NATURAL MOISTURIZING AGENT IN SKIN 357 Table III Analysis of Skin Scrapings Subject Per Cent Moisture Binding at PCA-Na 95% R.H. (%) /•-Moles Amino Acid (Ms Keratin) 1 110 2 113 3 132 4 94 5 123 6 86 7 78 8 93 9 78 10 102 11 89 12 89 13 96 14 85 3.36 2.51 7.30 2.32 5.09 2.43 1 94 4 26 2 43 3 72 4 04 5 6o 3 16 1.63 3.00 1 90 3 71 2 07 2 24 2 01 1 29 2 11 1 77 1.96 1.76 2.38 1.40 1.65 DISCUSSION AND SUMMARY The earliest humans with whom we are acquainted, the cave men and their apish forebears, possessed hair which covered the entire surface of the body. It protected them from cold, heat, sun, rain and wind and also acted as a protective padding against injury. With the evolution and civilization of man, he shed his natural hairy covering to become in- creasingly hairless except for the scalp, axilla, and pubis. As a result of this diminished protection, the exposed skin has become more susceptible to the environmental conditions to which it is exposed. Even to the most casual observer, it is apparent that the onset of dry cold weather is rapidly followed by the appearance of dry and chapped skin. Indeed, correlations between environmental conditions and the onset of derma- titis hiemalis have been reported (25). The rapidity with which these changes occur suggests that the physical properties of the stratum cor- neum are to a large extent controlled by the environment to which it is exposed. The importance of moisture in controlling the flexibility of stratum corneum is now well accepted. When exposed to conditions of low tem- perature and humidity, the stratum corneum can rapidly become more brittle because of loss of moisture from the underlying layers. This in- crease in skin stiffness results in fissures and the general conditions seen in dry sldn.

358 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 130-- 120-- 110-- 1OO- 90- 80- 7O REG 1: REG 2: Estimated PCA from known values of Moisture B•nding Estimated Moisture Binding from known values of PCA I 1 i .0 2.0 3.0 4.0 5.0 % PCA Figure 1. Moisture binding vs PCA content for fourteen member panel Counteracting this tendency of the skin to dry out is the presence in the skin of naturally occurring humectants. It is the role of these hu- mectants to help preserve the moisture content of the skin and thereby maintain it in a soft and supple condition. The detailed chemical composition of all of the natural humectants has not been determined. Simple amino acids make up a large propor- tion of the water-soluble constituents of skin in which these humectants are found. However, it can be shown that at the pH at which these amino acids are found in skin they are relatively nonhygroscopic. The occurrence of PCA in the human body appears to be largely limited to the stratum corncure. It is found only in trace amounts in other tissues and organs. In this respect the amount found naturally