LACTIC ACID STINGING TESTS same panelists (10). Furthermore, discrepancies occurred when the right and left cheeks were simultaneously compared. Many contract laboratories now perform lactic acid stinging tests on facial products. Unfortunately, the results of inter-laboratory reports are disturbingly great (personal communication). In this report, we shall summarize the investigations that led to a rigorous refinement of the original method. MATERIALS AND METHODS Ten percent racemic D-L lactic acid is prepared in distilled water from 85% syrup (Sigma Chemical). Fifty microliters is then pipetted onto the absorbent pad contained in a 2-cm-diameter Hilltop chamber. This volume saturates the pad without leakage. The adhesive around the chamber is trimmed to leave two short 2-mm tabs on opposite sides, sufficient to seal the chamber to the skin (Figures 1, 2). The exposure time is ten minutes, after which we remove the chamber and briefly hand- wash the face with a mild liquid soap. During the exposure, the subject records stinging each minute on a 0 to 3 interval scale: 0 = none, 1 -- slight, 2 = moderate, 3 = severe. Skin care products are discontinued for two weeks prior to the test, except for lipstick Figure 1. A Hilltop chamber with its backing trimmed to minimize adhesive in contact with the skin Lactic acid solution is applied to the absorbent pad that fills.

4 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 2. A Hilltop chamber in place on the skin. and eye-area make-up. The panelists are also given a mild soap bar or liquid wash to be used only once daily. Coffee, tea, alcohol, and smoking is prohibited on the day of the test. The panelists report to the laboratory one hour before testing and remain in a quiescent state in a noiseless, pleasant room. The subjects are comfortably seated during the exposure. Two values are recorded: (1) The time in minutes when stinging is first unequivocally perceived (short times usually presage intense stinging). (2) The peak intensity reached during the ten-minute exposure, on a 0 to 3 scale. Occasionally, stinging declines before the end of ten minutes. An alternative system is to sum the ten scores and to calculate the mean. SUBJECT SELECTION We are persuaded that women are more likely to be stingers than men, but this difference has not been conclusively demonstrated. To eliminate ethnic differences, we recruit only normal white women. We restrict the age range to 25-50 since it is known that stinging diminishes in the elderly (6). Selection must be a very careful process. Women are often confused about sensory descriptors. We take pains to explain the distinction between itching, burning, and stinging, using common-place examples of each. For example, itching is characterized by