Air pollution removal by trees was found to be substantially higher in rural than urban areas, although the effects on human health were found to be substantially greater in urban areas.
Trees are known to affect the quality of the air by directly removing air pollutants. This process happens when trees intercept pollution particles and the harmful gases diffuse within their leaves, where they are absorbed by films of water to form acids.
Previous research has estimated that urban trees in the US remove about 711,000 tons of pollution a year. However, trees exist across the nation in varying densities. Although overall tree cover in the US is estimated at 34.2%, North Dakota has just 2.6% tree cover, while New Hampshire has 88.9% - so air pollution removal and impact on human health may vary from region to region.
The researchers behind the new study set out to assess the amount of air pollution permanently removed by trees.
Within the study, they assessed four pollutants for which the Environmental Protection Agency has established air quality standards: nitrogen dioxide, ozone, sulfur dioxide and particulate matter less than 2.5 microns (PM2.5) in aerodynamic diameter.
In the US, PM2.5 was linked to an estimated 130,000 deaths in 2005, while 4,700 deaths in the same year were related to ozone.
Using computer simulations to make their estimates, the researchers took into account four main variables:
- The total tree cover and leaf area index on a daily basis
- The hourly flux of pollutants to and from the leaves
- The effects of hourly pollution removal on pollutant concentration in the atmosphere
- The health impacts and monetary value of the change in pollutants.
Trees 'remove less than 1% of air pollution'
The researchers found that the air pollution removal offered be trees equates to an average air quality improvement of less than 1%, but even that small-sounding improvement has substantial impact.
The authors estimate that 850 human lives are saved each year as part of this air quality improvement, and that 670,000 cases of acute respiratory symptoms are prevented. They were also able to attach a monetary value to the improvement in human health from the reduced air pollution, which works out at nearly $7 billion each year.
The removal of pollution was found to be substantially higher in rural than urban areas, although the effects on human health from pollution removal were found to be substantially greater in urban areas.
Commenting on the results, Michael T. Rains, director of the Forest Service's Northern Research Station and the Forest Products Laboratory, says:
"With more than 80% of Americans living in urban areas, 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."
In 2013, Medical News Today reported on a study from researchers at the University of North Carolina at Chapel Hill that found trees may actually contribute to air pollution. The authors behind that study describe how a substance given off by trees, called isoprene, produces harmful airborne particles when it combines with man-made nitrogen oxides in the air.