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The CDC’s guidelines (and manufacturers’ instructions) will tell you to reapply sunscreen after two hours. There’s a belief that this is the case because sunscreen used to get “used up” as it does its job, and that modern sunscreen does not. That’s partially true; older sunscreen formulations did get less effective after you spent time in the sun.
But the real reason for reapplying sunscreen is that it moves around your skin as you go about your life. It can get rubbed off by clothing or towels, it can dribble down your face as you sweat, and even if you’re sitting comfortably in the shade, the movement of your body can result in the sunscreen working its way into tiny folds and wrinkles in your skin, leaving areas unprotected. Imagine wearing a coating of makeup all day; how do you think it would look after two hours? Sunscreen migrates the same way, so make sure to reapply.
