LACTIC ACID STINGING TESTS 5 the desire to scratch. Burning in the painful sensation that follows a thermal burn. Stinging occurs when alcohol is applied to a cut or abrasion. The reliability of the subject must also be checked. A woman who stings on the first exposure is subjected to a repeat test a week or two later, this time using distilled water. As many as 20% of initial stingers have to be disqualified because they report appreciable stinging to distilled water. Green et al. also identified false-positive reactors by prescreening with the vehicle only (4). Finally, we establish whether the candidate is able to discriminate between 5% and 10% lactic acid. REPRODUCIBILITY We applied 10% lactic acid to ten moderate stingers and repeated the exposure exactly one year later. The findings were virtually identical (Table I). We maintain a panel of experienced stingers. We found that stinging was greatest and most reproducible in the winter months (6). This also holds true for susceptibility to chemical irritants (11). We forego stinging tests in hot, humid summer months because of variability of results, especially if the subjects have recently sweated. RESULTS AND DISCUSSION EFFECT OF PRIOR DAMAGE Frosch and Kligman found enhanced stinging when lactic acid was applied to facial skin one day after administering 2 MEDs of ultraviolet radiation (1). Likewise, sharper and faster stinging occurred on skin that had been mildly damaged by a chemical irritant, benzalkonium chloride. We preselected a panel of mild-to-moderate stingers to evaluate the influence of prior Table I Reproducibility of the Lactic Acid Sting Cheek Test: Stinging Grades in Moderate Stingers Tested After a One-Year Interval Onset (minutes) Peak intensity Subject Nov. '92 Nov. '93 Nov. '92 Nov. '93 1 2 4 2 2 2 3 3 1 2 3 4 2 2 3 4 5 3 1 2 5 3 4 1 1 6 2 3 3 2 7 4 3 2 1 8 4 4 2 2 9 2 2 3 2 10 1 2 3 3 Median 3.0 3.0* 2.0 2.0'* Neither the median onset *, nor median intensity **, measured in 1993, was different from that measured in 1992. The Wilcoxon rank sum test was used for the statistical analysis.

6 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS chemical injury. Beginning on a Monday morning in February and repeated daily over the next four days, the face was washed with a highly detersive soap in the following way: A generous lather was worked up with the fingers on the moistened faces of ten subjects and was allowed to remain for five minutes before rinsing. By the fourth exposure the skin had become dry, scaly, and tight, with a variable mild redness. Lactic acid was then applied one day after the last exposure (Table II). In every case the onset was shortened and stinging reached a peak within a few minutes. Half of the subjects showed erythema that lasted a few hours. Enhanced stinging, but to a lesser degree, was still evident when the test was repeated one week later. We repeated this soaping procedure on a group of six nonstingers. An immediate slight stinging was experienced by four of the six after lactic acid was applied. However, there was little or no stinging after the first few minutes. Thus, nonstingers did not become stingers after being irritated by a detergent. REGIONAL DIFFERENCES The face is, of course, not a uniform territory. Among other differences, the density and size of the follicular orifices and sebum production vary in different regions, showing a mid-to-lateral decreasing gradient (12). We evaluated stinging responses on ten moderate stingers in the following regions: the mid-forehead, the chin, the nasolabial fold, the lateral cheek (pre-auricular), and the malar eminence (Table III). One site was tested per day. The greatest stinging occurred on the malar eminence, followed closely by the nasolabial fold, with a sharper decrease on the chin. The forehead and lateral cheek were the least reactive. Accordingly, the malar eminence is our preferred test site. An explanation for these striking regional Table II Exacerbation of Stinging by Vigorous Twice-Daily Soap Washings on Four Successive Days: Effect of Soap Washings on Mild Stingers Bas eli ne Post - wash i ngs Onset Onset Subject (minutes) Peak intensity (minutes) Peak intensity 1 2 1 1 2 2 2 1 1 2 3 3 1 1 2 4 4 1 2 3 5 2 2 1 3 6 2 2 1 3 7 5 1 2 3 8 3 1 1 2 9 4 1 2 2 10 3 1 1 2 Median 3.0 1.0* 1.0* 2.0'* The onset time was significantly shortened *, and the peak intensity significantly increased **, after the soap washing regimen. Statistical analysis by Wilcoxon rank sum test.