What is SPF? (Sun Protected Fun, right?)

Migraine

SPF is an acronym for Sun Protection Factor. The number you see associated with SPF represents the length of time you can stay out in the sun without burning, multiplied by the corresponding number. So, a person who would normally start to burn in 10 minutes, could theoretically have 150 minutes of sun protection with a sunscreen that has an SPF of 15. If you normally start to burn after 15 minutes of sun exposure, an SPF 15 product will let you stay in the sun for approximately 15 times longer (or 3 1/2 hours) without burning. 

Regardless of SPF number, testing of sunscreen does not really correspond to the amount of time you can stay out in the sun without burning because sweating, clothing rubbing against the skin, and water sports will all wear the sunscreen off. Tests also use much more sunscreen than do most people, so SPF number results can be inaccurate. Usually, an SPF 15 rated sunscreen will give your about an hour of protection before you should reapply sunscreen. This, of course, varies from person to person. It also helps to wear protective clothing, hats and sunglasses

What remains important to remember is that SPF strength is less important than frequent application, and avoidance of sun between 10 am and 4 pm, when UVB rays are strongest. SPF protection should always be combined with protective clothing and sun avoidance during the hottest and brightest parts of the day.

Some people do pay more for higher SPF protection, and this may make sense for those with lighter skin. Doctors generally recommend purchasing an SPF sunscreen of 15 or higher. There are lower ones, which will cause the skin to tan or burn in under an hour. It may be unnecessary to purchase an SPF higher than 15, except possibly for infants, if you can be vigilant about reapplying the sunscreen every hour, and after swimming or very vigorous physical activity.

Excerpts taken from www.wisegeek.com