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After spending time outdoors, you're advised to reapply sunscreen every two hours, as per FDA guidelines. But what if you’re not exposed to the sun the entire time? And what if you're using a mineral sunscreen, which doesn't deplete the way some chemical sunscreens do? It turns out, reapplication is still necessary.
As chemist Michelle Wong explains, the need to reapply sunscreen isn't just because it wears out—though some older sunscreen formulas did become less effective after absorbing sunlight. Nowadays, many chemical sunscreens are designed to withstand extended sun exposure. There are other factors at play as well.
Sunscreen wears off over time
Sunscreen is typically invisible, so it might be hard to picture what happens after it's been on your skin for a few hours. A good comparison is foundation, the skin-tone product used as a base for many makeup looks. Like sunscreen, it's applied evenly across the skin and, similarly, doesn't get 'used up' by sun or air exposure.
So, what happens to foundation after a few hours of wear? It certainly doesn’t look fresh because it doesn’t magically remain in a smooth layer. Every time something touches your face, like your hand or a tissue, a little bit rubs off. (For another example, think about how much less lipstick you have after eating a sandwich.)
While we may not touch our makeup-covered faces often, think about how you apply sunscreen. It's on your hands, which touch everything. It's on your legs, which get brushed by the edges of your shorts. When you sit, swim, or towel off, sunscreen gets rubbed off or displaced.
Sunscreen gradually thins out and clumps together, even without direct contact.
But what if you’re confident nothing has touched your face all day? Again, consider foundation. Even with careful application, it wouldn’t stay flawless by the end of the day.
Your skin naturally produces oils and sweat, and you’re constantly on the move. These factors combine to cause substances like makeup or sunscreen to shift around the skin. Eventually, you'll find bits of sunscreen (or foundation) collecting in the small creases and folds of your skin. Where does that come from? It comes from the surrounding areas, which now have a thinner layer or no product at all, leaving them unprotected from the sun.
This effect is even more pronounced on a hot, sunny day, says Wong: 'It gets worse with heavy sweating. The sunscreen layer also breaks up and clumps together as your skin moves—when you talk, eat, or yawn.'
Sunscreen can also evaporate into the air and be absorbed by your skin over time, further thinning the protective layer.
Yes, reapplication is necessary.
Now that you understand what happens to sunscreen after it's been on your skin for a few hours, the answer is clear: Yes, all types of sunscreen need to be reapplied. Not because the sunscreen is gone, but because effective protection depends on maintaining an even layer of it on your skin.
The FDA suggests reapplying sunscreen every two hours, or more frequently if you've been sweating or swimming. Water-resistant sunscreens are rated for either 40 minutes or 80 minutes of water exposure (including sweat), so use that as a guide for when to reapply.
It's also a good idea to reapply if you know you've wiped it off, like if you've been using your sleeve to wipe sweat from your face while jogging around the neighborhood.
