A new study by scientists at the U.S. Department of Energy's Brookhaven National Laboratory and colleagues in Norway suggests that the benefits of moderately increased exposure to sunlight - namely the production of vitamin D, which protects against the lethal effects of many forms of cancer and other diseases - may outweigh the risk of developing skin cancer in populations deficient in vitamin D.
The study will be published online in the Proceedings of the National Academy of Sciences the week of January 7, 2008.
"We know that solar radiation is the leading cause of skin cancer," said communicating author Richard Setlow, a Senior Biophysicist Emeritus at Brookhaven and a well-known expert on the link between solar radiation and skin cancer. Setlow's group was the first to establish that ultraviolet A (UVA) radiation and visible light are the primary causes of malignant melanoma, the deadliest form of skin cancer. He and his colleagues emphasize that people need to protect themselves from the harmful effects of sun exposure.
But solar radiation is also a major, if not the main, source of vitamin D in humans. In the presence of sunlight, the body converts certain precursor chemicals to active vitamin D.
"Since vitamin D has been shown to play a protective role in a number of internal cancers and possibly a range of other diseases, it is important to study the relative risks to determine whether advice to avoid sun exposure may be causing more harm than good in some populations." The concern, he says, is particularly great in populations from northern latitudes, such as Scandinavia, where sun exposure is extremely limited.
In the current study, Setlow and his colleagues used a model incorporating information on solar radiation intensity and a vertical cylinder shape to represent the human body's skin surface to calculate the relative production of vitamin D via sunlight as a function of latitude, or distance from the equator. The cylindrical model more realistically represents human body sun exposure than flat surface exposure measurements used in previous models. The scientists also examined the incidence of and survival rates for various forms of cancer by latitude.
According to the calculations, people residing in Australia (just below the equator) produce 3.4 times more vitamin D as a result of sun exposure than people in the United Kingdom, and 4.8 times more than people in Scandinavia.
"There is a clear north-south gradient in vitamin D production," Setlow says, "with people in the northern latitudes producing significantly less than people nearer the equator."
In populations with similar skin types, there is also a clear increase in the incidence of all forms of skin cancer from north to south. "This gradient in skin cancer rates indicates that there is a true north-south gradient in real sun exposure," Setlow says.