For most individuals, a sunscreen with a sun protection factor, or SPF, of at least 30 is optimal as it blocks out 97 percent of harmful UVB rays. With sunscreen, SPF numbers on a package vary and can be as low as two or as high as 100.
Although a sun lotion with an SPF of 30 or higher is generally recommended, a product with an SPF 15 can offer adequate sun protection as well. An SPF 15 sunscreen blocks out approximately 94 percent of UVB rays while an SPF 30 blocks out 97 percent, according to WebMD. That percent increases to 98 for an SPF 45 product. Applying the sun lotion when necessary, such as after two hours or after swimming, makes it most effective.