The question of whether sunscreen prevents skin cancer is an ambiguous one. Many epidemiological studies show an increased risk of skin cancer to the sunscreen user. Similarly, a review of studies on skin cancer and sunscreens by Science News, vol 153 1998, found that people who used sunscreen are more likely to develop basal cell cancer than people who did not. Science News also examined ten studies of melanoma, five of them concluded that people who used sunscreen were more likely than non users to develop melanoma. Three of the studies found no association between melanoma and sunscreen use, and two studies found that people who used sunscreen were the most protected. Science News, vol 153 1998, found that epidemiological data was not conclusive. This may be because people involved in the studies were not wearing effective sunscreen, or were staying in the sun longer because they thought they were protected or some other factors that were not considered in the studies. This highlights the difficulty in studying and interpreting studies of humans and the study of epidemiology.
More recently, results of a randomized controlled trial of adults using sunscreen daily found increased risk of basal cell carcinoma. Other results have shown a case-control study from southern Sweden of 571 patients diagnosed with malignant melanoma showed significantly raised odds ratio for developing malignant melanoma after regular sunscreen use.
Different factors must be taken in to account to elucidate the preventive effects or actual lack of preventative effects from sunscreen. This can include sunscreen misuse and actual application methods that provide a false impression of safety in the sun. Sunscreen needs to be reapplied every 2 hours as total skin absorption of most sunscreens is around 2 hours. It may also be that sun-sensitive persons typically are more likely to use sunscreens and are at a higher risk of melanoma, thus reports of increased risk for melanoma among sunscreen users may reflect an increased risk among sun-sensitive persons rather than an increased risk solely due to sunscreen use. Sunscreen use may also be leading to increased risk of melanoma by individuals assuming that the use of sunscreen allows them to be exposed to sun light for extended periods of time without any ill effects. The point is we just do not know.
Early sunscreens primarily protected against UVB radiation but not UVA radiation. 90% of melanomas are caused by UVA radiation. Since older sunscreens only blocked UVB radiation they would not have blocked the full range of wavelengths causing melanomas, so their protection against melanoma would have been minimal. Therefore prior to the addition of UVA absorbing compounds to sunscreen in 1989, by allowing increased exposures by preventing UVB burning, sunscreen users could actually increase UVA exposures and melanoma prevalence. This suggests that any studies prior to 1989 that found sunscreen to increase the likelihood of melanomas do not accurately portray the effects of sunscreens used today and further highlights the serious limitations of epidemiological studies..
The possibility still remains that these products may be contributing to skin cancers and this is supported by the fact that a potential mechanism for the development of skin cancers with some of the toxic chemicals used in sunscreens can occur and cannot be ignored.
My personal preference is to use less toxic chemicals on my skin and get more sun.