Study looks at relationship between trees and nitrogen dioxide

The people of Portland have to thank trees for more than their flashy colors this fall. According to a recent research project at Portland State, trees may play a part in reducing nitrogen dioxide, also known as NO2, an air pollutant that leads to certain health conditions.

“In [our] research, we noticed there was a relationship between the reduction of nitrogen dioxide, which is a pollutant that is associated with high temperature exhaust, like car exhaust or truck exhaust, and where there was vegetation,” explained Dr. Linda George, a professor of environmental sciences at PSU, who is trained as an atmospheric chemist. “So wherever there seemed to be trees, there seemed to be less NO2.”

Participants in the project placed sensors at various sites across Portland to study the relationship between the presence of trees and their target air pollutant.

“This last study that we did, we made a lot more measurements than anyone else has done in other cities in the country, and we think we were able to pick out the factor of trees,” George said.

Meenakshi Rao, a graduate research assistant at the School of the Environment at PSU, explained the process.
“We measured at 144 sites and built a model for that, then we applied the model to all of Portland. Once we had this model of air pollution all over Portland, we could start looking for things like, ‘is there a correlation of where there’s more trees, there’s less nitrogen dioxide?’” Rao said.

George said that the ultimate goal of this project is to relate the presence of trees to the health of the population.

“I’m more interested in human health, and what are the pollutants that we emit,” she said, “because what they do when they end up in the environment can affect people.”

Although there is more research needed before any conclusions can be drawn, the research team has discovered a correlation between population health and the presence of trees.

“It’s still a strong assumption that it is trees and not something else that has that effect,” Rao said.

Dr. Vivek Shandas, an associate professor in urban studies and planning at PSU, is helping the research team examine what their data means for the city of Portland.

“What we’re trying to do in the project is draw a real clear link between trees and health,” Shandas said. “We’re trying [to] work with the Oregon Health Authority to look at the relationship between actual health, not just modeled health, in relation to trees.”

“We did think there would be an effect, but we didn’t know that it would be big enough to make a difference in health. So that was very exciting, that it is actually big enough,” Rao said.

One of the next steps of this project include comparing data of the Portland air to specific health conditions.

“We want to know if there’s differing rates of cancer in relation to trees. We want to see differing emergency room visits based on the number of trees in a neighborhood, we want to see asthma rates,” Shandas said.

In addition, the team hopes to make use of their findings by developing a website or app for the public.

“We’re trying to create some tools. This is funded by the U.S. Forest Service, and part of the project is to help communities identify places where they can plant trees,” Shandas said. “So we’re creating an online system that allows a neighborhood association, a group of students, as well as anybody who’s interested in the relationship between trees and health to know what are the places they can improve quality of health in the easiest way possible. And we’re not just doing it for Portland, we’re doing it for 12 other cities in the country.”

“I definitely think more every time I turn on my car engine, because in the Portland area, the biggest source of NO2 is cars,” Rao said. “I also feel it makes me appreciate Portland even more. Because even though my research shows we still have a health impact of NO2, the air in Portland is very clean compared to a lot of areas.”

Click on the following title for the official study in the academic journal, Environmental Pollution is available online: “Assessing the relationship among urban trees, nitrogen dioxide, and respiratory health.”