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J. Soc. Cosmet. Chem., 41, 227-233 (July/August 1990) The simultaneous penetration of water and sodium lauryl sulfate through isolated human skin MARIE LOD•N, Medical Department, Research and Development, ACO AB, S-171 26 Solna, Sweden. Received September 28, 1989. Synopsis Surfactants can influence the barrier function of the skin and induce an increased transepidermal water loss. The simultaneous penetration of water and sodium lauryl sulfate (SLS) (0.1, 1.0, and 10.0% w/w) through isolated human skin was measured using tritiated water and 35S-labeled SLS. The amount of SLS penetrating the skin was 50-100 times higher from 1.0% SLS than from 0.1%. Increasing the concentration from 1.0 to 10% increased the amount penetrating ten times. The rate of penetration also increased with respect to time, indicating increasing damage to the skin over the duration of contact. Since no true steady state was obtained, only pseudopermeability constants, p-Kp, could be calculated (obtained by division of the penetration rate by the applied concentration). The p-Kp of SLS at the arbitrary times 21-24 hr was 0.1, 1.0, and 1.3 Ixcrn/min -• for 0.1, 1.0, and 10%, respectively. The p-Kp of water at the same arbitrary time was 55 •xcrn/min -• when 0.1% SLS was present and 120 Ixcm/min -• when 1.0 and 10% were present. The results indicate that skin damage from washing solutions is not linearily related to the surfactant concentration in the solution. The monomer activity of SLS was not constant above critical micelle concen- tration (CMC) but increases when the concentration of micelies increases. INTRODUCTION Human skin is daily exposed to surfactants. As a result of these exposures, the barrier function of the skin can be changed (1-6). For a certain surfactant, the degree of damage is dependent on the concentration in the skin, i.e., on the penetration rate. Sodium lauryl sulfate, SLS, is an anionic detergent frequently used in experimental dermatology to study the biological response to surfactants. Exposure of the skin to SLS changes the lipid composition of the stratum corneum (7). The composition of the intercellular lipids seems to be an important factor in the regulation of epidermal per- meability (8,9). SLS extracts lipids from the intercellular spaces in the stature corneum (10) and stimulates keratinization (11) and epidermal sterol and fatty acid biosynthesis (12). There are few data in the literature concerning the absolute amount of surfactant that will enter the skin upon exposure. It is believed that the present study is the first to 227