Why it's not worth using expired sunscreen - and how to tell when SPF becomes ineffective

• Sunscreen expires about 3 years after the date of purchase, or by the expiration date.
• Expired sunscreen may be less protective against harmful, cancer-causing UV rays from the sun.
• Sunscreen expires more quickly if it's not stored properly - keep it in a dark, cool environment.

Most sunscreens expire because their active ingredients degrade over time.

When you purchase a bottle of sunscreen, look for an expiration date. If there isn't a date on the label, FDA regulations require that sunscreen remain effective for three years after the date of purchase.

However, that doesn't guarantee your sunblock will be effective that long. Exposure to extreme temperatures or light - like leaving a bottle in your car over the summer - will speed up the sunscreen's degradation.

That's because the heat breaks down the active ingredients that protect against the UV rays. This degradation will also happen at room temperature, but much more slowly.

That's why "sunscreen, a lot of the time, needs to be maintained at room temperature and without significant exposure to light," says Henry Lim, MD, a board-certified dermatologist in the Dermatology Department of the Henry Ford Health System.

Why sunscreen expires

There are two main types of sunscreens: chemical and physical. Both protect against UV rays, they just use different ingredients to do so. And this largely effects how they will expire.

How chemical sunscreens expire

The active ingredients in most chemical sunscreens are variations on benzene rings, says Lim, which have a stable structure that allows the molecule to absorb ultraviolet light.

Over time and under extreme conditions, the bonds holding the ring together will begin to break apart, and the compound will no longer be effective at blocking harmful UV rays.

In addition to these active ingredients, chemical sunscreens contain inactive ingredients which help to:

• Stabilize the active ingredients
• Aid absorption
• Moisturize the skin, similar to a lotion

"All of the UV filters or active ingredients, are properly and very precisely mixed with the vehicle," Lim says. "The various other ingredients that are put in there make sure the sunscreen will work well." Time, heat, and light, he says, can "disturb that finely tuned balance and would make the sunscreen much less effective."

Important: If you open a bottle of chemical sunscreen, expired or not, and the consistency or color of the sunscreen has changed, these inactive ingredients have probably also degraded, and you should throw the bottle out.

How physical sunscreens expire

For physical, or mineral-based, sunscreens, the active ingredients reflect UV radiation instead of absorbing it.

These sunscreens usually come as a lotion, where the active ingredients, like zinc oxide or titanium dioxide, are suspended in the inactive ingredients that allow you to apply the sunblock to your skin.

However, the fine nanoparticles in zinc oxide and titanium dioxide are denser than their surroundings and, over time, will clump together, becoming unevenly distributed in the lotion.

As a result, when you apply the sunscreen, you won't get as even a layer of coverage - since you'll have clumps of protective minerals in some places, and no protection at all in others.

Note: One way to tell if your mineral sunscreen has expired is that these larger clumps will cause the lotion to appear speckled, with distinct white clumps, which some may consider unsightly, says Lim.

However, these minerals are extremely tiny, which means there's a chance that you may not see speckles or clumps, but still won't be evenly protected if the bottle has passed its expiration date.

The active ingredients in mineral sunscreens don't degrade the way the UV-absorbing molecules in chemicals ones do, but expired inactive ingredients in either type of sunscreen could lead to problems when you go to apply it.

It's "very similar to body lotion," Lim says. "you put it on the shelf for three years, and when you try to use it, it just doesn't work as well." When you're relying on these ingredients to help sunblock stick to your skin, he added, it's best not to take chances.

What to expect if you use expired sunscreen

The lower the SPF, usually the riskier it is to use expired sunscreen. "Once it is degraded, it is no longer going to be functioning properly," Lim says, "So definitely the SPF would drop."

A bottle of SPF 15 sunscreen may have the same active ingredients as a bottle of SPF 50, but the SPF 15 bottle will have a lower concentration of active ingredients. So, if it's expired, that SPF 15 bottle may have lost almost all of its ability to protect you from UV radiation, Lim says.

If you absolutely must use a bottle of expired sunscreen, it's nearly impossible to know by how much that SPF may have dropped. That all depends on how you've stored it.

Lim says he advises his patients to buy a new bottle every year if they can, to make sure you're getting something at full potency. "You're using it daily in the summer," he says, and at the beach "it's all over a large part of the body. [A bottle] shouldn't last too long."

Quick tip: Lim's advice is in line with the Center for Disease Control, which says you should buy a new bottle every year or two.

But a fresh bottle of sunscreen is just one way you should be protecting yourself from UV skin damage. Clothing, brimmed hats, staying in the shade, avoid the sun during peak UV intensity, and UV-filtering sunglasses are also effective ways to protect your skin. And - bonus - they don't expire!

Insider's takeaway

Sunscreen does expire - typically about 3 years after the date of purchase. Always make sure to check the expiration date on your sunscreen, and if it has passed, don't use it. It's likely that the active ingredients in your sunscreen have degraded and it will no longer be as effective.