How does sunscreen work?

Dear Cecil:

I've always wondered how sunscreen works. As I understand it, it keeps your skin from burning (and, to an extent, tanning), but it doesn't seem to reflect the light away (you don't shine that much with sunscreen on). So the light is getting through, but it's not affecting your skin. This doesn't make sense to me. Any straight dope on that?

Cecil replies:

There are two kinds of sunscreens, physical and chemical. Physical sunscreens are opaque formulations such as zinc oxide and titanium dioxide. They completely block sunlight but their cakey appearance tends to make you look like a clown in the circus so they’re used mostly by lifeguards and such.

Chemical sunscreens reflect or absorb the ultraviolet light that causes tanning and burning. The absorbed energy is reemitted in a harmless form, either heat or fluorescence, neither of which is perceptible.

The effectiveness of a chemical sunscreen is indicated by its SPF (sun protection factor): a sunscreen with an SPF of 15 will allow you to stay in the sun for 15 times as long as you could if you didn’t use sunscreen. Thus, if ten minutes in the sun without sunscreen will produce a modest tan, a sunscreen with an SPF of 15 will produce the same results in 150 minutes. But there are numerous qualifications and exceptions, so don’t push your luck.

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