Solar radiation is the main cause of
skin cancers. However, it also is a main source of
vitamin D for humans. Because the optimal status of
vitamin D protects against internal
cancers and a number of other diseases, a controversy exists: Will increased sun exposure lead to net health benefits or risks? We calculated the relative yield of
vitamin D photosynthesis as a function of latitude with a radiative transfer model and cylinder geometry for the human skin surface. The annual yield of
vitamin D is 3.4 and 4.8 times larger below the equator than in the U.K. and Scandinavia, respectively. In populations with similar skin types, there are clear latitude gradients of all major forms of
skin cancer, indicating a north-south gradient in real sun exposure. Surprisingly, the incidence rates of major internal
cancers also increase from north to south. However, the survival prognosis also improves significantly from north to south. Reasons for these findings are discussed in view of the role of
vitamin D. In Norway,
melanoma rates increased by
a factor of 6 from 1960 to 1990, while the prognosis improved in the same period. After 1990,
melanoma rates have remained constant or even decreased in age groups <50 years, whereas the prognosis has not improved further. These data, together with those for internal
cancers and the beneficial effects of an optimal
vitamin D status, indicate that increased sun exposure may lead to improved
cancer prognosis and, possibly, give more positive than adverse health effects.