When you shop for sunscreen, you probably look for the SPF number on the product label. However, many people don't understand what the SPF number really means.
SPF (the letters stand for Sun Protection Factor) is a measure of how much ultraviolet radiation it takes to produce a sunburn on skin that is protected with the particular sunscreen product, as compared to the amount it takes to produce sunburn skin that is not protected with any sunscreen. A higher SPF factor means greater protection against sunburn.
Many people think that the SPF tells you how long you can stay out in the sun while wearing the product without getting burned. For example, some people think that if they can stay in the sun for a half-hour without getting burned if they are not wearing sunscreen, then they can safely stay in the sun for 15 times as long - or seven and a half hours - if they are wearing sunscreen with an SPF of 15. But this is not correct. The SPF number refers to the amount of exposure to ultraviolet light, not to the duration of exposure. You can get as much ultraviolet exposure from a brief period of time out in the sun in the middle of the day as from a much longer period outdoors early in the morning or late in the afternoon. The intensity of ultraviolet exposure also varies geographically, increasing as you move closer to the Equator. An hour's exposure to the sun in Minnesota or Maine is a very different thing from an hour's exposure to the sun in Florida or Texas.
Instead of thinking of SPF numbers as relating to the amount of time that you can safely spend in the sun, it is best to use them primarily for comparing products. A sunscreen with an SPF of 15 provides more protection than a sunscreen with an SPF of 8, for example. Most experts recommend that people should choose sunscreens with an SPF of 15 or more.
It's also important to realize that the SPF number only indicates how well the sunscreen protects against ultraviolet B (UVB) rays, which are the kind that cause sunburn. It does not give you any information about how well the sunscreen protects against ultraviolet A rays (UVA), which do not cause sunburn but do contribute to skin cancer. The American Academy of Dermatology and other experts recommend that consumers should select "broad-spectrum" sunscreen products that protect against both UVA and UVB.