SUNCARE PRODUCT PHOTOSTABILITY - A KEY PARAMETER FOR A MORE REALISTICIN-VITRO EFFICACY EVALUATION

There is little dispute that modern suncare products should provide pr otection against exposure to both UVB and UVA radiation, and that the product should maintain this broad-spectrum protection throughout the period of exposure to the sun. We have described previously an in vitr o methodology for the assessment of suncare product efficacy, by trans mission spectroscopy. We now describe a modification to the method whi ch takes into account the influence of the exposure of the product to solar UV by introducing pre-irradiation prior to the transmission meas urements. By this means we can examine the influence of UV exposure on the protection efficacy of the product. UV pre-irradiation was achiev ed with a xenon are source optically filtered to achieve a spectrum si mulating that of summer sunlight with a UV irradiance at the sample of 5 mW/cm(2). Different irradiation doses were applied to thin films of the suncare product, spread onto a UV transparent and chemically iner t substrate. Spectral transmission was measured in both non-irradiated samples and samples irradiated with UV doses of 18, 36 and 54 J/cm(2) , and sun protection factors (SPF) calculated. We found that the SPFs were only slightly modified when the active ingredients contained phos tostable UV filters (e.g. butyl methoxydibenzoylmethane/4-methylbenzyl idene camphor/terephthalylidene dicamphor sulfonic acid). However, wit h photo-unstable ingredients, the SPFs dropped significantly following UV exposure. For example, chemical filters containing butyl methoxibe nzoylmethane/octyl methoxycinnamate suffered protection efficacy losse s up to 50-60% of their initial value. The addition of photo-stabilizi ng UV filters did not prevent these reductions in protection efficacy. This study has demonstrated the importance of taking into account the photostability of suncare products when evaluating their protection e fficacy by an in vitro technique. With sample pre-irradiation, it is p ossible to evaluate the efficacy of suncare products in more realistic experimental conditions akin to their use in vivo.