Trees do more than just clear the air and provide shade from the hot summer sun. Though no panacea, they can make cities like Baltimore healthier, a recent study suggests.
Using computer modeling to quantify the health benefits of trees in 10 cities, including Baltimore, researchers with the U.S. Forest Service and a private think tank say leafy foliage in urban areas can scrub enough soot out of the air to reduce asthma attacks, emergency room visits and even deaths.
"It's the first time we've actually been able to tie it to human health, which is pretty exciting," said David J. Nowak, a federal research forester and lead author of the report. Collaborating on the study was the Davey Institute, the research arm of an Ohio-based tree care company.
Scientists have long scrutinized how trees affect smog, or ozone pollution. This study, published online in the journal Environmental Pollution, focused on another pollutant — fine particles, or soot — and estimated the benefits that trees provide in breathing problems avoided and lives saved.
In Baltimore, researchers figured, trees remove roughly 14 tons of pollution annually. As a result, they concluded, there's one less premature death, nearly 140 fewer asthma attacks and 240 cases of labored breathing avoided in Baltimore each year.
Urban air is swimming in fine particles from car and truck exhaust, power plant emissions, wood smoke and various industrial processes. The smallest particles, those measuring less than one-seventh the width of a human hair, can become lodged deep in the lungs. There, they can aggravate respiratory or cardiovascular conditions and lead to premature death. Fine-particle exposure has been linked to a variety of health problems, including asthma, heart attacks and lung cancer.
Laboratory studies have shown that tree leaves can act as filters, collecting particles from the air. Every so often, rainfall washes the buildup off the leaves to the ground. But stiff winds also can knock particles off the leaves back into the air.
"It's like when you have a dirty shirt and shake it real hard," Nowak said.
For cities like Atlanta with abundant tree canopy, the pollution-removing effect of leaves is greatest. But the impact on people's health is more pronounced in the most populous cities, such as New York, where the study estimated that up to eight deaths were avoided.
The health benefits of trees are about average in Baltimore. But that's not necessarily going to stay that way. The city has been losing trees and canopy over the past decade at least, Nowak said, and they now cover just 27 percent of the landscape. Depending on who's counting, 16 percent to 20 percent of what's left is considered distressed, diseased or dying.
The city has vowed to increase tree cover to 40 percent of the landscape by 2040. But the city's tree-planting efforts are less than half what would be needed to reach that goal, according to the Baltimore Tree Trust, a nonprofit dedicated to re-leafing the city. Groups like the trust are trying to supplement the work of the cash-strapped municipal government.
Amanda Cunningham, the trust's executive director, said that "in the microscopic world of atmospheric pollutants, you can see and feel the difference a tree makes in improving the air you breathe and the temperature of that air."
The group is focusing planting efforts on urban neighborhoods with the highest asthma rates and the fewest trees. Its first project is McElderry Park in Southeast Baltimore, where volunteers have put in 154 big trees in the past year, Cunningham said. The trust hopes to add 650 more in the next few years — enough to boost tree cover there from 13 percent to the citywide goal of 40 percent.
"With one of the city's highest rates of emergency room visits by children for treatment of asthma," Cunningham said in an email, "the residents of McElderry Park, will hopefully breathe a little easier as they take action to change the environmental conditions where they live."
But trees are not a cheap fix for air pollution, Nowak cautioned. A dense canopy of trees may trap emissions, he said, preventing them from being dispersed by winds. Pollution can damage trees, undermining their environmental benefits. Ozone, a highly reactive gas in smog, can burn foliage, while fine particles can lodge themselves in the stomata, or pores, of leaves, preventing them from taking in carbon dioxide and giving off oxygen and water vapor.
Another recent study has suggested that trees exposed to air pollution can actually make it worse under certain circumstances. Isoprene, an organic compound given off by trees to protect their leaves, reacts with nitrogen oxides from vehicle exhaust, power plants and other sources to produce fine particles, the study found.
Nowak said he hopes this line of research can help urban communities plan better where and how to plant trees, to maximize their benefits.
"We know trees en masse have this removal effect," he concluded. "But at a local level they can either remove pollution or increase it, depending on the design."