Sunscreen can be confounding. Some formulations are thick and chalky; others are clear and “lightweight.” Some come as a lotion; others are a spray, stick or gel. Some are labeled “broad spectrum” and “water resistant.” And then there’s the question of sun protection factor (SPF): How high is high enough?

Ahead of Memorial Day, the unofficial start to summer, Scientific American spoke with dermatology and cosmetic chemistry researchers about the science of sunscreen: how it works, how to use it properly, and how formulations in the U.S. could be improved—and sooner than you think.

How sunscreen protects you from the sun

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Let’s start with how sunscreen works. Ultraviolet (UV) radiation from the sun can cause skin cancer by damaging the DNA in our skin cells, and it can also 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 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 on skin and don’t leave a white sheen, says Kerry Hanson, a research chemist at the University of California, Riverside.

SPF measures a sunscreen’s ability to protect you from burning. But here’s the rub—it specifically measures protection from ultraviolet B (UVB) light, the part of the UV spectrum with a wavelength 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 ultraviolet A (UVA)—UV light with a wavelength 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 keep you from getting burned but still put 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 Ph.D. 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.

How U.S. sunscreen formulations are changing

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, and some spray-on sunscreens have been recalled for contamination with the carcinogen benzene, the FDA notes. Research also suggests that some chemical sunscreen ingredients may damage marine life, such as coral reefs.

In 2019 the FDA released a study showing that certain molecules in chemical sunscreens—such as avobenzone, ecamsule, octocrylene and oxybenzone—may be absorbed into the bloodstream, calling for further investigation. Experts have told Scientific American that it’s still unclear what, if any, risk those chemicals may pose to health.

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

As that research continues, health experts emphasize that the risk of skin cancer is real—and stress the importance of wearing sunscreen to protect against it.

How to choose the best sunscreen

The answer to this is simpler than you might think.

“The best sunscreen is the sunscreen that you will wear,” Addae says. She recommends finding a cream or lotion of at least SPF 30 with a look and feel that you like. “If you don’t like wearing it, you’re not going to wear it,” she says. Addae is developing zinc oxide sunscreens that don’t leave as much of a white pallor by changing the shape of the chemical’s particle structure, an advancement that may be particularly beneficial for people with darker skin tones.

If you’re going to the beach or doing an activity that makes you sweat, Hanson recommends a water-resistant, broad-spectrum sunscreen for the greatest protection. Apply it liberally—and reapply often, she says: to meet intended protection levels, the average American needs about a shot-glass-worth of sunscreen to cover their whole body. “Your goal is to make a nice film as if it’s latex paint,” she says.

Sunscreens that are available in other parts of the world such as Europe and Asia may offer a wider variety of active ingredients. That’s because the U.S. FDA regulates sunscreens as a drug, a classification that carries strict rules, instead of as a cosmetic, Hanson says. Last year the FDA proposed allowing a new ingredient called bemotrizinol—commonly found in Asian and European sunscreens—to be used in the U.S. If approved, it’d be the first new ingredient to be allowed in sunscreen since 1999.

And of course, sunscreen isn’t the only way to protect yourself from the sun. Avoiding peak sunlight hours, wearing a hat, covering yourself with clothing and sticking to the shade are safe and effective ways to reduce your risk of harmful sun exposure, Fisher says. “I cannot overemphasize the value of physical means of sun protection,” he adds.