SPF (S-P-F), or sun protection factor, is a measure of how much longer a product allows you to stay in the sun without burning than if you were wearing no sunscreen at all. For example, if you usually start to burn after 10 minutes in the sun, an SPF 10 sunscreen would theoretically let you stay out for 100 minutes before getting red. Of course, the protection factor is dependent on using generous amounts of the product and re-applying it after swimming, sweating, or prolonged exposure. Experts recommend wearing a sunblock of at least SPF 15 every day. Applied adequately, a lotion with SPF 15 stops about 92 percent of UVB (U-V-B) radiation. SPFs higher than 15 give slightly more protection, but the gains are small and incremental. An SPF 30 keeps out about 96.7 (ninety-six point seven) percent of UVB rays, while a product with SPF 40 absorbs around 97.5 (ninety-seven point five) percent. However, the SPF rating refers only to the amount of protection from UVB radiation, the wavelength most likely to burn you and most often associated with skin cancer. SPF doesn't address the amount of UVA (U-V-A) protection a given product may offer. To guard against UVA rays and the premature aging they can cause, a sunscreen should also contain zinc oxide, titanium dioxide (dye-AWK-side), or avobenzone (av-oh-BEHN-zone).