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Urban Trees May Be Counterproductive For Air Quality, Says UMass Researcher

New research suggests there are benefits to efforts to plant more trees in urban areas. But it also finds the jury is out on whether those trees help air quality. 

Theodore Eisenman is the lead author of two articles on the subject, one in Landscape and Urban Planning and another in The Lancet.

Eisenman is an assistant professor in the Department of Landscape Architecture and Regional Planning at UMass Amherst. He joined us to talk about how trees can affect air quality in cities one way or another.

Theodore Eisenman, UMass: On the one hand, urban trees can potentially reduce air pollution by having particulates and gases deposit on their leaves and branches.

On the other hand, trees can potentially reduce air quality through a range of mechanisms, one being emission of organic compounds that can lead to ozone formation.

Another is the restriction of air circulation in street canyons, which is a common condition in cities around the world. In this case, trees can actually concentrate pollutants in the places where people walk, bike and work by restricting air circulation in urban street canyons. And they can produce pollen. And a smaller but emerging body of research shows synergistic interactions between pollen and air pollution.

So really it's a complex range of mechanisms.

Adam Frenier, NEPR: How could the negative impacts affect the rates of asthma and other respiratory illnesses?

One of the things that our study does, I think, that is fairly unique is that we brought together both natural scientists and epidemiologists. When I started looking into the scientific literature on the benefits of urban trees, I was first struck by how numerous and complex the mechanisms are between urban trees and air quality.

But I was even more surprised by the substantial differences in how environmental scientists and public health researchers — also known as epidemiologists — address this issue.

Natural scientists, for example, tend to focus on the mechanisms whereby urban trees affect air pollution levels, whereas public health researchers start with actual observations of human illness and death, and then seek to understand the causes for those observed illnesses and death.

In the case of urban trees, most epidemiological research highlights asthma risks associated with pollen production rather than reduced asthma via decreased air pollution. And importantly, based on our review, there is currently little empirical evidence of asthma reduction, or other respiratory health benefits, owing to air pollution reduction by urban trees. And in some circumstances, urban trees even reduce the air quality and increase asthma.

A street in West Springfield, Massachusetts.
Credit Rusty Clark / Creative Commons / flickr.com/photos/rusty_clark
Creative Commons / flickr.com/photos/rusty_clark
A street in West Springfield, Massachusetts.

So should there be fewer trees planted in urban spaces?

Well, that is definitely not one of the conclusions of this article. We actually state very emphatically that our research does not in any way suggest that trees should not be planted in cities.

Trees are essential for parks and people-friendly streets, and urban flora provides numerous benefits to wildlife and human health and well-being that are unrelated to air quality.

Is there more research that needs to be done on this idea?

Absolutely. We really end our article — we have two articles that came out — by really reinforcing the need for more interdisciplinary research, and in particular, more empirical research by epidemiologists.

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