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According to Shari Lipner, dermatologist at Weill Cornell Medicine and NewYork-Presbyterian, sunscreen’s SPF (sun protection factor) does indeed tell you how much sun protection it can offer you. SPF is a measurement of how much ultraviolet radiation – short, invisible wavelengths of light from the sun – is necessary to burn the sunscreened skin as compared with the unadorned flesh.

The numbers themselves refer to how long you can bake in the sun without burning: If it takes 20 minutes for your bare skin to start reddening, using an SPF 15 sunscreen is meant to let you have fun in the sun for 15 times longer – about five hours. But you still need to reapply it every two hours because of sweating.

There’s a common misunderstanding that the protection increases linearly, and it definitely doesn’t. SPF 15 blocks about 93 percent of UVB rays, but doubling up to SPF 30 blocks just 97 percent. The improvements drop off considerably from there, with SPF 50 blocking 98 percent of UVB rays and SPF 70 blocking less than one per cent more.

“Basically, it just doesn’t go up tremendously,” Lipner said. So she generally recommends SPF 30 across the board. That doesn’t mean there’s no reason to pick a higher number. “Most people just don’t apply enough,” she explained, “So if you use a higher SPF, it can make up for using a little less than you should.”

SPF isn’t the only thing to worry about. SPF only measures a sunscreen’s ability to block out UVB rays. Scientists used to think these were the only solar rays that contributed to skin cancer, but it’s become clear that UVA rays — the rays associated with giving you a tan — are dangerous as well. They’re less intense, but much more common — and they penetrate deeper into the skin. Look for a sunscreen that has ‘broad spectrum’ protection.

“Wear sunscreen every day,” says Lipner. It’s not a beach thing or a sunny day thing. Sunscreen should be a constant accessory. “We know that sun exposure causes wrinkling, skin aging, sun damage and skin damage, and we know that UV radiation is a risk factor for skin cancer,” she said. “Even on cloudy days.”
Washington Post