The cuticle consists of 6 to 8 layers of overlapping scale-like cells arranged so that their free edges point toward the tip of the shaft.16 It protects the cortex beneath and acts as a physical barrier against external insult.2 It consists of 2 parts, the exocuticle and the endocuticle.

External to the cuticle is the F-layer, tightly joined to the cuticle by covalent bonds.5 It consists of a single-molecule–thick layer of 18-methyl eicosanoic acid, a highly hydrophobic branched-chain fatty acid,5 considered by some to act as a natural hair protector or natural conditioner.14

Healthy cuticle is soft to the touch; reflects light, causing hair to shine; and limits shaft friction against other hair shafts.16,19 It is responsible for the lustre and texture of hair.10 What makes hair appealing to the sight and touch is chiefly the way the scale-like cuticle cells are arranged, allowing each hair shaft to slide gently over neighboring shafts.1

Friction from brushing and combing, application of alkaline products, and use of hair dryers all disrupt and damage the cuticle5,10,20 and may compromise its protective role.15

The cortex accounts for most of the keratinized hair shaft and is the main component responsible for its mechanical properties (hair strength and elasticity) and for hair color.2,16 It is therefore the site of most cosmetic changes. Permanent hair coloring, waving, and straightening all cause physical and chemical changes to the cortex.15

The cortex is made up of densely packed elongated cortical cells containing keratin filaments that run parallel to the hair shaft16 as well as an amorphous matrix of high-sulphur proteins in which the α-helical keratin chains are embedded.19

Keratins are a family of complex proteins.2 Acidic keratin polypeptides pair with basic keratin polypeptides forming polypeptide chains of hard keratin known as protofilaments.2,21 Multiple protofilaments make up a keratin strand with a large number of sulfur-containing cysteine residues2,15 that form covalent disulfide bonds to adjacent chains, so that neighboring keratin chains are strongly crosslinked,22 thus conferring high physical and chemical stability.2

These disulfide bonds are largely responsible for hair shape, stability, and texture,15 and confer a degree of elasticity that allows hair wet with water to be stretched without damage up to 30% beyond its original length.1

Keratin chains are also more weakly bonded to each other by van der Waals forces, hydrogen bonds, and saline bonds.22

Only terminal hairs can have a core medulla.15 If present, the medulla may be a single core strand extending continuously through the shaft or only an intermittent strand.19 The medulla is composed of specialized cells with intervening air spaces.15 It either plays no cosmetic role at all16 or a very minor one.2

Temporary synthetic hair dyes are applied on clean hair. Shampoo-like products are rinsed in water whereas other cosmetic formulations (lacquers, sprays, and so on) remain in place until the hair is washed.

Nonoxidative semipermanent hair dyes are used to enhance color and to modify or soften the effect of gray hairs,14,15 but cannot lighten hair color because no bleaching agents are included.10 One salient characteristic is low molecular weight, which enables these products to diffuse into the middle layers of the cuticle without binding firmly to the hair protein itself.14

The most widely used semipermanent hair dyes are nonionic nitro dyes,25 which are barely affected by the negative charges on the hair surface and also relatively small. They are thus able to get past the cuticle14 with no oxidative reaction whatsoever.15

This type of dye is typically color-fast through 6 to 8 shampooings.14 Washing opens up the cuticle and releases the dye, which is water soluble.14

Oxidative demipermanent synthetic dyes contain 2% hydrogen peroxide and low levels of alkalinizing agents (generally monoethanolamine rather than ammonia), so hair penetration is more efficient than with nonoxidative semipermanent dyes, but less so than with permanent dyes.14 Owing to their greater coloring power, these dyes are used to enhance natural color, brighten it up, or cover up to 50% of gray hair, but have little hair-lightening potential.14

Application of permanent synthetic hair dyes requires mixing of separate product components14:

• -

  Tint: dye precursors (an oxidant, usually hydrogen peroxide, and an alkalinizing agent, typically ammonia or monoethanolamine) in a surfactant vehicle. Generally available in cream or liquid form.14

• -

  Developer: a stabilized solution of hydrogen peroxide,14 generally in liquid form, to promote oxidation of the dye precursors.

