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updated 5/18/2009 12:45:47 PM ET 2009-05-18T16:45:47

Silently lining the roads, shading lush parks, and softening the hard angles of an urban landscape, trees seem passive garnish to a bustling, humming human society. But they are also sentinels, their leaves taking careful measurements of the microscopic particles of pollution that humans generate.

They are so precise in this task that Barbara Maher and a group of scientists at the University of Lancaster in the United Kingdom are turning to them as reliable, street-level pollution monitors.

Particulate air pollution is an insidious, deadly cocktail of chemicals leftover when power plants or car engines burn fossil fuels. Noxious compounds like volatile organic compounds (VOCs), polycyclic aromatic hyrdrocarbons (PAHs), and toxic heavy metals combine to form fine dust that inflames the lungs and invades the bloodstream, liver, even the brain when inhaled.

Maher's team measured pollution based on magnetic signatures from tiny bits of iron in the particles.

They found that lime trees scattered throughout Lancaster gradually collected particles over a period of seven to 10 days, until they accurately reflected ambient levels of pollution. Leaf samples were analyzed from 30 trees as a pilot study, but Maher sees every roadside tree in the city of 133,000 people as a pollution monitor.

"We haven't measured all of the 1,650 trees, but we plan to," she said. "We're going to generate a map of particulate pollution for the whole of Lancaster."

Maher will present the team's finding later this month at the American Geophysical Union's joint assembly in Toronto.

Monitoring particulate pollution on small scales is crucial, because concentrations can fluctuate by a factor of 10 or more over a few city blocks. People living near a traffic light or busy intersection may therefore have much higher exposures to the toxins than those who live on a quiet street.

Across the United States and Europe, particulate air pollution stealthily claims hundreds of thousands of lives each year. Reactions to the particles can impact every stage of life, from birth defects and impaired lung development in children, to asthma, heart attack and stroke in adults.

The problem is even worse in the developing world, where lax emissions laws are common and people rely heavily on wood fires for heat and cooking.

But traditional pollution monitoring stations are expensive to deploy and operate; Lancaster has just one, in the center of town.

"If trees prove useful as biomarkers for pollution, this could result in a major breakthrough for pollution sensing," Michael Jerrett of the University of California, Berkeley said.

Jerrett was cautious in his enthusiasm, pointing out that it's still early days for this science.

The trees they may protect people from pollution, too. In the study, trees with front row seats to traffic screened 15 to 20 percent of particulates out of the air.

Maher thinks that number can be increased by positioning plants to capture pollution in high-traffic areas, and by using evergreens, whose high number of needles and lack of seasonal foliage cycle make them perfect for the job.

© 2012 Discovery Channel

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