New research shows how cities can optimize the environmental benefits of trees

Trees can have a significant impact in combating the effects of climate change because they provide relief from extreme heat and clean the air. But research has shown that a chemical reaction between some trees and nitrogen oxides can have harmful air quality effects.

Some trees, notably oak and poplar trees, naturally emit a chemical called isoprene, which helps them cope with heat stress. The compound is innocuous to humans on its own, but it creates ground-level ozone when combined with nitrogen oxides and sunlight. Nitrogen oxide compounds are naturally present in the atmosphere, but they are made more prevalent by burning fossil fuels to power vehicles and buildings. During heat waves, and especially at temperatures higher than 95°F, isoprene production is especially heightened, elevating the risk of ground-level ozone exposure.

A recent study published in Environmental Science and Technology looked at six-by-six-inch squares of tree canopy data in five boroughs of New York City and modeled ground-level ozone concentrations by changing tree species and tree canopy cover variables. In a scenario where oak trees covered 40% of Manhattan, ground-level ozone levels could increase more than 30%. When the same scenario is run with a mixed canopy of trees, peak ground-level ozone could increase by about 10%.

Surface-level ozone is responsible for an estimated 365,000 deaths globally by causing lung damage, especially in vulnerable populations and those with pre-existing conditions like asthma. Other symptoms of ground-level ozone exposure are coughing, scratchy throat and shortness of breath. (The EPA has a number of printable pamphlets on ozone effects on air quality and health.)

An important caveat in the study is that if nitrogen oxide levels fell with the use of alternative fuels or electrification, ground-level ozone exposure would be greatly reduced and trees would not contribute to higher concentrations of the pollutant. The author of the paper said "changing fossil-powered emissions would be much more effective than changing the trees."

Health care journalists need to be cautious about how they convey the role of trees in the creation of ground-level ozone, with the burden of change being on the fossil fuel combustion that creates nitrogen oxides. Trees provide tremendous health and environmental benefits to humans, plants and wildlife by mitigating flood risk, providing migration corridors and habitat for wildlife and reducing the effects of the heat island effect. It is critical that these benefits are not lost on audiences when reporting on ground-level ozone.

Experts to contact

• American Lung Association Helpline - This free service connects you with lung experts. Language interpretation services are available.
• Contact EPA about ozone pollution - Drop-down inquiry form for questions and concerns around ozone pollution.
• Thomas D Sharkey, University Distinguished Professor in the Plant Resilience Institute at Michigan State University - Sharkey researches the relationship between isoprene and ozone.

Additional resources

• Ground-level ozone is getting worse. Here's what it means for your health. - National Geographic.
• The best trees to reduce air pollution - BBC.
• Overheated trees are contributing to urban air pollution - NewScientist.
• New York City's Trees Work Hard. Still, They Could Use a Little Help. - The New York Times.
• The climate gains of urban trees - Axios.