Wear Sunscreen in the Office? Maybe

Getting some sun exposure clearly has its benefits; it increases your vitamin D levels and helps ward off insomnia and the blues. But nobody wants skin cancer, cataracts, wrinkles, sagging skin, and age spots. Most of us know enough to lather on the sunscreen before heading off to the pool or beach, but we may not use it when we're, say, sitting in the shade for hours under a beach umbrella.

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However, between 10 a.m. and 4 p.m., when the sun is at its strongest, you can get a fair dose of ultraviolet rays even when you're not bathed in sunshine. Ultraviolet A rays, which tan and age the skin and raise skin cancer risk, are strong throughout the day and can reflect off of beach sand and water, exposing you even when you're in the shade. They can also pass through office and some car windows. Sunburn isn't a reliable guide to UV exposure. Sunburn-causing ultraviolet B rays are weak in the off-peak hours and don't penetrate glass or reflect much off of surfaces, so you could be getting high amounts of UVA exposure even if you're not getting a sunburn.

"We don't really how much radiation is safe to get," says Henry Lim, chairman of the department of dermatology at Henry Ford Hospital in Detroit. He says that getting outside without sunscreen for 10 to 15 minutes in the midday sun three times a week "is adequate to maintain healthful vitamin D levels and is probably safe." If you're outside for longer durations, you've got to slather on a broad-spectrum sunscreen that blocks out UVA and UVB rays, including when you're:

• Sitting in the shade: It's best to use sunscreen, Lim says, if you're sitting on a shaded porch or under a cabana at the beach for an hour or longer.

• Taking a long car trip: Newer car windshields block out nearly all UV radiation, but you can still get some UVA rays through side and rear windows—especially those that aren't tinted. If you're in the car for, say, four or five hours on a sunny day, says Lim, you could get an unhealthful dose of UVA. Sunscreen isn't necessary for the daily commute, unless you're driving with the windows down.

• Sitting directly in front of an office window all day: Since UVA rays pass through glass, you might need to protect yourself if you're situated right by a window with strong sun exposure. However, Lim says that sunscreen isn't necessary if your desk is more than a few feet from a sunny window.

• Hiking in high elevations: For every 1,000-foot increase in elevation, you get about 8 to 10 percent more UV radiation, according to Lim. That's because the atmosphere is thinner.

• Doing outdoor activities, even in the early morning or late evening: Unlike UVB radiation, which is strongest at midday, UVA rays are strong throughout the day. "You can go swimming at 6 p.m. and still get a fair amount of radiation," says Craig Austin, an assistant clinical professor of dermatology at Mount Sinai Medical Center in New York. "So you should apply sunscreen even then."