Sunscreen can be confusing. Some formulations are thick and chalky, others are clear and “light”, some come in lotion form, others in sprays, sticks and gels. Some are labeled “broad spectrum” and “water resistant.” And then there’s the question of SPF: how high is enough?
As Memorial Day approaches, the unofficial start of summer, Scientific American spoke with dermatology and cosmetic chemistry researchers about the science of sunscreen: how it works, how to use it correctly, and how formulations in the United States 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) rays from the sun can cause skin cancer by damaging the DNA in our skin cells, as well as causing premature skin aging and wrinkles. Sunscreen usually protect our skin from these harmful rays in two ways: by blocking UV light or absorbing it (or both).
“Mineral” or “physical” sunscreens (zinc oxide and titanium dioxide formulations) are considered UV blockers, although research shows they can also absorb UV light. They act like nanoparticle-sized “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. This is also why they tend to be pasty white in color.
“Chemical” sunscreens are above all UV absorbers. “Chemical sunscreens essentially use molecules that can absorb different wavelengths in the UV part of the spectrum,” says Fisher. Then they convert UV light into heat. These sunscreens are typically clear and don’t leave a white cast 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 thing: it specifically measures protection against UVB light, the part of UV spectrum with a wavelength between 280 and 315 nanometers. While UVB exposure has some benefits (e.g., it aids the synthesis of vitamin D), it can also cause sunburn, redness, and inflammation, Fisher says.
For years, sunscreen manufacturers have prioritized products that protect against UVB. Scientists believe now that UVA (ultraviolet light with wavelengths between 315 and 400 nanometers) can also increase the risk of skin cancer. This risk is not taken into account by a measure like the SPF. If your sunscreen protects against UVB but not UVA, it may be protecting you from burning while exposing you to the sun’s harmful rays, Fisher says.
There is, however, another way. Sunscreens labeled “broad spectrum” theoretically protect against both UVB and UVA rays. But there is no SPF-like measurement for UVA in the United States, which means it can be difficult to know how much UVA protection a “broad-spectrum” sunscreen can offer, Fisher says.
Other regions of the worldincluding in Asia and Europe, use a metric called UVA-PF, or “UVA protection factor” that measures protection against UVA, for example, with a rating scale such as PA+ to PA++++, says AJ Addae, a cosmetics scientist and doctoral student 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 sunscreen formulas in the United States are evolving
In recent years, health advocates and consumer protection groups have raised safety concerns about certain ingredients in some sunscreens. Sprayed sunscreen may be flammable; others were recalled for contamination with benzene, a carcinogen, Notes from the United States Food and Drug Administration. Research also suggests that certain chemical ingredients in sunscreens may damage marine lifelike coral reefs.
In 2019, the FDA published a study showing that some molecules in chemical sunscreens, such as avobenzone, ecamsule, octocrylene, and oxybenzone, can be absorbed into the bloodstream, requiring further research. Experts said Scientific American that it remains unclear what risk, if any, these chemicals may pose to health.
“Insufficient data does not mean that we conclude that they are not safe,” Theresa Michele, then director of the FDA’s Office of Nonprescription Drugs, said: said Scientific American in 2023. “It just means we’re asking for additional data.”
As this research continues, health experts emphasize that the risk of skin cancer is real and stress the importance of wear sunscreen to protect yourself from it.
How to choose the best sunscreen
The answer to this question is simpler than you think.
“The best sunscreen is the one you’ll wear,” says Addae. She recommends finding a cream or lotion that’s at least SPF 30 with a look and feel you like. “If you don’t like wearing it, you’re not going to wear it,” she says. Addae is development zinc oxide sunscreens that don’t leave as much of a white cast by changing the shape of the chemical particle structure, an advancement that may be especially beneficial for people with darker skin.
If you’re going to the beach or doing an activity that makes you sweat, Hanson recommends a water-resistant, broad-spectrum sunscreen for optimal protection. Apply it generously — and reapply it often, she says: To achieve intended levels of protection, the average American needs about one shot glass of sunscreen to cover their entire body. “Your goal is to make a beautiful film that looks like it’s latex paint,” she says.
Sunscreens available in other regions 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, subject to strict rules, rather than as a cosmetic, Hanson explains. Last year, the FDA propose allow use in the United States of a new ingredient called bemotrizinol, commonly found in Asian and European sunscreens. If approved, it would be first new ingredient be authorized in sunscreens since 1999.
And of course, sunscreen isn’t the only way to protect yourself from the sun. Avoiding peak sunlight hours, wearing hats, clothing, and staying in the shade are safe and effective ways to reduce your risk of harmful sun exposure, Fisher says. “I cannot stress enough the value of physical means of sun protection,” he adds.