What does SPF stand for, exactly?

Our sunscreens may not be protecting us as well as we think. In part, we have ourselves to blame – we often use too little, don’t rub it in properly or don’t reapply enough or at all.

You should have all of the information you need to be safe and comfortable in the sun this summer, says Consumer Reports.

What does SPF stand for, exactly?

SPF literally means sun protection factor. It’s a measure of how well a sunscreen shields your skin against ultraviolet B rays (UVB), the chief cause of sunburn. Usually, the number is explained as the amount of time it takes an individual’s skin to burn when it’s covered in sunscreen compared with when it’s not. For example, an SPF 30 would theoretically allow you to stay in the sun 30 times longer than you could without protection. That means if you typically burn in 15 minutes, wearing an SPF 30 would extend that time to 7½ hours.

Here’s the catch, though: The level of SPF protection is calculated assuming you’ve applied the right amount of sunscreen. For the average-sized person, that’s an ounce (about 2 tablespoons, enough to fill a shot glass or a blob the size of a golf ball) to adequately cover your face and body. Most people apply about half that amount. Also, sweating and swimming decrease the amount of sunscreen on the skin over time.

For the best protection, apply sunscreen 15 to 30 minutes before going outdoors so that it has time to fully absorb into your skin. Then – regardless of the SPF number on the bottle – reapply it every 2 hours, or immediately after swimming or perspiring heavily.

“I see patients daily who got burned because they didn’t reapply,” says Mona Gohara, M.D., associate clinical professor of dermatology at Yale School of Medicine.

Don’t rely on a tinge of burn on your skin as a sign you should reapply, either. Sunburn often takes several hours after sun exposure to fully develop, so you could be burning even if your skin doesn’t look red right away.

Is higher better when it comes to SPF?

“It’s not true that sunscreens with higher SPFs block double or triple the rays as those with lower ones. They really only provide slightly more protection,” Gohara warns. The breakdown: SPF 30 blocks 97 percent of UVB rays, SPF 50 blocks 98 percent, and SPF 100 blocks 99 percent. Not realizing that may lead people to think that if they use a higher SPF, they don’t need to reapply or practice other sun-savvy behaviors, such as seeking the shade and covering up.

For example, in a 2014 nationally representative survey of 1,000 adults from the Consumer Reports National Research Center, 40 percent of people said they wait three hours or longer before reapplying when they are wearing a sunscreen with a higher SPF than they usually do, and 31 percent never reapply. That is especially problematic when, as mentioned, people tend not to apply enough sunscreen in the first place. In previous tests, Consumer Reports has found that if you use half of the recommended amount of sunscreen, you’ll get half the protection – an SPF 30 becomes an SPF 15, for instance.

That illustrates the advantage of using a higher-SPF sunscreen: Even if a product doesn’t deliver its claimed SPF, you’ll have a better chance of getting a minimum level of coverage. In Consumer Reports’ tests this year, of the 36 sunscreens labeled SPF 40 or higher, 26 met their claim or tested above an SPF 30.