LACTIC ACID STINGING TESTS 9 scaling over the next few days. The onset of stinging was reduced by at least two minutes in eight of the ten subjects tested with 20%. It thus appears that a near plateau in peak stinging is reached at 10%, with only modest increases with 20%. However, this last concentration can induce objective signs of irritation such as redness and desquamation. •(•e conducted a preliminary investigation with 50% lactic acid in five moderate sting- ers. Increases in the intensity scores were not impressive, though stinging began almost immediately. More noteworthy was the appearance of moderate erythema in four of the five, still present in three subjects 24 hours later. A mild spotty folliculitis was noted in two, 24 hours after exposure. High concentrations thus can provoke a visible dermatitis after a ten-minute exposure. INFLUENCE OF AREA No attention has been paid to the size of the area of exposure. This turned out to be a surprisingly influential factor. On separate days we compared the stinging scores of five subjects to 10% lactic acid using small Hilltop chambers having a diameter of 1.2 cm and larger ones of 1.7-cm diameter. The subjects included mild and moderate stingers. In nearly every case, both the onset of stinging and peak intensity was greater with the larger chamber. Stinging was even further enhanced when we placed 3 ml of 10% lactic acid in a 3-cm-diameter glass cylinder hand held to the skin of these five subjects. Swabbing the entire face of these subjects with 10% lactic acid enormously increased the stinging response. The onset was almost immediate, increasing to peak intensity within one minute. In four, stinging was so severe as to require rinsing with water to obtain relief. Moreover, objective signs of irritation, notably erythema, appeared, lasting for hours or even the following day. •(•e routinely use 1.7-cm -diameter Hilltop chambers. This amplification of subjective and objective signs of irritancy by application to larger areas is an intriguing phenomenon that deserves further study. Incidentally, in unrelated studies we were able to demonstrate more intensive inflammatory reactions to 24-hour occlusive applications of irritants like sodium lauryl sulfate and benzalkonium chloride when the patch test area was increased fourfold. RESERVOIR EFFECTS Stinging ceases quickly when the test area is washed and rinsed. The consequences are quite different if the area is not washed after removing the chambers. Stinging dimin- ishes slowly, usually within one to two hours. Then, reapplication of a chamber filled with distilled water will elicit stinging, though of somewhat lesser intensity. •(•ater can elicit this response in some persons even one day later. Reactivation of stinging by water does not occur when the test area is washed and rinsed in the routine way. Clearly, without rinsing, a reservoir of lactic acid is established in the stratum corneum. •(•ater results in a release of the stored acid. •(•e have made similar observations with strong soaps applied in a chamber. When the site was not washed, reexposure to water caused reappearance of erythema. Testers need to be aware of this phenomenon and insist

10 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS on a wash-out period before assessing subjective and objective reactions to potentially irritating materials. TACHYPHYLAXIS? This esoteric term applies to a well known phenomenon in which a predicted response to a chemical fails to occur upon repeated challenge. For example, the vasoconstriction induced by a six-hour exposure to a corticosteroid will not occur after three to four such exposures daily. The site becomes refractory. We were cautious not to repeat the lactic acid test too frequently on the same site for fear of intensifying the stinging response. Previous reports have warned against repeated exposures in order to prevent intensified reactions (9). However, reexposures every other day or every week on the same site did not, as we expected, augment the response. The scores either remained nearly the same or the reactions actually diminished, suggesting tachyphylaxis. More work is needed to define the dimensions of this phenomenon. How tachyphylaxis occurs is unknown. Perhaps receptors become saturated, the horny layer becomes less permeable, or follicular shunts become blocked in some way. Desensitization to the sensations induced by repeated exposures to capsaicin is well known (4). CONCLUSIONS The ability to identify stingers and classify them into mild, moderate, and severe opens up new clinical applications. There is tentative evidence that stingers are more likely to have "sensitive" skin. Muizzuddin et al. found that women who reported higher intol- erance to various products had lactic acid stinging scores that were more than three times that of persons without "sensitive" skin (13). These same individuals were also much more likely to develop irritation after exposure to Balsam of Peru. It would seem worthwhile to test new products on stingers prior to marketing. Our practice is to apply finished formulations twice daily for two weeks to the entire face of moderate stingers. Lactic acid stinging is scored at baseline and one to two days after two weeks of product usage. An appreciable increase in stinging scores would signal a potential for the occurrence of disagreeable reactions in users who have "sensitive" skin. On the other hand, a reduction in stinging scores would indicate a beneficial, protective effect. We have witnessed both outcomes in preliminary trials of proprietary "moisturizers." The stinging phenomenon is still mysterious. We are ignorant of the neurophysiologic mechanisms that underlie it. Why some apparently normal women sting severely and others not at all is a question worthy of serious investigation. Stingers cannot be recognized by phenotype or by clinical examination. Many factors are undoubtedly at play and can only be brought to light by focused research. Among the variables that might affect stinging are age, ethnicity, sex, atopic background, barrier function, cosmetic practices, and degree of oiliness or dryness. We are looking into all of these. A huge variety of skin care products and cosmetics have been designed for facial application. The predictive value of the stinging test must take into account the in- tended use of the product. For example, Grove et al. determined that a commercial formulation did not cause adverse effects in a panel of stingers (9). Nonetheless, the