Air pollution is a serious problem in the United States. As a young child growing up in suburban Los Angeles, I remember days in which we were not allowed to play outside because of the air quality. Kids in other states had snow days, or so I was led to believe, but we had smog days.

LA is doing better now. We don’t have smog days anymore. But air pollution still causes quite a bit of problems, both for public health as well as for the cost of health care. It’s been implicated in diseases like bronchitis, asthma, and other respiratory conditions. It impacts the cardiac, vascular, and even neurological systems. It leads to emergency room visits, hospital admissions, and sometimes mortality. Kids miss school, adults miss work. Thankfully, we have trees. It is not an exaggeration to say that trees save both lives and money. That’s because they scrub the air of pollution.

The Clean Air Act required the U.S. Environmental Protection Agency to set air quality standards for six “criteria pollutants” that are both common throughout the country and detrimental to human health and welfare: carbon monoxide, nitrogen dioxide, ozone, lead, sulfur dioxide, and particular matter, which includes tiny little bits of stuff less than 2.5 microns in diameter. In 2005, particulate matter was implicated in some 130,000 deaths, and 4700 were related to ozone.

But the truth is, those numbers could have been much worse if not for the trees. That’s according to a new study by U.S. Forest Service researcher David J. Nowak and colleagues, published this week in the journal Environmental Pollution.

Trees remove air pollution in two ways. Some pollutants get stuck to the plant surface itself. That’s just a temporary removal, as wind can shake the pollutants back into the air, and rain can wash it down into the soil. The primary way in which trees remove pollution is by sucking up the gases through tiny openings on leaves called stomata, which literally means “mouths.” That’s especially true for ozone, sulfur dioxide, and nitrogen dioxide.

In the current study, Nowak’s team set out to determine what the health impacts and economic benefits are of air pollution removal by trees in the continental United States. Using public records available for 2010, the researchers determined that trees and forests removed more than 17 million metric tons of pollution from the air that year. In terms of saved health care expenses, that is associated with a value of $6.8 billion dollars.

More than 96% of pollution removal due to trees occurred on rural land, simply because most of the United States is classified as rural. However, the health effects were more heavily weighted in urban areas – more than 68% – simply because more people live in cities. Therefore, Novak concludes, “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,” like Manhattan.

It’s worth pointing out that the reason that cities are weighted so heavily is that Novak’s analysis reflected the EPA’s “primary quality standards,” which are aimed at human health. If he also considered the EPA’s secondary standards, which describe protections for animals, crops, vegetation, and buildings, then the relative value of rural trees would certainly increase.

Taken together, the pattern is very straightforward: “the greater the tree cover, the greater the pollution removal; and the greater the removal and population density, the greater the value.” Of course, trees also benefit human welfare in other ways. They reduce air temperatures, for example, which can in turn reduce the need for air conditioning.

And here’s the kicker: the 17 million tons of pollution removed by trees, and the associated $6.8 billion dollars saved, represents less than one percent of the pollution swirling around our air. Imagine how much healthier and richer we could all be if we simply planted more trees in our cities. – Jason G. Goldman | 01 August 2014

Source: Nowak D.J., et al. 2014. Tree and forest effects on air quality and human health in the United States, Environmental Pollution, 193 119-129. DOI: http://dx.doi.org/10.1016/j.envpol.2014.05.028

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