STINGING CAPACITY OF APPLIED SUBSTANCES 199 Those who experience sharp stinging for at least 3 to 5 min are identified as stingers. An alternative screening test, which can be carried out at room temperature, involves the application of 2 ml of 90 per cent aqueous dimethylsulfoxide (DMSO) in a small glass cup on the cheek for 5 min. This produces intense burning in stingers however, the disadvantages of DMSO are disqualifying. The substance smells and tastes badly, not to mention that a solid tender wheal and persistent erythema develop. By contrast, lactic acid produces no visible changes. II. STINGING ASSAY This is carried out on groups of 5 to 10 stingers, depending on the degree of accuracy required. The subject enters the environmental chamber (110øF, 80 per cent R.H.) and after the face has begun to sweat copiously (generally after 15 min) a liberal amount of the test material on a cotton swab is thoroughly rubbed over the nasolabial fold and cheek. Stinging is evaluated immediately (10 sec) and at 2.5, 5.0, and 8.0 min on a 4-point scale: 0 = no stinging 1 = slight 2 = moderate and 3 = severe. The time se- quence was formed empirically. Some substances cause slight to severe stinging im- mediately after application with disappearance of the sensation within 5 to 30 sec. De- layed stinging is generally not preceded by a transient phase and usually becomes evident within a minute or two. The delayed stinging score for an individual is the mean of the three readings at 2.5, 5.0, and 8.0 min. We arbitrarily regard substances with average scores falling between 0.4 and 1.0 as having slight stinging potential. The range 1.1 to 2.0 signifies moderate stinging, and 2.1 to 3.0 signifies severe stinging. A second agent can be simultaneously evaluated on the opposite cheek. Within limits, a series of agents can be evaluated during 1 sweating period. EXPERIMENTAL OBSERVATIONS I. PREVALENCE or STINGERS Thirty young adult student volunteers (20 whites, 10 blacks, equally divided as to sex) were evaluated using both lactic acid and DMSO probes. Both tests gave concordant results. There were 4 stingers among the whites, only one of whom was a male. Only 1 black, again a female, exhibited stinging. With lactic acid stinging began after a few seconds and in 4 of 5 subjects persisted strongly for about another 5 min, subsiding slowly over the next 10. DMSO produced peak stinging within 3 min this rmnained at high intensity for another 10. The nonstingers did not perceive significant stinging with either test. It is worth noting that DMSO induced considerably less whealing and erythema in the latter. All 5 sting- ers thought they had unusually "sensitive" facial skin having had past trouble with cos- metics, soaps, etc. Three had a history of atopy (but not atopic dermatitis). The white stingers had light complexions, sun-burned easily, and tanned poorly. All 5 blushed easily. The sample is too small for any but the most tentative judgments. Females seem to be more susceptible. It remains to be shown whether this is a true sex difference or

200 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS