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Risks Associated With Hair-Care Formulations

In all mammals, hair develops as an epidermal structure from papillae deep in the skin and acquires characteristic patterns on the scalp, eyebrows, eyelashes, and elsewhere on the body. In humans, hair growth is continuous throughout life (declining with advancing age), occurs in cyclic patterns, and is influenced by androgens, thyroid hormones, and dietary factors. Human hair is composed largely of keratin and consists of a narrow central medulla surrounded by a thick envelope (cortex) of elongate cells, which contain numerous melanin granules that determine the natural color. The hair is ensheathed in a multilayered cuticle of overlapping cells that become progressively imbricated with continued growth. These cuticular cells are rich in cystine and become rough or show a weathered appearance through exposure to environment or poor health.

Chemically, human hair contains approximately 85 percent protein, 7 percent water, 3 percent lipid , 4.7 percent protein-bound sulfur (as cystine), and low concentrations of trace minerals (e.g., iron, zinc, copper). The phosphorus content is approximately 80 milligrams per 100 grams of hair. Hair is normally associated with sebum and exocrine secretions from skin glands that confer greasiness but influence its water content and mechanical and physical properties.

Hair follicles are determined prenatally; about 100,000 hairs are found on the scalp region of most adults. Hair density, color, and condition vary according to age, race, and genetic background. Natural hair colors vary from albino or white to blond, red, or intense black and reflect ethnic origin, age, diet, and health. While hair color is closely related to the density of melanin granules, impairment in a person’s health or substances in the diet that influence the availability of trace minerals are potential causes of changes in hair color or condition. The configuration of the hair shaft (i.e., straight, wavy, spiral, or peppercorn) is attributable to the number and distribution of disulfide bonds. Hair straightening requires reduction of these disulfide bonds and fiber cross-linkages.

Hair serves to eliminate toxic materials (e.g., lead) and metabolites from the body, and may be used to monitor environmental contamination. For example, copper deficiency is a cause of Menke’s “kinky” hair syndrome; protein deficiency leads to hair loss and discoloration. Hair keratin carries a strong negative charge and binds inorganic materials; it becomes prone to discoloration through exposure to environmental chemicals (e.g., cobalt, tar in cigarettes, picric acid, trinitrotoluene, etc.). Prolonged exposure to copper in diet, tap water, or swimming pools is a cause of green hair.

Melanin granules are secreted by melanocytes in the hair papilla and distributed to keratin in the hair cortex and inner layers of the hair sheath during normal development. Melanogenesis is subject to hormonal control and has been the focus of intensive genetic studies. Two main forms of melanin exist in human skin—eumelanin and phaeomelanin, both of which are derived from tyrosine through the action of tyrosinase (a cupro-enzyme) and possibly other key enzymes (with nickel, chromium, iron, and manganese as cofactors). Tyrosine is converted to dihydroxyphenylalanine and, via a series of intermediate steps, to indole-5,6-quinone, which polymerizes to eumelanin. Phaeomelanins are produced by a similar mechanism but with the incorporation of sulfur (as cysteine) by a nonenzymatic step in the oxidation process.

Hair color is a balance between these two melanins. Albino or white-haired individuals have latent melanocytes, but possibly show defects in tyrosinasemediated events. Graying of the hair is age-related and possibly results from declining melanocytic function or retarded hair growth resulting from atrophy or degenerative changes in hair papillae. Hair melanin absorbs insufficient ultraviolet (UV) light energy to afford protection for most individuals against sunburn.

A wide variety of dyes, dressings, and conditioners are available to men and women to enhance the color of hair or to alter its condition, providing the “feel good” factor. Natural hair dyes such as henna and mineral salts are still used, but hair dyeing increasingly involves careful chemical manipulation of the chemistry of hair fibers through bleaching or enhancement of natural colors. Additionally, social and cultural customs have led to the increasing demand for exotic colors. Hair coloring is a well-defined science with intense study of the interaction between hair keratin and highly reactive organic dyes, oxidizing agents, and conditioners.

The dyeing process provides for temporary, semipermanent (direct dyes), and oxidation-type reactions (semipermanent or permanent colors). It may involve absorption or adsorption (electrostatic) of the colorant into/to the hair structure, bleaching or otherwise masking the natural melanin colors, or alteration of the structure of the hair shaft, allowing deep penetration of the colorant. The hair cuticle provides a barrier to the absorption of hair dyes, particularly those of high molecular weight, but damaged fibers exhibiting higher negative charges and reduced phospholipid content are more porous. Dye uptake is determined by the partition coefficient between the hair and the dye carrier (water, alcohol, etc.), pH, and chemical charge (dye-fiber interaction). Thickeners and surfactants can also influence dye uptake.

Henna is the oldest and most widely used vegetable dye utilized in hair coloring. A temporary chestnut color is produced in blond or auburn hair by applying a paste of henna flowers and leaves ground in hot water immediately before use. (The dye is unstable in aqueous solution.) The addition of indigo achieves darker blue-black shades; extracts of walnut shell or logwood enhance brown coloration. Hair dyes and other hair treatments have been popular for centuries, but in recent years have been more chemically sophisticated, taking into account the chemical makeup of hair.

