Next time you pass a tree, you might want to give it a second thought. Maybe even a hug. One day, that tree might just help save your life.
Let me explain. In a new study published in the Environmental Pollution journal, researchers found that the positive impact that trees have on air quality translates to the prevention of more than 850 deaths each year as well as 670,000 incidences of acute respiratory symptoms. In 2010 alone, the study found that trees and forests in the contiguous United States removed 17.4 million metric tons of air pollution, which had an effect on human health valued at $6.8 billion. The results are even more impressive when considering that trees’ pollution removal only resulted in an average air quality improvement of less than 1 percent. Every year in the U.S., poor air quality is responsible for about 50,000 premature deaths and $150 billion in health care costs.
Fortunately, trees can help — they intercept particulate matter and absorb gaseous pollutants, effectively removing pollution from the air we breathe. Researchers calculated the health-saving effects through analyzing four county-level characteristics: daily tree cover and leaf area index; the hourly flux of pollutants to and from leaves; the impact of hourly pollution removal on pollutant concentration; and the health effects and financial impact of changing levels of nitrogen dioxide, ozone, particulate matter less than 2.5 microns (also known as PM2.5) and sulfur dioxide. They finally concluded that more tree cover means greater air pollution removal, and more removal coupled with a more densely populated area results in greater value to human health.
Study co-author David Nowak, a project leader with the U.S. Forest Service, told me that while previous studies have examined the local impact that trees have on air quality, this is the first to take that question to a national scale. Nowak said that while he expected some effect on human health based on previous studies, he was surprised by the impact that trees had on human mortality.
“To be honest, I really didn’t know to expect,” he said.
In addition to reducing mortality and acute respiratory symptoms, the study found that trees and their pollution removal powers prevented 430,000 incidences of asthma exacerbation and 200,000 school absences. The study also found that tree-related air pollution removal was substantially greater in rural areas (that’s where most of the forests are), but the monetary value of pollution removal was greater in urban areas (that’s where most of the people are). California, Texas and Georgia were home to the greatest pollution removal, while Florida, Pennsylvania and California reaped the greatest value from pollution removal. Nowak and co-authors Satoshi Hirabayashi, Allison Bodine and Eric Greenfield write:
As human populations are concentrated in urban areas, the health effects and values derived from pollution removal are concentrated in urban areas with 68.1 percent of the $6.8 billion value occurring with urban lands. Thus, in terms of impacts on human health, trees in urban areas are substantially more important than rural trees due to their proximity to people. The greatest monetary values are derived in areas with the greatest population density (e.g., Manhattan).
However, trees’ pollution capturing ability isn’t always a positive, Nowak tells me. If pollution is coming in from outside of a city, the more leaves the better. However, a street or highway with a thick canopy of leaves may simply trap pollutants and prevent them from dispersing — “and we don’t want to trap pollutants where we breathe,” Nowak said.
Nowak noted that trees are just one piece of the puzzle when it comes to improved air quality and he hopes the study findings can help local officials make informed decisions about managing vegetation in and around where people live. Next, Nowak is examining the link between trees and reduced emissions from power plants via variations in energy use linked to residential buildings. (In other words, how do trees and their effects on outdoor temperatures affect how we use energy?)
“I really hope that policy people will pick it up in terms of understanding that vegetation does have an impact on human health,” Nowak said of the study. “This is just one of the many services provided by trees…they provide so much from just one system and at one cost.”
To read a full copy of the tree study, click here. And to learn more about managing a community’s vegetation and calculating the value of trees, check out this free set of software tools that Nowak and colleagues developed known as i-Tree.
Kim Krisberg is a freelance public health writer living in Austin, Texas, and has been writing about public health for more than a decade.