UVB protection does not actually increase proportionately with a designated SPF number. For example, an SPF of 30 screens 97 percent of UVB rays, whereas an SPF of 15 screens 93 percent of UVB rays, and an SPF of 2 screens 50 percent of UVB rays. However, inadequate application of sunscreen may result in a lower SPF than the product contains.
Whichever SPF you choose, wearing sunscreen should not provide a false sense of security about protection from UVB exposure. No sunscreen can provide 100 percent UVB protection. Using a higher SPF provides greater UVB protection than a lower SPF, but it does not mean that you should stay out in the sun longer.