The first big offense? Leaving your sunscreen in your beach bag or car trunk all summer and not storing it in a cool place. "Keeping sunscreen in the heat and exposing it to light will cause it to break down faster," says cosmetic dermatologist Dr. Fredric S. Brandt. "When sunscreen is stored in a high temperature, the effectiveness decreases, and the sunscreen becomes less stable and reliable. When it gets hot or is stored above 77 degrees Fahrenheit, the potency is destroyed, and the sunscreen will degrade." So, remembering to keep sunblock in the shade when we're outside — and removing it from our bag once we head indoors — is one of the easiest ways to maintain its potency.
Using SPF that is degraded in any way is dangerous simply because you're not getting the protection you think you are. "It's not that the active ingredients in sunscreen start to deactivate once they're expired — they just start to break down," says dermatologist Dr. Paul Jarrod Frank. A common misconception is that when SPF expires, its "number" decreases, so you just have to apply more. According to Frank, this is total phooey. "You don't know exactly what the SPF is once sunscreens are expired, so you're just putting yourself at an unnecessary risk," he explains.
The easiest way to remember your expiration dates? Brandt suggests writing a "use by" date on your bottle when you buy it. While Brandt says the shelf life of a sunscreen is three years, Frank states the SPF you buy this season won't be good next summer. There's also a pretty simple way to figure out when your sun protection's a goner: If your sunscreen starts separating or takes on a consistency that's different from when you bought it, it's time to toss.
Both Frank and Brandt stress that no matter what your SPF number is, you should absolutely be applying it at least 15 minutes before you step foot into the sun. Then, once you're out there, you should be reapplying every two hours. "If you're in the water for more than 20 minutes, you should reapply once you get out, though," Frank advises.
Ultimately, the best way to ensure your sunscreen is safe is through education — and, you know, actually wanting to protect yourself. "The biggest decision is not following specific rules. It's using your head and making the decision that you want to protect yourself from the sun," Frank says. "You can't apply too much of it, so just apply and reapply, and you'll stay safe." Roger that.
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