Have you ever slathered yourself in sunscreen for a day at the beach, on the boat, or by the pool, only to go to the bathroom post-dip and look in the mirror to find yourself burnt to a crisp? It’s an unfortunate event but it’s happened to the best of us—and believe it or not, the shelf life of your sunscreen could be to blame.

How long does sunscreen last, you might ask? The answer depends on the intent of your question. After all, sunscreen has an expiration date but it also has a set protection period, which is why dermatologists are so adamant about telling their patients—and anyone else who will listen—the importance of SPF reapplication.

All this to say, if you’ve been religiously applying sunscreen but still experience the tingling, tightness, and redness of over-exposure to harmful UV rays, it’s high time you learn the reality of some of your most burning questions surrounding SPF. How long does sunscreen last once applied? How long is sunscreen good for? And can you use expired sunscreen? Uncover the answers to all these questions and more, below.

How long does sunscreen last on skin?

The longevity of your SPF will depend on the specific product you’re using. That said, according to the American Academy of Dermatology (AAD), you should reapply sunscreen every two hours while spending time in direct sunlight for optimal protection. And that goes for all SPF levels, too. While experts at the AAD (and dermatologists in general) recommend using SPF 30 or higher for solid sun protection, whether you use SPF 30, SPF 15, or SPF 50, reapplying is key to keeping your skin safe. That’s because sun exposure breaks down the ingredients in the formula, so once it hits that two-hour mark, it won’t continue delivering optimal protection.

Even though the UV protection of sunscreen only lasts for a few hours on the surface of the skin, the product itself sinks into pores and can irritate your complexion if left on overnight. So if you’ve been pondering whether or not it’s okay to sleep with sunscreen on, do yourself a favor and treat it like makeup—wash it off already!

Does sunscreen expire?

Like all skin-care products, sunscreen does have an expiration date. According to board-certified dermatologist Christina Lee Chung, MD, FAAD, at Schweiger Dermatology Group in Philadelphia, PA, sunscreen typically goes bad in three years.

“So if you purchase a sunscreen and notice there isn’t an expiration date, the best practice is to mark the date of purchase [or when you first peel the seal off] and toss it before you reach the three-year mark—which hopefully you don’t reach before using it all,” she says.

If you’re someone who likes to stretch the rules a bit and use products beyond their marked expiration dates, Chung suggests always keeping the rule of three in mind. “If you purchase a sunscreen and the expiration date is at the two-year mark, chances are you’re good for another year after that,” she says.

Which sunscreens break down the fastest?

While sunscreens have a general lifespan of three years, they break down differently based on the formula. “Chemical sunscreens disintegrate faster due to their relatively unstable active ingredients, such as octinoxate and avobenzone,” says board-certified dermatologist Michelle Henry, MD, FAAD.

Research also shows that oxybenzone (another popular chemical sunscreen ingredient) oxidizes particularly quickly, rendering it less effective over time. So, not only is it incredibly important to reapply every two hours like clockwork, it’s also important to not accidentally leave in your car on a hot summer day, Chung says, as heat and sun exposure will degrade its ingredients.

Physical sunscreens (aka ocean-friendly sunscreens), like zinc oxide and titanium dioxide, on the other hand, don’t break down. “Rather the formulation that stabilizes the product and allows it to disperse evenly degrades which indirectly affects their sun protective properties,” Dr. Chung says.

Can sunscreen go bad before the expiration date?

It’s possible for sunscreen to lose its effectiveness before the expiration date. Like food and other sensitive skin-care products, sunscreen fares best “when stored properly—in cool, room-temperature environments, away from heat exposure,” Dr. Henry says.

Can you use expired sunscreen?

Can you? Yes. Should you? It’s not advisable. “There comes a time when the product changes and, if used, may have unintended negative effects,” Dr. Chung says. There are a few reasons for this. For starters, expired ingredients can trigger allergic and irritant reactions. This can show up in the form of general inflammation, rashes, or full-blown breakouts from the degraded ingredients, Dr. Henry says.

Another reason you could have an adverse reaction to expired sunscreen? It could have mold, which means your face would be covered in bacteria if you were to apply it. “Sunscreens contain preservatives to keep them sterile,” Dr. Chung says. “These preservatives will diminish over time, increasing the risk of bacterial overgrowth, which can lead to acne breakouts if applied to the skin.”

When sunscreen goes bad, the ingredients aren’t the only things affected—the very texture and consistency of the product can be, too. According to Chung, mineral sunscreens, in particular, get grittier with age. Because of this, they can be more difficult to apply, which can result in uneven coverage. “And no one wants to come back from having fun in the sun looking like a Jackson Pollock painting,” she says.

The biggest risk of all, though, is that once expired, your sunscreen may lose its very purpose. “The challenge with expired sunscreen, while it likely still retains some measure of sun protective capability, no one can know how much,” Dr. Chung says. “So you could get a good hour or two of protection or you could be applying a product with a SPF equivalent of plain moisturizer—zero. And without a general sense, the risk of excess sun exposure increases significantly.”

How to tell if sunscreen has expired

Not sure if your SPF is expired? A good way to tell is by examining its color, texture, and scent. “Expired sunscreens can become grainy or clumpy in texture and display a distinct change in smell or scent if contaminated with bacteria,” Henry says.

Another way to determine if your SPF has lost its luster? Consider the state of your skin after you use it. “With expired sunscreens, our skin becomes instantly vulnerable to UV ray damage,” Henry says. “With reduced SPF efficacy, we can experience sudden sunburn from the lack of essential UV ray protection.”

The takeaway

To avoid any unnecessary skin irritation, Henry says that it’s best to keep tabs on your sunscreen’s expiration date and to toss even your most beloved SPF if that date arrives.

If the idea of throwing out your favorite sunscreen is cringe-worthy, may we introduce you to the AAD’s recommended sunscreen dosage guideline? For optimal UV protection, dermatologists suggest using an ounce (aka a shot glass’s worth) of sunscreen to adequately cover bare skin from head to toe (more specifically, derms recommend two finger-lengths worth of SPF for your face). If done daily (or even just during summer), you should easily hit empty before the three year-mark is up.

If you do have to toss your SPF, though, don’t take that as a sign that it’s a wasteful investment. Instead, treat yourself to a formula you’ll be happy to apply again and again. For me, nothing compares to the Tatcha Silken Pore Perfecting Sunscreen ($70)—it goes on so smoothly, never irritates my sensitive skin, and wears well under makeup. For my body, I keep my beach/lake/pool bag (I’m no gatekeeper! The A New Day Seasonal Tote Bag, $30, is my current obsession, so much so that I bought it in two colors.) stocked with Supergoop! PLAY Everyday Lotion SPF 50 with Sunflower Extract ($34), PLAY Antioxidant Body Mist SPF 50 with Vitamin C ($21), and Unseen Sunscreen Body SPF 40 ($42)—which is one of Well + Good’s top-rated sunscreens—are my current go-tos.