Of all the air pollutants, one of the most dangerous to human health is fine-particulate matter (PM_2.5)—solid or liquid particles less than 2.5 micrometers in diameter. The health risk for PM_2.5 is that these particles can be inhaled deep into the lungs, where they can be trapped and can impair lung function. The smallest of these particles, with diameters of less than 0.1 micrometer, can pass from the lungs into the bloodstream and can affect organs such as the heart and brain or cause cancer. The most common source of PM_2.5 in urban areas is fuel combustion by power plants, diesel-powered vehicles, and oil-burning furnaces; the average diameter of diesel exhaust particles is in the range of 0.05 to 0.08 micrometers. Two recent studies found that urban tree leaves can reduce PM_2.5 concentrations. See also: Air pollution; Combustion; Environmental toxicology; Particulates; Reduction of diesel engine particulate emissions

Researchers from Lancaster University in the United Kingdom placed a screen of 30 trees in planters in front of houses and then looked at the effect on the concentrations of fine-particulate air pollutants inside the homes. They reported that, compared to houses without the trees, the screened houses showed only half the indoor concentrations of particulate matter, ranging in size from 1 to 10 micrometers (PM₁, PM_2.5, and PM₁₀). Analysis of the trees' leaves showed that the surfaces of their leaves were trapping particles similar to those found inside the houses. See also: Air pollution, indoor; Air pollution monitoring site selection

In another study by the U.S. Forest Service, the State University of New York College of Environmental Science and Forestry, and the Davey Institute in Syracuse, New York, researchers estimated that trees removed a total of 223 tons per year of PM_2.5 from Atlanta, Baltimore, Boston, Chicago, Los Angeles, Minneapolis, New York City, Philadelphia, San Francisco, and Syracuse. They then used the estimates from each city to model the health benefits value of PM_2.5 removal, reported in U.S. dollars per year. The top three cities in terms of health benefits value were New York City ($60 million/year), Chicago ($26 million/year), and Los Angeles ($24 million/year). Although further work needs to be done, urban forests appear to be important for maintaining human health. See also: Environmental engineering; Forestry, urban