Sunscreen isn’t simple—experts explain the real limits

Ahead of Memorial Day, dermatology and cosmetic chemistry researchers break down how sunscreen blocks ultraviolet light, why SPF doesn’t cover UVA risk, and what could improve formulations in the U.S. soon—without losing sight of the simplest rule: wear what y

On the edge of summer—when Memorial Day lands and the days start stretching—the bottle in your bag can feel like a maze. Some sunscreens are thick and chalky. Others are clear and “lightweight.” Some come as lotions, others as sprays, sticks, and gels. Some promise “broad spectrum” protection and claim to be “water resistant.” And then there’s SPF. the number that’s meant to make the choice easy.

It doesn’t. Not fully. Dermatology and cosmetic chemistry researchers say the science of sunscreen is real, but the labels you see on store shelves can leave gaps—especially when it comes to UVA light, the kind that doesn’t show up in the simple SPF metric.

Ultraviolet radiation from the sun can cause skin cancer by damaging DNA in skin cells. It can also drive premature skin aging and wrinkles. Sunscreen generally protects in two ways: by blocking UV light, absorbing it, or doing both.

“Mineral” or “physical” sunscreens use zinc oxide and titanium dioxide. Research suggests they can both block and absorb UV light. Their job is often described as acting like nanoparticle-size “mirrors,” reflecting UV radiation and visible light. David Fisher. a professor in dermatology at Harvard Medical School and a physician at Massachusetts General Hospital. points out a practical clue: mineral sunscreens tend to be pasty white in color.

“Chemical” sunscreens work differently. Fisher describes them as using molecules that absorb different wavelengths across the UV portion of the spectrum. Those sunscreens then convert the UV light into heat. Kerry Hanson. a research chemist at the University of California. Riverside. says chemical sunscreens are typically transparent and don’t leave a white sheen.

SPF—the “sun protection factor”—is where the confidence can slip. It measures how well a sunscreen protects against burning from UVB light. That UVB band runs from 280 to 315 nanometers. Fisher notes that UVB has some benefits. including aiding vitamin D synthesis. but it can also cause sunburn. redness. and inflammation.

For years, sunscreen products prioritized UVB protection. Now, experts say UVA—ultraviolet light with wavelengths between 315 and 400 nanometers—can also raise the risk of skin cancer. And that risk isn’t captured by SPF. If a sunscreen protects you against UVB but not UVA. Fisher says it may prevent burning while still leaving you exposed to harmful rays.

Broad-spectrum claims are meant to address that. In theory, broad spectrum protects against both UVB and UVA light. But Fisher says there’s no UVA equivalent to SPF in the U.S. That means consumers can struggle to understand how much UVA protection a “broad spectrum” sunscreen actually provides.

Some regions outside the U.S. use a metric that attempts to measure UVA protection: UVA-PF. or “UVA protection factor. ” with rating scales such as PA+ to PA++++. AJ Addae. a cosmetic scientist and a PhD candidate at the University of California. Los Angeles. explains how that system works. But UVA-PF isn’t required or regulated by the U.S. Food and Drug Administration.

While scientists work on making labels more meaningful. regulators and researchers have also been dealing with concerns about sunscreen ingredients and formulation safety. Spray-on sun protection can be flammable. The U.S. Food and Drug Administration has noted that some products were recalled due to contamination with benzene, which is a carcinogen. Research also suggests certain chemical sunscreen ingredients may harm marine life, including coral reefs.

In 2019. the FDA released a study finding that certain molecules in chemical sunscreens—avobenzone. ecamsule. octocrylene. and oxybenzone—may be absorbed into the bloodstream. prompting calls for additional investigation. Experts speaking to Scientific American said it’s still unclear what risks, if any, those chemicals pose to health.

Theresa Michele, then director of the FDA’s Office of Nonprescription Drugs, told Scientific American in 2023: “Insufficient data does not mean that’s a conclusion by us that they’re unsafe,” adding, “It just means we’re requesting additional data.”

For all the uncertainty around ingredient-level risk, researchers emphasize that the core threat—skin cancer risk from UV exposure—is real. The takeaway, they say, starts with wearing sunscreen, but it doesn’t end at purchase.

Choosing the best sunscreen, Addae says, is simpler than it sounds: “The best sunscreen is the sunscreen that you will wear.” She recommends a cream or lotion with at least SPF 30 that feels right to you. If you don’t like wearing it, she says, you won’t use it.

Addae is also developing zinc oxide sunscreens designed to reduce the white pallor by changing the shape of the chemical’s particle structure. She says that could be especially beneficial for people with darker skin tones.

If you’re heading to the beach or doing something that makes you sweat. Hanson advises using a water-resistant. broad-spectrum sunscreen. She emphasizes applying it liberally and reapplying often. To reach the intended protection levels. Hanson says the average American needs about a shot glass-worth to cover their whole body—describing the goal as making “a nice film as if it’s latex paint.”.

Regulation affects more than messaging—it shapes the ingredient options. Sunscreens available in other parts of the world may offer a wider variety of active ingredients. Hanson attributes this to a difference in how the U.S. regulates sunscreens: the FDA regulates sunscreens as a drug, which carries strict rules, rather than as a cosmetic.

Hanson points to a specific possible shift. Last year, the FDA proposed allowing bemotrizinol, a new ingredient commonly found in Asian and European sunscreens, to be used in the U.S. If approved, bemotrizinol would be the first new ingredient allowed in sunscreen since 1999.

And even the best sunscreen, experts say, isn’t the only line of defense. Fisher recommends practical sun-protection measures: avoiding peak sunlight hours, wearing hats and clothing, and sticking to the shade. He adds, “I cannot overemphasize the value of physical means of sun protection.”

In the rush toward summer, it’s tempting to treat sunscreen like a single number you can trust. But the science is more exacting than that—separating UVB and UVA. questioning ingredient details. and turning “broad spectrum” into something you still have to think about. The safest choice may still come down to the most human one: finding protection you’ll actually use. and pairing it with the physical habits that don’t need a label to work.

sunscreen SPF UVA UVB broad spectrum zinc oxide titanium dioxide avobenzone ecamsule octocrylene oxybenzone FDA bemotrizinol skin cancer water-resistant sunscreen