RABBIT SKIN STUDIES IN EVALUATING COSMETIC SAFETY 383 and the per cent sensitivity can only be evaluated after many thousands of packs are used, and set off against the complaints of sensitization received. Sensitization in the human may depend not on one exposure to a specific product but upon several exposures to different products at the same time. It is impossible from animal experimentation, if the results in the animal are negative, to postulate how the human population will respond. It is necessary to perform experiments with animals in order to prove that, at least in the laboratory, the matehal under study will not develop primary irritation, or sensitization, in animals. MR. A. W. POND: I would not want you to think that I was making a plea for the abandonment of all sorts of testing--I would deprecate this as much as you would. I was stating that it would be desirable if we could have the type of testing which would be predictive in the sort of sense that we are required to be predictive. THE LECTURER: I am afraid that it is impossible to predict sensitivity of a low order. This type of sensitivity can only be evaluated after the product has been on the market, and has been used by many thousands. It is impossible for a dermatologist to test a preparation on two hundred humans, and after obtaining no response in each case to state that the next ten thou- sand people receiving the same matehal will show no response. In any population, the range of sensitivity to chemicals, and to cosmetic products, is tremendous and there will always be some individuals who will respond adversely to almost anything which is marketed. It is remarkable that the number of individuals, who react to the millions of cosmetics sold each year, is so small. Miss H. THORPE: What significance does the skin thickening observed in the rabbits have with regard to effects of the compound in the human ? Could this perhaps be the result of mechanical trauma ? Did you get any effect with controls ? THE LECTURER .' The skin thickening which we obtained in the rabbit was, we believe, a response to a specific chemical, the identity of which we do not know. The material was a leaf extract and may have contained many active ingredients. The response was such that the epidermis was three to four times thicker at the point of application than in the surrounding skin. Concerning the significance of the thickening I would say that any material which, after placing upon the skin, causes the response found may result, if applied over a long period of time, in a dangerous overgrowth of epithelial tissue. The thickening was not the result of mechanical trauma since control areas on the opposite side of the animal, treated with water and

384 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS massaged in the same way, did not show this thickening. The response of the skin was due directly to the application of the extract, and not due to any mechanical irritation.