Are You Wearing Enough Sunscreen? Most People Don't Apply It Correctly, Study Says

Most people wear sunscreen incorrectly, applying a thin layer that provides only 40 percent of its purported SPF, or sun protection factor, according to new research. But rather than teach consumers how to properly administer a lotion or spray, researchers recommend using a high-SPF sunscreen to better prevent ultraviolet radiation that can damage cells and cause skin cancer.

In a U.K. study published this week in the journal Acta Dermato Venereologica, scientists divided a group of 16 fair-skinned Brits into two groups: one that received a single exposure to ultraviolet radiation, and another that was exposed to 30 times the amount of UV rays on five consecutive days to simulate the amount of sunlight in popular vacation destinations like Florida and Brazil. Both groups wore SPF 50 sunscreen, but the vacation participants applied a thicker layer, closer to the manufacturer-suggested two milligrams per square centimeter.

Even after prolonged exposure to the sun, the faux-vacationers who wore a thicker layer of sunscreen were better protected from UV-caused DNA damage than their counterparts, who applied less than 0.75 milligrams per square centimeter, the average amount of sunscreen users apply. The American Academy of Dermatology recommends adults apply one ounce—enough to fill a shot glass—of sunscreen to ensure full coverage.

To determine a sunscreen's SPF rating, manufacturers test its ability to protect users' skin when they applied two milligrams of sunscreen per square centimeter. Most users apply less than half of that amount, lead author Antony Young said.

"Given that most people don't use sunscreens as tested by manufacturers, it's better for people to use a much higher SPF than they think is necessary," Young said in a statement.

A man walks to the beach in July in Ouistreham, France. A new study found most people apply sunscreen poorly and receive only 40 percent of its purported SPF protection as a result. (Photo by Charly Triballeau/AFP/Getty Images)

SPF indicates the amount of time a sunscreen user can spend in the sun without obtaining a sunburn. So, SPF 30 translates to 30 times more sun protection for sunscreen wearers compared to unprotected sunbathers. But while an SPF 15 sunscreen should suffice for a few hours in the sun, poor application means users will likely reap less than half its UV-protecting benefits, Young said.

Another proven way to prevent shoddy coverage? Apply sunscreen more than once. A March study found that after a second application, sunscreen users applied between 13 to 100 percent more sunscreen, and many covered spots on their body they'd missed entirely in their first dose.

The type of sunscreen (lotion vs. spray) is less important than how well it's applied and its SPF rating. Though spray sunscreen is more common and easier to apply, users are more likely to apply a thinner, less protective layer of a spray sunblock than lotion or cream users. The Food and Drug Administration also warned against inhaling spray sunscreen or applying it near an open flame, as some products contain flammable ingredients.

Sun protection is in dire need of improvement: skin cancer rates doubled between 1982 and 2011 and have continued to steadily increased over the last several years, the CDC reported. More than 90 percent of cases of melanoma, the most aggressive and dangerous form of skin cancer, are caused by UV radiation damage to skin cells. Ultraviolet light triggers a reaction in thymine, a DNA base, which can disrupt cellular processes and kill healthy cells or create cancerous ones in their place.

But some skin cancers develop from DNA damage without UV exposure. A 2012 Nature paper found that melanomas can develop in people with "red hair/fair skin" phenotype without excessive sun exposure. The pigmentation of those cells can produce a carcinogenic reaction caused by the body's inability to detoxify free radicals, which can damage DNA the same way UV radiation can.