By Ingfei Chen

Q I know I should always wear sunscreen in the snow, but which one is best?
A Use one that shields out sunburning UVB rays and also UVA, which not all sunscreens do, says National Cancer Institute dermatologist John DiGiovanna. Both kinds of ultraviolet light contribute to skin damage and cancer. The product's SPF—a rating of UVB protection—should be at least 15. (UVA protection is not yet rated.)

According to a study by the Environmental Working Group, four out of five sunscreens are ineffective or unsafe. The study reported that many don't block UVA or they use oxybenzone, an ingredient that has raised some health concerns.

But that analysis wasn't reviewed by outside experts. And while some animal and lab tests suggest that oxybenzone may weakly disrupt hormone systems and release free radicals, harm to sunscreen users has not been shown. "We feel that the risks of not using a sunscreen outweigh the concerns," says Len Lichtenfeld, the American Cancer Society's deputy chief medical officer. Still, the FDA is overdue with rules for rating UVA protection.

For now, buy "broad-spectrum" sunscreens with ingredients such as avobenzone and mexoryl. Zinc oxide and titanium dioxide are also good, especially if you're wary of oxybenzone. Above all, find a sunscreen you like; apply it liberally and frequently. Wear long sleeves, a hat, and shades, too.