How Sunscreen SPF Is Measured

Mom applying sunscreen to daughter
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SPF (Sun Protection Factor) is a multiplication factor you can use to determine how long you can stay out in the sun before getting a sunburn. If you normally can stay out 10 minutes before burning, a sunscreen with an SPF of 2 will let you stay out twice as long, or 20 minutes, before feeling the burn. An SPF of 70 will let you stay out 70 times longer than if you had no protection (or 700 minutes in this example, which would be over 11 hours or a full day).

How Is SPF Determined?

Think SPF is a calculated value or an experimental lab value, based on how much ultraviolet light penetrates a coating of sunscreen? Nope! SPF is determined using human experimentation. The test involves fair-skinned volunteers (people who burn most quickly). They apply the product and bake in the sun until they start to fry. 

What about water resistant?

For a sunscreen to be marketed as 'water resistant', the time required to burn must be the same before and after two consecutive 20 minute soaks in a Jacuzzi. The SPF factors are calculated by rounding down the time required to burn; however, you may get a false sense of protection from an SPF because the amount of sunscreen used in the tests is a lot more product than the average person uses. The tests use 2 milligrams of formula per square centimeter of skin. That's like using a quarter of an 8-oz bottle of sunscreen for a single application.

Still... a high SPF confers more protection than lower SPF. 

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Helmenstine, Anne Marie, Ph.D. "How Sunscreen SPF Is Measured." ThoughtCo, Mar. 24, 2017, thoughtco.com/how-sunscreen-spf-is-measured-3976036. Helmenstine, Anne Marie, Ph.D. (2017, March 24). How Sunscreen SPF Is Measured. Retrieved from https://www.thoughtco.com/how-sunscreen-spf-is-measured-3976036 Helmenstine, Anne Marie, Ph.D. "How Sunscreen SPF Is Measured." ThoughtCo. https://www.thoughtco.com/how-sunscreen-spf-is-measured-3976036 (accessed November 24, 2017).