APPRAISAL OF METHODS FOR DETFCTING PRIMARY SKIN IRRITANTS 741 perfumes, sunburn preventitives and special creams and lotions. Some of these products remain on the surface of the skin for prolonged periods, whereas others, such as shampoos, are rinsed off quickly. Some of the problems encountered in testing for primary irritants were mentioned by Fisher (2), Rostenberg (3, 4) and Rieger and Battista (5). In a brief discussion of the many methods available to a dermatologist evaluat- ing drugs for use on the skin, Calnan (6) referred to four possible approaches: (1) An examination of the gross macroscopic appearance of the skin. (2) A determination of the effect of the substance on separate elements of the skin, e.g. keratinization, sweating, hair growth and pigmentation. (3) An observation of the structure of individual cells. (4) An investigation of biochemical pathways in the skin. These parameters of investigation are applicable also to a safety testing programme for cosmetics. In this review, the tests frequently employed in the safety evaluation of compounds used in skin preparations are discussed with reference to the various modifications which have been suggested for overcoming the difficulties experienced in using them. STANDARD TESTS The patch test, as originally described in 1895 by Jadassohn (7) and later adapted by Bloch (8, 9) and by Draize, Woodard and Calvary (10), appears to be the most widely adopted test for detecting primary skin irritants (1). The technique is simple and has been found to give reproducible results with a wide range of materials both in animal tests and in clinical trials. In practice, initial tests for the irritancy of substances are performed in animals and then, if warranted, tests on human subjects are carried out. Animal patch tests The patch test as described by Draize et al (10) involves single applica- tions of the test substance to the shaved skin on the back of albino rabbits. Accurately measured quantities (0.5 ml or 0.5 mg) of the substance are applied to the skin under 25.4 mm square gauze patches which are secured with adhesive tape. To prevent any volatile constituents from evaporating, the trunks of the rabbits are wrapped in rubberized cloth. The animals are placed in restrainers and the patches left intact for 24 h. This procedure ensures that the skin is in contact with the test substance for the entire period

742 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS of treatment, and that the patches are not dislodged by the movement of the animals. In addition to applying the patches to areas of intact skin, patches are applied to areas of skin which have been abraded by scoring through the stratum corneum, but not sufficiently to cause bleeding. Test areas are examined at 24 and 72 h after treatment and scores allocated according to the degree of erythema, oedema and eschar formation. The average of the combined scores given for the areas of intact and abraded skin is referred to as the Primary Irritation Index. The test as described has been widely adopted and found to give useful information for placing sub- stances in order of their irritancy. The use of positive and negative controls is essential in animal tests (11, 12). Sodium lauryl sulphate is regarded by some workers as a satisfactory positive control for primary irritants, since it does not produce an allergic reaction (13, 14). Although some workers believe that sufficient evidence is available to indicate that tests of this type are suitable for showing the irritancy of substances to the human skin (15), their views are not generally accepted for the reasons discussed below. A disadvantage of the test as recommended, is that the results rely entirely on subjective assessments of the appearance of the test sites. No objective measurements are made nor is any microscopic examination of the skin carried out to detect changes which are not visible to the naked eye. Schmid (16) suggested that alterations in the thickness of the skin might be taken as an indication of the degree of irritations. He indicated that histo- logical studies might be carried out to substantiate this conclusion. Where it is possible to examine areas of treated skin histologically, a more detailed assessment of the damage caused by an irritant can be made (17). The standard patch test as outlined above, has been modified on numer- ous occasions, but in general, these modifications concern the size of the patches, the quantity of test substance applied to the skin, the duration of the exposure period, and the animal species employed (18, 19). The degree of occlusion and the pressure with which the patch is applied are other variable factors which have been investigated as possible sources of error (2, 20-25). A semi-occlusive patch test described by Holland (20) resembled the Draize test in having the test materials applied on absorbent pads, but the rubberized cloth or Cellophane used to prevent any volatile materials escaping, is omitted. Using detergents, these authors obtained different scores from those given by the standard patch test. A pressure patch test was devised by Fernstrom (26, 27). This patch consisted of an 8 x 8 mm square of blotting paper impregnated with the test substance which was applied to the skin under a sheet of rubberized cloth. On this