314 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS mance, irritancy potential, and cost. Data regarding (a) the relative effectiveness of various added surfactants in reducing SLS irritancy and (b) the SLS:cosurfactant propor- tions needed to achieve specified levels of irritancy reduction would be valuable for developing product formulations. In this study, an in vivo method devised for studying reduction of SLS irritancy by added cosurfactants 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 laureth-5 carboxylate (SLEC-5) or sodium laureth-13 carboxylate (SLEC-13) in molar ratios ranging from 1:0.08 to 1:1.00 were used for patch application. ,. By using this test method together with a data reduction system, SLEC-13 (i.e., the hig'h.e•"'•thoxylated surfactant) was demonstrated to be more effective than SLEC-5 in reducing the irritancy potential of SLS. The method appears to provide the requisite specificity and sensitivity to discriminate differences in the irritancy potential of sur- factant mixtures containing SLS. METHODS STUDY PARTICIPANTS Twelve healthy Caucasian female volunteers participated in the study. Six panelists were treated with mixtures of SLS and SLEC-5 (mean age 32 years range 22 to 51), and six were treated with SLS and SLEC-13 (mean age 32 years range 24 to 39). The participants were briefed on the study procedures, and each gave written informed consent. They exhibited no erythema on the lower back at the initial evaluation. Use of creams, lotions, or other cosmetic products was restricted on the test area throughout the study. The panelists were otherwise permitted to follow their usual bathing proce- dures. TEST MATERIALS Surfactants used were: sodium lauryl sulfate (CH3(CH2) •oCH2OSO3Na) (Sigma Chemicals Co., St. Louis, MO) and sodium laureth-5 carboxylate (RO(CH2CH20) 4- CH2CO2Na, where R is a C•2--C•4 coconut alcohol) (Hostapur © TP-2505) and sodium laureth- 13 carboxylate (RO(CH2CH20)• •.4CH2CO2Na) (Hostapur © TP-2506) (both from Hoechst Celanese Corp., Charlotte, NC). The degrees of ethoxylation (4 and 11.4) represent an average (9). The following aqueous solutions were prepared for patch application: 1.0% (w/w) SLS 1.6% (w/w) SLEC-5 (molar equivalent of 1.0% SLS) 2.8% (w/w) SLEC-13 (molar equivalent of 1.0% SLS) and mixtures of 1.0% SLS:SLEC-5 or -13 in molar ratios of 1:0.08, 1:0.10, 1:0.13, 1:0.17, 1:0.25, 1:0.50, and 1:1.00. Distilled water was used as the diluent and the control.

ASSESSING REDUCTION OF SLS IRRITATION 315 EXPOSURE AND EVALUATION SCHEDULE For each application, 300 }xl of the test solutions were applied to the Webril © pad of 25-mm diameter Hill Top © chambers (Hill Top Research, Inc., Cincinnati, OH). For each day of patching, up to 10 patches (1.0% SLS 1.6% SLEC-5 or 2.8% SLEC-13 1.0% SLS:SLEC-5 or -13 in molar ratios of 1:0.08, 1:0.10, 1:0.13, 1:0.17, 1:0.25, 1:0.50, and 1:1.00 and distilled water) were applied for two hours onto marked sites on the lower back. Patches (separated by at least one inch) were applied on the same sites from Day 1 through Day 5 (Monday through Friday) or until an erythema score •2, as defined below, occurred. Upon removal of each patch, the skin was rinsed with distilled water and gently dried with a paper towel. Visual grading and instrumental measure- ments were performed by the same technician throughout the study, beginning one hour after patch removal. Panelists were evaluated while lying prone on an examination table. A final evaluation was performed on Day 8 (Monday of the following week). Approx- imately 72 hours separated the Day 8 evaluations from the removal of the last patches on Day 5. In the case of the test sites where patching was discontinued after Day 3 or 4 (i.e., sites patched with 1.0% SLS or molar ratios of 1:0.08, 1:0.10, or 1:0.13), the recovery period ranged up to five days. Measurements were taken after an acclimatiza- tion period to the room environment of approximately 30 minutes. The laboratory temperature was maintained at 70-73øF, and the relative humidity was 35-55%. The initial instrumental values measured on Day 1 prior to patch application were used as baseline values. VISUAL GRADING The skin was graded visually for erythema according to a scale modified from that of Frosch and Kligman (10): 0.5 = slight redness 1 = well-defined redness 2 = moderate redness 3 = marked redness and 4 = intense redness with epithelial damage. TRANSEPIDERMAL WATER LOSS (TEWL) MEASUREMENTS Quantitative measurements of evaporative water loss were performed using the Ser- voMed © Evaporimeter EP-1C (ServoMed, Stockholm, Sweden) (11,12). TEWL was expressed as the amount of water evaporated per unit of surface in an hour (g/m2/hr). The instrument's probe was hand held perpendicular to the skin surface and allowed to equilibrate for 20 seconds. A computer was then used to collect 10 measurements over a 20-second sampling period and record the mean value. ELECTRICAL CONDUCTANCE MEASUREMENTS Electrical conductance, indicating the hydration state and the integrity of the stratum corneum, was measured with the Skicon©-200 skin surface hygrometer (I.B.S., Ltd., Shizuoka-Ken, Japan). The measuring principle of the instrument has been described in detail by Tagami and co-workers (13). Electrical conductance was expressed in I•S (microSiemans). The spring-loaded probe was set at 30 grams. Measurements were made in triplicate and the mean value was calculated.