How Do They Measure SPF?With a solar simulator.

How are SPF ratings for sunscreens determined?This summer, Neutrogena is marketing a high-end sunscreen with an SPF of 70.* The company boasts that its Ultra Sheer Dry-Touch is "revolutionary." How do scientists determine sun protection factor ratings?

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Human guinea pigs. A product's SPF refers to its ability to deflect ultraviolet rays. To calculate this figure, scientists gather 20 human volunteers who are especially susceptible to sunburn. According to FDA guidelines, volunteers must have a skin type of I, II, or III on the Fitzpatrick phototyping scale. (The categories correspond to the amount of pigment present in the skin: Very fair blonds or redheads are Type I, while those with dark brown or black skin are Type VI.) Using a device called a "solar simulator," experimenters irradiate a small patch of skin on each subject and then record the UV dose required to produce mild redness (in scientific parlance, the "minimal erythematic dose"). After applying a thick layer of sunscreen, the experimenters repeat the test. Then they divide the MED needed to redden the protected skin by the MED needed to redden bare skin. The result, rounded down to the nearest five, is the SPF.

Since Neutrogena's new product has an SPF 70 rating, someone who normally burns after 10 minutes can apply Ultra Sheer Dry-Touch and then luxuriate in the sun for more than 11 hours. By way of contrast, one of the first manufactured sunscreens, Gletscher Crème, had an SPF of 2. This means that a melanin-challenged person would become vulnerable after 20 minutes.*

Scientists have also developed a test that determines whether SPF numbers remain steady upon exposure to water. After applying sunscreen, volunteers immerse themselves in a Jacuzzi for 20 minutes, air-dry, and then take another 20 minute dip. Next, scientists irradiate a small area of skin and calculate the MED. If the post-immersion MED equals the dry laboratory MED, then the manufacturer has the legal right to call its product "water resistant."

Here's the catch: Although SPF tests are quite rigorous, the number on a sunscreen bottle rarely corresponds to real-world effectiveness. Why? First and foremost, people with different complexions burn at different rates. Type I skin burns faster than Type VI, for example. Second, experimenters use a whole lot more formula in labs than average people do when they lie out at the beach. The FDA requires that manufacturers test their products at 2 milligrams per square centimeter of skin. That's roughly equivalent to 2 ounces of sunscreen to cover your whole body, or one-quarter of a standard 8-ounce container every time you sit by the pool.

Explainer thanks André Garner of the Skin Cancer Foundation and Beth Lang of the Schering-Plough Research Institute.

*Correction, Aug. 13, 2007: This article originally stated that sunscreens with an SPF of 70 were first introduced this year. It also suggested that regularly reapplying sunscreen with an SPF of 2 would prevent sunburn. (Return to the corrected sentences.)

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Juliet Lapidos is a Slate assistant editor.

Photograph of sunscreens by Joe Raedle/Getty Images.

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