Your sunscreen might be giving you less than half the protection you think

why you're still getting burnt with sunscreen on

First, the good news: a new study confirms that SPF does, in fact, protect your skin from the damaging rays of the sun, so making the effort to slather on some sunscreen is worth the effort.

Related: The best sunscreens for runners

But on the flipside, you might not be getting as much protection as you think, as researchers from King’s College London discovered.

In the study, researchers looked at how much sun protection people actually receive, based on typical use of sunscreen versus how much they would receive if they actually followed the recommended use. They found that applying sunscreen with an SPF of 50 in the typical way only provides about 40 percent, at best, of the expected protection.

The SPF level of a sunscreen in a lab is determined based on the density of 2 milligrams (mg) of sunscreen on the skin. In order to achieve this in the real, sunny world, you should be using about a 10p coin sized amount for your face, and one ounce—the amount in a standard shot glass—for the rest of your sun-exposed body, Joshua Zeichner, M.D., director of cosmetic and clinical research in the department of dermatology at The Mount Sinai Hospital, told Bicycling US by email.

But most people don’t slather on nearly enough: the significant gap in protection between typical use and recommended use is most likely caused applying sunscreen more thinly than what manufacturers suggest using to achieve the SPF rating listed on the product.

Related: 10 tips to make running in the heatwave tolerable

Another issue is reapplication, according to Zeichner. Most people don’t reapply sunscreen every two hours, which is the recommended strategy.

“If you’re not applying enough sunscreen, either because you’re applying less or you’re not reapplying, the SPF value essentially gets diluted out,” he said.

As the UV light hits your sunscreened skin, it gradually reduces the chemical blockers in the product, he adds, which is what makes reapplication necessary. And if you’re sweating a lot outside when you run, that can make even more frequent application necessary.

That dilution factor is one reason why you might want to up your SPF—though that’s no excuse for applying less than the recommended amount, or forgetting to reapply. Choose a sunscreen with an SPF of at least 30, and make sure it’s broad-spectrum—meaning it offers protection against both UVA and UVB rays, according to the American Academy of Dermatology.

Breaking a sweat outside? Zeichner suggests a stick sunscreen, especially for the face. This type can easily be carried in your pocket, and doesn’t tend to drip when you sweat the way liquid sunscreens can.

“Ultimately, the best sunscreen is the one you are actually using,” he says. “Stack the cards in your favor by using high SPF sunscreen, and doing frequent reapplication.”

A version of this article originally appeared on Bicycling US