Definition of Sun protection factor (SPF)

Sun protection factor (SPF): A number on a scale for rating sunscreens. SPF stands for Sun Protection Factor.

SPF numbers on a package can range from as low as 2 to as high as 60. These numbers refer to the product's ability to screen or block out the sun's burning rays.

The SPF rating is calculated by comparing the amount of time needed to produce a sunburn on protected skin to the amount of time needed to cause a sunburn on unprotected skin.

Let us take for instance a fair-skinned person who would normally turn red after 10 minutes in the sun. Ten minutes is their "initial burning time." If that person uses a sunscreen with SPF 2, it takes 20 minutes in the sun for that person's skin to turn red. Now, if that person uses a sunscreen with SPF 15, it multiplies the initial burning time by 15, so it takes 150 minutes, or 2 and half hours, for that person's skin to turn red.

Sunscreens with an SPF of 15 or higher are generally thought to provide useful protection from the sun's harmful rays.