Definitions for "Sun Protection Factor"
Keywords:  spf, uvb, lotion, melanoma, unprotected
rating which determines how many times longer your skin can tolerate UV rays when sunscreen is applied. For example, if it takes you 5 minutes to burn and you use an SPF 15 sunscreen, it should take you 75 minutes to burn (5 minutes times 15 SPF equals 75 minutes).
How many times longer a person wearing sunscreen can stay in the sun before beginning to burn than they would without any sunscreen at all.
A number quantifying the effectiveness of sunscreens. For example, if you can normally stay outside for about 15 minutes before you start to burn, then applying a sunscreen with an SPF of 20 allows you to stay out for 20 times 15 minutes before burning—or rather, 300 minutes. The SPF is measured in the laboratory under standardized conditions, so caution has to be applied when using it to estimate the time one can actually stay in the sun. As many sunscreens only block UV-B radiation, they are not as effective as the SPF suggests in protecting the body from harmful effects caused by UV-A radiation, like photoaging and possibly melanoma.