Nanotechnology is the engineering of particles on an extremely tiny scale — between 50,000 and 100,000 times smaller than the width of a human hair. On paper, the potential is captivating. Such tiny particles can encapsulate food molecules so fats pass through the body without being digested. Meanwhile, supplements that contain nano-size vitamins are more easily absorbed by the body. These very capabilities, however, are causing opponents to question the new science and its unknown risks to human health.

“The human body hasn't ever encountered nanoparticles before,” says Ronnie Cummins, national director of the Organic Consumers Association (OCA). “The technology has tremendous potential, but we've got to know exactly what we're doing, especially before we put it into food and health care.” Following an announcement by the United Kingdom's Soil Association that it would refuse organic certification to any food that contains engineered matter smaller than 125 nanometers, the OCA is petitioning the FDA to enforce similar guidelines. Still, cosmetics companies are jumping on the nano bandwagon. The Environmental Working Group (EWG) has identified more than 9,500 personal care products that contain nanoparticles, with zinc oxide-based sunscreens leading the charge.

“This particular material doesn't penetrate beneath the layers of the skin,” says Sonya Lunder, a senior analyst at the EWG who recently conducted an audit of sunscreens. “Of course if there's any indication that it's more penetrating than we thought, we might change our recommendation to consumers.” Companies aren't currently required to indicate nanoparticles on labels. Lunder's tip: If your sunscreen is invisible on your skin, it probably contains nanoscale zinc.

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