Canada's Condominium Magazine

City trees clean the air and save lives: study

Everyone “knows” in a general sort of way that trees and other plants are good for the environment, especially dense urban environments like Toronto. Plants absorb carbon dioxide and give back oxygen, which we breathe. It’s absolutely fundamental: we couldn’t live without them. But trees do more than just pump out oxygen, and now, for the first time, a study has shown in detail how city trees actually save lives by helping to control pollution.
New study places monetary and human health value on pollutant-removing function of urban trees. Toronto faces losing thousands of ash trees to the emerald ash borer. Loss of ash trees has been linked to increased human deaths by US researchers.

The US Forest Service studied the impact urban trees have on very fine particulate air pollution—the tiny, invisible particles of dust, mold spores, burnt fossil fuel residues and other things that float around us and can cause respiratory illnesses like asthma, bronchitis, and even heart attacks. That invisible mix of burnt coal dust and various oxides can, and does, kill us. Even slight increases in concentrations of the stuff add to the health risk to humans by 6 or 8 per cent. As the study says, urban particulate air pollution is a serious health issue.

What they found is that city trees remove those fine particles from the atmosphere, and thus improve the air quality. They accomplish this in a number of ways: affecting air temperature and energy use (shade), affecting wind speed, emitting their own organic compounds, trapping the particulates in their leaves. As you would expect and have probably noticed, the air is cleaner in a forest or in a heavily treed area of a city than it is outside the treed area. That is because there are lower concentrations of particulate matter floating in the air.

Trees can make cities healthier. While we need more research to generate better estimates, this study suggests that trees are an effective tool in reducing air pollution and creating healthier urban environments.

Researcher David Nowak

And the study was able to put numbers and values on the trees’ air-cleansing function. A smaller city like Syracuse, New York, for example, had 4.7 tonnes of particulates removed by trees, with an annual cost benefit of $1.1 million. In New York City, on the other hand, trees saved the city $60.1 million, mostly in reduced health care costs associated with pollution. A hectare of tree cover was found to be worth, on average, about $1,600 in health benefits, but much higher in New York City, where it was estimated at $3,800. The health benefits value of removing pollutants from the air, for the cities studied, averaged a total of $1.2 billion.

Even more dramatically, they found that the removal of particulate pollution from the air resulted in a typical “reduction in human mortality” of 1 person per year; it was as high as 7.6 people per year in New York City. In other words, trees save lives.

This is especially pertinent to Toronto at this time. The city is facing the loss of tens of thousands of its ash trees because of an infestation of the emerald ash borer. US researchers found a correlation between the loss of ash trees and increases in human deaths from cardio-vascular and respiratory illness. The results are startling: more than 20,000 deaths were associated with emerald ash borer penetration in the study area.

The lead researcher on that study, which was published in the American Journal of Preventive Medicine, said on PBS News Hour that maybe it was time to start thinking of trees “as part of our public health infrastructure.”


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