Lungs of the City: Urban Trees Help Filter Out Air Pollution

Dear EarthTalk: Do urban trees really help reduce pollution and clean the air?

—Peter Grainger, Queens, NY

trees as lungs
Urban trees play a vital role in taming air pollution and helping counteract the urban heat island effect caused by so much concrete in the hot summer sun. Credit: Reynaldo #brigworkz Brigantty from Pexels.

Back in 1872 Frederick Law Olmsted, the granddaddy of American landscape architecture and the designer of New York’s Central Park, proclaimed that trees were the “lungs of the city.” While Olmsted’s statement may have been more philosophical than scientific, researchers have since found that city trees do indeed perform important environmental functions like soaking up ground-level pollutants and storing carbon dioxide, which helps offset global warming.

Each year in Chicago, for example, the windy city’s urban tree canopy removes 15 metric tons of carbon monoxide, 84 metric tons of sulfur dioxide, 89 metric tons of nitrogen dioxide, 191 metric tons of ozone and 212 metric tons of particulates, according to David Nowak, project leader of the U.S. Forest Service’s Urban Forest Ecosystem Research Unit. Trees absorb these gaseous pollutants via the tiny pores in their leaves and break them down into less-harmful molecules during photosynthesis.

In Sacramento, California, a public-private partnership called Sacramento Shade spearheaded the planting of more than 200,000 trees around the city in the mid-1990s. In a study assessing Sacramento’s bolstered tree cover, Greg McPherson of the Western Center for Urban Forest Research found that the region’s urban forest removes more than 200,000 metric tons of carbon dioxide from the atmosphere each year, saving taxpayers as much as $3 million annually in pollution cleanup costs.

Meanwhile, the tree cover in New York City helps remove enough airborne toxins to save taxpayers as much as $10 million a year in pollution mitigation costs, according to Nowak. The Big Apple’s five boroughs are home to more than five million trees, covering nearly 17 percent of its public and private land, he adds.

Gary Moll, a vice president at the nonprofit group American Forests, asserts that trees are the “ultimate urban multi-taskers,” absorbing carbon dioxide and acting as filters, sponges, humidifiers, heat shields and wind blockers. Under Moll’s supervision, American Forests is assessing the costs and benefits of city tree cover across the country. The group uses a combination of satellite data, field surveys and computer modeling technology to measure regional tree canopy and calculate its dollar value.

All told, Olmsted was right in his assessment of the importance of city trees. Indeed, planting trees in urban environments may be one of the best medicines available to help restore our ailing cities.