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How Long Does Sunscreen Last Before It Expires?
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Next time you're heading to the beach, remember to check whether your sunscreen is still effective. Although sunscreen may last longer than milk and restaurant leftovers, it still has an expiration date, experts told Live Science.

That date depends on how the sunscreen is stored, said Georgios Imanidis, a professor of pharmaceutical technology at the University of Applied Sciences Northwestern Switzerland.

If sunscreen is stored in a cool and dry place — in a closet, for instance — it can last for years, maybe even five to 10 years, the experts said. Most sunscreen manufacturers say that sunscreen is effective for three years, so long as the product is stored in optimum conditions. [How Does Sunscreen Work?]

But people often take sunscreen to the beach, leave it in their hot cars or stuff it in their backpacks while outdoors. When sunscreen gets hot, its components break down faster, making it expire sooner than it would normally, maybe even in six months to a year, Imanidis said.

Sunscreen contains inorganic compounds, such as zinc oxide or titanium dioxide, which prevent sunburn by absorbing or reflecting ultraviolet (UV) radiation that would otherwise penetrate your skin.

But sunscreen also contains ingredients that give the lotion a fragrant smell and make it easy to apply, said Rigoberto Advincula, a professor in the Department of Macromolecular Science and Engineering at Case Western Reserve University in Cleveland. These components include natural oils and aloe vera, as well as additives such as emulsifiers (substances that help oils and water mix into a single substance).

The first component to break down is usually the emulsifier, Advincula said. Without this ingredient, the water and oil separate. This can make the sunscreen runny or grainy, or simply not stick to the skin as well as it used to, the experts said.

To avoid this problem, users should simply give the sunscreen a good shake and then put it on, so long as it's not too old, the experts advised.

As sunscreen ages (or is exposed to too much heat and moisture), its other ingredients degrade and interact with one another, leading them to lose some of their sun-blocking properties, Imanidis said.

"[But] after all this, the sunscreen does not completely  lose its properties," he said. "It may lose its potency to some extent, but it's still a sunscreen."

For example, a sunscreen with a sun protection factor (SPF) of 55 might age into a sunscreen with a 40 or 30 SPF over time, Imanidis said. The SPF number refers to how long a person can stay in the sun without getting sunburned. If a person normally gets sunburned in 10 minutes, then with SPF 30, they can stay out 30 times as long, or 300 minutes (5 hours).

However, few people put on as much sunscreen as manufacturers advise. The gold standard is 0.00007054 ounces per 0.15 square inches (2 milligrams per square centimeter). So you don't have to pull out the scale on the beach, put the equivalent of a full shot glass of sunscreen over the exposed parts of your face and body, according to the Skin Cancer Foundation.

What's more, because people might use expired sunscreen (which is less effective) and fail to put on enough sunscreen in general, it's advisable to re-apply it often, about every 2 to 3 hours, Imanidis said.

"You may go through a tube that has 2 or 3 fluid ounces [about 60 or 88 milliliters] in a week," Imanidis said. [Why Does the Body Tan?]

He added that sunscreen sprays are less efficacious than thicker products, such as creams.

"We find that sprays do not last as long as others in terms of protection," Imanidis said. "If you have a cream that's relatively thick, and you go out in the sun, you can apply it twice or three times a day. If you have a spray, then probably use it every hour."

Original article on Live Science.