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Outdoor air pollution is a serious environmental health risk linked to both chronic and acute health conditions, including stroke, heart disease, lung cancer, asthma, chronic obstructive pulmonary disease, and respiratory infections.

According to the World Health Organization (WHO), ambient (outdoor) air pollution in both cities and rural areas caused an estimated 3.7 million premature deaths worldwide in 2012, the majority of which were due to heart disease and strokes.1

As WHO noted:2

“Most sources of outdoor air pollution are well beyond the control of individuals and demand action by cities, as well as national and international policymakers in sector like transport, energy waste management, buildings and agriculture.”

This is largely true, but there is one environmental change that could have a dramatic influence on air pollution, and its effects on human health, independent of these other factors, and that is planting more trees (especially in urban areas).

A new study actually quantified the benefits to human health from trees’ impacts on outdoor air pollution, and they were quite remarkable.

Trees Save Close to 1,000 Lives, and Billions in Health Costs, Each Year

In the first broad-scale estimate of air pollution removal by US trees nationwide, researchers found that trees and forests in the US removed 17.4 million tons of air pollution in 2010, with human health effects valued at $6.8 billion.3

Although this pollution removal equated to an average air quality improvement of less than 1 percent, its effects on human health were significant, especially in urban areas. The health impacts included the
avoidance of more than:

• 850 deaths     
• 670,000 cases of acute respiratory symptoms     
• 430,000 incidences of asthma exacerbation     
• 200,000 school days lost

As you might suspect, most of the pollution removal occurred in rural areas (where tree cover can be as high as 88 percent) but most of the health impacts were within urban areas (where air pollution tends to be worse and population levels are higher).

Previous research has also shown that pollution removal by trees impacts human health. One study found that a 10-by-10 kilometer space (approximately 6-by-6 miles) with 25 percent tree cover in London could remove more than 90 tons of particulate matter annually, which would lead to the avoidance of two deaths and two hospital admissions per year.4

Trees remove air pollution primarily by uptake of pollutants via leaf stomata (pores on the outer “skin” layers of the leaf). Some gaseous pollutants are also removed via the plant surface.

Once inside the leaf, the gases “diffuse into intercellular spaces and may be absorbed by water films to form acids or react with inner-leaf surfaces.” According to the researchers:

“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  [air pollution] formation.”

Adding trees to urban areas was deemed to be particularly important, given the trees’ close proximity to people. The researchers added:

” 96.3 percent of pollution removal from trees occurred on rural land. However, as human populations are concentrated in urban areas, the health effects and values derived from pollution removal are concentrated in urban areas with 68.1 percent of the $6.8 billion value occurring with urban lands.

Thus in terms of impacts on human health, trees in urban areas are substantially more important than rural trees due to their proximity to people. The greatest monetary values are derived in areas with the greatest population density (e.g., Manhattan).”