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Dear Science: What does sunscreen SPF mean, and what happens if you mix them?

(Rachel Orr/The Washington Post)

Dear Science: I'm gearing up for a summer vacation, and I've been burned before — so I want to make the most of all these half bottles of sunscreen left in my cabinet. If you mix equal amounts of SPF 10 sunscreen and SPF 30 sunscreen, do you get SPF 10, 20, 30 or 40? And what SPF should I be aiming for? 

Here's what science has to say:

Step away from the mixing bowl, friend. Mixing sunscreens is a no-no. But it's important to understand why. First, let's get one thing out of the way: What the heck even is SPF?

According to Shari Lipner, dermatologist at Weill Cornell Medicine and NewYork-Presbyterian, your sunscreen's SPF (sun protection factor) does indeed tell you how much sun protection it can offer you. Researchers test sunscreen on bits of skin, comparing it with totally unprotected skin as they're both exposed to sunlight. 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 and all that, so don't actually pay attention to the whole time thing.

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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 1 percent 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: Folks with sensitive skin may find that these tiny increments actually help, and Lipner notes that the increased protection of SPF 50 can help make up for the fact that people are just generally bad at putting sunscreen on.

"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."

The difference between the high numbered SPF sunscreens is explained. (Video: ACS via Youtube)

And anyway, she added, 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, or you're leaving yourself vulnerable to UVAs.

Back to your mixology question, which is one Lipner hears a lot.

"Some people think they can, you know, mix an SPF 15 and 30 and get an SPF 45, which sort of makes sense, but not if you look at the science behind it," Lipner said.

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To figure out the new SPF of a sunscreen cocktail, Lipner said, you'd have to measure how much of the active ingredient was in the mix and do a few calculations.

"But suffice it to say that if you mix sunscreen, the maximum SPF you’ll get is the higher of the two you started with," she said. "And you’ll likely end up diluting it instead."

In fact, if the two sunscreens use different active ingredients to protect you from the sun, they could end up compromising one another. Most sun protection products contain some "physical" sunscreens that block the sun's rays, well, physically. But these tend to show up as bright white and opaque on the skin, so unless you're rocking a classic lifeguard look, your product likely has a few "chemical" sunscreens in it as well. These work by creating a chemical reaction that turns UV rays into harmless heat, so you definitely don't want to mix a couple of unrelated sunscreens into one bottle — you might end up interrupting the chemistry that keeps you from burning.

We'll leave you with one last piece of advice from Lipner: Wear sunscreen every day. It's not a vacation thing, a beach thing, a sunny day thing or a summer 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 in winter and on cloudy days."

So pick a sunscreen and stick with it. One sunscreen.