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Previously asked Questions:

Q: How does sunscreen work?

A: Sunscreens block or absorb ultraviolet light. You can block UV with opaque creams like the white zinc oxide cream that you see lifeguards putting on their noses. You can also absorb UV radiation in much the same way that melanin does. The first and most common of the absorption chemicals is PABA (para-aminobenzoic acid). It absorbs UVB. Other chemicals include:

Cinnamates absorb UVB.
Benzophenones absorb UVA.
Anthranilates absorb UVA and UVB.

All sunscreens are labeled with an SPF, or Sun Protection Factor. The SPF acts like a multiplying factor. If you would normally be OK in the sun for 10 minutes and you apply an SPF 10 sunscreen, you will be OK in the sun for 100 minutes. In order for the sunscreen to work, however, you have to apply plenty and it has to stay on. You should apply it about half an hour before going out in the sun (or the water) so it can bind to your skin – if you don’t, then it is very easy for the sunscreen to wash off.

The SPF rating, by the way, applies only to UVB radiation.