Examining the Link Between Geographic Location and the Sun's Rays

The ways in which a person’s pigment impacts the likelihood of achieving a tan or getting certain diseases may actually be a result of where on the globe his or her ancestors lived, and not by the use of sunscreen and protective sun behaviors, according to Penn State researchers.

Nina Jablonski, professor of anthropology and chair of Penn State’s anthropology department, and George Chaplin, senior research associate in anthropology and an expert in geographic information systems, examined the way that the sun illuminates different parts of the earth, paying attention to levels and angles of incidence of both ultraviolet A and B radiation at various latitudes. The researchers found that, “in the latitudes between 23 and 46 degrees, an area that encompasses North Africa, South America, the Mediterranean and most of China, ultraviolet B radiation, which produces vitamin D in human skin but can destroy folate, is much more variable.

"What we now recognize is that some of the medical problems seen in darkly pigmented people may be linked at some level to vitamin D deficiency," said Jablonski. "Things like certain types of cancer in darkly pigmented people and in people who use a lot of sunscreen or always stay inside could be partly related to vitamin D deficiency."

According to the researchers, because sunburn and most skin cancers do not impact an individual’s ability to reproduce, they are not selection factors. They also note that the body has a mechanism to prevent the overproduction of vitamin D.

"Past arguments about the selective value of dark pigmentation focused on the protective effects of melanin against sunburn, skin cancer, and overproduction of vitamin D,” Jablonski and Chaplin report in Proceedings of the National Academy of Sciences. “These factors can no longer be considered significant selective pressures.”