APPRAISAL OF METHODS FOR DETECTING PRIMARY SKIN IRRITANTS 743 was placed a plastic sponge and the whole secured with bandages. Rosten- berg (22) suggested that in such a test the skin will be kept in contact with the test substance more consistently than if the standard test is used. Despite any advantages that this method may have in animal tests, Bett and Calnan (28) considered that it offered no improvement over the standard procedure in clinical trials. Another modification, aimed at improving the contact between the skin and the test compound, is the cup technique (29, 30). Cups made of impervious material are filled with the test substance (liquid) and applied to the skin. Rostenberg (22) reported that this method was particularly satisfactory for screening substances for irritancy in human tests but it had the disadvantage of being more cumbersome and slower than the standard procedure (31). The reproducibility of the results obtained using this technique is not known. False positive reactions in the occluded patch test have been related to the effects of the patch on the microclimate of the skin at the test site (4) and the irritancy caused by the patches themselves. Although in safety testing, it is the aim of the experimentalist to exaggerate the conditions under which the test substance ultimately will be used, Idson (25) holds the view that the conditions used in patch testing are too stringent and the results obtained likely to be misleading. With occlusive patches, the skin is under duress and is unable to function normally, in terms of sweat evapora- tion, cooling and cutaneous respiration (4, 32). Furthermore, volatile solvents are prevented from evaporating as they would do in normal use of the product (32, 33) and may modify the irritant response (34). A reduc- tion in the evaporation of sweat may result in an increased hydration of the skin (35), an effect which is likely to increase skin permeability and thus increase sensitivity. An example of misleading reactions given by patch tests was provided by Holland (20), who exposed volunteers to solutions of detergents and found very different results using occlusive and non-occlusive conditions of testing. These observations emphasize the importance of testing substances under conditions which resemble the intended conditions of use (25). The components of the patches themselves have been shown to bring about changes in the skin, irrespective of whether irritants are present or not. Rubberized fabric, adhesive plaster and other materials commonly used, have been shown to produce changes in the skin which include inflammation, increased hydration and damage to the stratum corneum (36-38). Hence, adequate precautions should be taken to ensure that the materials used in patch tests are of minimal irritancy to the skin.

744 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Another feature of the patch test which may give misleading results is the pressure used in applying the patch (2, 22). Fisher (2) pointed out that the very application of the patch to the skin is sufficient to cause an im- pregnated test substance to be forced to the periphery of the patch. In this way, uneven distribution of the test substance would be formed, giving rise to stronger reactions in the skin at the periphery of the patch than at the centre. For a mild irritant this could result in a score being given which would be consistent with that of a moderate or even a severe irritant. Evidence that certain substances appear less irritant when applied to the skin under occlusive patches (,24) and that other irritants such as solvents are unlikely to be detected using open patch tests (32) suggests that in a realistic assessment of primary irritants both open and closed patch tests are required (5, 18, 22, 39). Open tests involve an application of standard amounts of the test compounds to the skin followed by an evalua- tion of the cutaneous responses after a given period, e.g. 24 or 48 h. Such tests are carried out routinely in the evaluation of epidermal carcinogens. In this instance, applications are often repeated at frequent intervals for 30-80 weeks--or until tumours appear. In studies of early changes produced by carcinogens, an evaluation of the cutaneous response is occasionally carried out within 24 or 48 h (40-42). Repeated administration for testing irritancy has also been recommended for medicaments (17). However, no actual recommendations for repeated application appear to have been made concerning the testing of cosmetic chemicals for primary irritation. The Draize patch test in animals, although widely used for the detection of primary irritants, can duly be regarded as an approximate guide to the safety of a particular substance (1, 3, 5, 11, 43-45). Whereas tests in animals are usually satisfactory for detecting severe irritants, they are of considerably less value in tracing mild irritants due to their lower sensitivity (1, 25, 46). To increase the sensitivity of animal skins to optically applied substances, Finkelstein and his co-workers (47) pre-treated the skin with 20•o formalde- hyde solution. In their view, the sensitivity of the animal skins was increased such that the responses given were of a similar order to those given by human skin. Brown (48) reported recently that there was noticeably good agreement between the results obtained with occluded patches on formalin treated rat skin and those on untreated rabbit skin with a range of ionic and non-ionic surface active agents. It is considered, however, that formalin is sufficiently irritant to obscure the responses of an animal's skin to a mild irritant. Thus, the technique is of questionable relevance as a possible modification of the standard patch test.