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For years people have known that trees produce the oxygen humans breathe in order to survive. Despite knowing how trees convert carbon dioxide into oxygen, researchers have yet to estimate just how many people trees affect. In the first ever broad-scale study that accounted for air pollution and tree removal, researchers with the U.S. Forest Service found that trees save more than 850 human lives and prevents 670,000 cases of acute respiratory symptoms annually.

For this study, the research team composed of Dave Nowak and Eric Greenfield with the service's Northern Research Station, and Satoshi Hirabayashi and Allison Bodine of the Davey Institute examined the effects of air pollution removal and people's health. They found that air pollution removal tends to be higher in rural areas than urban areas. However, the effects of pollution removal were greater in urban areas than rural ones.

The team reported that air pollution removal translated to an increase in the average air quality by less than one percent. Even though this rate is extremely small, the effects are more substantial. The team stated that air pollution removal could save lives and prevent respiratory diseases, which can then improve overall life quality.

"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," said 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."

The researchers had taken into account four specific pollutants, which were nitrogen dioxide, ozone, sulfur dioxide and particulate matter. The team looked for health improvements in the pulmonary, cardiac, vascular, and neurological systems.

"In terms of impacts on human health, trees in urban areas are substantially more important than rural trees due to their proximity to people," Nowak said in the press release. "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."

The study, "Tree and forest effects on air quality and human health in the United States," was published in the journal, Environmental Pollution.