Geography and radiation determine how you tan

WASHINGTON - Getting a tan is a matter of radiation and geography as there is great variation of ultraviolet radiation, says a study.

“The variation of ultraviolet radiation, especially in the middle and high latitudes is great,” said Nina Jablonski, study author and professor of anthropology, who heads the Penn State’s anthropology department.

“Tanning has evolved multiple times around the world as a mechanism to partly protect humans from harmful effects of ultraviolet radiation,” added Jablonski.

Jablonski, working with George Chaplin, senior research associate in anthropology and an expert in geographic information systems, looked at the way the sun illuminates different parts of the Earth.

They looked at levels and angles of incidence of both ultraviolet A and B radiation at various latitudes. Ultraviolet B radiation is much more variable than ultraviolet A as latitude increases due to atmospheric scattering of the light and absorption by oxygen.

Ultraviolet B radiation produces vitamin D in human skin. Ultraviolet radiation can, however, destroy folate. Folate is important for the rapid growth of cells, especially during pregnancy where its deficiency can cause neural tube defects.

“What we now recognise 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,” she added.

Scientists have understood for years that evolutionary selection of skin pigmentation was caused by the sun. As human ancestors gradually lost their pelts to allow evaporative cooling through sweating, their naked skin was directly exposed to sunlight.

In the tropics, where human ancestors evolved and where both ultraviolet radiations are high throughout the year, natural selection created darkly pigmented individuals to protect against the sun.

As humans moved out of Africa, they moved into the subtropics and eventually inhabited areas up to the Arctic Circle.

Ultraviolet radiation in these areas is neither consistent nor strong. North or south of 46 degrees latitude, which includes all of Canada, Russia, Scandinavia, Western Europe and Mongolia, there is insufficient ultraviolet B through most of the year to produce vitamin D.

Populations in these areas evolved to have little skin pigmentation, said a Penn State release.

The tanning process evolved for humans who by and large were naked all the time. As the ultraviolet B radiation began to increase in the early spring, the skin would begin to gradually darken.

As the sun became stronger, the tan became deeper. During the winter, as ultraviolet B waned, so did the tan, allowing Vitamin D production and protecting folate.

These findings were published in the Proceedings of the National Academy of Sciences.