What does SPF mean?

SPF stands for Sun Protection Factor. It is primarily a measurement of the protection a sunscreen provides against ultraviolet B (UVB) radiation.  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.

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