Most evidence about ultraviolet (UV) radiation exposure and the prevention of skin cancer comes from observational and analytic epidemiologic studies, not from experimental studies in humans. Such studies have consistently shown that increased cumulative sun exposure is a risk factor for nonmelanoma skin cancer.[1,2] Individuals whose skin tans poorly or burns easily after sun exposure are particularly susceptible.[1]

It is not known, however, if reduction of exposure to UV radiation through the use of sunscreens and/or protective clothing or through limitation of exposure time can reduce the incidence of nonmelanoma skin cancer in humans. One study has shown that regular sunscreen use can reduce the incidence of solar keratoses (precursors of squamous cell carcinoma) and increase remissions of existing lesions.[3] In Australia, 588 persons aged 40 years and older who attended a free skin cancer screening clinic and had one to 30 solar keratoses were enrolled in a randomized, controlled trial assessing the effect that the regular use of sunscreen (sun protection factor 17) could have on solar keratoses; 431 persons completed the study. Persons in the sunscreen group developed significantly fewer new lesions and had significantly more remissions of existing lesions than persons in the base-cream group. Furthermore, the amount of sunscreen used was related to development of new lesions and remission of existing lesions in the sunscreen group; no such effect was observed in the base-cream group. A different Australian randomized study, however, showed that after 4.5 years of follow-up, there was no significant difference in incidence of squamous cell carcinomas after regular sunscreen use. Although a post hoc subgroup analysis showed a reduction in the frequency of carcinomas on the sites of daily sunscreen application, the validity of that finding is questionable because of the possible effects of multiple testing.[4] An 8-year post-trial observational follow-up demonstrated statistically significant reductions in both the frequency and the overall incidence of squamous cell carcinomas in the regular sunscreen-use arm, but the reliability of these findings is uncertain given their occurrence outside of the controlled-trial environment.[5]

The relationship between UV radiation exposure and cutaneous melanoma is less clear. Rather than cumulative sun exposure, it is intermittent acute sun exposure leading to sunburn that seems to be more damaging;[6] such exposures in childhood or adolescence may be particularly important.[7] Results from a collaborative European case-control study and one animal study, however, suggest that sunscreens that protect against sunburn may not protect against UV radiation-associated cutaneous melanoma.[8,9] Nonmodifiable host factors, such as propensity to burn, a large number of benign melanocytic nevi, and atypical nevi may also increase the risk of developing cutaneous melanoma.[7] A meta-analysis of 18 studies that explored the association between melanoma risk and previous sunscreen use illustrates widely differing study qualities and suggests an absence of an association.[10]