More about: Sunscreen use and duration of sun exposure: a double-blind, randomized trial.

The recent article by Autier et al. (1) invites some critical evaluation, since it has important public health implications. The authors assessed the influence of the sunscreen sun protection factor (SPF) on the duration of sun exposure and sunburn episodes in two groups of young people given unlabeled products of either SPF 30 or SPF 10. They concluded that sun exposure was 25% longer in volunteers who used SPF 30 than in those who used SPF 10. This finding is not surprising, but it is difficult to understand the basis of this increased exposure, since there was no difference in sunburn or skin-reddening episodes in the two study groups. Thus, an important question remains: What caused the SPF 30 group to sunbathe for longer periods? In other words, what clinical end point determined sun exposure behavior? The authors state that both SPF groups used the same amount of sunscreen, which was estimated to be 25% to 33% of the amount required for SPF assessment. It is recognized that people use less sunscreen than under SPF test conditions (2). However, each volunteer was given a total of 300 mL (approx. 370 g) sunscreen, which is enough for a maximum of 10 whole-body applications per person if applied correctly. This amount is at best a 5-day supply, assuming two daily applications, whereas the mean holiday period was about 20 days, 8 of which, on average, had sunbathing activity. Such a limited supply of sunscreen might have encouraged rationing. In fact, the volunteers used much less than recommended— about 70 g—enough for two wholebody applications or 1 day’s supply. This observation suggests that the sunscreens were either hardly used at all or just applied to restricted areas. Application of the product at a lower level than under SPF test conditions reduces the SPF (3). Assuming an approximately comparable degree of SPF reduction for both products (3) and allowing for 25% increased exposure time with the higher SPF product, the data predict that the SPF 30 group would have received less than 50% of the erythemal ultraviolet radiation (UVR) dose than the SPF 10 group on the sunscreentreated sites. In other words, one would have expected the higher SPF group to be better protected from sunburn even though they spent 25% more time in the sun. Surprisingly, however, the cumulative sunburn and skin-reddening data suggest that this was not the case. Given the very low level of sunscreen use, it would have been useful to have assessed whether sunburn occurred on protected or unprotected sites. The possible modifying effect of sunscreens on sun exposure is an important public health issue, and its study is most welcome. It would not be surprising if high SPF products were used by sunsensitive people, especially the young, to allow longer sunbathing. However, the most likely determinant of long-term risk is the UVR exposure dose at the basal layer, rather than exposure time per se. Comparably applied, use of an SPF 30 product for twice as long as use of an SPF 10 product (with comparable UVR absorption profiles) will result in 1.7-fold better acute, and probably better chronic, photoprotection of sunscreen-treated skin. In this context, the observations by Autier et al. (1) are interesting but are rather difficult to explain, since there seemed to be no differences in acute photoprotection. One possible explanation is that the goal of the study population was a tan and that sunburn was seen, incorrectly, as required for tanning. Overall, the data obtained by Autier et al. (1) suggest that the public still requires considerable education on the correct use of sunscreens.