COSMETICS ?ERSUS TOPICAL THERAPEUTIC AGENTS 435 Fourth, although thick stratum corneum, like that on the palms and soles, is relatively inflexible when dehydrated, thin stratum comeurn may be relatively flexible even when its water content is low. Fifth, the concentration of water is probably not uniform throughout the stratum comeurn. In an environment of low relative humidity, water continuously diffuses from the moist layers of the epidermis through the relatively dry stratum corneum into the atmosphere. It has been shown that the lipid film on the surface of the stratum comeurn is not the rate- limiting barrier to the diffusing water. Either the entire stratum comeurn serves as a relatively homogeneous barrier or the more compact layer at the base of the stratum comeurn serves as the rate-limiting barrier. In either case the outward movement of water can occur only when the water content of the inner layers is greater than the water content of the outer layers. The exact water content of the different layers of the stratum corneum under various environmental conditions has not been determined. Sixth, while too little is known about potential alteration of the water- holding capacity of the stratum comeurn in diseased states, it is known that disease often alters the thickness and texture of the cutaneous surface. Actually, even the slight toughening of the surface in mild chapping may always follow subclinical inflammation and may not develop simply as the result of abnormal environmental conditions without pre-existing inflam- mation. A type of emollient more commonly found among cosmetics than among therapeutic agents is an aqueous solution or suspension of a strongly hygroscopic agent such as glycerin or sorbitol. It is not entirely clear just how such hygroscopic substances exert their emollient action. If an- hydrous glycerin is put on the skin, it will at first withdraw water from the stratum comeurn and thus temporarily make the skin more "dry." The water requirements of an applied hygroscopic agent can, however, be supplied from the environment as well as from the skin so that at equilib- rium the requirements of both the stratum comeurn and the hygroscopic material are satisfied (2). At any given humidity, the total amount of water retained equals the sum of the amount held by the stratum corneum and the amount held at that relative humidity by the hygroscopic material (3). Bath oils may be of two varieties: those which are immiscible with water and form a layer on its surface and those which can be emulsified in water. When either type is used in the bath, it is likely that some oil will be deposited on the skin from the bath water (4). Although it is evident that a film so deposited acts as a lubricant, it is not yet known whether such a film is sufficiently occlusive to favor retention of water by the stratum corneum. Evidence is still inadequate to show that lanolin or any purified fraction

436 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS of lanolin aids retention of water by the stratum corneum more effectively than do other oily materials. 2. Keratolytic ,'tgents Salicylic acid is the keratolytic agent most commonly used in products for topical therapy. This substance does not actually dissolve keratinized tissue as its name might imply even 50% salicylic acid in an anhydrous vehicle has no observable keratolytic effect on a piece of dry callus. When salicylic acid ointments or plasters are applied to the surface of the skin, they cause retention of increased amounts of water in the stratum corneum because they are occlusive and because the low pH of the acid favors water retention (5). After this occurs, the softened cornified tissue can be easily removed by mechanical means. The keratin molecules probably undergo little chemical change during this process. For cosmetic purposes, keratolytic agents are used to soften hair so that it can be removed (depilatories) or reshaped (waving preparations). To accomplish this, more potent keratolytic agents, which attack the keratin molecule itself, are needed it is obvious that salicylic acid cannot do this. Formerly, inorganic sulfides at high alkalinity were employed for this, but alkaline thioglycolates have come into use since it was learned that these compounds soften hair. Thioglycolates attack the disulfide group of cystine (6) subsequent oxidation "neutralizes" the action of the thio- glycolares. After neutralization, the hair becomes relatively normal again if the action of thioglycolate has not gone too far. Thus, it is seen that the action of the keratolytic agents present in cosmetic preparations used on hair differs appreciably from that of the keratolytic agents present in preparations used for topical therapy of the skin. $ Cleansers The dermatologist usually recommends relatively simple cleansing pro- cedures. During the acute phase of a dermatitis, he may recommend that cleansing procedures be reduced to a minimum and that wet compresses be used. At the present time, there is a divergence of opinion among derma- tologists as to whether soap does or does not aggravate a pre-existing dermatitis. Suskind (7) and Stoughton et al. (8) have presented evidence to indicate that patients with mild dermatitis can cleanse their skin with soap and water without causing further irritation. If a nonsoap cleanser is desired, nonalkaline synthetic detergents are available. Currently, however, the alkalinity of a soap is not thought necessarily to be injurious. An aqueous solution of sodium laurate is less irritating to the skin at pH 9,5 than at pH 7.5 (9). Cleansing techniques which do not involve the direct application of water or water and detergent are not infrequently recommended by both