Sunscreen
Sliced Bread
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Does SPF 30 Offer Double the Protection of SPF 15?
SPF is based on what's called your minimal erythema dose. That's the minimal amount of ultraviolet that's going to cause your skin to go red. SPF 30 allows 3.3% of ultraviolet B through, and SPF 15 allows 6.7% of UVB through. The ideal sunscreen should protect right across that spectrum in as uniform a manner as possible. In doing so, you're going to be hopefully getting something that reduces the amount of UVA by almost as much as the amount of the UVB.
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