ASSESSING REDUCTION OF SLS IRRITATION 325 Table VI Correlation Coefficients Between Evaluation Methods Erythema Electrical scores TEWL conductance Laser Doppler flowrnetry Skin color reflectance Eythema • 0.48* scores (p 0.0172) TEWL 0.83$ • (p 0.0001) Electrical 0.675 0.553 conductance (p 0.0003) (p 0.0055) Laser Doppler 0.613 0.483 flowmerry (p 0.0017) (p 0.0168) Skin color 0.763 0.783 reflectance (p 0.0001) (p 0.0001) 0.32* (p 0. 1268) 0.72* (p 0.0001) 0.773 (p 0.0001) 0.683 (p 0.0002) 0.39* (p 0.0564) 0.46* (p 0.0241) 0.37* (p 0.0731) 0.773 (p 0.000 0.50* (p 0.0138) 0.38* (p 0.0651) 0.29* (p 0.1665) 0.94* (p 0.0001) Values represent Pearson correlation coefficients. * Correlation coefficient for SLS/SLEC-5 treated group. 3 Correlation coefficient for SLS/SLEC-13 treated group. For the panelists treated with SLS and SLEC-13 (Table IV), skin response •50% of the SLS value was caused on Day 2 by a mean molar ratio of 1:0.09. The ratio decreased steadily to 1:0.11 on Day 3, 1:0.16 on Day 4, and 1:0.18 on Day 5. The mean molar ratio of 1:0.14 on Day 8 was still significantly different from the Day 2 ratio. Statistical differences were found between the two groups-of panelists on Days 3, 5, and 8 (Ta- ble V). DISCUSSION APPROACHES TO IRRITANCY TESTING OF SURFACTANT MIXTURES CONTAINING SLS Skin irritation potential of SLS as measured by various instrumental methods has been most commonly assessed by means of a single application for a 24-hour period (14, 16, 17). At 1.0% to 5.0% concentrations under occlusion, SLS causes drastic irri- tation. These conditions generally are not optimal for demonstrating reduction of SLS irritancy by the addition of milder surfactants. Shorter and repeated exposures of ap- propriate SLS concentrations that induce a step-wise progression of irritation (18-20), together with the use of multiple evaluation methods, appear to have the requisite sensitivity for such testing. A rangefinding study (20) using a test design combining 0.25%, 0.5%, and 1.0% aqueous SLS solutions with one-, two-, and four-hour occlu- sive patch applications on five consecutive days indicated that a two-hour patch exposure with 1.0% SLS would provide suitable experimental conditions. In both the rangefind- ing and the 1.0% SLS:SLEC-5 or -13 mixture studies, a dramatic increase of all the assessment parameters of skin irritancy occurred on Day 3 at the SLS-treated sites in all panelists. Addition of SLEC-5 or -13 to 1.0% SLS delayed the irritation response to Day 4 or 5, thus showing reduced irritancy.

326 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS RATIONALE FOR THE USE OF MOLAR RATIOS Molar ratios rather than percentage weights of surfactant mixtures were used since surfactants with different molecular weights can be more fairly compared. In this study, the molecular weights of SLEC-5 and -13 are 470 and 796, respectively (9). If the mixtures of SLS and SLEC-5 or -13 were prepared on a weight basis, there would have been almost twice the number of moles of SLEC-5 as compared with SLEC-13. How- ever, it should be pointed out that since product formulations are usually based on weight, conversion of the molar ratios to a weight basis may be necessary if, for example, the relative costs of the cosurfactants were being compared. RATIONALE FOR THE DATA REDUCTION SYSTEM A vast amount of data was generated for each surfactant mixture, since the protocol involved six panelists, ten molar concentrations, five evaluation methods, and seven sets of measurements. A data reduction system was an absolute necessity to provide man- ageable data for interpretation. 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 ratio of 1.0% SLS:SLEC-5 or -13 that induced •50% of the SLS value was used as the basis for further calculations. The molar ratio that caused only half the response as that caused by SLS was arbitrarily selected as the breakpoint. In the event of severe irritation that necessitated discontinuance of patching, the value obtained for the last patch application was used for all subsequent days' values. The •50% values used thus represented a conservative level that did not overstate the effectiveness of the modifying surfactant in overcoming SLS irritancy. Further, the stopping of patch application at SLS sites on Day 3 biased the molar ratio extrapolation in the direction of requiring greater amounts of SLEC-5 or -13 to achieve the effect of attaining •50% of the SLS response. If SLS patching had been continued, the •50% SLS irritancy effect may have been maintained at the ratios found on Day 3. However, the use of the Day 3 SLS values showed that continued patching did result in progression of the irritancy effect of the mixtures. This system of data transformation, with its final expression as molar ratios, reduced the recorded values to numbers that could be statistically analyzed and ultimately to a single number for each evaluation method that allowed comparison of different surfactant mixtures. VALIDATION OF THE TEST METHOD: EFFECT OF VARIOUS MOLAR RATIOS OF 1.0% SLS:SLEC-5 OR -13 ON THE SKIN IRRITATION RESPONSE The sensitivity of the test procedure and evaluation methods was demonstrated by the results, which generally showed a progression in irritancy with 1) decreasing amounts of SLEC-5 or-13 (molar ratios of 1.0% SLS:SLEC-5 or-13 from 1:1.00 to 1:0.08), and with 2) each successive day of patch application. This is illustrated by the consistent pattern of responses shown in Figures 1A-E and 2A-E. The skin irritation response was inversely related to the molar percentage of ether carboxylate within the mixtures. The greater the amount of SLEC-5 or -13, the greater the reduction in irritation. This effect occurred in spite of the fact that the total surfactant concentration in the mixtures was increased by the added cosurfactant. Although some variation in the intensity of the