Every report has a genesis, an initial conversation that sprouts an idea that grows into a research study. For me, one of those moments was a phone interview I had with a professor at King’s College in London, about the somewhat goofy idea of gluing pollution to roads.
London, like a lot of cities, struggles with its air quality, especially having too much particulate matter (PM) in the air. PM is essentially small particles (dust) that can be inhaled into the lungs, where they do major damage, contributing to asthma, strokes, and heart attacks. Outdoor PM causes 3.2 million deaths every year.
London got so desperate to get their PM concentrations down (which they needed to do to avoid fines from the European Union) that back in 2012 they were experimenting with spraying sticky substances on roads, and hoping that enough PM stayed there to remove atmospheric concentrations. It didn’t work too well, apparently.
During the conversation, I brought up the role of trees in potentially reducing PM concentrations. A lot of empirical studies in particular sites had reported a reduction in PM downwind of trees, which essentially function like giant filters. PM will settle out on the leaf surfaces, just as dust will settle out on furniture in your house. The professor, who was trained in environmental health, was a bit dismissive of the potential for trees to meaningfully clean the air. To be convinced they might have a role, he would have to see evidence that trees had enough scope to make a difference, and that tree planting was cost-effective , relative to other ways cities try to remove PM (e.g., putting scrubbers on smokestacks).
Now, here it is four years later, and The Nature Conservancy is putting out a new report, entitled Planting Healthy Air, that tries to answer these questions. The Planting Healthy Air report, done in collaboration with the C40 Climate Leadership Group, tries to estimate the potential for trees to clean and cool the air. For 245 major cities globally, we evaluated the current and potential future removal of PM by trees, as well as the mitigating effect on temperature.
Trees cool the air in two main ways. They cast shade that keeps the sun’s rays from hitting concrete and asphalt, which can absorb and later release heat (the so-called urban heat island effect). They also transpire water, which cools the atmosphere, just as you feel cooler on a hot day when your sweat evaporates.
What we found after a comprehensive review of the literature is that trees can provide a local but meaningful benefit to those nearby. Just downwind of a planting, PM concentrations may be reduced by 7 to 24 percent, and temperatures may be reduced by 2 to 4 degrees Fahrenheit. We emphasize “local” benefits, because most of the improvement is within 100 meters of a planting. Farther away than that, the cooler and cleaner air from the tree canopy has rediluted with the background air in the city, and PM concentrations and temperatures are not different than those background levels.
Moreover, we found the trees are a cost-effective way to provide these benefits. Measured in $ per ton, trees removed a similar amount of PM per dollar spent as other commonly used strategies for reducing PM. Similarly, compared to other ways to cool outdoor air temperatures, such as cool roofs, trees deliver similar benefits per dollar spent.
Trees, of course, supply both these benefits at once, as well as other co-benefits: helping mitigate stormwater, provide habitat for birds and other biodiversity, and provide aesthetic beauty. Trees, therefore, deserve to be a tool in the toolbox of urban planners as they struggle to clean and cool the air.
But we take pains in the report to make clear that tree planting is not a replacement for the other strategies society might do to reduce air pollution, for instance. While trees can provide meaningful reductions in PM, the majority of PM reduction will have to come from other strategies. Tree planting, in other words, is a complementary strategy, not a replacement.
We show in the report that trees are already doing a lot in our studied cities, providing tens of millions with significantly cleaner and cooler air. But there is potential scope to do a lot more. For roughly $4 annually per person, cities around the world could plant and maintain enough additional trees to save 11,000 to 36,000 lives annually from air pollution and heat-related mortality. And for every death avoided, there would be many more who would benefit from better health, or simply a more enjoyable life, where strolling down the street on a hot day is pleasant.
So the answer to the professor’s question seems to be a qualified “yes.” Trees are a cost-effective way to help clean and cool the air for potentially hundreds of millions of people.