Trees are good for your health. By absorbing pollution, they clean the air and help people avoid sickness. In 2010, trees in the U.S. saved 850 lives, and prevented 670,000 cases of severe respiratory problems, according to one recent study.
Understanding the exact role of trees is still at an early stage, however. While researchers like David Nowak at the U.S. Forest Service have mapped the benefits of trees at a national level, and developed tools to help planners price the benefits of trees, we still don’t have models to understand the relationship of trees to health at a block-by-block level. We don’t know what happens, say, if we plant 10 trees in a street, when there are other pollution sources and sinks nearby.
But researchers are trying to get to that kind of granularity. A group at Portland State University recently mapped Portland’s nitrogen dioxide levels using 144 sensors placed at strategic points around the city. It then looked at possible causes of that pollution (including traffic, rail and industrial sites) and possible sources of remediation (like trees). From that, the group was able to develop a “land use regression” model–a way of understanding relationships between pollution and its causes.
Methodological detail aside, what’s interesting is that the study showed that trees may have even more benefits than shown in previous studies.
Based on its model, the P.S.U. research, published in the journal Environmental Pollution, says that every 10 hectares of trees within 400 meters of a site reduces 0.57 ppb of NO2 in the summer months. The research then estimates that Portland’s trees reduce “asthma exacerbation” incidences for 4 to 12 year-olds by 21,000 cases a year, and that the elderly require 46 fewer hospital visits overall. In all, trees lead to healthcare savings of about $7 million.
More important than these numbers, the study showed there’s an almost 10-fold difference in pollution between the city’s best and worst areas. That means talking about Portland’s situation as a whole–or any other city’s situation–isn’t that useful. “There is a tremendous variability within the city. One could argue that intra-city variability is as great as country variability,” says Vivek Shandas, one of the researchers.
At the moment, officials base their responses to pollution on “airshed models” covering tens of kilometers. Shandas and his colleagues hope to shift cities to a more intimate scale that reflects circumstances on the ground, including the role played by trees in particular places. That, in turn, should benefit lower-income neighborhoods that historically have faced the worst pollution and have had had the least means of dealing with it (including lots of trees).
Using a National Science Foundation grant, the team now plans to translate the Portland model to 13 other U.S. cities. “We want to help public health decision-makers and urban foresters to see where you might get the biggest health benefit from expanding tree canopies in particular areas of a city,” Shandas says.