SPF numbers (Sun Protection Factor) indicate the level of protection against UVB rays, which are primarily responsible for sunburns. SPF measures how long you can stay in the sun without getting burned compared to the time it would take for unprotected skin to start reddening. An SPF of 30 filters 96.7% of radiation, an SPF of 50 filters 98%, and an SPF of 15 offers 93% protection against UVB rays. It’s important to choose broad-spectrum sunscreen that also protects against UVA rays, which can cause premature skin aging and other cellular damage.