Trees remove air pollution by the interception of particulate matter on plant surfaces and the absorption of gaseous pollutants through the leaf stomata.  However, the magnitude and value of the effects of trees and forests on air quality and human health across the United States remains unknown.  Computer simulations with local environmental data reveal that trees and forests in the conterminous United States removed 17.4 million tonnes (t) of air pollution in 2010 (range: 9.0-23.2 million t), with human health effects valued at 6.8 billion U.S. dollars (range: $1.5-13.0 billion). This pollution removal equated to an average air quality improvement of less than one percent.  Most of the pollution removal occurred in rural areas, while most of the health impacts and values were within urban areas.  Health impacts included the avoidance of more than 850 incidences of human mortality and 670,000 incidences of acute respiratory symptoms.

Greenfield, Eric (USDA Forest Service)

Environmental Pollution. 193: 119-129.