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j. Soc. Cosmet. Chem., 48, 289-295 (November/December 1997) In vitro assay of high-SPF sunscreens R. P. STOKES and B. L. DIFFEY, Regional Medical Physics Department, Dryburn Hospital, Durham DH1 5TW, UK. Accepted for publication December 1, 1997. Synopsis In vitro spectral transmission measurements using excised human epidermis as the substrate were used to determine the photoprotection provided by physical and organic chemical sunscreens encompassing a wide range of sun protection factor (SPF). The measured SPFs were in good agreement with the quoted SPFs of the products. This in vitro technique using human epidermis could prove reliable for evaluating the SPF of high-protection sunscreens for which in vivo assay is problematic due to the impractically long irradiation times to achieve erythema on sunscreen-protected skin. We also compared our calculated SPFs, which assumed a natural sunlight spectrum, with those that would have been obtained assuming a xenon-arc solar simulator spectrum. We found that for products with relatively low UV-A absorption, the use of a solar ß simulator for in vivo testing overestimates the SPF that would be expected in sunlight. INTRODUCTION The photoprotection provided by a sunscreen product is assessed in terms of its sun protection factor (SPF). Sunscreen SPFs are generally measured by in vivo assay and defined as the ratio of the ultraviolet (UV) dose required to cause minimal erythema in protected skin to that required for unprotected skin. Internationally agreed procedures (1,2) define protected skin as that to which a 2-mg/cm 2 layer of sunscreen has been applied. In vivo assay is problematic for high-protection sunscreens (SPF 25) because of the impractically long UV irradiation times and variability of results (3). Conse- quently, it would be particularly desirable to use a reliable in vitro assay for these products, whereby the transmission of UV radiation is measured first through a substrate and then through the substrate with applied sunscreen. The ratio of UV transmission without sunscreen to that with sunscreen gives a measure of photoprotection. A wide range of substrates has been used for in vitro assay, including wool, pig skin, hairless mouse epidermis, human epidermis, human stratum corneum, synthetic skin casts, and surgical tape (4). Unfortunately, most substrates are least reliable when products offering high protection are assayed. The substrate expected to give results closest to in vivo assay is human epidermis. In the study reported here, we have applied physical sunscreens, with varying concen- trations of the same active ingredient, and organic chemical sunscreens to excised human epidermis, and using a spectral transmission technique (5), measured the SPF at an 289