The term SPF on your bottle of sunscreen stands for Sun Protection Factor. It is primarily a measure of the protection a sunscreen provides against ultraviolet B (UVB) radiation (it also measures protection from some shorter ultraviolet A rays but does not reflect defence against longer ultraviolet rays). An SPF is the ratio between the time it takes for the skin to redden or sunburn to start (known as the MED or minimal erythema dose) in skin protected with sunscreen and the MED in unprotected skin. For example, if it takes 20 minutes to develop redness without sunscreen, a sunscreen with an SPF of 15 might prevent reddening for 15 times as long...about 5 hours.
There has been some controversy over what level of SPF is necessary. In vitro (laboratory based) tests have shown that SPF 15 sunscreens filter out 93% of UVB rays, while SPF 30 sunscreens protect against 97%. However, studies in humans have shown that most people apply sunscreen too sparingly, typically using only 25-50% of the amount that is needed. Because of the way sunscreens work, a sunscreen with an SPF of 15 applied improperly will often provide an SPF as low as 2.
Studies in people outside of a laboratory (with real sun exposure) have shown that those using a sunscreen with an SPF of 30 had greater protection from sun damage than those using one with an SPF of 15. In order to achieve adequate protection, the Canadian Dermatolo
gy Association and the American Academy of Dermatology recommend using a sunscreen with an SPF of 30. Look for a sunscreen that says "broad spectrum" in order to choose a product that protects against both UVA (ultraviolet A radiation) and UVB. Better yet, look for ingredients such as Zinc Oxide, Mexoryl™, Avobenzone, and Tinosorb™, all of which are excellent UVA blockers.
Sunscreens should be applied liberally; a half teaspoon-full should be used for the face and ears and a shotglass-full should be used for the body. It should be applied 15-30 minutes before sun exposure and should be reapplied every two hours (and more frequently if you are exposed to water or are sweating).