Human irritant response to different qualities and concentrations of cocoamidopropylbetaines: a possible model of paradoxical irritant response.

Cocoamidopropylbetaines are surfactants frequently used in cosmetics. We have evaluated the irritant capacity of 3 different qualities of cocoamidopropylbetaine, using the following method: patch tests have been carried out with 5 different dilutions in 67 patients and the results read at 2 days by noninvasive methods (direct visualization, transepidermal water loss (TEWL), laser Doppler flowmetry (LDF)). The results with the 3 methods were concordant. However, the results with the different concentrations were paradoxical, as irritancy did not increase at higher concentrations. We have tried to explain this by the fact that these substances contain by-products (free amidoamine and sodium monochloroacetate), the concentrations tested all being above the critical micelle concentration and therefore containing both micelles and monomer. Finally, we believe that noninvasive methods such as TEWL and LDF could be of great use in the evaluation of irritant contact dermatitis.