I use sunscreen every day, but I get confused about this reapplication thing. My dermatologist said I need to put it on again every two hours, but what's the chemistry behind that? Does the product actually disappear from the skin or does something chemical happen that reduces the protection?
Beyond the chemical degradation that the other person mentioned, there's a practical side nobody talks about: you sweat, you get in the water, you wipe your skin with a towel, and a good chunk of the product just comes off physically. Not all that loss is molecular transformation - it's literally coming off your skin. So reapplying every two hours (or sooner if you get in the water) isn't just about the product losing its strength, it's also about replacing what's gone. The catch is that a lot of people think they can put sunscreen on once in the morning and stay protected all day, but that's just not how it works if you're moving around, sweating, or at the beach.
Sunscreen degrades when exposed to sunlight because UV filters (especially chemical ones) absorb the radiation and transform into other substances, losing their ability to protect! Plus, the product wears off your skin through sweat, friction with clothes, water or sand, even if you don't notice it. Your dermatologist is right: every two hours (or sooner if you're in the water or sweating) you need to reapply because the protective barrier becomes compromised and your skin is left exposed to UV rays!
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