New approach for a reliable in vitro sun protection factor method - Part II: Practical aspects and implementations

Our previous paper (Part I: Principle and mathematical aspects) presented a new reliable in vitro sun protection factor (SPF) method and demonstrated it to be reproducible and correlated with the in vivo method. Nevertheless, the relevance of an international method should to be adaptable to all products on the market and demonstrated with a blind test. Thus, the aim of this second article was to focus on the practical aspects and implementation (Part II) of a large population of different commercially available sunscreen formulations to obtain similar in vivo SPF results for the purpose of labelling.

The method uses the spectroradiometric measurement of residual ultraviolet (UV) through the sample that was applied on a substrate with a robotic appliance. The method has been demonstrated to be highly reliable, and it is based on a multisubstrate solution with a single UV pre-irradiation dose. Furthermore, different categories of the product were studied to identify a reliable and universal in vitro SPF method.

Based on different sunscreens products classified into 5 different groups (emulsion, oil, alcohol, stick and powder), it was demonstrated that our method has good reproducibility and accuracy compared with the clinical SPF method. Indeed, the mean coefficient of variation (CV%) was approximately 7%, and the coefficient of correlation reached approximately 0.8-1.0 for different types of tested products.

Our second paper concludes that the new in vitro SPF method (based on 113 sunscreen products from the Parts I and II) is clearly adaptable for the SPF labelling purpose on any product type because it is non-invasive, less expensive, more practical and more reliable if performed under strict conditions.