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So, it might be a good idea to use sunscreens with a higher sun-protection factor (SPF) to begin with, the British researchers advised.
"What this research shows is that the way sunscreen is applied plays an important role in determining how effective it is," said study author Antony Young of King's College London.
In its study, Young's team assessed DNA damage skin samples from 16 fair-skinned people who used different amounts of sunscreen. The skin samples were exposed to levels of ultraviolet (UV) light similar to that occurring during a day out in the sun, or during a long beach vacation.
The study showed that typical amounts of SPF 50 sunscreen applied by people -- less than the recommended coverage manufacturers use to determine their SPF rating -- provided only a maximum of 40 percent of the expected protection from the sun's harmful rays.
The research showed that even at thinner applications, sunscreen did prevent at least some UV damage to exposed skin. But that level of protection rose as the amount of sunscreen applied reached manufacturer-recommended levels.
However, "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 university news release.
One U.S. dermatologist said the findings are in line with the advice he gives to his patients.
"Dermatologists typically instruct patients that they should use sunscreens of SPF 30 and above, because if they use an SPF 15 on vacation they are likely only getting a third to one-half of the SPF protection as specified on the label," said Dr. Scott Flugman. He's a dermatologist at Northwell Health's Huntington Hospital in Huntington, N.Y.
But even if applied more thinly than recommended, high-SPF sunscreen provides "significant protection from DNA damage, even though this sunscreen dose was much less protective than the heavier applications," he said.
Dr. Michele Green practices dermatology at Lenox Hill Hospital in New York City. She recommends that people "reapply sunscreen every two hours if outdoors. In addition, if you are fair-skinned with a predisposition to skin cancers, you should wear SPF of 50+ and protective clothing."
Clothing can be a key factor, she added. "There is an abundance of sun-protection clothing on the market which will provide additional protection against the damaging rays of the sun," Green said. "It also helps as most people only apply sunscreen on their face and do not spend the time to apply on their body as well."
The study was published July 25 in the journal Acta DV.
-- Robert Preidt
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SOURCES: Scott Flugman, M.D., dermatologist, Northwell Health's Huntington Hospital, Huntington, N.Y.; Michele S. Green, dermatologist, Lenox Hill Hospital, New York City; King's College London, news release, July 25, 2018