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caw | 5 years ago

The standard Coppertone sunscreen says

Directions: Apply liberally 15 minutes before sun exposure. Reapply: after 80 minutes of swimming or sweating; immediately after towel drying; at least every 2 hours

I’ve always heard this as time to absorb to become waterproof, but I’m not sure if it’s actual skin absorption or the liquid used to convey the sun blocking compounds to dry or evaporate. Either way, apply and then wait.

discuss

buran77|5 years ago

When you film it in UV, sunscreen appears completely black from the first drop out of the bottle, meaning it absorbed all UV. The 15 minutes might be for applying or getting absorbed before there's a chance to get wiped off.

This is a good video to highlight this [0], the sunscreen part is in the second half. Probably many more like it.

[0] https://www.youtube.com/watch?v=o9BqrSAHbTc

SiVal|5 years ago

I always (mis?)interpreted that as a way to get you to finish putting it on before you went into the sun, so you wouldn't get burned while possibly putting it on (a group of kids, for ex) too slowly while standing in the mid-day sun.

anonAndOn|5 years ago

I think the 15 mins is to allow sufficient time to dry so that you don't immediately sweat it off and/or wash if off in the pool/lake/ocean. It's got to stay on your skin to be effective.