514 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS taneous absorption is probably independent of that of the vehicle. Concen- tration of the compounds in the vehicle appears to be important for aqueous solutions of electrolytes and less so for other types of compounds and vehicles. These features appear to be applicable to compounds in pure solvents or in creams and ointments. The physiological state of the stratum corneum considerably influences percutaneous absorption. An increase in its temperature or in its normal water content considerably increases the rate at which substances, particu- larly water and water-soluble substances, pass across. Alteration of pH affects percutaneous absorption through an alteration of the state of ioniza- tion of the test substance or through damage induced by very high or low pH. Measurement of percutaneous absorption has been made more sensitive and accurate by the employment of radio-isotope techniques. The older techniques employing chemical analysis and histochemical demonstration of topically applied substances are less widely used but may be of value in certain specific investigations. The method of 'disappearance measurement' of a compound, labelled with a suitable isotope, has found a wide applica- tion in experimental studies of percutaneous absorption in vivo but has only a limited application to human studies. This method needs to be com- bined with autoradiographic studies in order to allow for any error arising from stray (v-emitters) or inaccessible ([i-emitters) sources of radiation if the test compound is retained in the skin appendages. Excretion in urine, faeces and expired air of the topically applied compound or its metabolite provide a good qualitative evidence of percutaneous absorption but with few exceptions are a poor guide for quantitative studies. The same comment may be applied to biochemical estimations (such as enzyme levels), measure- ment of storage in internal organs and assessment of the percutaneous LD50. Despite this reservation such tests may provide invaluable information in some fields of toxicity studies, e.g. pesticides. The pharmacological effects chiefly employed in human percutaneous studies are vasomotor effects and inhibition or stimulation of sweat produc- tion under physiological conditions. In vitro studies employ both chemical and isotopic techniques for measuring the transepidermal passage of a particular test compound on skin samples removed from animals or man and mounted on a suitable chamber. These techniques give useful com- parative information on the percutaneous absorption of different chemicals or the same chemical in different solvents. They also provide basic informa- tion on physiological and pathological skin conditions which affect trans-

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