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March 31, 2011

An Emerging Pollutant Circles The Globe

Air Pollution: The first global study finds widespread distribution in air of compounds used in personal care products

Sarah Everts

THEY'RE EVERYWHERE Cyclic volatile methylsiloxanes find use in many personal care products. Shutterstock
THEY'RE EVERYWHERE Cyclic volatile methylsiloxanes find use in many personal care products.
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Many personal care products such as deodorants, shampoo, conditioner, cosmetics, and lotions owe their quick-drying, silky feel to ingredients known as cyclic volatile methylsiloxanes (cVMS). But the compounds are currently under regulatory scrutiny in Canada, Europe, and the U.S. because scientists think they may persist and bioaccumulate in the environment.

Although researchers have previously measured the presence of cVMS at several sites, a team of regulatory, academic, and industry researchers now reports the first global-scale study of cVMS distribution in air (Environ. Sci. Technol. DOI: 10.1021/es200301j. It found the compounds in 20 sites around the world, including pristine Arctic environments and industrial areas in the U.S. Northeast.

One of the cyclic volatile methylsiloxanes, D5, appears in high levels in the eastern U.S. and Europe.
One of the cyclic volatile methylsiloxanes, D5, appears in high levels in the eastern U.S. and Europe.

The team, led by Tom Harner, a senior research scientist at Environment Canada, measured the presence of four kinds of cVMS: D3, D4, D5, and D6, which stand for hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, and dodecamethylcyclohexasiloxane, respectively. Harner's team found cVMS to be "widespread globally," he says. The compounds are "present at relatively high air concentrations compared to other classes of air contaminants such as persistent organic pollutants," he adds, "including polychlorinated biphenyls and organochlorine pesticides."

The researchers also found that concentrations of all four cVMS were not geographically uniform. They measured high concentrations of D3 and D4 on the West Coast of North America. Harner suspects these chemicals potentially come from industrial Asian sources such as China and travel across the Pacific in air streams. The team found high levels of D5 and D6 in urban areas in the eastern U.S. and Europe, which are "most likely due to personal care product use," he says.

Whether the presence of these chemicals causes problems in the environment is hotly debated. Studies have reached opposing conclusions about potential health effects for aquatic wildlife such as fish. An industry association, the Silicones Environmental, Health and Safety Council of North America, argues in an online position paper that "siloxanes do not pose a risk to human health or the environment."

The study's finding of the extensive presence of cVMS in the air around the world was no surprise, comments Nicholas Warner, an environmental scientist at the Norwegian Institute for Air Research's Polar Environmental Centre, in Tromsø, because computer modeling of global concentrations had predicted it. However, he appreciates the agreement between data and models because it raises confidence in recently developed models of how these emerging pollutants move around the world. The models required new algorithms to accommodate unusual properties of vCMS: their rapid evaporation and extreme hydrophobicity.

Although 90% of all cVMS emissions end up in the air, hydroxyl radicals quickly break down the molecules to silanols, Warner says. Compared to vCMS, the silanol break-down products "are less volatile and therefore more likely to be deposited to surfaces from air," Harner says. His team argues for research into possible environmental impacts of silanols.

The remaining 10% of cVMS emissions end up in water systems, likely from personal care products flushed down the drain. Most of the concerns about the environmental impacts of cVMS stem from the water-based cVMS rather than the air-based cVMS, Warner explains. A recent study reported cVMS levels in watersheds.

Measuring cVMS concentrations without contaminating samples is challenging because the molecules exist in in silicon-based lab equipment as well as in a potpourri of personal care products used by researchers handling study samples. Some researchers go to great lengths to avoid sample contamination, such as eschewing all personal care products while in the field. Warner notes that Harner's team adsorbed cVMS on polyurethane foam disks, which are known to contain cVMS. The team did preclean the equipment to remove possible contaminating cVMS, but Warner would like to see proof that there was no sample contamination. If the Environment Canada team's precleaning method successfully removes all cVMS, Warner says, it would be a great tool for further field work.

Chemical & Engineering News
ISSN 0009-2347
Copyright © 2011 American Chemical Society
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