Greening isn't always the most effective solution to pressing urban problems. But that doesn't mean the problems can't be solved.
Almost every mention of urban trees in the popular media includes the phrase "trees clean the air." But how does this actually work? Trees produce oxygen, of course, as all plants do, which makes the planet habitable. But there's currently no shortage of oxygen in the atmosphere inside or outside of cities. More likely, these statements refer to the influence of trees on air pollutants that are of concern for human health: particulates, NOx, ozone, and other oxidants.
For many years, field measurements of air pollution near and away from urban trees were pretty scarce in the scientific literature. Estimates of the effects of trees on air quality came from models or calculations of possible effects based on a small handful of studies. But in recent years there have been more direct measurements of canopy effects on urban pollution. And epidemiologists are starting to weigh in, with studies of respiratory illness near and away from trees.
So what do these studies have to say? Recently, my colleague Theodore Eisenman at the University of Massachusetts led a synthesis of all the evidence. Let's briefly sum it up:
Trees can filter some particulate pollution, but sometimes they make it worse
Small particles suspended in air can reach our lungs and trigger numerous health problems. Particles are removed from the air when they land on a surface, and trees can have a relatively large surface area. Some of the larger estimates of the ability of trees to reduce particulate pollution are based on estimates or models of particle deposition onto leaves or canopies. However, trees also affect the movement of air, so they can change the way particles are dispersed. Dispersion effects can either increase or decrease particulate concentrations depending on the conditions. For example, tall trees can trap pollution below the canopy on busy roads. Because of the varying effects of deposition and dispersion, the results of direct measurements of particulate pollution are very mixed, and tree canopies have been reported to modestly increase, decrease, or have no effect on particulates in different locations.
The gases that cause smog can also be removed, trapped, or even produced by trees
There's a similar story for the air pollution caused by reactive gases. Gaseous pollution causes health problems because it chemically reacts with living cells. When these gases react with the cells in plant leaves they're removed from the air. This is the mechanism by which plants "clean" NOx and ozone pollution. However, NOx can also be trapped under tree canopies that line roads in the same way as particulates. In addition, ozone is produced by the reaction of NOx with other compounds in the presence of light. And unfortunately, these other compounds can be produced by trees. This is a major wrench in the story about how trees "clean the air:" the volatile organic compounds that are pre-cursors to ozone formation are emitted from plants (there are human-caused sources too, like volatilizing gasoline).
Trees have mixed effects on rates of asthma, in part because of pollen allergies
In the end, the reason we want to reduce air pollution is because it causes major public health problems. But the evidence that trees consistently reduce rates of asthma is weak, and some epidemiological studies find increases in pollen sensitivity near tree canopies. There are ways to reduce the effects of pollen allergies on human health, by selecting the right species and by diversifying the tree canopy in general. But overall, trees have a relatively small effect on air pollutant concentrations and associated illness. (Note: there's a big difference between concentrations, which are what affect our health, and the mass of pollutants removed or added to the atmosphere. You can't directly translate pollutant mass into human health outcomes.) And sometimes, trees can make pollution or asthma a little worse.
The punchline: We can solve the air quality problem, but not with trees
To be clear, there is evidence that specific, carefully placed vegetation, like low roadside hedges, can measurably reduce air pollution. But more to the point, in my opinion air pollution is an example of a problem that has totally tractable solutions we can implement right now. But they don't need to include urban greening to be effective.
In most cities, vehicle exhaust is a significant source of air pollution. There may be industrial, smoke, or dust sources as well, but in general air pollution is most commonly a fossil fuel problem, and we already know that fossil fuels must be eliminated now to solve both climate change and urban air quality. We can absolutely do this with current technology: for example, by subsidizing the switch to electric vehicles charged with renewable energy.
This doesn't mean that greening can never contribute to improved air quality, or that we should stop trying to imagine and design nature-based solutions to pollution problems. Maybe dense greenwalled buildings like One Central Park in Sydney (where I am sitting typing this right now) reduce concentrations of pollutants entering the building. These new designs haven't been adequately studied.
This is not a drill: We must reduce urban air pollution now
It's almost universally true that cities have very limited resources to combat pollution and climate change, and they must choose their investments very carefully. This is why evidenced-based solutions are so crucial: we can't afford to invest in large-scale solutions that don't work. In the case of air quality, this is literally a matter of life and death. Where I live in Salt Lake City, researchers estimated that 2017 saw 72 excess deaths, 7 excess cases of lung cancer, 118 other serious illnesses, and 156,242 missed school and work days due to air pollution.
A common argument I hear is that it's okay to keeping pushing the "air cleaning" benefits of trees even when we know they won't be effective because trees have "co-benefits," meaning they help people in other ways. But I strongly disagree with this argument. There are real lives affected by pollution in real ways right now, and the problem is urgent. We should absolutely work toward a goal of greener cities, but when urban greening can't deliver the results we desperately need, we have to invest in the solutions that can.
In the case of urban air quality, this mean reducing and ultimately completely eliminating fossil fuel combustion in urban areas. No exceptions and no excuses.
What do you think? Should we promote trees that "clean the air"? Do all urban problems have nature-based solutions? Leave a comment and let us know.