Ultraviolet (UV) radiation from the sun can cause skin cancer by damaging the DNA in our skin cells as well as cause premature skin aging and wrinkles. Sunscreen typically protects our skin from those harmful rays in two ways: by blocking UV light or by absorbing it (or both).
“Mineral” or “physical” sunscreens—formulations made with zinc oxide and titanium dioxide—are thought to be UV blockers, although research shows they can also absorb UV light, too. They act as nanoparticle-size “mirrors” to reflect UV radiation and visible light, says David Fisher, a professor in the department of Dermatology at Harvard Medical School and a physician at Massachusetts General Hospital. That’s also why they tend to be pasty white in color.
“Chemical” sunscreens are primarily UV absorbers. “Chemical sunscreens are essentially utilizing molecules that can absorb different wavelengths within the UV portion of the spectrum,” Fisher explains. Then they convert the UV light into heat. These sunscreens are typically transparent, and don’t leave a white sheen on the skin, says Kerry Hanson, a research chemist at the University of California, Riverside.
SPF, or “sun protection factor” measures a sunscreen’s ability to protect you from burning. But here’s the rub—it specifically measures protection from UVB light, the part of the UV spectrum with a wavelength of between 280 and 315 nanometers. While UVB exposure has some benefits—it aids in the synthesis of Vitamin D, for instance—it can also cause sunburn, redness and inflammation, Fisher says.
For years, sunscreen manufacturers have prioritized products that protect against UVB. Scientists now believe that UVA—ultraviolet light with wavelengths between 315 and 400 nanometers—can raise the risk of skin cancer, too. That risk isn’t captured by a metric like SPF. If your sunscreen protects against UVB but not UVA, it may be protecting you from getting burnt but still putting you at risk from the sun’s harmful rays, Fisher says.
There is another way, however. Sunscreens labeled as “broad spectrum” in theory protect from both UVB and UVA light. But there isn’t an SPF-like measure for UVA in the U.S., which means it can be difficult to know how much UVA protection a “broad spectrum” sunscreen may provide, Fisher says.
Other regions of the world, including Asia and Europe, use a metric called UVA-PF, or “UVA protection factor” which does measure protection against UVA, say, with a rating scale such as PA+ to PA++++, explains AJ Addae, a cosmetic scientist and a PhD candidate at the University of California, Los Angeles—but UVA-PF is not something the U.S. Food and Drug administration, which oversees sunscreen, requires or regulates.In recent years, health advocates and consumer protection groups have raised some safety concerns about certain ingredients in some sunscreens. Spray-on sun protection can be flammable; others have been recalled for contamination with benzene, a carcinogen, the U.S. Food and Drug Administration notes. Research also suggests that some chemical sunscreen ingredients may damage marine life, such as coral reefs.
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