Trees save the lives of around 850 people each year, according to a new study from the U.S. Forest Service.

Air pollution affects human health both indirectly and indirectly, according to the agency. Effects of airborne pollution are felt in human health when directly inhaled, as well as through its effects on crops and the health of ecosystems. Contaminants in the air can also play a role in climate change, presenting additional dangers to human beings.

Concentrations of air pollution and density of tree populations vary around the United States. This creates a patchwork of miniature ecosystems around the United States, making climatic modeling challenging.

"Trees affect air quality through the direct removal of air pollutants, altering local microclimates and building energy use, and through the emission of volatile organic compounds (VOCs), which can contribute to [the formation of ozone and particulate matter]" Forestry Service officials wrote in an article announcing the findings.

Computer simulations using data from a myriad of these mini-ecosystems were run, modeling the effects of trees on airborne-contaminants. Researchers found that trees removed over 19 million tons of air pollutants from skies over the United States during 2010. The Forest Service estimated the removal of these pollutants from the air saved Americans around $6.8 billion during that year alone in health care costs.

Gases like ozone are dangerous pollutants at ground level, where they form into smog, along with hydrocarbons. This pollution, usually found in urban areas, can lead to respritory problems, especially in young people, the elderly, and those with lung conditions.

"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," Michael Rains, Director of the Forest Service's Northern Research Station and the Forest Products Laboratory, said.

Particulate matter, tiny flecks of material suspended in the air, can also play havoc with human lungs. Smaller pieces of this pollutant can lodge deeper in the organs than larger pieces, which are caught in natural filters. The smallest, and potentially hazardous, of these particles are classified as PM 2.5, meaning particulate matter under 2.5 microns, about 0.0001 inches across.  

" Trees directly affect particulate matter in the atmosphere byintercepting particles, emitting particles (e.g.., pollen) and resuspension of particles captured on the plant surface. Some particles can be absorbed into the tree, though most intercepted particles are retained on the plant surface. The intercepted particles often are resuspended to the atmosphere, washed off by rain, or dropped to the ground with leaf and twig fall," the report states.

Investigation of the role trees play in reducing air pollution and saving human lives was published in the journal Environmental Pollution

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