Are you protecting your skin from the sun? Do you know what SPF (sun protection factor) to use? We spoke with Piedmont dermatologist Jodi Ganz, M.D., to find out what those SPF numbers really mean. “The SPF number means it would take you that much longer to burn than you normally would without sunscreen,” Dr. Ganz says. “If it takes you 10 minutes to burn, then using an SPF5 means it would take you five times longer, or 50 minutes, to burn. An SPF10 means you could stay in the sun 10 times longer, or 100 minutes.”

But Dr. Ganz cautions that most sunscreens “break down” on average in two hours, meaning they lose their ability to protect you. So, you must reapply sunscreen every two hours. These days, sunscreens offer SPFs higher than 90.

Dr. Ganz says an SPF90 is better than an SPF30, but the difference is only minimal. “As a rule, anything over 30 is good,” she says. “No matter what the number, it will only last two hours before you need to reapply.” Dr. Ganz says you should apply one ounce (a shot glass) of sunscreen over your entire body every two hours. Of that, about a tablespoon should go on your face. “If you’re at the pool for a whole day, you can easily go through a bottle of sunscreen if you’re applying it properly,” says Dr. Ganz, who recommends using sunscreens that block both UVA and UVB rays.

UVA rays have long-term aging effects and cancer effects on the skin. UVB rays are the ones that burn. She says your goal with wearing sunscreen should be to protect yourself down the road from aging and skin cancer. If your goal is to look tan, she recommends self-tanners. “The newer ones do a good job of not being orange and streaking,” she says. “If you want an all-over tan, the spray tans at salons are good.”

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