If it says SPF 70, it's marketing hype. SPF measures the percentage of UVB rays blocked: SPF 15 blocks 93 percent of rays, SPF 30 blocks 97 percent, and SPF 50 blocks 98 percent. Anything higher adds little extra protection, which is why the FDA is considering banning labels over SPF 50 from store shelves. Dermatologists recommend an SPF between 30 and 50 for optimal protection.