"Women can tell you how many degrees (Fahrenheit and Celsius, to say nothing of Kelvin) it was outside." -Optimistic. on first kisses
Question #1191 posted on 10/29/2003 4:02 a.m.
Q:

Dear 100 Hour Board,

How do they calculate SPF?

- Will Sun Block Help if the World is Over? -

A: Dear Will,
They (meaning the dermatology community) calculate an average skin protection level, and then the SPF is however many times more protection the sun block provides. Thus, SPF 30 sun block has 30 times the UV protection than your normal skin. I'm assuming that they use a fairly sensitive skin level as average, because burnable people are the ones that use sun block.

-Benvolio