Many of us are, shall we say, rather haphazard sunscreen users -- a fact that makes dermatologists apoplectic.
But, in this era of evidence-based medicine, three nagging questions remain: How much do we really need to worry about skin cancer? How tightly linked are sun exposure and skin cancer? And how good is the evidence for the cancer-protective effect of sunscreens?
The answers are complex.
To be sure, skin cancer is not among the 10 leading causes of cancer deaths, according to 2006 projections by the American Cancer Society. The most dangerous type of skin cancer, melanoma, will strike 62,190 people this year and kill 7,910. It would kill many more if it weren't caught and treated early. And though cases of melanoma are still rising, they're rising more slowly than in the past.
More than 1 million cases of basal and squamous cell cancers will be diagnosed this year, occurring most often in sun-exposed areas of skin, such as the face, neck and hands. Most of these are curable because they too are usually caught early. Of course, removing cancers, especially from the face, can be disfiguring.
As for the sun exposure link: For squamous cell cancer, that's a no-brainer. "There is a very direct relationship between the amount of sun exposure over a lifetime and the development of squamous cell cancer," said Marianne Berwick, chief of epidemiology at the University of New Mexico.
Dr. Martin A. Weinstock, a Brown University dermatologist and chairman of the Skin Cancer Advisory Group for the American Cancer Society, said squamous cell cancer can be triggered by two kinds of ultraviolet light, UVB and, to a lesser extent, UVA. Moreover, actinic keratoses -- reddish patches on the skin linked to sun exposure -- are now seen as precursors to squamous cell cancer.
The links between sun exposure and the other two skin cancers are trickier to sort out.
"Nobody knows much about basal cell cancer," Berwick said. But the real mystery is melanoma. "There is definitely a debate about how much melanoma is linked to the sun. The majority of dermatologists feel it is partially linked to the sun," said Dr. Rebecca Kazin, an assistant professor of dermatology at Johns Hopkins University. "It has a large genetic component as well," with melanoma often running in families.
In a combined analysis of three large studies published in the Journal of Clinical Oncology last year, Harvard researchers listed the risks for melanoma in the following order: older age, male sex, family history of melanoma, high number of moles, history of severe sunburn and light color hair. In other words, sun is one factor among many.
So, it seems, is the pattern of exposure. Some researchers say intermittent exposure confers the most risk -- such as when pale workaholics spend a week in the Caribbean scorching their skin. But a person's age at which sun exposure occurs is important too. When people migrate before age 15 from rainy Britain to sunnier Australia, they end up like native Australians with a higher risk of melanoma, Weinstock said. If they migrate later in life, their risk is more like that of the British, low, suggesting that childhood exposure is a major factor.
And now, sunscreen. The evidence for sunscreen's protective benefits varies depending on the type of skin cancer.
For squamous cell cancer, the evidence that sunscreen protects "is so strong that there is essentially no room for reasonable doubt," Weinstock said. A 1999 randomized controlled trial of nearly 1,400 people in Australia found that sunscreens were protective against squamous cell cancers.
For basal cell protection, Weinstock characterized the evidence for sunscreens as "substantial but indirect and not sufficient to provide proof."
For melanoma, he said, the case for sunscreen efficacy "is strong, but not definitive proof."
Over the years, European research has suggested that sunscreen use could actually increase the risk of developing skin cancer by delaying sunburns, which perhaps encourages people to spend more time in the sun.
A meta-analysis published in 2002 in the American Journal of Public Health looked at data on 9,000 people and concluded that there was no increased risk linked to sunscreen use. A separate review of 18 studies in 2003 went further, finding no relationship, good or bad, between sunscreen use and melanoma.
Further muddying the waters on sunscreens is a class-action lawsuit now in Los Angeles Superior Court. The suit alleges that some sunscreen makers are misleading consumers with labels that promise protection against UVB and UVA light when their products protect against only part of the UVA spectrum. But if you're worried, make sure the sunscreen you buy contains titanium dioxide, zinc oxide or avobenzone, which protect against the whole UVA spectrum. Sunscreens clearly help prevent sunburns, and "any bad, blistering burn in your lifetime increases your risk for skin cancer, including melanoma," said Dr. John Williams, a dermatologist at Brigham and Women's Hospital in Boston.
So even if the evidence for sunscreen is imperfect, using it takes little effort and money.
And at the very least, it smells good and is a nice moisturizer.