There are practical challenges to running a controlled experiment to test the effects of plants on local air pollution levels. Barbara Maher, an environmental researcher at the University of Lancaster in the UK, should know—for a study she’s running at four primary schools set along busy streets in nearby Manchester, for example, she and her colleagues had to rent expensive air quality monitoring equipment. They also had to get administrators at the four schools on board, which meant waiting until school let out for summer to install the plants, even though the timing was far from ideal for collecting data. At the same time the plants went in the ground last year, the volume of traffic dropped precipitously due to the summer holiday, she says. 

For Maher, though, these challenges were worth taking on because she sees the plant installations—which she calls tredges, a...

Trees are known to filter some particulate pollution, and Maher says she hopes they can be deployed to shield schoolchildren from harmful effects of spending their days just meters away from a continuous parade of emissions sources. She and others have found that leaves pick up and store particulates from the air. And when she and colleagues lugged potted silver birch trees to the edge of a busy road near their campus in Lancaster, they saw levels of particulate pollution inside the houses just behind the trees drop by more than 50 percent

While they’re no substitute for reducing emissions themselves, Maher says, tredges could be “an interim measure that can actually make a measurable difference almost straight away.” Her new study aims to improve air conditions for children in the targeted schools and test different tredge setups to inform similar efforts in the future.

A tredge installed by Maher’s group
barbara maher

Yet in the literature as a whole, the evidence for trees as pollutant filters is not as dramatic or clear-cut as in Maher’s earlier research. Some modeling studies have suggested that as trees grow tall over high-traffic roads, they can create “street canyons” where pollutants are trapped by stagnant air, worsening local air quality. In addition to the size of the trees, factors such as the species and even the frequency of rainfall affect the plants’ filtering capacity. 

In a study published last year, Steven McNulty of the United States Department of Agriculture Southeast Regional Climate Hub in North Carolina and his colleagues at Beijing Forestry University in China took a close look at how effectively different intensities and durations of simulated rainfall cleansed particulate matter from leaves on branches they collected from four different tree species and one shrub in Beijing. To be a good filter, McNulty explains, “you want to make sure you have a leaf that not only can take up the particulate matter but also can be cleaned and reused.” Without the washing away of collected particulate matter, leaves lose their air-purifying capacity as their surfaces fill up. 

See “Trees Tested as Pollutant Traps

The team found that the London plane tree (Platanus acerifolia) accumulated the most particles of the species tested—a result consistent with studies by other groups that found rough leaves like the plane tree’s tend to make the best filters. Leaf texture affected how leaves were cleaned by water, too, with rougher surfaces washed most effectively by short, intense bursts of rainfall, and smooth leaves by long, gentle showers. McNulty says the researchers tested a wide variety of rainfall types with the hope that their results can inform planting plans not just in relatively arid Beijing, but also in Chinese cities with far wetter climates.

These particles are able to gain access to just about every organ in the body, where it’s very likely they’re causing a lot of damage.

—Barbara Maher, University of Lancaster

Although his study aimed to provide practical data on effectively using trees as filters, McNulty won’t hazard a guess as to how much trees, on the whole, might be lowering levels of particulate pollution in cities such as Beijing. That’s a complex question, he says, and the calculation would “be very difficult to do.”

Paul Monks, an atmospheric chemist at the University of Leicester who conducts modeling studies on the local effects of trees on air pollution, says he doesn’t think plants filter enough to make a big difference to cities’ air quality. “You’d have to reforest huge amounts of areas to even get marginal effects,” he says. In a 2017 study, Monks and his colleagues modeled air quality for part of central London and found that tree leaves reduced the localized concentration of fine particulate matter by up to 2 percent. But that effect was overshadowed by trees’ effect on air flow and thus dispersion of pollution, which, depending on the direction and velocity of the wind, could make air quality either better or worse.

For Maher, Monks’s study exemplifies what she views as a common flaw with modeling studies: the use of an unrealistically low value for the rate at which particulate matter is deposited onto leaves, which in turn leads to underestimates of trees’ filtering capacity. “The value [modeling studies] conventionally use is often more than 10 times lower than the value that we use, which we have measured,” she says. Monks concedes that using a different value for what’s known as the deposition velocity, “you’ll get a slightly different number”—but only slightly, he says. The main problem with relying on plants as filters is, he says, that there’s only so much air, and thus particulate matter, that can come into contact with leaves.  

Despite their differences, Monks says that he thinks constructing a “green wall” of vegetation to physically block pollution—outside schools, for example—can be effective, as long as it’s done in a way that doesn’t trap pollution. And Maher agrees that tredges need to be managed carefully in order to use them effectively for pollution mitigation. Her current study aims to get at what that management entails. Using sensors along the roads, as well as in playgrounds and classrooms, the team will compare a school that has no added vegetation with three schools planted with ivy, western red cedar, or a mix of western red cedar and Swedish birch. Once the results are in this spring, the control school will get the tredge type found to be most effective. [Update, August 25, 2022: Results of the study have now been published in Scientific Reports.]

For his part, Monks thinks the emphasis should still be on halting pollution at the source. He compares trying to filter pollutants from the air by any means to stirring milk into one’s coffee and then trying to get it back out again. “There are really, really good reasons to plant trees, and we must plant more of them,” he says. “But let’s not pretend that’s going to solve our air pollution problem.”

Shawna Williams is a senior editor at The Scientist. Email her at swilliams@the-scientist.com or follow her on Twitter @coloradan.

This story is part of a series by The Scientist on how natural environments affect human health. 

Click through to find out more about some of the other proposed mechanisms linking spending time in nature to physical and mental health benefits.

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