328 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS skin there should be allowed to elapse a period of time. This period varies with different observers and in searching the literature one finds that this period varies anywhere from five days to twenty-one days. It is this period during which time the individual is not in contact with the substance that is called the sen- sitizing period since the initial ap- plication of the tested substance to the skin has resulted in the probable deposition of small quantities of the test substance in the skin. The bio- logical effects of this become appar- ent on the second application. On second application the substances are again applied to the skin of the same subjects and in the same man- ner as done on the first application. This patch is kept in contact with the skin for a period ranging any- where from twenty-four to forty- eight hours. At the end of this time, these patches are removed and the reactions are noted. The second application of a substance tested, such as wearing apparel, would be re-wearing the same arti- cle of clothing so that it would be in contact in the same parts of the body, as on the first application. If on first application there has been no evidence of irritating effect on the skin and after the subsequent layoff period and a reapplication of the material to the skin there is a reaction, then the skin has been sensitized to that substance. The substance is therefore known as a sensitizing substance. Ira material gave very mild reactions on first application and the second appli- cation elicited much stronger re- actions in a significant number of subjects using the same concentra- tion of this test substance then the material once more can be consid- ered as being a sensitizer. If a material produced a marked reac- tion on first application, it is only proper to assume that the reaction would have been obtained in the second application if the same con- centration were used. Therefore where strong reactions are obtained on first application of the material, then on second application a much higher dilution is applied to the skin. If the reaction to the diluted mate- rial is as great as, or greater than, that obtained with a more concen- trated material on first application then this substance may also be said to be a sensitizing substance. Thus far we see that if a sub- stance is a primary irritant, we can control this irritating effect either by varying the time during which the material is kept in contact with the skin or by diluting it suffi- ciently with an innocuous substance and thereby reaching the point where the material is no longer irritating regardless of the time it is kept in contact with the skin. Therefore, with primary irritants we can regulate the action of the material or we can eliminate its irritating effects entirely. The situation is quite different with the substance which is a sen- sitizer. This substance on first ap- plication may give absolutely no reaction, or a very mild reaction, or in some cases it may also be a

EXPERIENCES WITH HUMAN PATCH TEST METHOD 329 strong primary irritant. However, the sensitizing substance on second application, even if there has been a mild reaction or no reaction on the first application, will elicit a severe reaction. In the case where it is a primary irritant, a high dilution of this material will elicit as severe a reaction as the concentrated ma- terial in the first application. We have thus far considered merely the concentration of thesema- terials. Now the time factor enters. In order to definitely rule out a substance as being a sensitizer, observations must be made of the skin of the subject for many days after the skin patch is removed. In our own experience we have seen sensitizing reactions appear imme- diately, within several hours after the second application and we have seen them with some substances delayed so that they did not appear for approximately three weeks. At the end of three weeks after the second series of patches had been removed, the site of the second application became inflamed and a severe reaction set in. With some materials the primrose reac- tion is apparent. In this reaction, if the sensitizer is placed on one arm on first application and on the other arm on second application, the sensitizing reaction obtained will appear not only at the second site of application but will produce a flare-up on the original site of application. The patch method of Dr. Schwartz should be, by his own admission, called the "human skin test method." This method as outlined above involves initial application of a substance, a period of layoff, and a second application of the substance. The number of subjects which should be studied again depends on the information which is desired. Primary irritation may be deter- mined on a few subjects. Ten subjects to 25 subjects should give all the information necessary as to whether a material is a primary irritant or not. It is only where it is important to determine whether a material is a sensitizer or not that the group of 200 subjects (unselected subjects, males and females approxi- mately equally divided as to sex and preferably those which are not patients in a clinic where they are being treated for some dermatitis or dermatological condition) is re- quired. It is only when a material or a substance shows up negatively, that it does not tend to appear to be a sensitizer on 200 subjects, that the prophetic patch test issues its prophecy that it is now probably safe to have the material come in contact with a larger group of sub- jects. It must always be remem- bered, however, that regardless of large numbers of cases where a substance has been used safely there is always a possibility that some individual may be allergic to that substance. This, however, does not incriminate the substance itself as being a sensitizing sub- stance. It is only the idiosyncrasy of that person who has that individ- ual allergy or sensitivity that makes him react to this material.