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j. Soc. Cosmet. Chem., 43, 313-330 (November/December 1992) A human in vivo method for assessino reduction of the irritation potential of sodium lauryl sulfate by mild surfactants: Validation with an ether carboxylate with two different deorees of ethoxylation SOPHIE ZEHNDER, RAYMOND MARK, SCOTT MANNING, ADEL SAKR, J. LEON LICHTIN, and KARL L. GABRIEL, Biosearch Incorporated, P.O. Box 8598, Philadelphia, PA 19101 (S.Z., R.M., S.M., K.L.G.) and Cosmetic Science Program, College of Pharmacy, University of Cincinnati, Cincinnati, OH 45267 (S.Z., A.S., J.L.L.). Received July 20, 1992. Synopsis An in vivo method devised for studying reduction of the irritation potential of sodium lauryl sulfate (SLS) by added surfactants was validated using an ether carboxylate with two different degrees of ethoxylation. The method involved a two-hour occlusive patching procedure on five consecutive days accompanied by multiple instrumental measurements. Various aqueous solutions combining 1.0% SLS with sodium lau- reth-5 carboxylate (SLEC-5) or sodium laureth- 13 carboxylate (SLEC- 13) in molar ratios from 1:0.08 to 1:1.00 were applied using Hill Top © chambers on the panelists' lower backs. Measurements of transepi- dermal water loss, electrical conductance, laser Doppler flowmetry and skin color reflectance were per- formed. In addition, the skin was visually graded for erythema. A data reduction system was devised to handle the large number of data points. To accomplish this, the degree of skin response caused by 1.0% SLS was used as the reference standard. For each evaluation method, on each day and for each panelist, the molar ratio which induced •50% of the SLS value was used to derive a mean molar ratio for each group of six panelists. The mean molar ratio (i.e., the proportion of SLEC-5 or -13 to "neutralize" 1.0% SLS) reflects the effectiveness of the ether carboxylate in reducing the SLS irritancy potential. The molar proportions that induced signs of irritation changed with successive patch applications, and increasing amounts of the ether carboxylate were needed to overcome the effects of SLS. Although both SLEC-5 and -13 exerted beneficial effects in reducing SLS irritancy potential, it was clearly demonstrated that SLEC-13 (i.e., the higher ethoxylated surfactant) was more effective in modifying the irritancy potential of SLS than SLEC-5. The various evaluation methods generally showed good correlation but differed somewhat in sensitivity. The method appears to provide the requisite specificity and sensitivity to discriminate differ- ences in the irritancy potential of surfactant mixtures containing SLS. INTRODUCTION The irritancy potential of sodium lauryl sulfate (SLS) can be modified by the addition of a variety of mild surfactants such as ether sulfates, sulfosuccinates, or amphoterics (1-8). Such surfactant mixtures are common in formulations of soap and cosmetic products. Considerations in selecting cosurfactants to be added to SLS include product perfor- 313