Higher SPF ratings generally mean better protection from the sun's ultraviolet B rays, so SPF 30 sunscreens provide protection from 97 percent of UVB rays and SPF 45 or higher sunscreens block 98 percent. The SPF rating, however, does not signify how much protection someone gets from ultraviolet A rays, which are also linked to skin cancer, according to the Skin Cancer Foundation.
By simply multiplying the SPF number by 30, anyone can calculate how long he or she can stay in the sun while wearing an SPF 30 sunscreen. This is because without sunscreen, the average time it takes someone to get sunburn is approximately 30 minutes. So, if someone wears an SPF 15 sunscreen, he or she can stay out in the sun for 450 minutes, or 7 1/2 hours, before getting burned.
For people wearing a SPF 30 sunscreen, they can stay in the sun for 900 minutes, or 15 hours. The same idea applies to SPF 50 sunscreen, although skin doctors warn that the overall protection from the sun's ultraviolet rays aren't substantially higher, blocking only an extra 1percent of UVB rays.
Also, people tend to put on only a third of the recommended amount of sunscreen, meaning that they are only getting a third of the protection. Doctors recommend that staying out of direct sunlight, even when wearing sunscreen, is preferable, as there is no sunscreen that can protect the skin from 100 percent of the sun's harmful rays.