LACTIC ACID STINGING TESTS 7 Table III Regional Differences in Stinging (peak intensity) Subject Forehead Chin Nasolabial Lateral cheek Malar eminence 1 1 2 2 0 2 2 0 2 3 1 3 3 1 2 2 0 3 4 0 2 3 1 3 5 1 2 2 1 2 6 1 3 2 0 1 7 0 2 2 0 2 8 0 3 3 0 3 9 1 2 2 1 3 10 0 2 2 0 2 Median 0.5 2.0' 2.0' 0 2.5' The malar eminence*, nasolabial region*, and chin* show a modest rank order, but this was not statistically different. All were significantly greater than the forehead and lateral cheek. Statistical analysis by Wilcoxon rank sum test. differences is not obvious. Innervation, blood supply, and permeability may all be playing a role. CONCOMITANT TESTING ON OPPOSITE SIDES Mayne et al. noted asymmetry of the reactions to swabbings with 5% lactic acid (10) on opposite sides. We examined disparate responses on opposite sides by performing cham- ber tests on ten moderate stingers on a single day. Ten percent lactic acid was applied to the right side of five subjects and to the left side in the remaining five. Two hours later, the test was repeated on the opposite sides. We found exceptional concordance in regard to onset and peak intensity (Table IV). The right and left cheeks are evidently symmetrical by chamber testing. Table IV Lactic Acid Stinging on Contralateral Cheeks: Symmetry of the Stinging Response Onset (minutes) Peak intensity Subject Right Left Right Left 1 2 4 2 2 2 3 2 3 2 3 1 1 2 3 4 4 1 1 1 5 2 1 2 2 6 1 1 3 3 7 3 4 1 1 8 2 1 2 2 9 5 3 1 1 10 1 3 2 1 Median 2.0* 1.5* 2.0'* 2.0** The onset * and peak intensity ** were not statistically different.

8 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Frosch and Kligman thought that concomitant testing on opposite sides was feasible (1). Thus, a comparison of two products or chemicals could be made at the same time (1). Experience has convinced us that concomitant testing yields inconsistent and misleading results. Striking interactions in perception occur when materials of differing stinging potential are applied to opposite sides. We demonstrated this by simultaneously ap- plying 10% lactic acid on one side and distilled water on the opposite side of ten strong stingers. Three of these recorded moderate stinging on the water side. In another test on this same panel, 5% and 10% lactic acid were applied on opposite sides. The same three subjects reported equally severe stinging on both sides, and another three could not distinguish between the sides. We have repeatedly found on simultaneous testing that the side which provokes more intense stinging elevates the stinging score on the opposite side. Moreover, some degree of perceptual confounding occurs even when the tests are separated by 30 to 60 minutes, if the material producing more intense stinging is applied first. We perform unilateral testing only. STINGING AND IRRITANCY Like Frosch and Kligman (1), we have not been able to establish a strict correlation between chemicals that induce inflammatory irritant reactions and their ability to provoke stinging. Some strong irritants such as sodium lauryl sulfate and benzalkonium chloride do not cause stinging, while many substances that are toxicologically innocuous can elicit stinging. A variety of acids, such as lactic, citric, and phosphoric acids, readily induce stinging, as do strongly alkaline agents such as sodium hydroxide. Stinging capacity cannot be reliably predicted by chemical structure. One has no recourse except to test empirically. By contrast, Muizzudden et al. have found that facial stinging correlates well with hyperreactivity to irritants placed on the back and forearm (13). It is known that some persons have a generally hyperirritable skin sensitivity to chemical insults such as DMSO, ammonium hydroxide, and cantharidin (11). However, it remains to be dem- onstrated whether hyper-reactors are generally stingers. EFFECT OF CONCENTRATION Routine stinging tests were performed on ten subjects who were moderate stingers, using the following concentrations: 2.5, 5.0, 10.0, 15.0, and 20.0. We started with the lowest concentration, and exposed only one cheek per day. On each following day, the next higher concentration was applied, on the opposite cheek. The end point was the peak intensity score. The results may be readily summarized. Two of ten subjects experienced slight stinging to 2.5% lactic acid. Four had mild stinging to 5%. With 15% the intensity scores were approximately the same as with 10%, though stinging tended to begin earlier. With 20%, four reported more severe stinging, while the remaining six showed no increase. These same four showed mild erythema that lasted for a few hours, followed by mild