Air Purification

Tree and forest effects on air quality and human health in the United States


  • “Trees remove gaseous air pollution primarily by uptake via leaf stomata, though some gases are removed by the plant surface. For O3, SO2 and NO2, most of the pollution is removed via leaf stomata. Once inside the leaf, gases diffuse into intercellular spaces and may be absorbed by water films to form acids or react with inner-leaf surfaces. Trees directly affect particulate matter in the atmosphere by intercepting particles, emitting particles (e.g., pollen) and resuspension of particles captured on the plant surface.”
  • “Pollution removal by urban trees in the United States has been estimated at 711,000 tonnes (t) per year.”
  • “The total amount of pollution removal in 2010 by trees and forests in the conterminous United States was 17.4 million t (range: 9.0 million t to 23.2 million t), with a human health value of $6.8 billion (range: $1.5 billion to $13.0 billion) (Table 2).”
  • “Most of these benefits were dominated by the effects of reducing human mortality, with a national reduction of more than 850 incidences of human mortality (range: 184e1634) (Table 4). Other substantial health benefits include the reduction of more than 670,000 incidences of acute respiratory symptoms (range: 221,000e1,035,000), 430,000 incidences of asthma exacerbation (range: 198,000e688,000) and 200,000 school loss days (range: 78,000e266,000).”
  • “Modeling broad-scale effects of pollution removal by trees on air pollution concentrations and human health reveals that while the percent reduction in pollution concentration averages less than one percent, trees remove substantial amounts of pollution and can produce substantial health benefits and monetary values across the nation, with most of the health values derived from urban trees.”

The Effects of Urban Trees on Air Quality

  • “In 1994, trees in New York City removed an estimated 1,821 metric tons of air pollution at an estimated value to society of $9.5 million. Air pollution removal by urban forests in New York was greater than in Atlanta (1,196 t; $6.5 million) and Baltimore (499 t; $2.7 million), but pollution removal per m2 of canopy cover was fairly similar among these cities (New York: 13.7 g/m2/yr; Baltimore: 12.2 g/m2/yr; Atlanta: 10.6 g/m2/yr).”
  • “Large healthy trees greater than 77 cm in diameter remove approximately 70 times more air pollution annually (1.4 kg/yr) than small healthy trees less than 8 cm in diameter (0.02 kg/yr).”