EXPERIENCES WITH HUMAN PATCH TEST METHOD 325 as a standard basic technique in de- termining the effect of a substance upon the skin. In brief, this basic technique consists of: 1. Application of the substance directly to the skin for a defi- nite period of time. 2. Removal of the substance from the skin. 3. A recuperation period of a definite length of time. 4. Reapplication of the sub- stance directly in contact to the skin of the same subject. 5. Removal of the second appli- cation. The technique of this basic method may be modified depending upon circumstances, that is, the nature of the material to be tested--its volatility, solubility, pH, and os- motic pressure, to name but a few. The substance is placed in contact with the skin by means of a patch. The material used for the patch must be one which is inert in regard to the skin. For this purpose many investigators use sterile cotton gauze pads or lintine discs. If the sub- stance to be tested is a solid, a meas- ured amount is placed on the disc and spread uniformly over the en- tire surface. If the substance is a liquid, the disc is saturated with the substance. This disc is then placed directly in contact with the cleansed skin of the human subject. The disc is then covered with a water- proof film such as cellophane or waxed paper. It is important that the film of cellophane or waxed paper cover an area larger than that of the disc. This is done so that the adhesive tape used to kee p the disc in place does not come in con- tact with the disc itself or the skin immediately surrounding the disc, so as to leave a definite margin of skin between that which is exposed to the substance and that which is exposed to the tape. That is im- portant because many of the adhe- sive materials used in tapes produce skin reactions by themselves. Fundamentally, the substances coming in contact with the skin may be divided into two types, namely, primaryirritants and second- ary irritants. A primary irritant is a substance which produces irri- tating effect on the skin on first contact. A secondary irritant is a substance which produces irri- tation of the skin only after the skin has previously been exposed to the substance. The fact that there are both primary and secondary irritants serves as the rationale behind the method of the patch test as outlined above. The patch test procedure is planned to detect both types of irritants. A primary irri- tant may be detected at the second step of the procedure as outlined above, that is, when the first appli- cation is removed from the skin and the results determined. A sec- ondary irritant is detected at the fifth step of the patch test procedure. The mechanism of irritation is radically different. The primary irritant produces its effect by virtue of the inherent irritating qualities of the substance. This is a non- specific type of irritation, the effect is similar to that caused by heat, low pH, rapid volatilization, or

326 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS even mechanical irritation. Second- ary irritation is a specific reaction of the body as a whole to a sub- stance. It is an allergic response of the body based upon an allergen- antibody reaction. A secondary ir- ritant shows no irritation at the end of the first application. How- ever, on reapplication skin irritation results. In the interim between the first application and second application, the body's defensive mechanism begins to form anti- bodies as a reaction to contact. When the substance is reapplied to the skin a second time anti- bodies are already present and the antagonism between these anti- bodies and the substance produces a reaction. This reaction may be local, that is, confined only to the site of the reapplication, or it may be general, that is, over the entire skin surface as well as mucous membranes and the viscera. Thus step 3 of the patch test procedure, the recuperation period, must be of duration sufficiently long to enable the body to produce the antibodies necessary to react with the sub- stance when it is reapplied. There are many properties of primary irritants which are factors in the testing of these irritants. These are concentration, volatility, duration of contact, area of contact, pH, and osmotic pressure. The concentration of a substance which is placed on the skin is an important factor, since, if a substance is known to be irritating in high concentra- tions it may not be in lower con- centrations. In studying a sub- stance which shows a tendency to- ward being a primary irritant, it is wise to work with high dilutions so that reactions obtained are not violent. Concentrations of the sub- stance may gradually increase as the solvent vaporizes or evaporates and produce a misleading result. Just as much, can be learned about irritating properties of a primary irritant from a mild reaction in higher di- lution as can be learned from a severe reaction in low dilutions. The boiling point and vapor pres- sure of the solvent are also very im- portant since highly volatile sub- stances are irritating by virtue of their burn-producing capacity for example kerosene is a widely-used solvent yet the number of cases of dermatitis produced by kerosene is relatively small. However, im- proper use of kerosene produces an irritation of the skin. Thus if a substance such as kerosene is used as a solvent, the patch would be removed within an hour after application. If we permit it to stay on the skin for twenty-four hours, such as is done with water- solvent patches, a large blister similar to a first degree burn would result. However, the reaction which is produced on the skin with kero- sene in an hour disappears within a very short time. The irritating qualities of a highly volatile sub- stance such as kerosene can be mitigated to a great extent by dilut- ing it with an innocuous substance such as heavy mineral oil and there- by permitting a longer period of contact with the skin. Eventually