SPF 30 sunscreen blocks around 97% of UVB rays, while SPF 50 blocks 98%. No sunscreen prevents 100%. As such, experts suggest a minimum of SPF 30, but there’s little use going over SPF 50, as it can lull you into a false sense of security. Of course, any level of sun protection needs to be applied well and correctly in the first place and reapplied every couple of hours or after swimming and sweating.