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According to an About.com report, SPF is actually determined indoors by exposing human subjects to a light spectrum to determine how the light affects their skin.
The study said that sunscreen with an SPF of 15 filters 92 percent of UVB rays. In other words, a sunscreen with a SPF of 15, will “delay the onset of sunburn in a person who would otherwise burn in 10 minutes to burn in 150 minutes. The SPF allows a person to stay out in the sun 15 times longer.”
Unfortunately there is currently no measure of UVA absorption that has been determined. Most sunscreens offer protection from UVA and UVB rays, although the time factor that a person can stay out in the sun and not be affected by UVA rays cannot be determined like that of UVB rays.
Either way, it is extremely important to wear sunscreen of at least a SPF of 15 when you are outdoors and reapply every two- to three hours.