sunscreens:what is SPF,exactly?

SPF--The higher the number, the better, right? Before you throw caution to the wind,puma femmes though, you should know a few things about SPF. What is it, exactly? What do the numbers mean, and how high can they go? We use sunscreen to block ultraviolet light from damaging the skin. There are two categories of UV light -- UVA and UVB -- that we consider in terms of sunscreen. UVB causes sunburn, and UVA has more long-term damaging effects on the skin, like premature aging. SPF, or sun protection factor, numbers were introduced in 1962 to measure a sunscreen's effect against UVB rays.
SPF numbers start at 2 and have just recently reached 70. To figure out how long you can stay in the sun with a given SPF, use this equation:Minutes to burn without sunscreen x SPF number= maximum sun exposure time ,For example, if you burn after 10 minutes of sun exposure, an SPF of 15 will allow you to be in the sun for up to 150 minutes without burning. But before you grab your calculator and head for the beach, you should know that this equation is not always accurate. People usually use far less sunscreen than the amount used in testing. In the real world, the average sun worshipper uses half the amount of sunscreen used in the laboratory, which could result in a sunburn in half the time.