How Trees Save at Least One Life Each Year in Major Cities

In urbanity, trees mop up aerosolized baddies like dust and smog.

Trees Save Lives By Mopping Up Particulates

Central Park's trees, it turns out, are literal life savers. (Photo: citypass.com)

A staff writer for LiveScience, Doug has written for the NYTimes.com. He lives in New York City.

What's green, tall and saves many lives each year? If you guessed a tree, you guessed right. 

Trees act to remove tiny particles from the air that would otherwise end up in people's lungs and cause respiratory and heart problems that can shorten lives and ultimately be fatal, new research shows.

The study, to be published in the July issue of the journal Environmental Pollution, found that on average the trees save one human life in each of the 10 cities surveyed by causing a decrease in particulate matter. Trees remove as much as 64.5 tons of these tiny particles in Atlanta, and as little as five tons in Syracuse, the study found.

To arrive at these figures, researchers looked at simulations of weather patterns and hourly concentrations of particulate matter and matched those to locations of trees and urban forests. They then came up with an estimate of what volume of dust the trees take out, said study co-author David Nowak, a researcher and project leader with the U.S. Forest Service in Syracuse, New York.

The study looked at the PM2.5, the class of particulates that are smaller than 2.5 microns in diameter. These are so small that once suspended in the atmosphere, they are difficult to get out, as they tend to hover about on winds and currents. They are much more damaging than larger particulates because they can make their way deep into the lungs, Nowak said.

Trees remove these particles by trapping them on their surface, and to a much smaller extent, absorbing them into their pores. Trees with hairy or sticky leaves remove even more than most. "It's like how you'd catch more with a fur jacket than under armor," Nowak said. Although other objects and even buildings can remove particles by trapping particles, they are not as effective as trees, whose leaves create layers with ample surface area.

The study used values from the Environmental Protection Agency to quantify the health benefit of the trees' dust-removal activities. Trees provide $60.1 million in savings in New York City, nearly $29 million in Chicago and $23.7 million in Los Angeles.

Perhaps surprisingly, the trees only account for a relatively small improvement in air quality, at least when it comes to these tiny particulates. In Atlanta, for instance, they remove 0.24 percent of the fine particulates from the air. But even small improvements in this category add up to real differences, Nowak said.

Particulates are emitted upon combustion in factories and in automobiles; they can also be created by forest fires. Roadways often have high levels of particulate matter. After leaves trap these particles, they are usually removed by rainwater, which transports the particles to the ground. In some cases this could be a cause for concern, depending upon what exactly the particulates consist of.

Coal power plants, for example, have been known to emit particulates that contain heavy metals. Trees may then not solve the pollution problem, but just transfer the pollution to the ground. "That's why we'd like to keep it out of the air in the first place," Nowak said.

The study should help future city planners select the best places to put trees to maximize their public health benefits. Besides particulates, trees also absorb noxious gases like ozone and nitrous oxide.

"It's better to have trees breathing this stuff in than people," Nowak said.

Nowak is currently working on a paper looking at the total health benefits of trees on a nationwide level, benefits which he says are substantial.

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