How SPFs are Calculated

If you’ve ever wondered exactly how those SPF numbers are calculated on your sunscreen, you’ve got some sunburnt volunteers to thank.

SPF manufacturers test their products’ sun protection factor by slathering it on volunteers, at a rate of 2 mg of sunscreen per square centimeter of skin, and then blast them with UV rays that simulate sunlight until a burn appears. They then calculate the time it takes to create the burn on the protected skin and divide it by the time it takes to burn unprotected skin. The result is an approximation of the amount of time you can spend in the sun wearing a specified SPF until you burn.

But because most people use sunscreens improperly, sunburns still happen. According to recent research, people apply much less than what is used in the SPF calculations—as little as 1/5 of the amount used on the test subjects. Because of the complex calculations used in the SPFs, this exponentially cuts down the sunscreen’s effectiveness. According to the Mayo Clinic, the average adult should use about 1 oz of sunscreen per application, about the size of a shotglass. And make sure you use at least a teaspoon on your face.

So the higher the SPF the better, right? Well, not always. “Higher SPFs give people a false sense of security,” says an esthetician at Pharmaca Santa Fe. Indeed, studies show that people who use high-SPF sunscreens feel safer staying in the sun longer, and therefore do. After swimming, washing or toweling off, even adequate amounts of high-SPF sunscreen can and will lose effectiveness.

“The trick is to reapply,” the esthetician says. “All chemical sunscreens break down after a few hours.” Our estheticians usually recommend nothing higher than an SPF 30, reapplied every two hours. Mineral sunscreens can also be washed or rubbed off, so reapply as frequently as you would a chemical sunscreen.

For more on finding the right SPF for you, visit with a practitioner or esthetician at a Pharmaca near you, or check out our online selection of favorite sunscreens from brands like Goddess Garden, Supergoop, Alba and Badger.

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