Ask a Top Doc: Do the new 100+ SPF sun blocks really work?

William James, MD, a Top Doctor and dermatologist at the Hospital of the University of Pennsylvania, tells us if we should toss our old bottles

Everyone knows it by now: Shielding yourself from the sun is the first step toward protecting yourself from skin cancer and a face full of wrinkles. But when it comes to buying a block, the number of choices can be overwhelming — especially when it seems that new, higher SPFs are coming out faster than Conan O’Brien burns at the beach.

SPF stands for the “sun protection factor,” says William James, MD, a Philly Mag Top Doctor and dermatologist at the Hospital of the University of Pennsylvania, and the number only refers to the amount of UVB rays — the ones responsible for turning you into a lobster — that are blocked. “Sunscreens with an SPF of 15 block about 93 percent of the UVB rays,” says James, and anywhere from 30 to 100+ SPF only block about 97 percent. Translation? Even a small jump in price might not be worth it, as you’re not adding a big sun-blocking benefit when opting for a high SPF.

So which block should you buy? “Your best bet is 30 SPF,” says James, stressing that the studies done to measure a sunscreen’s blocking power usually overestimate the amount a real consumer will apply by half — which means an SPF of 15 might really only result in a true SPF coverage of about seven. And if you’re concerned about aging in the sun, the SPF isn’t the number to go by. Instead, James recommends opting for a block that contains helioplex or escamule, two ingredients that act as broad-spectrum shields and offer the best protection from UVA rays, the ones responsible for premature aging.