According to the Skin Cancer Foundation, SPF 15 blocks approximately 93% of UVB rays. That rate jumps to 97% for SPF 30 and 98% for SPF 50, but nothing blocks 100%. Remember, the SPF rating only pertains to UVB (burning) rays, which are present when it’s sunny, but it’s the UVA (aging) rays that are present during ALL daylight hours that are responsible for accelerating photoaging.