SPF: Sun Protection Factor. SPF measures the UVB protection of a sunscreen. (You can assume that UVA protection holds strong, while UVB defense wavers.) Unprotected skin takes 20 minutes to redden and burn in the sun, so an SPF 15 sunscreen or moisturizer will extend that 15 times longer, or roughly 5 hours. (You should play it safe and reapply after 2-3 hours, though.) They also explain it in terms of percentages, which explains why you might experience more of a skin tan using SPF 15 than SPF 30: SPF 15 blocks about 93 percent of UVB rays during this time. SPF 30 doesn’t block double the rays; it simply ups the ante to 97 percent, whereas SPF 50 takes it to 98 percent. UVB is the chief culprit behind sunburn, while UVA rays, which penetrate the skin more deeply, are associated with wrinkling, leathering, sagging, and other light-induced effects of aging (photoaging).UVA: Ultraviolet “A” rays can penetrate deep into the skin. This leads to premature aging, or “photoaging” — fine line
Everything Hair And Skin