How do urban forests impact our health?

A new study examines some of the benefits--and an unexpected drawback--of urban forestry projects

Photo by Henry Dixon on Unsplash

Summer has come to a close here in Boston, where we’ve dusted off our fall jackets to head out in the morning. But this past summer, like the one before it, was punctuated by severe weather events, from days of extreme heat to drenching storms—the types of events expected to increase in the coming decades with rising global temperatures.

One strategy Boston and other cities around the world are employing to help prepare for the future is urban forestry, the practice of proactively adding trees (and other greenery) to the concrete and asphalt landscape. Last fall, the City of Boston released its Urban Forest Plan to improve the city with additional greenery, stating:

The Urban Forest Plan is a major milestone in our efforts to meet our environmental goals while establishing a blueprint for the preservation, cultivation, and expansion of our urban forest over the next two decades. This plan is the first of its kind in Boston and explores the many, interconnected benefits of a robust urban forest.

A living network of trees controls drainage and flooding, filters groundwater, cools and purifies the air we breathe, beautifies our city, and mitigates the dangers of extreme heat.

The City of Boston’s urban forest assessment of the Charlestown neighborhood, showing current living trees in green and possible sites to plant new trees in red, with priority zones outlined in black.

It can be easy to overlook the environmental benefits of our cities’ trees. Once mature, they provide shade, protecting pedestrians from the sun’s UV rays, reducing temperatures, and reducing need for energy-intensive air conditioning. Trees directly remove gaseous air pollutants (like SO₂, NO₂, CO, and ozone) in their leaves, and they trap harmful fine particulate matter in the air (PM_2.5) until it can be washed away by the rain.

Are there any downsides?

While planting more trees in cities sounds pretty appealing, there are some challenges and tradeoffs. For starters, trees have to be pretty big—and at least a few decades old—to be able to provide many of the benefits listed above. They require a lot of support to grow in urban conditions. They take up space that some might want to use for other purposes. And when they successfully improve neighborhoods by some measures, researchers have voiced concern that they can create other problems, such as making housing less affordable.

But from the perspective of direct effects on the body, it seems to be only upside at first blush. High levels of pollutants and high temperatures are both known causes of breathing and heart problems, particularly for people with asthma, COPD, and heart disease (which is why public health warnings are often issued on hot days with poor air quality). Trees can address these problems and look nice while doing it.

But trees—regardless of where they are—can also cause problems because they’re not just an inert part of the environment. Trees make pollen, and pollen can cause problems. For some, pollen is a nuisance that periodically leads to runny noses, itchy eyes, and sneezing; for others, pollen can trigger asthma attacks, sending people to the hospital.

A new National Bureau of Economic Research working paper by researchers in China and the U.S.—Jianwei Xing, Zhiren Hu, Fan Xia, Jintao Xu, and Eric Zou—examined the impacts of a massive urban forestation program in Beijing. Beijing began their urban forestry program over a decade ago in an effort to improve urban air quality.

In a decade, the project has added 2 million mu of greenery – equivalent to 515 square miles, roughly the size of Los Angeles – spread throughout the city, with many forest patches situated near the city’s most densely populated areas.

Using a number of different research methods and data sources, including air quality measurements, satellite measurements, and healthcare data, the researchers arrived at number of encouraging findings. Throughout their analyses, they take advantage of the as-good-as-random variation in factors like wind direction and the precise timing of a season’s peak pollen count. Here’s a summary of what they found:

• A 30% increase in greenery from 2001 to 2020, much of it driven by the urban forestation program, confirmed by satellite imagery

• Improvements in air quality where greenery was added using satellite pollution measurement methods

• Improvements in particulate matter pollutants in areas that were downwind from areas where greenery was added; those same areas did not see improvements when the wind was blowing in other directions

• Looking at Beijing as a whole, there was an estimated 4.2% reduction in fine particulate matter pollution (PM_2.5) attributable to the urban forestation program.

But they also found something unexpected. Tree pollen became an issue. They found:

• Increases in pollen counts in areas that were downwind from areas with increased vegetation; these increases weren’t present in those same areas when the wind was blowing in another direction

• In Beijing as a whole, there was an estimated 7.4% increase in exposure to pollen.

The question, then, is what were the net health impacts of these changes—one good, one bad. Here’s what they found:

• The increase in the number of emergency department visits attributable to the increase in pollen exposure was similar in magnitude to the decrease in the number of emergency department visits attributable to improvements particulate matter caused by the urban forests.

• However, the increase in health care spending attributable to the pollen was only about a third of the decrease in spending attributable to improved particulate matter pollution.

The most logical explanation for this is that unlike those affected by particulate air pollution—which tends to be elderly patients with chronic heart and lung disease—pollen sensitivity affects patients of all ages (and younger patients more so). Younger patients with allergic reactions or asthma brought on by pollen are, on average, going to be less likely to require prolonged care or hospitalization. Put simply, the people impacted by pollen, on average, can handle the exposure better than the people impacted by particulate pollution.

What does this study mean for urban forestry?

It’s very hard to measure all of the potential benefits and all of the potential costs of implementing an urban forestry program. Most city residents, we imagine, would appreciate the many benefits of additional greenery around town. But if oak trees trigger your asthma, and the city plants an oak tree right in front of your apartment or place of work, you might not be so keen on the urban forestry program.

What this study tells us is how important it is to measure the impacts of the various interventions we make with the goal of a healthier future. Beijing’s urban forestry program seems to show a net benefit when it comes to emergency department visits, but there is also clear evidence of a tradeoff being made between those sensitive to particulate matter and those sensitive to tree pollen.

Of course, these may be tradeoffs citizens are willing to make, but it’s important to be aware of them—not only to make more informed decisions, but so that ongoing public health efforts can address the new problems, like pollen, that arise.