the cuticle of the strands, reconstruct the hair fibre, reduce frizz and assist in growth, but, if done gradually, gives the effect of straightening [45]. Therefore, many hairdressers use this product in processes such as ‘heat sealing’, ‘deep hydration’, ‘shielding’ and ‘plastic hair’. However, for the product to act, a process of rearrangement of cystine bonds is indispensable, which can be obtained with the use of the iron (high temperatures) or with glyoxylic acid [46].

Chemical straighteners and their potential damage to the hair fibre

  1. Top of page
  2. SynopsisRésumé
  3. Introduction
  4. Structure of the hair fibre
  5. Keratins
  6. Intermolecular bonds, stability and strength of the hair fibre
  7. What determines the curl of the hair fibre?
  8. Products and techniques for hair straightening
  9. Chemical straighteners and their potential damage to the hair fibre
  10. Health risks caused by chemical straightening
  11. Legal aspects of the use of chemical hair straightening
  12. Conclusions
  13. Acknowledgements
  14. References

The common side effect of all chemical strengtheners is damage to the hair shaft [3, 35]. One of the damages that occur is the removal of the monomolecular layer of fatty acids covalently bound to the cuticle, including 18-methyl eicosanoic (18-MEA) acid. This hydrophobic layer retards water from wetting and penetrating the hair shaft and changing its physical properties. Removal of the fatty acid layer decreases the brightness of the hair, making it more susceptible to static electricity and frizzing induced by humidity. The second damaging event is the breaking and rearrangement of disulphide bonds, which preferentially affects the amino acids containing sulphur. In damaged hair that is not relaxed, there is a reduction of 21% cystine and a 50% decrease in methionine from the root to the tip of the hair shaft. These changes are magnified with chemical exposure [35].

Among the least harmful chemicals are the ammonium bisulphite creams and carbocysteine. The former contain a mixture of sulphide and disulphide, in variable proportions, depending on the pH of the lotion, but can only produce short-term smoothing [35], while the use of products with carbocysteine only serves to settle the hair and reduce the volume [46].

Health risks caused by chemical straightening

  1. Top of page
  2. SynopsisRésumé
  3. Introduction
  4. Structure of the hair fibre
  5. Keratins
  6. Intermolecular bonds, stability and strength of the hair fibre
  7. What determines the curl of the hair fibre?
  8. Products and techniques for hair straightening
  9. Chemical straighteners and their potential damage to the hair fibre
  10. Health risks caused by chemical straightening
  11. Legal aspects of the use of chemical hair straightening
  12. Conclusions
  13. Acknowledgements
  14. References

Chemical relaxants have found wide use by individuals with wavy/curly hair, both for the ease of care of relaxed hair and for the aesthetic aspect it provides. However, the number of reported side effects has health implications and draws attention to a higher emphasis on safer ways of application, as well as tougher legislation on these chemicals. The straightening process presents risks for users. The adverse results of repeated and regular professional applications of chemical hair straighteners for a period exceeding 1 year may include itching, burns and scars on the scalp, thinning and weakening of the hair shaft, discoloration and hair loss, apart from allergic reactions to chemicals [1, 4-6].

Legal aspects of the use of chemical hair straightening

  1. Top of page
  2. SynopsisRésumé
  3. Introduction
  4. Structure of the hair fibre
  5. Keratins
  6. Intermolecular bonds, stability and strength of the hair fibre
  7. What determines the curl of the hair fibre?
  8. Products and techniques for hair straightening
  9. Chemical straighteners and their potential damage to the hair fibre
  10. Health risks caused by chemical straightening
  11. Legal aspects of the use of chemical hair straightening
  12. Conclusions
  13. Acknowledgements
  14. References

Although the definition of a cosmetic product may vary, even slightly, among the regions of the world, in general, the current regulatory framework for cosmetics in the U.S.A., Japan, Canada, Europe and Mercosur (Argentina, Brazil, Paraguay and Uruguay) has a broad definition of cosmetics, with safety ensured through control over ingredients in the form of positive lists, prohibited and restricted lists, specific requirements concerning safety testing and maintenance of data files on safety [47, 48]. The U.S.A., Japan and Canada also have a narrow definition, with few restrictions on the ingredients that can be used and the type of safety testing to be undertaken as determined by manufacturers. In the U.S.A., products can be categorized as either cosmetics or drugs and are therefore subject to both sets of regulations. Japan has an additional product category of quasi-drugs whose regulation is less rigorous than that of drugs but still requires pre-market approval and registration of ingredients. The Association of Southeast Asian Nations (ASEAN), Mercosur and the Comunidad Andina (Andean Pact) regions have used the European model in drafting their own cosmetic regulations, which are closer to the Japanese model but without the category of quasi-drugs [48]. Nevertheless, in Europe, the cosmetic field distinguishes itself from other sectors through some challenging particularities, nearly all driven by European cosmetic legislation, where in the first instance, the term ‘risk assessment’ is replaced by ‘safety evaluation’, because a cosmetic product is a priori considered to be safe [49].

