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After spending some time in the sun, you should reapply your sunscreen – every two hours, according to FDA guidelines. But what if you are out of the sun all the time? And what if you are using a mineral sunscreen that doesn’t “wear out” like some chemical sunscreens do? It turns out that you still have to reapply.
As Chemist Michelle Wong explainsReapply isn’t just about using up sunscreen – although some older sunscreens became less effective the more sunlight they absorbed. (Today, however, many chemical sunscreens can hold sunlight for hours without breaking down.) There are other reasons as well.
Sunscreen is (ideally) invisible, so it can be difficult to imagine what will happen if it’s on your skin for a few hours. As a visual example, think of the foundation, the skin-colored cosmetics that form the basis of many facial make-up looks. Like sunscreen, it should be applied to the skin in an even layer. Just like sunscreen, it is not “consumed” by exposure to sunlight or air.
So what happens after you’ve put the foundation on for a few hours? It certainly doesn’t look brand new as it doesn’t magically stay in an even layer on your skin. Every time something touches your face, like your hand or a handkerchief, a little bit of it rubs off. (As another cosmetic example, consider suddenly having a lot less lipstick on your lips after eating a sandwich.)
We don’t like to touch our painted faces much, but think about how you apply sunscreen. It’s up to your hands and your hands touch everything. It’s on your legs and the edges of your shorts are brushing against it. If you sit in a chair, frolic in the water, or take off a towel after swimming, sunscreen can rub off.
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Sunscreen will thin and clump even if you don’t touch it
Okay, but what if you are sure that nothing has touched your face all day? Think again about the foundation. You still wouldn’t expect it to look perfect at the end of the day even if you were very careful.
Your skin produces oils and sweat, and so do you exercise. These factors combine to make a substance – be it makeup or sunscreen – wander around the skin. You will end up using sunscreen (or foundation) in the tiny wrinkles and folds of your skin. Where does it come from? From neighboring areas of the skin that now have a thinner layer or no layer at all and are therefore no longer adequately protected from the sun.
This happens even more in the heat of a sunny day, explains Wong: “It’s worse when you sweat profusely. The layer [of sunscreen] also breaks and clumps when your skin moves – when you talk and eat and yawn. “
Sunscreen can also evaporate into the air and be absorbed by the skin over time, further thinning the layer of product on your skin.
Yes, you have to reapply
Now that you know what happens to sunscreen after it has been on your skin for a few hours, the answer should be obvious: yes, all types of sunscreen need to be reapplied. Not because the sunscreen has been used up, but because the protection depends on an even layer of the material on the skin.
FDA guidelines recommend putting on sunscreen every two hours or “more” if you’ve sweated or swum. Waterproof sunscreens are rated for either 40 minutes or 80 minutes of water exposure, including sweat, so you can use this as a guideline for reapplication.
It’s also advisable to reapply it if you know you’ve rubbed it off, for example if you wiped the sweat off your face with your sleeve while jogging around the neighborhood.