A member asked:

What are spf scores, and how long do they last in the sun?

A doctor has provided 1 answer

SPF: Spf stands for sun protection factor. The number is determined experimentally indoors by exposing human subjects to a light spectrum meant to mimic noontime sun. Some subjects wear sunscreen and others do not. The amount of light that induces redness in sunscreen-protected skin, divided by the amount of light that induces redness in unprotected skin is the spf. It is mainly a measure of uvb protec.

Answered 5/11/2012

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