vacationers relax under the shade of umbrellas on a pool deck.

Does sunscreen expire? How much SPF is enough? Your burning questions, answered

Experts weigh in on everything to know about sunblock—from the difference between mineral and chemical varieties to when and how often you should be applying it.

Vacationers relax under the shade of umbrellas on a pool deck in Palm Springs, California. Experts say that skin cancer is on the rise—making it more important than ever to protect your skin from harmful ultraviolet rays. 
Photograph by Jenn Emerling, Nat Geo Image Collection

We’ve all felt it: that moment the warmth of the sun’s rays becomes a bit too warm and the reality hits that we’ve got a sunburn. Most of the time, it leads to several hours or days of pain and discomfort. Over time, however, this sun exposure causes premature skin aging and cancers like melanoma.

In the last several decades, skin experts have stopped wondering if we should be wearing sunscreen (that answer is a resounding yes), but how much, when, and under what circumstances. More questions swirl about the benefits of different types of sunscreens and their relative safety for us and the environment.

“Skin cancer is on the rise. I diagnose it every day in my practice,” says Gregory Papadeas, a dermatologist based in Denver. He cites not only the depletion of the ozone layer, the part of the Earth’s atmosphere that helps absorb harmful ultraviolet rays, but also the increasing popularity of outdoor activities such as hiking, kayaking, and cycling.

National Geographic talked to a range of dermatologists to get the lowdown on everything you need to know about sunblock, from A to Zinc oxide.

How important is wearing sunscreen?

Very, says Bruce Brod, a dermatologist at the University of Pennsylvania. But sunscreen is only one part of a more holistic strategy to prevent harmful sun exposure. The most effective strategy is to avoid the sun when it is most intense, typically between 10 a.m. and 2 p.m.

Other options include wearing hats with brims at least four inches wide and wearing sun protective clothing. Although most clothing provides more sun protection than going out in bare skin, Brod recommends buying clothes with an Ultraviolet Protection Factor (UPF) of at least 30.

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To check if an item you already own provides protection, hold it up to a light bulb. If it lets light through, it probably won’t provide much protection, especially if it’s gauzy or gets wet.

Burns aside, why should I be concerned about sun exposure?

For starters, that’s because UV rays are the leading cause of premature aging in skin, which includes wrinkles, sagging, and dryness, says Mona Gohara, a dermatologist at Yale University. Individuals with darker skin may also notice hyperpigmentation, or an uneven increase in skin color.

But what about getting a base tan? Will that protect me?

No. “Tans are your body's way of telling you that you've damaged the skin,” Gohara says. “There’s no such thing as a good base tan. That’s a fallacy.”

When it comes to SPF, is bigger always better?

A sunscreen’s SPF (short for Sun Protection Factor) is a measurement of how much solar energy (in the form of UVA and UVB rays) it takes to cause a sunburn in comparison compared to untreated skin when it’s used as directed. So wearing adequate amounts of sunscreen with an SPF of 15 means it will take 15 times more solar energy to cause a burn; an SPF of 30 would take 30 times more.

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Current advice from the American Academy of Dermatology and the Skin Cancer Foundation recommends wearing sunscreen with at least SPF 30.

The important caveat is using the product as directed, and most of us don’t.

So when should I apply sunblock—and how much should I put on?

The amount of sunscreen applied under laboratory conditions is about 1 ounce. It’s about the size of a golf ball, or enough lotion to fill a shot glass. The average person uses around one-third that amount, according to Dawn Marie Davis, a dermatologist at the Mayo Clinic.

Although mineral sunscreens (more on them below) are effective immediately, chemical sunscreens need to be absorbed before they become effective. Besides not applying enough in the beginning, most people don’t reapply sunscreen frequently.

The Skin Cancer Foundation advises reapplying sunscreen every two hours, or after swimming or heavy sweating, whichever comes first. This holds true regardless of a product’s SPF. Sunscreen of SPF100 provides more protection, not longer protection.

Do I need to wear sunscreen if I’m staying indoors?

DiAnne Davis, a medical and cosmetic dermatologist in Dallas, Texas, urges her patients to apply sunscreen every morning, as part of their normal, daily routine. Protecting your skin is as essential as brushing your teeth, she says, and making sunscreen a habit is the best way to ensure that you are protected 24/7/365.

