What SPF should I use in summer?

SPF 30 blocks 97% of UVB rays and is sufficient for daily use. SPF 50 blocks 98% the difference is marginal. What matters more than SPF number is: applying enough (a full shot glass for your body, a nickel sized amount for your face), reapplying every 2 hours during sun exposure, and choosing broad spectrum protection (covers both UVA and UVB). A SPF 30 you reapply beats a SPF 70 you apply once and forget.