A person applying sunscreen to a child.

If you diligently apply—and reapply—your sunscreen to all exposed skin when you’re outside, you probably think your skin is safe. But dermatologists warn that few people actually use enough sunscreen to get anything even close to the SPF stamped on the bottle.

“Sunscreen testing in the lab does not always correlate with how well it performs in the real world, mainly because consumers are applying about half as much sunscreen as they should,” says Joshua Zeichner, MD, director of cosmetic and clinical research at Mount Sinai Hospital in New York City.

So what actually happens if you skimp on your sunscreen? CR’s sunscreen tests have shown that using half the sunscreen cuts the SPF protection by about half. A recent study from King’s College London came to a similar conclusion but also examined the effect less protection would have on skin. The study was published in the journal Acta Dermato-Venerelogica.

More on Sun Protection

The standard test manufacturers use to determine a sunscreen’s SPF calls for applying sunscreen at a thickness of 2 mg per square centimeter of skin. That translates into using a shot-glassful for your whole body. Then the area is exposed to UV light from a sun simulator.

In the King’s College study, researchers looked at how well a sunscreen labeled SPF 50+ protected skin when it was applied at different levels of thickness. At the typical amount people use—the equivalent of 0.75 mg per square centimeter of skin—the SPF protection level dropped by about 60 percent, according to the lead author, Antony R. Young, Ph.D., a professor at St. John’s Institute of Dermatology, King’s College London. That means that the SPF 50+ worked like an SPF 20.

And skin biopsies taken by the researchers show you pay a price for not slathering on an adequate amount of sunscreen. When Young and his team examined the samples for the kind of DNA damage that could lead to skin cancer, he says they found “the thicker the sunscreen application, the better protected they [the study subjects] were from DNA damage.” However, even the thinnest layer of sunscreen was better at shielding skin than no sunscreen at all. “That was encouraging, because it suggests that even a little sunscreen made a difference,” says Young.

Tips for Better Coverage

One takeaway from this new research is that sunscreen with a higher SPF might help make up for less-than-perfect application. “I look at high SPF sunscreen as like an insurance policy to help guarantee the best level of protection,” says Zeichner. “The higher SPF you start out with, the higher level you’ll end up with, no matter how much you put on.”

That said, the best protection still comes from proper application. “You need a blob the size of a marble to cover your face and ears, then another blob the size of a golf ball for the rest of your body,” says Joel Cohen, M.D., a Denver-based dermatologist on the teaching faculty at the University of Colorado and the University of California at Irvine. That adds up to about an ounce of sunscreen to cover your whole body.

Given that you also need to reapply every 2 hours or after swimming or heavy sweating, a 6- or 7-ounce tube of sunscreen should be empty at the end of couple of days (not the end of a week-long vacation). And be especially careful with spray sunscreens, because it's harder to determine how much you're using than it is with a lotion. 

But being sun smart is about much more than properly applying sunscreen.

Wearing sun-protective clothing means you can skip the sunscreen on those covered body parts. Hats with broad brims will help protect the face, neck, and ears. But because they can’t provide 100 percent coverage, they should be used with sunscreen. “A hat won’t cast a full shadow over the face, plus there is still significant UV radiation from the reflection of the sun off of cement, sand, or water,” says Cohen. And whenever possible, seek out shade, especially between the hours of 10 a.m. and 2 p.m., when the sun’s rays are at their strongest.