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Yeah, it's the last. "The English," says Stephen Schwartz wistfully. He's president of a sunscreen-rating company, International Research Services. "They're classic peaches and cream -- beautiful, light skin that burns very easily." |
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Well, the bunnies finally get a break! But then, bunnies don't need sunscreen. . |
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Pale, furless creatures need sunscreen because the ultraviolet wavelengths of light from the sun beat
through the cell walls of our skin and wreak havoc on our DNA. When we figured this out a few years
back, chemists began tinkering with molecules that would foil the ultraviolets.
One method is to grind zinc oxide, or something equally reflective, into teeny, tiny bits and add it to a lotion base. The UV light hits these bits and bounces away. The more ingenious method is to build molecules held together by rugged, double bonds between its atoms. When a particle of UV energy collides with such a burly molecule, it exhausts its destructive energy bashing apart the double bonds and has no strength left to burn a bod. (Who knew your skin hosted such dramas?) |
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But how do they figure out which molecules actually work? And what is an SPF, anyway? . |
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When you go into the sun, your particular skin takes X minutes to turn pink. This interval is your
personal Minimal Erythemal Dose (MED), and a light-skinned person's is less than 15 minutes. An SPF,
or sun protective factor, simply indicates how many times your MED you can safely linger outside: If
your MED is 10 minutes and you're wearing SPF 20, you're protected for 200 minutes. If my MED is 15
minutes and I'm wearing SPF 20, I'm protected for 300 minutes.
So if SPF is relative to the individual, how do scientists determine the SPF of a given bottle of ooze? Schwartz calls 20 or 25 of his stable of "subjects," selected for their pale skin, and they come to the lab for a date with a "solar simulator." This device holds a xenon bulb that pumps out concentrated UV and visible light that can deliver a sunburn in a matter of minutes. Each guinea person's naked skin is first zapped with the solar simulator to establish her personal Minimal Erythemal Dose. Now, by the time manufacturers come to Schwartz for a final SPF rating, they're pretty sure of where their ooze falls, and they set that as the target. So if the guinea person's MED is 10 (10 minutes to pink), then to achieve a rating of 20, the sunscreen has to prevent him from burning in 200 minutes (10 times 20), or just a few minutes from the solar simulator. So on each person's back, a patch the size of a playing card is marked off and smeared evenly with sunscreen. The simulator presses its tubular nose against the subject's skin and blasts fake sunbeams in a line of five small circles across the King of Diamonds. The beam irradiates each spot for a different length of time, some longer, some shorter than the target time. If, by the next day, at least one spot below the target number remains white and at least one spot above the target has turned red, your SPF is, well, on target. |
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Occasionally manufacturers overestimate their protection factor and the subjects go home with five-aspirin-sized sunburns. . |
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It's all a vast improvement over the days when sunscreen testing took place in the great outdoors
where, after all, the sun is located. There, testers found that no two sunny days are alike in terms
of cloud cover and haze. And, even if you could keep track of 150 fully greased subjects at a beach,
you could still lose your client to sunstroke. Schwartz recalls one manufacturer who pooh-poohed his
warning that haze doesn't stop the sun, and went oozeless during a beach test.
"His forehead looked like a stop sign," he recalls with scholarly appreciation. "He burned really nicely."
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 Vocabulary in vivo, adv. Tests such as the sunscreen challenge are done in vivo, or "in a living thing" (from the Latin vivere, to live). In vitro, or "in glass," refers to test tubes, petri dishes and other lab gear in which life is more or less imitated.
Check out more of "The Skinny On ..." stories:
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Hannah Holmes usually lives in Portland, Maine, but this month she's putting her sunscreen
to the test in Mongolia's Gobi Desert for Discovery Online's Dinosaurs in the Dunes special. She
also writes for Escape, Outside, Sierra, Backpacker, Eco Traveler and Women's Sports and
Fitness. Write her at skinny@online.discovery.com.
Main WebLinks Picture: Brian Frick | Copyright © 1997 Discovery Communications, Inc. |