758 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS variations which occur in sweat and sebaceous gland secretions in different rodents (150) and in different subjects (125). Also the effects of agents such as tetracycline on these secretions have been examined (124). Although as yet not widely used, it is possible that such investigation could provide useful information as to the mechanism of irritancy produced by chemicals such as detergents. A more useful assessment, not of the content but of the actual quantity of surface lipid present on the skin, was described by Roth (152). The apparatus known as the 'Sebograph' used the principle that the fat present in sebum would disperse or spread out an oil droplet. Using a stylus, the surface lipid is collected from 1 mm • of skin and transferred to an inked oil droplet. The degree to which this droplet speads out ('hs-value') is indicative of the lipid content of the sebum sample. The technique was used to examine the lipid content of the sebum from various parts of the body and to demonstrate the effects of washing with ordinary toilet soap. The interesting and consistent variations which were seen with a panel of volunteers illustrated the value of this test in routine investigations. Although several techniques are available for determining distribution and functional capacity of sweat and sebaceous glands, none has apparently been used in investigating the effects of cosmetics on the skin appendages thus the value of the techniques cannot be assessed. Measurement of the pH of the skin surface The oily secretion of the sebaceous glands, etc. constitutes an initial barrier against agents applied to the skin (153). The fatty acids present, known as the 'acid mantle' are largely responsible for the pH of this surface layer of the skin which in man varies from 4.04 to 6.6 (153-155) depending on the age, race or sex of the individual and the area of skin examined. The pH of animal skins is higher and ranges from 4.8 to 6.6 for the guinea pig to 6.2-8.6 for dogs. The interspecies variability of the acid mantle is attri- buted to the differences in the composition of the sebaceous secretion. This could also account for individual variations of pH occurring in man (156, 157). The acid mantle is able to buffer small quantities of acid or alkali applied to the skin and reduce their corrosive effects. Strong acids or alkalis which are not readily buffered are severe irritants (2, 68, 158-160). Another potential adverse effect of irritants is an alteration of the buffering capacity of the skin. Some soaps and detergents have been shown to bring about

APPRAISAL OF METHODS FOR DETECTING PRIMARY SKIN IRRITANTS 759 such a change (161). Although not extensively used, a measure of the dermal 'neutralizing' capacity of the skin may be a useful guide to the effects of a substance on the function of the sebaceous glands. Methods for measuring the acidity of the skin surface date back to 1892 when Heuss (162) used simple colorimetric indicators. Using this technique, the human skin was shown to have a pH of approximately 5.5 (163). More recent investigators have used pH meters and as the instruments have become progressively more accurate the techniques have become more sensitive (154, 164-167). Saitoh and Inomato (166), in discussing the use of pH meters in measur- ing the pH of the skin surface layer, emphasized the importance of main- taining a constant contact between the skin and the electrodes for obtaining consistent readings. They also considered that the pressure with which the electrodes were applied to the skin and the period of contact were potential sources of error. Using short periods, with pH readings taken at frequent intervals and an apparatus designed to give a constant contact with regular pressure, these authors studied the response of human skin to treatment with solvents which removed the surface lipids. With substances which had a relatively high acidity, neutralization was achieved within about 30 min, but with alkaline substances the reduction in pH was delayed. These observations suggest that the time taken for the pH to return to normal is a measure of the harmful effect of a particular substance. Although the use of pH neutralizing ability of the skin appears to be limited to a few studies in which the effects of soaps and detergents have been examined, the technique would seem to have a wider application in the evaluation of the safety of cosmetic substances. It has the advantage of being usable in vivo with the minimum of difficulty and does not require to be carried out by medical personnel. For this reason studies into its applica- bility in this field would appear to be warranted. The mechanical properties of the skin Normal skin is capable of being stretched by up to 10-50•o depending upon the age, sex and species of animal (167, 168). Rodent skins have the greatest extensibility of those so far examined. Female human skin is weaker than that of males and skin taken from the trunk is less elastic than that derived from the limbs (169). Also the extensibility of the skin declines with age. If the skin is stretched for only a few seconds the extension is reversible,