Ranking of surfactant skin irritancy in vivo in man using the plastic occlusion stress test (POST)

Irritant effects and disturbance of water-holding capacity induced by surfactants were investigated using the plastic occlusion stress test (POST) technique. 4 surfactants (sodium lauryl sulphate (SLS), sorbitan monolaurate, cocoamidopropyl betaine and benzalkonium chloride) and an amphoteric/nonionic (A/N) mix were investigated. 1 x daily for 3 days, test surfactants were applied to marked sites (open) on the volar forearm. After 24 h occlusion with a plastic chamber on the 4th day, skin surface water loss (SSWL) (g/m2h) was recorded continuously for 25 min with an evaporimeter. The differences between the surfactants were statistically significant (Friedman's test p less than 0.01). SLS and the A/N mix revealed significant differences compared to the other compounds (p less than 0.05), both in decay constants and in values recorded at the beginning and at the end of the decay curve. The study suggests a mechanism for the high irritant potential of sodium lauryl sulphate compared to other surfactants: sorbitan monolaurate and benzalkonium chloride seem to influence (at the concentrations tested) to a lesser extent the water-holding capacity of the stratum corneum. The clinical and biological relevance of these observations requires long-term clinical correlations with final product use tests.