Past, Present, and Future of Sun Protection Metrics.

Since the beginning of the development of sunscreen products, efforts have been made to measure and quantify the protection performance of such products. Early on an in vivo method was established that allowed statements on the sun protection performance in humans. Later, by establishing defined basic and experimental conditions, the method became internationally standardized delivering the well-known sun protection factor (SPF). The method was widely used and is nowadays regarded as a gold-standard method. Further standardized methods were added shortly thereafter. However, shortcomings such as the confined radiation spectra used by the methods, the invasiveness, the complexity in their application, as well as their time- and cost-intensity promoted the development of alternative methods. The shortcomings were recently followed by another, namely, the large interlaboratory variances of the sun protection metrics SPFISO 24444. This all together shows that there is a justifiable need to explore the potential of alternative methods, to complement the existing methods, to serve as equivalents, or even to replace it in the future. Based on the work of Uhlig and coworkers, the authors propose to test the suitability of the alternative methods and their possible equivalency to the reference methods in a broad-based investigation, taking into account possible interlaboratory variances. A research program - developed by a consortium - is in public planning where stakeholders from research, industry, authorities, and the public can come together to facilitate and further advance standardization of the measurement of the sun protection performance. The authors give an insight into historical, technical--conceptual, and future developments of methods for -determining the protective performance of sun protection products.