Sun protection factor measurement of sunscreens is dependent on minimal erythema dose

This study investigates the influence of skin colour and minimal erythema dose (MED) on the in vivo determination of sunscreen sun protection factors (SPFs). The MEDs of groups of 10-20 subjects were measured on the lower back with a 1000-W solar-simulated xenon arc lamp. Five sunscreens, with commercially measured SPFs ranging from 4 to 30 + were then tested on the different groups, and their SPFs were correlated with volunteers' MEDs. We found that the sunscreens had higher SPF values when tested on subjects with lower MEDs and paler skin. The SPF values obtained with our ultraviolet (UV) source were lower than the SPF values reported with commercially used solar simulators. We conclude that while SPF tests with artificial UV sources and pale-skinned volunteers can and should be used to rank the efficacy of various sunscreens in preventing sunburn, they should not be interpreted as measures of a sunscreen's absolute level of sun protection. Factors such as the differences in skin colour and MED between subjects used for SPF testing and the general population, the spectral differences between sunlight and artificial UV, as well as the tendency of the public to apply only small amounts of sunscreen and to re-apply it infrequently, mean that laboratory and sunlight SPFs may be markedly different.