HAIR COLORANTS 381 Despite the significance of these investigations, it has to be remembered that hair is not a passive substrate. While it allows the smaller dye precursors to diffuse in, it limits the diffusion of the larger dye molecules out. In addition, it provides a catalytic surface for peroxide decomposition, accelerating dye forming reactions but apparently protecting the dyes inside from the oxidative decolorization reactions they readily undergo in solution. Finally, hair can be dyed selectively in the presence of the scalp, although the larger pore sizes in the skin may allow for easier removal of dye during subsequent rinsing and shampooing. Thus, there are major areas of this dyeing process that are still incompletely understood. DIRECT DYES The chemistry of these dyeing systems is relatively simple since no chemical changes are involved during the dyeing process, and the dye color is transferred directly to the hair. Typical shades contain 10-12 different dyes, of which the most common are 2-nitro-p-phenylenediamines. A variety of N • and N 4 substituents has been added to the wide range of substituents already patented. Despite this, none seems to have a major advantage over the methyl or hydroxyethyl substituents previously claimed and widely used. Some substituted nitro-aminophenols have also been claimed (20). Of particular interest with this class of dye is their resistance to shampoo and rinsing. This property is carefully balanced in commercial shades and appears to be related to the dyes' structure and molecular weight (21). There has been a study of impurity content of these dyes in relation to product stability (22). Surprisingly, the nature of these impurities has not been determined, nor, in general, have purification techniques been widely applied in dye synthesis. It seems likely that purity of the dyes must affect many properties of the dyes, and more attention will be given to it in the near future. OTHER HAIR COLORING SYSTEMS Most hair coloring is performed with products of the previous two types. However, there is a significant market for products which perform more specialized functions and involve different chemistry. Development of a permanent hair coloring system based on direct dyes has been the subject of many patent claims over the years. An obvious advantage to the consumer is the direct transfer of solution color to the hair (unlike conventional permanents where the solution is initially almost colorless). A common approach is to use the reactive dyes developed for other fibers, e.g., wool (25), with curing conditions moderated for use on live heads. Dye is thus bonded covalently to the hair and cannot be removed during subsequent shampooing. The practicality of such systems remains to be proven since except for the relevant patents there is a distinct lack of published scientific information. Progressive dyeing systems are appropriate for the user who needs a gradual change in hair color. Most systems rely on aerial oxidation of the reactants. For example, a recent

382 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS patent covers the auto-oxidations of 1,2,4-benzentriol with p-diaminobenzenes added to modify the shades (24). However, the metal salt-based compositions (e.g., lead acetate) are apparently still the only product for this type of dyeing effect, since no organic dye-based system is on the market. Patents have been issued at a steady rate for dyeing systems incorporating either colored resins (25) or natural melanin precursors (26). The lack of commercialization in these areas suggests major drawbacks both in performance and cost of such systems. In any case, the materials are not available in quantities where objective testing is possible. Significant advances in the technology are needed to overcome these problems. HAIR DYE TOXICOLOGY Over the last few years, this area has received substantial literature coverage which has recently been summarized by Corbett (2). During this time, most dye ingredients have been subjected to extensive toxicological evaluation including animal feeding studies. In general, adverse effects have not been observed and only three important dye ingredients were found to be carcinogenic in bioassay: 2,4-diaminoanisole, 2-nitro- p-phenylenediamine, and 2-nitro-4-aminophenol. Even if such studies are assumed to be valid, conservative risk assessment and human experience seem to support the position that there are no chronic untoward effects from the use of hair dyes. Thus, while such studies may reflect inherent properties of a dye ingredient, usage conditions must be considered. For example, the concentration of 2,4-diaminoanisole during the dyeing process has been monitored chromatographically (27). With a black shade, the concentration falls to around 10% of its original level within 15 minutes and to 5% within 30 minutes. This reflects rapid utilization of such intermediates and suggests that much less than the formulation amount is available during most of the dyeing step. In addition, dye must penetrate the scalp to cause many of the adverse effects. Skin penetration data on radiolabelled dyes after topical application show relatively low dye permeation rates which, however, depend on the area of application (28). Translating such results into actual dye usage is difficult. More recent work (29) has attempted to quantirate dye penetration under actual usage conditions. In essence, the data shows that with p-phenylenediamine, 2,4-diaminoani- sole, or HC Blue//1 less than 0.2% of the applied dye was absorbed by human subjects during the coloring process. The remainder was located in the hair (5-20%), or in the rinse water. One hopes that this type of study is extended to other common dye ingredients, so that a realistic picture of the toxicological risks associated with hair dye usage can be determined. CONCLUSIONS A theoretical understanding of the properties of dyes and the dyeing process has continued to develop as investigations of more complex and more realistic systems are undertaken. However, the interactions of such systems with hair and skin are still incompletely understood.