Sun protection factor (SPF) refers to a product’s level of protection from exposure to ultraviolet (UV and UVB) radiation, which can redden skin in as little as 10 minutes without sunscreen.
Applied properly, sunscreen with SPF 15 (the recommended minimum) can allow the wearer to stay in the sun without burning up to 15 times longer than bare skin alone. SPF 15 filters out 93 percent of incoming UVB rays.
SPF 30 can absorb 97 percent UVB rays, and SPF 50 can absorb 98 percent of UVB rays.
Products with SPF greater than 50 provide almost no added protection from UVB—no sunscreen can block all the sun’s radiation.
Sunscreen should be worn even on cloudy days when up to 40 percent of the sun’s UV radiation can still reach the ground.
