Ultraviolet (UV) radiation from sunlight can cause many adverse effects in human skin—particularly in the 290–320 nm UVB range, which is the most harmful. Public concern about skin damage induced by sun exposure also has increased, leading to more specific regulatory requirements as well as controls for sun protection products. Evaluations for the level of UVB protection afforded by sunscreen products are based on in vivo methods.1, 2 These consist of comparing the UV radiation dose required for the appearance of a biological endpoint, in this case skin redness, with and without protection.
As in most other fields, industrial laboratories and health authorities require that such in vivo methods be substantiated by in vitro methods for ethical, economical and practical reasons. The determination of the in vitro SPF by means of a spectrophotometer was initially described by B.L. Diffey and J. Robson,3 then modified and improved to evaluate the level of UVB protection brought by the product. In vitro methods typically are based on the assessment of UV transmittance through a thin film of sunscreen sample spread on a roughened substrate. Many studies4–6 have been published in which the product quantity, spreading and substrate have been varied to improve the reproducibility and correlation of in vitro SPF evaluations with in vitro methods. Although repeatability can now be made relevant for very specific protocols, correlation with in vivo values is still a challenge for some products. This is clearly because a key parameter has not been considered: the affinity between the plate and the product, owing to the surface properties of the substrate.