[Characterization of sun protection performance: Quo vadis?]

The task of the first sunscreens was to prevent the development of sunburn and, following the spirit of the 1950/1960s, to not impair the tanning of the skin. The need to quantify the protective performance soon arose. Originally with the help of natural-nowadays artificial-sunlight, a method was developed to determine a sun protection factor (SPF). It is formally defined as a ratio between minimum erythema-effective UV dose on sunscreen-protected skin and minimum erythema-effective UV dose on unprotected skin (ISO 24444:2019). Three observations question the suitability of the method. (1) Interlaboratory variability: Despite strict standardization, results of SPF determinations from different laboratories are subject to large variations. (2) Natural vs. artificial sunlight: The radiation spectrum of artificial sunlight differs from that of natural sunlight. SPFs determined with artificial sunlight (as depicted on all sunscreens currently on the market) are significantly too high compared to SPF determination with natural sunlight. (3) Erythema burden: When determining SPF, subjects are exposed to potentially harmful radiation. Against this background alternative methods-in vitro SPF, hybrid diffuse reflectance spectroscopy (HDRS) and in silico calculations-are presented. These have the potential to replace the current method. As an immediate measure, it is recommended to return to the comprehensible description of low, medium, high, and very high protection and in the future to take into account the spectrum of natural sunlight.