And the “365” isn’t just for show. Davis says that people always need to consider sun protection, even in the winter, on cloudy days, and even, yes, when you’re indoors and near a window. While normal light bulbs and computer screens don’t damage your skin, certain types of UV rays can get through glass windows in cars and homes. Even if you’re not outside, your skin can still get damaged.

“Anyone can get skin cancer, regardless of age, gender, or race,” she says.

Will sunblock prevent me from getting enough vitamin D?

Don’t worry about sunscreen use causing low vitamin D levels, says Dawn Marie Davis. As little as 15 minutes of sun exposure each week—the kind you’re likely to get walking to and from your car, on parts of your skin that are less than 100 percent protected—is adequate for vitamin D protection. Fortified foods and nutritional supplements are a far safer way to raise vitamin D levels than sun exposure, she said.

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What about mineral vs. chemical sunscreen. Are they safe?

Both mineral and chemical sunscreens prevent UV rays from harming your skin, but they do so in different ways. Mineral sunscreen lotions contain zinc oxide and/or titanium dioxide. These minerals sit on top of the skin and reflect UV rays before they can cause damage. Chemical sunscreens contain oxybenzone or octinoxate, compounds then absorb the UV rays instead of your skin.

Mineral sunscreens have historically left a white cast on the skin when applied, although newer formulations have dramatically improved that harmless but unpopular issue, says Adele Haimovic, a dermatologist in New York City. She recommends mineral sunscreens for people with sensitive skin. Because they aren’t absorbed, they are less likely to cause reactions.

The Environmental Working Group and other consumer organizations have raised concerns about the safety of oxybenzone. They point to evidence that the chemical is an endocrine disruptor (a catch-all term for compounds that affect hormones and other functions of the endocrine system), and cite a January 2020 study in the Journal of the American Medical Association that found that six active ingredients in chemical sunscreen were absorbed in the body.

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However, all the dermatologists who spoke with National Geographic affirmed the safety of chemical sunscreens. People who do have concerns about oxybenzone should use mineral sunscreen rather than refraining from any sun protection, Gohara says.

“If you're applying every two hours and applying in the right amount, it becomes a matter of personal preference,” she says. “The sunscreen that works best is the one that you're going to wear.”

Some sunscreen is labeled ‘reef-safe.’ What does this mean?

Some active ingredients in chemical sunscreen are also being identified as harmful to delicate coral reefs. It’s why Hawaii banned the sale of sunscreens containing oxybenzone and octinoxate in 2018. But Haimovic says that the issue is still an open question.

“More research is needed on this topic,” she says. With coral reefs under threat from so many sources, including climate change and pollution, it’s hard to know what impact sunscreen is having.

I’ve heard sunscreen is better in Europe and Korea. Is that true?

It’s complicated. But experts say that sunscreen in the U.S. is pretty good and most countries have similar rules to how they rate sun protection.

If I have darker skin, do I still need sunblock?

Absolutely, Davis says. Melanin is a skin pigment that serves as a natural sunscreen, she explains. The average SPF provided by melanin in people with more highly pigmented skin ranges from an SPF of 3 to an SPF of 13, far below recommended levels of SPF 30. What this means is that people of color have some extra sun protection but not enough to forgo sunscreen.

Do I need to worry about expiration dates on sunscreen?

Here, our expert advice was mixed. Mayo’s Davis says that consumers should pay close attention to expiration dates. Using expired sunscreen, she explains, raises the possibility that it may have lost some efficacy. Another concern? The growth of harmful bacteria.

Still, Gohara and Papadeas say that the dates on bottles should be taken with a grain of salt.

“If your sunscreen expires today, you can probably use it for another year,” Papadeas says. However, that’s only as long as the ingredients don’t begin to separate, which is a sign your sunscreen is losing efficacy.

So if you have a bottle that’s a little past its use-by date, it’s probably fine. The biggest concern Gohara has about expired sunscreen is that it’s a sign you’re probably not using enough.

“One bottle of sunscreen shouldn’t last you more than a couple of months if you’re using it correctly,” she says.

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