SPF stands for sun protection factor, a measure of how well sunscreen protects against UVB rays. (UVA protection isn't rated.) Manufacturers calculate SPF based on how long it takes to sunburn skin that's been treated with the sunscreen as compared to skin with no sunscreen. Experts recommend using a sunscreen with an SPF of at least 30. Sunscreens with SPFs greater than 50 provide only a small increase in UV protection. High-number SPFs last the same amount of time as low-number SPFs.