We recommend a broad-spectrum sunscreen with SPF 30 or higher. We know that SPF 15 blocks about 93 percent of UVB rays, SPF 30 blocks 97 percent and SPF 50 blocks 98 percent. No sunscreen blocks all UV rays yet, which is why it is important to practice other sun-protective measures. While sunscreens with higher SPF block slightly more UV rays, they last the same amount of time as a lower-number SPF sunscreen and should be reapplied after two hours.