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Sun Protection Factor or SPF is a number on a scale for rating sunscreens. 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. For instance, a fair-skinned person who's "initial burning time" which means they normally turn red after 10 minutes in the sun uses an SPF 2. It will take 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.
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