Researchers have attempted to estimate the direct impact of trees on air pollution and human health. What they found was that trees improved overall air quality in th United States by only one percent. However, that one percent improvement prevents 850 premature deaths, 670,000 incidences of acute respiratory symptoms and saves $7 billion in health care spending.

The study looked at four pollutants monitored by the Environmental Protection Agency (EPA): Nitrogen dioxide, ozone, sulfur dioxide, and particulate matter. Health effects related to these pollutants included pulmonary, cardiac, vascular and neurological problems. In 2005, 130,000 deaths were attributed to particulate matter less than 2.5 microns (PM2.5) in aerodynamic diameter and 4,700 deaths were attributed to ozone air pollution.

Unfortunately, the benefits of the pollution removal were not distributed evenly. Overall, the U.S. tree cover is estimated at 34.2 percent. However, in North Dakota the cover is only 2.6 percent while in New Hampshire the estimate is 88.9 percent. There is also a sharp divide between urban and rural areas.

“In terms of impacts on human health, trees in urban areas are substantially more important than rural trees due to their proximity to people. 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,” said Dave Nowak of the U.S. Forest Service’s Northern Research Station in a statement.

This study is just the latest to stress the importance of urban forestry. However, it is the first in the U.S. to quantify the potential savings. In 2014, most urban centers have urban forestry plans and the study, which is available in full at the Forest Service’s website, could help city planners make the case for expanding park land and other tree planting efforts.

“With more than 80 percent of Americans living in urban area, this research underscores how truly essential urban forests are to people across the nation. Information and tools developed by Forest Service research are contributing to communities valuing and managing the 138 million acres of trees and forests that grace the nation’s cities, towns and communities,” said Michael T. Rains, Director of the Forest Service’s Northern Research Station and the Forest Products Laboratory.