In Brazil, the legislation of Mercosur was internalized by ANVISA, through specific Resolutions and Ordinances pertaining to the verification of compliance with good manufacturing practices and control of industrial establishments for the manufacture of toiletries, cosmetics and perfumes; those dealing with the definition of cosmetic products, requirements for registration, nomenclature for ingredients; handbook of good manufacturing practices for cosmetics; microbiological control parameters, among others, including those related to positive lists, prohibited and restricted lists; and specific requirements concerning safety testing and maintenance of data files on safety [47]. For hair straightening, for example, as previously mentioned, ANVISA does not register hair straighteners based on formaldehyde, as to achieve straightening, this product has to be used in concentrations of 20–30%, which is totally banned [33]. The sanitary legislation permits the use of formaldehyde and glutaraldehyde in cosmetics only with the function of preserving (with a maximum limit of 0.2% and 0.1%, respectively, according to Resolution RDC No 162 of September 11, 2001) or as nail hardener at a maximum limit of 5% (Resolution RDC No 215 of July 25, 2005) [33, 40, 50, 51]. On the other hand, according to the United States Department of Labor's Occupational Safety and Health Administration (OSHA), hazards associated with formaldehyde must be listed if it is present in the product at 0.1% or more (as a gas or in solution) or if the product releases formaldehyde into the air above 0.1 parts per million [43]. The risk of this substance being misapplied increases as concentrations and frequency of use rise and occurs through gas inhalation and by skin contact, being dangerous both for professionals who apply the product and for users [33].

There are other substances registered in ANVISA which can be used for straightening hair. These include ammonium thioglycolate, sodium hydroxide, potassium hydroxide, calcium hydroxide, lithium hydroxide and guanidine carbonate, but subject to restrictions and established conditions, according to Ordinance No 71 of May 29, 1996 and its updates [47, 50, 51]. Any straightener can burn the scalp if used incorrectly, because lye found in many hair relaxants can burn the skin and even those without lye still need to be safe and used correctly [52]. Thus, according to the United States Federal Food, Drug, and Cosmetic Act (FD&C Act), one of the two most important laws pertaining to cosmetics marketed in the United States, the caution ‘This product contains ingredients which may cause skin irritation on certain individuals and a preliminary test according to accompanying directions should first be made’, must be placed on the product label displayed, for the cosmetic is not deemed to be adulterated [53].

Conclusions

  1. Top of page
  2. SynopsisRésumé
  3. Introduction
  4. Structure of the hair fibre
  5. Keratins
  6. Intermolecular bonds, stability and strength of the hair fibre
  7. What determines the curl of the hair fibre?
  8. Products and techniques for hair straightening
  9. Chemical straighteners and their potential damage to the hair fibre
  10. Health risks caused by chemical straightening
  11. Legal aspects of the use of chemical hair straightening
  12. Conclusions
  13. Acknowledgements
  14. References

Any chemical process of cosmetic treatment for hair straightening affects the chemical structure of the keratin fibres, because these processes target links that provide stability for the fibre. Although hair fibres are composed of dead epidermal cells, when they emerge from the scalp, there is wide variation in their natural wave and their response to hair cosmetics. Moreover, the development of hair is a dynamic and cyclic process, where the duration of growth cycles depends not only on where hair grows, but also on issues such as the individual's age, dietary habits and hormonal factors. Thus, although it is possible to give hair a cosmetically favourable appearance through the use of cosmetic products, it is necessary to take into account these issues and understand first the morphology of hair so that treatment can be done appropriately, avoiding the undesirable side effects mentioned above. Also, it is advisable to be familiar with the current regulatory framework for cosmetics to know whether the product to be applied is within the norms of current legislation in the country, which serve to protect the client from adverse health effects.

Acknowledgements

  1. Top of page
  2. SynopsisRésumé
  3. Introduction
  4. Structure of the hair fibre
  5. Keratins
  6. Intermolecular bonds, stability and strength of the hair fibre
  7. What determines the curl of the hair fibre?
  8. Products and techniques for hair straightening
  9. Chemical straighteners and their potential damage to the hair fibre
  10. Health risks caused by chemical straightening
  11. Legal aspects of the use of chemical hair straightening
  12. Conclusions
  13. Acknowledgements
  14. References

The authors gratefully acknowledge the Brazilian National Council for Technological and Scientific Development (CNPq) for scholarships for the Training Program of Human Resources in Strategic Areas (RHAE) – Researcher in Company, for Ana Luisa Miranda-Vilela, as well as CNPq and the company ‘Beleza Natural’ (Natural Beauty) for financial support for Nanodynamics.

References

  1. Top of page
  2. SynopsisRésumé
  3. Introduction
  4. Structure of the hair fibre
  5. Keratins
  6. Intermolecular bonds, stability and strength of the hair fibre
  7. What determines the curl of the hair fibre?
  8. Products and techniques for hair straightening
  9. Chemical straighteners and their potential damage to the hair fibre
  10. Health risks caused by chemical straightening
  11. Legal aspects of the use of chemical hair straightening
  12. Conclusions
  13. Acknowledgements
  14. References