Urban Trees Help Us Breathe

Rebecca Nisley is a science writer and editor for the USDA Forest Service’s Northern Research Station.

We know that trees provide ecosystem benefits even in cities. The oxygen, cooling shade, and carbon storage are commonly understood, but did you know that trees also remove air pollution?

Trees do this by catching small particles of soot and dirt on their surfaces and also by absorbing gaseous pollutants (such as ozone, O3) through the leaf pores.  

With more than half the United States and world’s populations now residing in cities, urban forests and their ecosystem benefits are more valuable than ever. The many city and national planning and action groups working to maintain, improve, and enlarge urban forests need accurate information, facts, and numbers to justify and improve their programs. 

In the first broad-scale estimate of air pollution removal by trees nationwide, U.S. Forest Service Northern Research Station scientists David Nowak and Eric Greenfield, and their collaborators Satoshi Hirabayashi and Allison Bodine of the Davey Institute, calculated that trees are saving more than 850 human lives a year and preventing 670,000 incidences of acute respiratory symptoms. The study is the first to directly link the removal of air pollution with improved human health effects and associated health values.

Air pollution has direct effects on human health. Small particles, ozone, and other pollutants worsen chronic respiratory diseases such as asthma, bronchitis, emphysema, and chronic pulmonary obstructive disease (COPD) and can bring on acute cardiac and pulmonary incidents, possibly leading to premature death. These problems affect about 1 in 7 Americans middle-aged or older according to the National Center for Health Statistics at the Centers for Disease Control.

The scientists found that although pollution removal is substantially higher in rural than in urban areas because of the greater number of rural trees, the effects on human health and monetary savings are substantially greater in urban than in rural areas.

The study considered four pollutants for which the U.S. EPA has established air quality standards: nitrogen dioxide (NO2), ozone (O3), sulfur dioxide (SO2), and particulate matter less than 2.5 microns (PM2.5) in aerodynamic diameter.

In the United States, about 130,000 PM2.5-related deaths and 4,700 ozone-related deaths in 2005 were attributed to air pollution. Using computer simulations—the Forest Service’s i-Tree and the Environmental Protection Agency’s BenMAP—with local environmental data, they found that trees and forests in the conterminous United States removed an average of 17.4 million tonnes of air pollution in 2010, with human health effects valued at an average of 6.8 billion U.S. dollars.

Although pollution removal by trees brought about an average air quality improvement of less than 1 percent, these health effects demonstrate that planting urban trees not only beautifies cities but also helps make them healthier places to live in and helps us breathe a little easier. 

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