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The key could lie in the amount of ultraviolet B (UVB) light the skin absorbs -- enough to stimulate a healthy, vitamin D-linked immune response in the skin but not so much that it boosts skin cancer risk.
"I do think that a little bit of sunlight is good for people, but I think that one of the problems that the American Cancer Society and dermatologists have is, how do you define what a little bit is?" said skin cancer researcher Marianne Berwick, chief of epidemiology at the University of New Mexico's Cancer Research and Treatment Center. "How do you tell people that it's OK to have a little bit of sunlight but not too much?"
In 2005, Berwick's team published a controversial study that found that melanoma patients with higher levels of daily sun exposure actually had better survival than patients who spent less time in the sun.
"I've been searching for an explanation for that ever since," she said.