Trees breathe by taking in carbon dioxide and releasing oxygen. As they help keep our air well supplied with oxygen, they also filter it, removing pollution as they respirate.

By removing toxins and particulates from the air, trees are saving lives, scientists with the U.S. Forest Service say. According to a new study by USFS ecologists and collaborators, the pollution-removing qualities of trees save approximately 850 human lives a year, prevent roughly 670,000 cases of acute respiratory symptoms and save some $7 billion. The study -- the first of its kind -- was published last week in the journal Environmental Pollution.

"With more than 80 percent of Americans living in urban areas, this research underscores how truly essential urban forests are to people across the nation," explained Michael T. Rains, Director of the Forest Service's Northern Research Station and the Forest Products Laboratory.

"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," Rains added.

The study measured trees' ability to filter four main pollutants, which are most influential in dictating the EPA's air quality standards: nitrogen dioxide, ozone, sulfur dioxide, and particulate matter less than 2.5 microns (PM2.5) in aerodynamic diameter.

Of course, the pollution fighting capabilities are trees are just one of trees many tangible benefits. Previous studies have shown the ability of trees and forests to moderate temperatures, control erosion, filter water, and even promote more peaceful neighborhoods in inner cities.