While trees are just standing around – swaying in the breeze and providing shade – they are also up to some seriously noble work. While most of us know that trees take in carbon dioxide and release oxygen, they are quietly providing another important service as well. Namely, they remove deleterious pollutants from the air, according to new research from the United States Forest Service.

The study, led by Dave Nowak and Eric Greenfield of the Forest Service's Northern Research Station along with collaborators from the Davey Institute, found that trees are actively diminishing air pollution. The researchers examined four pollutants in particular: nitrogen dioxide, ozone, sulfur dioxide and particulate matter less than 2.5 microns (PM2.5) in aerodynamic diameter. Health problems from these pollutants include harm to pulmonary, cardiac, vascular, and neurological systems. In the U.S., an estimated 130,000 deaths were related to PM2.5 and another 4,700 to ozone in 2005.

The scientists looked at how trees remove air pollution by the interception of particulate matter on plant surfaces and the absorption of pollutants through the leaf stomata; they concluded that trees in the U.S. are saving the lives of more than 850 Americans a year. And that’s not all – they are preventing 670,000 incidents of acute respiratory symptoms and saving us a whopping $7 billion a year in health costs by reducing respiratory illness.

And all that just by improving air quality by less than 1 percent.

Not surprisingly, the amount of trees in an area has an affect on the benefits. While the average tree cover in the country is estimated at 34.2 percent overall, it ranges by area from 2.6 percent in North Dakota to 88.9 percent in New Hampshire.

“In terms of impacts on human health, trees in urban areas are substantially more important than rural trees due to their proximity to people,” Nowak said. “We found that in general, the greater the tree cover, the greater the pollution removal, and the greater the removal and population density, the greater the value of human health benefits.”

The study was published in the journal Environmental Pollution.

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