Sun exposure: Do the benefits outweight the risks?

Key Points

National report - The increased risk of skin cancer due to exposure to sunlight, and the benefits of sun exposure in enhancing production of vitamin D, were brought into sharper focus in a paper published in the Proceedings of the National Academy of Science in January.

The short version is that "sunlight could be good for you, but too much of a good thing could be bad for you," says Richard B. Setlow, Ph.D., the communicating author of the paper. He is a researcher at Brookhaven National Laboratory, Upton, N.Y., who has studied the effects of various wavelengths of visible and nonvisible light on biological systems.

His earlier work established that longer wavelengths, commonly known as UVA, induce melanoma. Shorter UVB wavelengths induce basal and squamous cell carcinomas.

But, surprisingly, the survival prognosis also increases as one moves from north to south. Increased sunlight-induced vitamin D appears to play a major role in that survival.

Patients benefit

Dr. Setlow says, "Melanomas, if left to themselves, often result in metastasis and death. However, individuals with melanoma exposed to a lot of sunlight - their death rate goes down by 40 to 50 percent.

"That's a situation where the shorter wavelengths are good for you and the longer wavelengths are not so good for you, because they induce melanoma."

That line of argument is reinforced by the observation of where skin cancers are likely to occur on the body. Basal and squamous cell carcinomas are widely distributed, but melanomas are less likely to occur on areas such as the face and hands that are regularly exposed to sunlight and hence have sustained high levels of local production of the active form of vitamin D.

His most recent study used a model incorporating information on solar radiation intensity and, rather than the traditional flat surface, 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.

Dr. Setlow and his colleagues found that people living in Australia produce 3.4 times more vitamin D as a result of their sun exposure than people in the United Kingdom, and 4.8 times more than people in Scandinavia.

The Tropic of Capricorn runs through the middle of Australia, while all of the United States, except Hawaii, lies to the north of the Tropic of Cancer.

Nonmelanoma cancers

Dr. Setlow says the incidence of nonmelanoma skin cancer is a function of latitude.

"If you went to Australia, the incidence of nonmelanoma cancers went up dramatically. However, the incidence of melanoma didn't go up that much, because the latitude dependence is different. So, here was a case where latitude dependence can tell you something about the ratio between melanin and melanoma incidence and vitamin D.

"Vitamin D production would be much larger in Australia than in Norway. But melanoma wouldn't be that much larger, maybe just a factor of two instead of a factor of four.

"If it were the same wavelength doing that, there would not be much you could do about it, but the wavelengths are different," he says.

According to Dr. Setlow, in principle, an ideal sunscreen would absorb UVA and allow beneficial UVB through to generate vitamin D. That type of sunscreen has not been developed.

He suggests 10 to 60 minutes of sun exposure every few days, depending on the latitude and time of year, to generate vitamin D, "and if you want more sun, put on a sunblock."