Recently, an increased number of methods for determining the ultraviolet (UV) protection factor have been proposed. In fact, the past five years have seen the validation of three major methods 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).1 Nevertheless, despite great effort, some methods based on photostability percentage are still not validated, and while several methods are published, none is generally used.2–5 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.3 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.
Sunscreen products are intended to protect the wearer from harmful UV rays. In some cases, the molecules of UV filters that provide this protection by absorbing energy are fragmented. Consequently, the filters no longer play their role of absorbing UV radiation, thereby affecting the protective efficacy of the sunscreen.6–8 The degree of photostability is difficult to predict; some UV filters are photostable while others are unstable—not to mention that certain mixtures of filters can even be used to enhance the photostability of unstable filters.9, 10 It is therefore necessary to determine the photostability of UV filter combinations to develop efficient sunscreen products.