No one, or at least no one reasonable, has ever been of the opinion that trees are a bad thing. Sure, maybe if you’re a land developer, or your vehicle’s on the losing end of a battle with a fallen limb, you might have an axe to grind with some particular trees. Otherwise, good things. Thanks to a study released by the U.S. Forest Service, we now know just how beneficial trees are: They save lives, to the effect of 850 every year.

No, trees aren’t fighting crime in the night or delivering babies – their lifesaving capabilities are thanks in large part to their pollution-filtering characteristics.

“Computer simulations with local environmental data reveal that trees and forests in the conterminous United States removed 17.4 million tonnes (t) of air pollution in 2010 (range: 9.0 e 23.2 million t), with human health effects valued at 6.8 billion U.S. dollars,” says the study.

In terms of human impact, that equates to the prevention of 670,000 incidences of acute respiratory symptoms.

What might be surprising is that the relatively sparse trees in urban areas have far more impact than their far more plentiful brethren. That’s not just because cities happen to be where much of the pollution is, but also because that’s where the people are.

“In terms of impacts on human health, trees in urban areas are substantially more important than rural trees due to their proximity to people,” researcher Dave 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.”

According to the press release, the study considered four pollutants for which the U.S. EPA has established air quality standards: nitrogen dioxide, ozone, sulfur dioxide, and particulate matter less than 2.5 microns (PM2.5) in aerodynamic diameter. Health effects related to air pollution include impacts on pulmonary, cardiac, vascular, and neurological systems.