SKIN REACTIONS TO COSMETIC PREPARATIONS 9 Even if there were no adverse effects in 30,000 persons tested, 1 in 10,000 (0.01%) would still be liable to skin reactions (10). Wilson (11) tested four groups of 50 persons, normal subjects, patients with atopic eczema, patients with nummular eczema, and those with contact eczema. 500 tests were performed in each group. Normals gave 6.8% positive reactions. Atopics gave 15% Nummu!ars gave 14% Contact eczema gave 23.7% The conditions of any test or even trial may not resemble the exact conditions in which the preparation will be used. No manufacturer can legislate for the personal idiosyncrasies and perverseness of his customers, particularly those with difficult or abnormal skins. PATCH TESTING The technique of patch testing is too well known to require description. The interpretation of results may be very difficult, and there is some con- fusion over the purpose and limitations of the test. Bett and Cainan (12) sum up the position admirably by stating that the test is designed to demonstrate the presence or absence of antibodies, and not to reproduce the patient's disease in miniature. A positive test means that specific antibodies are present which may be (1) the cause of the dermatitis, (2) the result of the dermatitis, and (3) unrelated to the dermatitis. It follows that the test substance must never be used in sufficient con- centration to act as a primary irritant, and that the patient must not be in status eczematicus. Sensitivity should be reserved for allergic sensitivity to an allergen (sensitizer), the reaction to a primary irritant being termed irritant or toxic to avoid confusion. The characteristics of the former are erythema, oedema, closely set even sized vesicles of the latter, erythema, oedema and necrosis of stratum comeus or deep layers, sometimes with bu!la formation. Unfortunately the characteristic form is not seen at every test. The constituents of most cosmetics are, however, sensitizers rather than irritants, in the concentration used in cosmetics. Soaps, detergents, shampoos, oils. and organic solvents are mostly irritants. Sensitizers give graduated response with increasing dilution to 10% or' less, whereas an irritant at 50% gives a faint or negative result. Sensitivity lasts for a long time, and test results tend to remain constant whereas aa

JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS irritant reaction may vary considerably, or be negative after an interval of some weeks. False positives These occur frequently in an acute generalized phase of eczema (status eczematicus). Repetition an adequate time after clearance in this case, should be negative. Patches near a strong positive or piaster reaction may react. If repeated separately they will be negative. Pressure from solids, e.g. dentures, may produce a false traumatic positive, to be repeated with shavings or powder. False negatives These occur with inadequate concentrations. In fact most of the stan- dard lists of concentrations tend to be too high. But eosin may need a concentration of 50% to produce a positive reaction. Inadequate penetration may produce a false negative response, e.g. lip- stick on skin thicker than the lip, or solids which cannot penetrate (repeat with powder if in doubt). In occasional failure to leach out the agent from material, the test conditions may be more dry than clothing in the axilla. Face powders give a very feeble erythema. False negatives because of the so-called refractory phase, or because the sensitivity is localized must be very rare. To avoid delayed positive reactions, tests should be read at 48 and 96 hours. Diluents Water, petrolatum, liquid paraffin, olive oil or castor oil are mainly used for dilution. Organic solvents are unsatisfactory and materials containing them should be applied after removal by evaporation or dilution, e.g. nail varnish is applied to lint and allowed to dry. N.B. A simple control test must not be forgotten. In cross sensitiza- lion, secondary allergens may not react in the same dilution as a primary allergen. Related substances, if they give negative results, should be re- tested in higher concentrations. REACTION PATTERNS TO COSMETIC IRRITANTS The skin, being a relatively simple structure, is only capable of a limited number of possible responses. These are: (1) Eczema--dermatitis. (2) Light Sensitivity