Recently, an increased number of methods for determining the ultraviolet (UV) protection factor have been proposed. In fact, three major methods have been validated by the ISO to evaluate sun protection, including in vivo SPF (ISO 24444), in vivo persistent pigment darkening (PPD, ISO 24442) and in vitro PPD (ISO 24443, to be published in 2012). Nevertheless, despite great effort, some methods based on photostability percentage are still not validated, and while several methods are published, none is generally used. This is mainly due to their lack of inter-laboratory reproducibility or their use of analytical chemistry techniques such as HPLC, which are excessively burdensome to implement. Clearly there is compelling interest in validating a method to determine the percentage of photo-protection that remains in a product after UV exposure, i.e., the photostability percentage.
The present article describes a reproducible method for determining the photostability of sunscreen products. This method is based in part on the in vitro determination of the UVA protection factor as proposed by Colipa for the irradiation aspect, and on the spectroscopy of a sunscreen in dilute solution for the absorbance measurement aspect.
This content is adapted from an article in GCI Magazine. The original version can be found here.