The mixture is preferably applied to dirty hair so that the sebum can protect the scalp from aggressive compounds, and the product is allowed to act for 20 to 40 minutes before rinsing with water.15

Liquid tints are the easiest to mix, but less product loss occurs with tint creams.14

Dyeing must be repeated every 4 to 6 weeks to cover new hair growth.10,14

While several studies indicate that the main allergens responsible for cosmetic-induced allergic contact dermatitis are methylisothiazolinones, p-phenylenediamine (PPD), and fragrance mixtures, in that order, the main agent of allergic contact dermatitis in hair dyes is PPD.30

In a recent study of 2939 consecutive patients recruited from 12 dermatology care centers who were tested for contact reactions to dye components, 4.5% tested positive for PPD and 0.1% for resorcinol.31

The main agents of hair dye–triggered allergic contact dermatitis are para molecules, especially PPD and its derivatives o-nitro-p-phenylenediamine and para-toluenediamine,32–34 which are highly sensitizing compounds. Darker dyes contain greater amounts of these substances.35

In normal hair-dyeing exposure, 1% of the PPD dose penetrates the skin, and 80% of that becomes monoacetyl-PPD and diacetyl-PPD by the action of N-acetyltransferase. Other derivative compounds bind to specific amino acids and form the complexes responsible for sensitization.36

Nevertheless, the published literature supports the inference that hair dyes cause very few adverse reactions (1 per million applications).37,38

Paradoxically, a number of studies show that the prevalence of allergic reactions to PPD seems to have been waning in recent years despite a worldwide increase in hair dye consumption.39 In the European Union (EU), PPD was described as the fifth most frequent allergen in the years 1985 to 1990, but ranked fifteenth between 1991 and 1996.40 This downward trend continued in subsequent years, and an overall reduction in prevalence was observed from 1969 to 2001.34,41

Contact dermatitis among professional hairdressers has also decreased, probably because neoprene gloves are used for protection.42,43

However, the rising fashion of body art, as well as the use of PPD-containing temporary tattoo products such as black henna, seem to be playing an important role in triggering allergic contact reactions to hair dyes.44–47According to the Cosmetic Ingredients Review Expert Panel, 2-amino-4-hidroxyethylaminoanisol and its salt, 2-amino-4-hidroxyethylaminoanisol sulfate, are safe when used in oxidative hair dyes as coupling agents, although they should not be used in cosmetic products in which they can form N-nitrous compounds.48

There are sporadic reports of allergic contact dermatitis caused by some components of temporary hair dyes, such as quinine.49

Eczema appears at the site of application and in sensitive nearby areas such as eyelids, and only in exceptional cases do patients experience severe and extensive symptoms such as facial edema or disseminated dermatitis.14

The potential carcinogenicity of hair dye ingredients has drawn the attention of toxicologists and epidemiologists for decades, as oxidative hair dyes are formulated using compounds that belong to the large chemical family of arylamines, which includes several powerful human carcinogens (benzidine, 4-aminobiphenyl, and 2-naphthylamine).

However, an in-depth review by Nohynek et al.39 of hair dye carcinogenicity studies conducted in humans and other mammals (including both laboratory studies and studies of occupational exposure) concluded that the combined findings strongly suggest that hair dyes do not pose a carcinogenetic risk. The absence of cancer risk to consumers arising from the use of oxidative hair dyes has been confirmed by the World Health Organization's International Agency for Research on Cancer as well as by the EU Regulatory Agency.38,50

However, a recent study by Couto et al.51 in Brazil appears to confirm a possible link between exposure of pregnant women to hair dyes and straighteners and development of leukemia in their offspring up to the age of 2 years.

As to legislation regulating hair dyes in order to prevent adverse effects, Spanish Order No. SSI/771/2013 of May 6, 2013, which modifies Annexes II and III to Royal Decree No. 1599/1997 of October 17, 1997 on cosmetic products, changed both the number of products allowed and their concentration levels.52

The 1997 Royal Decree consolidated all extant Spanish legal rules on cosmetics into a single legislative text and enacted EU Council Directive No. 76/768/EEC of July 27, 1976 on the approximation of the laws of the Member States relating to cosmetic products, along with its later amendments. Subsequent EU-wide rules produced by the European Commission, EU member states, and industry stakeholders were implemented in Spain over the following years, and further Orders have amended the Annexes. According to the latest amended version, as of September 1, 2013 no cosmetic product may be sold or in any way transferred to a consumer unless it complies with the provisions of the Order.52

Hair bleaches are applied for a variable amount of time, according to existing color and to the degree of lightening sought. The bleaching agent is neutralized when the intended amount of time has elapsed to prevent further toxicity to the hair fiber.