Step 1 : Introduction to the question "5. In the United States, sunscreens are classified based on their SPF. What does the term "SPF" mean?"
...1. Solar Positioning Factor 2. Sun Protection Factor 3. Sole Prevention Factor 4. Safe Partying Factor The Sun Protection Factor (SPF) is a rating system used to indicate the strength of a sunscreen. It theoretically indicates the amount of time that a person can stay out in the sun before burning. Thus an SPF of 15 indicates that a person can stay fifteen times longer in the sun than it would take to get a sunburn without sunscreen. This is confusing however, since it doesn't take into consideration sweating, water activity or other factors that can affect sunscreen performance. Perhaps the best way to use the SPF measurement is as the percentage of UVB rays blocked by the product, with an SPF of 15 blocking an estimated 94 percent of the rays and an SPF of 30 blocking 97 percent of the rays (American Academy of Dermatology).
