After spending some time in the sun, you’re supposed to reapply your sunscreen — every two hours, according to the guidelines. But what if you aren’t in the sun that whole time? And what if you use a mineral sunscreen, which doesn’t get “used up” the way some kinds of chemical sunscreens do? It turns out you still need to reapply anyway.
As chemist Michelle Wong explains, reapplying isn’t just about sunscreens getting used up — although some older sunscreen ingredients did get less effective the more sunlight they absorbed. (Today, though, plenty of chemical sunscreens can hold up to hours of sunlight without breaking down.) There are other reasons, too.
Sunscreen rubs off
Sunscreen is invisible (ideally), so it may be hard to imagine what happens when it’s been on your skin for a few hours. For a visual example, think about foundation, the skin-coloured cosmetic that forms a base for many full-faced makeup looks. Like sunscreen, it’s meant to be applied in an even layer across the skin. Also like sunscreen, it doesn’t get “used up” by exposure to sun or air.
So what happens when you’ve had foundation on for a few hours? It certainly doesn’t look brand new, because it doesn’t magically stay in an even layer on your skin. Every time something touches your face, like your hand or a tissue, a little bit gets rubbed off. (For another cosmetic example, think about how you suddenly have a lot less lipstick on your lips after you eat a sandwich.)
We may not touch our made-up faces very much, but think about how you apply sunscreen. It’s on your hands, and your hands touch everything. It’s on your legs, and the edges of your shorts will brush against it. When you sit in a chair, or cavort in the water, or towel off after swimming, sunscreen has opportunities to rub off.
Sunscreen thins out and clumps up even if you don’t touch it
OK, but what if you’re sure that nothing has touched your face all day? Think, again, of foundation. You still wouldn’t expect it to look perfect by the end of the day, even if you were very careful.
Your skin produces oils and sweat, and you also move around. These factors combine to allow a substance — whether makeup or sunscreen — to migrate around the skin. You’ll end up with bits of sunscreen (or foundation) in the little creases and folds of your skin. Where did that come from? From neighbouring areas of skin, which now have a thinner layer or none at all, meaning they are no longer sufficiently protected from the sun.
In the heat of a sunny day, this happens even more, Wong explains: “It’s worse if there’s any heavy sweating. The layer [of sunscreen] also breaks up and clumps into bits as your skin moves around — when you talk and eat and yawn.”
Sunscreen can also evaporate into the air and be absorbed into the skin over time, thinning out the layer of product on your skin even further.
Yes, you have to reapply
Now that you know what happens to sunscreen when it’s been on the skin for a few hours, the answer should be obvious: Yes, all kinds of sunscreen need to be reapplied. Not because the sunscreen is used up, but because protection depends on having an even layer of the stuff on your skin.
The FDA guidelines recommend reapplying sunscreen every two hours, or “more often” if you have been sweating or swimming. Water-resistant sunscreens are rated for either 40 minutes or 80 minutes of exposure to water (including sweat), so you can use that as a guideline for when to reapply.
It’s also smart to reapply if you know you’ve been rubbing it off, for example if you’ve been wiping sweat off your face with your sleeve while jogging around the neighbourhood.