By slathering sunscreen on yourself and your children this summer, you are fending off sunburn and cutting the odds of at least one kind of skin cancer.
But does sunscreen help prevent melanoma, the deadliest of skin cancers? Remarkably, scientists still aren't sure.
About 54,000 cases of Americans with melanoma are diagnosed each year -- triple the incidence of 30 years ago -- and 7,600 die of it, according to the National Cancer Institute. And the numbers keep climbing despite decades of widespread sunscreen use.
Some researchers, concerned about this apparent paradox, wonder whether sun lotions may have contributed to the problem. Older sunscreens absorbed ultraviolet B radiation, which is most responsible for sunburn, but did not block deeply penetrating ultraviolet A rays.
"The old sunscreens that were UVB blockers only may have been a bad idea," says Dr. James M. Spencer, vice chairman of dermatology at Mount Sinai School of Medicine in New York. "You didn't burn, so you stayed on the beach all day, getting massive doses of UVA."
Others disagree. Dr. Martin A. Weinstock, professor of dermatology at Brown University, says he believes sunscreens -- even those that block only ultraviolet B -- do help prevent melanoma. There is probably a lag time of at least 15 or 20 years between sun exposure and the cancer it causes, he says, and he thinks the melanoma numbers are beginning to turn around.
But Weinstock, too, believes sunscreen can offer false security -- because so many people fail to use it properly.
"They put it on too late, they put it on too thinly, and they don't reapply it as often as they should," says Weinstock, chairman of the American Cancer Society's skin cancer advisory group. In a 2002 survey, he says, 39 percent of teen-agers claimed they were using a broad-spectrum sunscreen when they got their worst sunburns.
In the sunscreen-melanoma conundrum, as in medical controversies involving diet strategies and hormone therapy, the desire of millions of people for practical advice collides with the ambiguity of scientific evidence.
"Physicians want to have something to tell their patients," says Marianne Berwick, an epidemiologist at Memorial Sloan-Kettering Cancer Center in New York. "But the question really is wide open."
Berwick should know. She's helping conduct an international study of 4,000 melanoma patients that may help answer the question.
Like other researchers, Berwick is quick to say it's a good idea to use sunscreen anyway. It prevents sunburn, slows the wrinkling of aging skin and clearly protects against one common kind of skin cancer, squamous cell carcinoma, which usually can be easily treated. Whether sunscreen helps prevent basal cell carcinoma, the most common skin cancer, is less certain, she says.
Yet it is melanoma that accounts for more than 75 percent of skin cancer deaths, so health experts are eager to find better ways to prevent it. Part of that quest has been the push for "broad-spectrum" sunscreens with "sun protection factor" ratings over SPF 15, because they contain chemicals that absorb longer-wave ultraviolet A as well as shorter-wave ultraviolet B. But experts are divided on whether even the most advanced formulas will defeat melanoma.
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