Planting Healthy Air: Can Urban Trees Help Clean Up Pollution?

October 31, 2016

Photo © Kevin Arnold

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.

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  1. What do you think are the implications for allergens and also urban forest fires?

  2. I think that the countries in develop , they do not appreciate the nature because they are thinking in how survive each day with the few money that they gain in their jobs.

    1. Luisa, You are probably right. Sometimes surviving takes all our attention.

    1. Hi CJ, Thank you for the question! That was not one of the cities covered in this study – I recommend contacting the Florida Native Plant Society for information on which trees are native to your area: and checking a resource like i-Tree to learn about the benefits of those trees:

  3. I would like to find a much more extensive list (than the one at the bottom of page 44) of the trees and bushes that are best at cleaning the air. Will you be producing one?
    Also, are there species of trees/plants that are actually harmful to indoor or outdoor air quality?

  4. Great report; thank you Dr. McDonald. We are a part of a neighborhood group fighting the museum of natural history’s efforts to slaughter 30 trees and replace them with a gigantic fossil-fueled building. Adding insults to proposed injury, the building and clear-cutting is slated for a NYC public park. And, a big part of the funding for this misguided project was steered from the public treasury to the museum (a private corporation) without any public input. Can u help us by weighing in at an environmentl review session yet unscheduled for this spring or provide a letter of support for our efforts to halt this toxic plan?

  5. thank-you; important research.
    When you say:
    “Just downwind of a planting, PM concentrations may be reduced by 7 to 24 percent, …
    most of the improvement is within 100 meters of a planting. ”
    But what is a “planting”. One tree per 100 x 100 meters or a small park? What could an urban forest accomplish, for example?

    1. Hi Walter, Thank you for the question! Rob says “For the purposes of this report, we focused on street tree plantings, so the numbers you quote are for the reduction just downwind of a street tree’s canopy.”

  6. Small particles in air may carry an electro-static charge. Is there any convincing evidence that trees attract and fix these particles the same way my wall soon goes black next to the air de-ioniser ?

  7. How can we protect our trees when developers swoop in and pay fines to cut down trees so they can build bigger buildings? I live in safety harbor, Florida and a big developer is cutting down140 trees in a wetland and another developer is planning on cutting down 200 plus old live oaks. Do you know of a conservancy group for trees? Any help would be appreciated. Love your article.

    1. Hi Kelly, I suggest contacting the city’s Building Division with your concerns: You might also check for tree advocacy groups at the local or state level. I couldn’t find any in your area, but here is an example from Atlanta: You could contact them to find out how they got started and see if they know of any similar organizations in your area. Thank you for the question and best of luck!

  8. […] that well-placed trees can reduce air-conditioning costs by about one-third. Trees also remove up to 24 percent of dust; studies show that kids who live near urban trees have lower rates of asthma. Trees can even make […]

  9. Does the height of the trees make a difference? Do tall trees clean more air?