CR83
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How Do They Measure SPF
I thought this was pretty interesting:
http://www.slate.com/id/2171789?GT1=10346
explainer: Answers to your questions about the news.
How Do They Measure SPF?
With a solar simulator.
By Juliet Lapidos
Posted Tuesday, Aug. 7, 2007, at 4:18 PM ET
How are SPF ratings for sunscreens determined?
Sunscreen with an SPF of 70 hit the marketplace for the first time this year. Hawaiian Tropic calls its Ozone Sunblock the "ultimate" in sun
protection, and Neutrogena boasts that its Ultra Sheer Dry-Touch is "revolutionary." How do scientists determine sun protection factor ratings?
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 Hawaiian Tropic's new product has an SPF 70 rating, someone who normally burns after 10 minutes can apply Ozone Sunblock and then luxuriate in
the sun for more than 11 hours. By way of contrast, one of the first manufactured sunscreens, Gletscher Cr?me, had an SPF of 2. This means that a
melanin-challenged person would need a fresh rubdown every 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 Andr? Garner of the Skin Cancer Foundation and Beth Lang of the Schering-Plough Research Institute.
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newbreedbrian
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interesting. thanks for that
The very existence of flamethrowers proves that sometime, somewhere, someone said to themselves, ?You know, I want to set those people over there on
fire, but I?m just not close enough to get the job done.? George Carlin
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