There’s a lot of confusion about what SPF really means. So let’s just clear something up: It is not an indicator of how long you can stay out in the sun.
“Studies have shown that SPF’s efficacy stays steady for about an hour, and then begins to drop after an hour because UV rays break down many sunscreen ingredients,” says Jill Weinstein, a dermatologist and instructor of clinical dermatology at Northwestern University Feinberg School of Medicine in Chicago. It’s why you should reapply sunscreen every two hours or after you sweat or swim (whichever comes first).
Here’s what SPF really means: It’s the percentage of UVB rays—which are the sun’s burning rays—that the sunscreen blocks. It’s not indicative of the percentage of skin-aging UVA rays that the formula protects against, which is another reason to reapply often and choose a sunscreen with broad-spectrum protection. To get really specific about what SPF means, the percent breakdown is this: SPF 15 protects against 93 percent of UVB rays; SPF 30 guards against 97 percent, and SPF 50 is about 98 percent. The difference sounds negligible, but not so much when you reverse it (because, duh, two percent of UVB rays get by SPF 50, while seven percent can get by SPF 15). And this next part is really key: Almost no one puts on enough SPF 30 (half a teaspoon for your face) to get the full SPF. “It’s impossible to get the SPF on the label without really caking it on,” says Darrell S. Rigel, a dermatologist in New York City. And since that’s not happening, go with SPF 50 or higher. We like Neutrogena Ultra Sheer Dry-Touch Sunscreen Broad Spectrum SPF 55, because it’s light and sheer for such a high SPF. “Be careful not to miss frequently overlooked spots, like between the eyebrows and around the nostrils and eyes,” says Vivian Bucay, a dermatologist in San Antonio.