How Does SPF Work?

So I went on vacation recently, which was nice.

One of the conversations that came up, which I'm sure does for many folks on vacation, was around the application of sunscreen. How often should you re-apply? How long will SPF 50 last vs. SPF 15? And then as we were talking, an even more fundamental question arose - what the hell is SPF anyway, and how does it work?

I'd always assumed in the past, like I assume many other people do, that it was a linear scale - so SPF 60 was 4x 'as good' as SPF 15. Someone in our group also said that the number was supposed to be a measure of duration for sun exposure. So, for SPF 60 you could go in direct sunlight for an hour longer than you would normally without burning whereas whereas for SPF 15 it'd only be a quarter of that.

Apparently as it turns out, neither of these things are true.

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