While trees' pollution removal equated to an average air quality improvement of less than 1 per cent, the impacts of that improvement are substantial.

Researchers valued the human health effects of the reduced air pollution at nearly USD 7 billion every year.

In the first broad-scale estimate of air pollution removal by trees nationwide, US Forest Service scientists and collaborators calculated that trees are saving more than 850 human lives a year and preventing 670,000 incidences of acute respiratory symptoms.

The study by Dave Nowak and Eric Greenfield of the US Forest Service's Northern Research Station and Satoshi Hirabayashi and Allison Bodine of the Davey Institute is unique in that it directly links the removal of air pollution with improved human health effects and associated health values.

The scientists found that pollution removal is substantially higher in rural areas than urban areas, however the effects on human health are substantially greater in urban areas than rural areas.

"With more than 80 per cent 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.

The study considered four pollutants for which the US Environmental Protection Agency (EPA) has established air quality standards: nitrogen dioxide, ozone, sulphur dioxide, and particulate matter less than 2.5 microns (PM2.5) in aerodynamic diameter.

Health effects related to air pollution include impacts on pulmonary, cardiac, vascular, and neurological systems. In the US, approximately 130,000 PM2.5-related deaths and 4,700 ozone-related deaths in 2005 were attributed to air pollution.

"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.

"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," Nowak said.

The study was published in the journal Environmental Pollution.