By the way, doctor: What's in sunscreen that protects against UVA rays?
Sun protection factor, or SPF, refers only to a product's ability to screen out UVB rays. SPF is calculated by comparing the time it takes to develop sunburn on sunscreen-protected skin to the time it takes on unprotected skin. If you usually turn red after 10 minutes in the sun without sunscreen, an SPF-15 sunscreen allows you to stay in the sun for 150 minutes (2.5 hours) before burning.
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