JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS detergent to screen his product with the utmost care for possible sensitizing or irritant effects. Dermatologists must invent tests to make such screening possible. These tests should have a high predictive value and be as easy to perform as possible. To design such tests, the dermatologist must consider what we know about the physiology and anatomy of the skin and the factors contributing to its optimum state. He then has to look for weak points in the defence mechanism. I need not remind you that the organ with which we are all concerned, and which has a surface of 1.5-2m2, is very thin. Starting from the surface, we find the epidermis and under it the corium above the subcutis. The epidermis, the principal topic of our discussion tonight, varies in thickness from about 0.1-0.4mm. It would carry us too far to discuss this whole thickness of the epidermis in any detail. We need only mention briefly that the epidermis is composed of the stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum germinativum. It must be kept in mind that these layers are not equally well developed every- where. The stratum spinosum and stratum germinativum together form the Malphighian layer. Our chief interest tonight is the stratum corneum, which varies in thickness from 0.006-0.08mm. Kligman (10) in his des- cription of the stratum corneum stressed the importance of this re- markable layer. He pointed out that it is thanks to this "miracle wrap" that man can live on dry land, and that it prevents the penetration of noxious substances into our "milieu int•rieur". Kligman certainly exag- gerated when he stated in another place that the Malphighian layer is only of interest as the producer of the stratum corneum. This Malphighian layer certainly has its own physiology and pathology. On the other hand, it must be conceded that damage to the Malphighian layer is often due to the defects in the protection provided by the stratum corneum. The remarkable stratum corneum originates from the cells of the Mal- phighian layer by keratinization, a complex intracellular process. It would however, be erroneous to think that the stratum ½orneum consists ex- clusively of keratin. Szakall (11), and Spier and Pascher (12) showed that up to 20-40% of the stratum corneum consists of water-soluble substances, the greater part being amino-acids. These water-soluble substances proved to be markedly hygroscopic and are considered to be important for keeping the stratum corneum supple. They are also supposed to play a part in neutralizing alkali on the skin. The conclusion that the stratum corneum is an important barrier

THE INTERACTION OF DETERGENTS AND THE HUMAN SKIN 9 against the loss of water from inside, and against the penetration of water from outside, was drawn from the observations that when one strips a skin of its horny layer by repeated applications of adhesive tape, the water loss does not vary greatly after a number of strippings but there is an abrupt increase in the loss of water through the epidermis the moment a con- spicuously glistening layer is reached. Szakall (11) postulated that only one particular layer in the stratum corneum acts as a barrier, but this con- ception has been criticized by Kligman and others. Concurring with Klig- man, we may safely state, that there is probably a barrier gradient of in- creasing resistance from above to below, corresponding to the increasing cohesiveness of the cells. It is another of Kligman's objections to much of the earlier experimental work on the stratum corneum, that there is a marked difference between the stratum corneum of the palms and soles, and that of the rest of the body. The horny layer of the palms and soles is much thicker, but, per unit of thickness, it is 100 to 200 times as porous as the stratum corneum of the abdomen. Unfortunately, much experimental work is done with the callus of the soles and then erroneously considered to apply to the rest of the stratum corneum. It has been known for some time that detergents may act in various ways on the skin. They may remove the lipid film from the surface of the stratum corneum, they may remove water-soluble substances, and they may react with the scleroproteins of the horny layer, causing an increase of the amount of free sulphydryl radicals. It is conceivable that detergents, by one or more of these mechanisms, could enhance the penetration of water and other substances, amongst them soaps, through the barrier zone. To test the effect of detergents on the skin, Bettley (3) tried to measure the penetration of potassium ions under the influence of soaps through the whole epidermis. He used two opposing Perspex chambers separated by cadaver epidermis. In one chamber the detergent under study is present with the addition of potassium chloride, in the other distilled water. The migration of potassium ions is measured. In doing so he made two assump- tions: 1. Once a substance has penetrated the barrier zone of the stratum corneum, the epidermis is no longer a serious obstacle. 2. The penetration of the K or Na ion (KC1 or NaC1 added to a soap solution) is a good parameter for the enhanced permeability of the stratum corneum. His experiments led to several interesting conclusions, e.g. sodium