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  Tree leaves monitor pollution levels
Urban trees act as sentinels using their leaves to measure microscopic particles of pollution, say UK researchers.

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

Particulate air pollution contains a mix of chemicals leftover when power plants or car engines burn fossil fuels. Noxious compounds like volatile organic compounds, polycyclic aromatic hydrocarbons and toxic heavy metals combine to form fine dust that can inflame the lungs and enter 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.
Pollution map

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 analysed 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 1650 trees, but we plan to," she says. "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.

In the developed world, particulate air pollution 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.
Cheaper alternative

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

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

But Jerrett is 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% of particulates out of the air.

Maher believes 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.
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