What is SPF
what does SPF mean?
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 suns