j. Soc. Cosmet. Chem., 47 1-11 (January/February 1996) An improved procedure for conducting lactic acid stinging tests on facial skin MICHAEL CHRISTENSEN, S.K.I.N. Incorporated, 151 East Ten• Avenue, Conshohocken, PA 19428, and ALBERT M. KLIGMAN•,-, of Dermatology, University of Pennsylvania School of Medicine, Philadelphia, PA 19104. Department Received November 15, 1995. • 0 3 1998 coe • •c• $y•o•$•$ The lactic acid stinging test was proposed as a means of identifying women with hyperirritable "sensitive" skin of the face. These are normal persons who, in the absence of clinical signs of irritation, experience various neurosensory discomforts after using certain cosmetics and skin care products unaccompanied by signs of irritation. The original test has come under sharp criticism for unreliability, because of inconsistent results among laboratories. The present studies were undertaken to improve the accuracy, reliability, and reproducibility of the test. It consists of a ten-minute exposure of the cheek to 10% racemic lactic acid in a Hilltop chamber. Two values are obtained: (1) the time required for stinging to be perceived and (2) the intensity of peak stinging on a 0 to 3 interval scale. The salient findings were: Identical results were obtained when the test was repeated on the same group after a period of one year. Increasing the concentration to 20% and greater may induce visible irritation without appreciably increasing the peak intensity scores. Deliberately damaging the skin by repeated washing with a highly detersite soap does not convert "non-stingers" to "stingers." Substances cannot be simultaneously tested on both cheeks. Strong stinging on one side enhances the perception of stinging on the opposite side. The revised test helps producers of skin care products to ascertain before marketing whether persons with "sensitive" skin will experience adverse subjective discomforts after daily use. A typical protocol involves twice-daily applications to the face for two weeks. Intensified stinging is a warning that consumers with sensitive skin may experience itching, burning, or stinging. In 1977 Frosch and Kligman published a method for appraising the capacity of topically applied substances to induce stinging on facial skin (1). An unusual experience stimu- lated their investigation. A sunscreen that contained amyl-dimethyl-p-aminobenzoic acid had passed all the conventional tests for topical safety. Nonetheless, after market- ing, the manufacturer received numerous complaints of stinging, necessitating recall. Intense stinging occurred in the complete absence of visible signs of irritation. This is sometimes called "subjective" irritation, to differentiate it from objective signs such as erythema and scaling (2). Producers of facial products have become increasingly aware that adverse neurosensory reactions such as itching, burning, and stinging are fairly common and are usually not

2 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS predicted by routine toxicologic procedures. Surveys have shown that many women who have no skin disorders describe themselves on questionnaires as having "sensitive skin" (3). Fragrances, sunscreens, and moisturizers are often singled out as causative. Green and Bluth have comprehensively reviewed the psychophysical methods for measuring adverse sensory reactions to chemicals (4). They suggest the term "chemosensory irri- tability," rather than subjective irritation. Since a substantial number of women report that they have "sensitive" skin (3), it is a daunting challenge to identify these women prior to marketing. By definition, "sensitive" skin refers to women without preexisting skin disease. Der- matologists are well aware that patients with rosacea, atopic dermatitis, ichthyotic disorders, and other dermatoses experience subjective irritation with much greater frequency (5) than other women. These patients can be identified by history and ex- amination. Identifying persons with sensitive skin has been the object of serious inves- tigation in our laboratory for many years. It is important to define stinging explicitly to avoid confusion with other subjective responses. A prototype for burning is the chloroform:methanol (20:80) model described by Soschin and Kligman (6). When applied to the cheek in a glass cup, intense burning develops in a minute or less. The end point is the time to experience intolerable burning. An example of itching is the sensation provoked by histamine, prompting an urge to scratch. Lactic acid stinging develops in a crescendo-type pattern. It begins to be perceived after a few minutes, gradually intensifying to peak intensity in about 3-5 minutes. It then disappears after 15 to 20 minutes. There are usually no clinical signs of irritation. A mild, transient erythema may appear in unusually susceptible persons. Stinging must be differentiated from pain, as exemplified in the model used by Arm- strong eta/. (7). They applied irritating chemicals to the bases of unroofed cantharidin blisters. Pain was rapidly induced. Green and Bluth have emphasized that individuals differ greatly in their responses to chemical stimuli (4): A response to one stimulus does not predict subjective irritation to others. Different sensations may occur concomitantly for example, stinging may be accompanied by itching. There is some evidence that strong stingers are more readily damaged by known irri- tants. Dimethylsulfoxide induces larger wheals in stingers (1). Lammintausta eta/. found that stingers were more vulnerable to irritating chemicals (8). Thus, the lactic acid test might provide a way to identify persons with hyperirritable or delicate skin. In the original model, stinging was most consistently provoked when the subjects were brought to a state of profuse sweating in an environmental chamber at 120øF. A 5% solution of lactic acid on a soaked cotton swab was then rubbed briskly over the nasolabial fold and cheek. Stinging was scored at 2.5, 5.0, and 8.0 minutes on a 0 to 3 interval scale. The mean of these three readings comprised the final score. Frosch and Kligman suggested a facial sauna as a convenient substitute for inducing sweating. Finally, they suggested that increasing the concentration to 10% would provoke equiv- alent stinging in the absence of sweating. Grove eta/. subsequently proposed guidelines to standardize performance of the test (9). The lactic acid stinging model has come under sharp criticism. Mayne et al. could not obtain reproducible results when the procedure was repeated at different times on the