Of the mineral dyes, only lead acetate is commercially available. In the United States, the FDA permits maximal concentrations of 0.6 percent; in the United Kingdom, less than 1 percent is allowed. (Kohl or “surma,” an eyelash and eyelid cosmetic used in Asian countries, contains up to 80 percent lead and is a known cause of anemia.) In contact with hair, the lead salt is poorly absorbed but interacts with keratin to deposit lead sulfide (at permitted levels [<0.5%] lead is absorbed into the skin without toxic implications). Silver nitrate has been used to color hair brown-black without significant absorption of metal into the circulation; silver is deposited in the hair cortex as silver sulfide. Metallic hair dyes tend to be long-lasting and are lost as hair grows and is shed naturally. Hair coloration is achieved as a gradual process through repeated application of rinses or pastes over several days.

Temporary dyes are frequently acidic and provide short-term coloration. They are of low penetration and do not involve melanin bleaching or structural changes in the hair fiber. They are commonly identified with a color index (CI) number; they are water-soluble, high-molecular-weight pigments. Temporary dyes are employed in water-thin color rinses, colored hair-setting lotions, colored styling gels, lotions, and shampoos. Hair dyeing is improved when the colors are applied under moderate heat. Hair dyeing and color balance is difficult to control with temporary methods because of the inconsistency of dye delivery systems and instability of the dye-keratin “bonding.”

Semipermanent dyeing systems are designed to last for several weeks. They impart darker colors without the use of oxidizing agents or structural changes in the hair shaft or papillae. Melanin may be partly masked but is not bleached. These dyes diffuse more readily into and out of the hair (aided by solvents and surfactants) and are progressively lost with hair shampooing and normal growth. The method employs dyes that are already colored, and in general is without the toxic risks associated with semipermanent or permanent methods, which involve de novo color production and strong oxidizing agents. When the dye is lost through shampooing, the hair resumes its natural color.

Oxidative hair dyeing systems involve the use of more toxic reagents. They are multistep processes leading to semipermanent or permanent coloration, according to the extent of bleaching involved. Surfactants and solvents influence the penetration of the active constituents; alkalizing agents determine pH. The resulting hair colorations are more stable against normal wearing processes than semi permanent preparations and involve an initial oxidation reaction, a coupling reaction, and production of a color reaction with dyeing of the hair fiber. The process requires a primary intermediate, a m -coupler or secondary intermediate (color modifiers), and hydrogen peroxide.

Hydrogen peroxide is commonly used as the oxidizing agent; it has the capacity to bleach melanin but it initiates the first coupling reaction and the ultimate development of the color. Initial oxidation of primary intermediates (e.g., p -aminophenol, p -phenylenediamine) by hydrogen peroxide is followed by coupling with an agent like m -aminophenols, or m -phenylenediamines. Further oxidation of this secondary intermediate leads to the formation of colored indamines, indolanilines, and indophenols. As a general rule, the higher is the electron-donating capacity of the coupling agent, the higher is the absorbance maximum of the indo-dye formed.  

Many of the organic aromatic amines used in hair dyes are strong sensitizers and oxidative dyes should be used with extreme caution. P -phenylenediamine and its derivatives, commonly employed in permanent or semipermanent hair colorings, are strong sensitizing agents and may damage the hair. Although a large number of possible combinations of primary intermediates and couplers leading to the production of exotic hair colors are possible, the cost of conducting regulatory toxicological evaluation is prohibitive in developing many interesting colors.

Lightening or removal of hair color without structurally damaging the hair shaft is a difficult process. Oxidizing agents, including hydrogen peroxide, can be extremely harmful to hair and will oxidize some cystine to cysteic acid, rendering the fibers less cohesive and susceptible to hydration and swelling under alkaline conditions. Bleached hair shows a loss of melanin granules at the periphery of the hair. Hair color is lost (platinum blond appearance) and the fibers become dry and fragile. Bleaching is inhibited by shampoos or acid rinses.

Hair conditioning includes permanent waving, straightening, and setting. In each case, sequential chemical treatments lead to modifications of the hair shaft with temporary or prolonged changes in disulfide bond distribution and behavioral characteristics. Softening, reshaping, and hardening are integral to permanent waving and involve an initial reduction in disulfide bonds and adhesiveness of adjacent hairs using heat or steam, thioglycollates, or ammonium hydroxide; styling with heated rollers; and setting or neutralizing the reaction. This last event is a reversal of the earlier reduction process with reconstitution of disulfide bonds from adjacent cystine moieties using hydrogen peroxide or a similar oxidizer. Hair straightening is a similar process, but may involve the additional use of hair dressings like gels, sprays, and creams (pomades) to hold the hair in place. Hot comb techniques may be used to disrupt the disulfide bonds followed by application of oils, petroleum jelly, or liquid paraffin.

Mousses are designed for hair styling and setting hair in position. Commonly, formulations are based on cationic conditioning polymers with quaternary ammonium salts, alcohols, perfumes, colorants, water, and preservatives. They may be in propellant, foam, or jelly form for direct application to hair after shampoo or cutting.