Once a sunscreen crosses the 30 SPF threshold, the added protection does not necessarily increase its protective factor. According to Florida dermatologist James M. Spencer, MD, an SPF 15 product blocks about 94 percent of UVB rays while an SPF 30 product blocks 97 percent of the harmful rays. The results are even more negligible the higher up the SPF scale you go. For example, an SPF 45 product blocks about 98 percent of the rays. Most dermatologists will recommend sticking with a product that delivers a protection of SPF 30.