Why Public Health Researchers Are Looking to Urban Trees

A global study finds they can help cool cities and reduce air pollution—for less money than high-tech answers

Simple, yet effective. (Peter Noyce ITA / Alamy )
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For all its comforts and conveniences, urban living can be hard on your lungs. Around three million people around the world die prematurely due to the effects of air pollution every year according to the UN, and studies suggest that number could grow to 6.2 million people per year by 2050. Most of these deaths are occurring in China, India and Pakistan, where cities are growing fast—but the problem extends to crowded cities globally, from London to Los Angeles.

What if there was a relatively simple, tech-free way to mitigate some of these lung-clogging effects? Public health experts seem to have found a partial solution. Introducing: Trees.

A new report by environmental nonprofit The Nature Conservancy lays out how trees could pave the way to cleaner air and cooler cities. Using geospatial information on forest cover paired with air pollution data and population forecasts for 245 cities, researchers found that trees have the biggest health payoffs in densely populated, polluted cities like Delhi, Karachi and Dhaka. The Conservancy and the C40 Cities Climate Leadership Group presented the findings of their global survey this week at the American Public Health Association meeting in Denver, Colorado.

Trees are already doing a lot of work to make cities cooler and healthier. First off, trees cool the area immediately around them through shade and transpiration, or the evaporation of moisture from leaves. That cooling is usually 3 degrees F in the area 100 feet around the trees. “It may not sound like much, but when heat waves hit, even a small drop in temperatures can add up to a reduction in lives lost,” says Rob McDonald, lead scientist for the Global Cities program at the Conservancy and an author on the new report. Trees also remove fine particulate pollution by grabbing it from the air before it reaches human lungs.

When the researchers examined the cost of cooling and cleaning from trees, they found it amounted to just $4 per person per year to get the maximum amount of cooling and cleaning per dollar spent—and that the return on investment for trees was higher than other ways to cool a city, like painting roofs white or scrubbing pollution from smokestacks. “If all these cities spent about $4 per person per year, we would save between 11,000 and 36,000 lives annually [because of the reduction in air pollution],” McDonald says. “Even as we move into an urban century, there is still a role for nature and trees.”

So what’s stopping this from happening already? While we know that trees can provide an important carbon sink and cooling system, the link between trees and public health is fairly new, says McDonald. As a conservation scientist presenting data at a public health meeting, he understands the challenges of bringing two worlds together. “In most cities, the parks department is separate from public health department, and they don’t think of their agendas as overlapping,” he says. But some cities are leading the charge: in Toronto, the public health department is already focused on climate change and heat waves, and the department is working closely with urban forestry officials to plant more trees.

Other studies have shown the surprising health benefits of urban trees. For example, the death of trees has been linked to a rise in cardiovascular disease, and having trees near a house is correlated with babies being born at a more healthy weight.

What’s not surprising about this study is that it’s more cost effective to plant trees in more densely populated and polluted cities, said Roland Ennos, a lecturer in the School of Biological Sciences at the University of Manchester, who was not involved in the research. “The real plus is that these are real numbers, and that trees can properly be compared with other methods. They seem to be as good as the other solutions posed, with the additional benefit that they also have other pluses—for example, reducing the chances of flooding, improving mental health and so on.”

Ennos pointed out that the study’s model for tree cooling may over-estimate the air cooling effects of street trees and the distance of their effect because it makes its estimates from experiments done on parks, not urban street trees, where trees may be further apart from one another. The study also assumes that trees have enough water to provide transpirational cooling, he explained—something that may be difficult in cities with hot and dry climates.

In fact, while urban trees make our lives easier, life for an urban tree is no walk in the park (so to speak). Besides a lack of water, studies have shown that urban trees’ lifespans are often shortened by pollution, pests and disease, which could worsen with the changing climate.

McDonald says water is certainly a consideration in determining the best type of tree for each city’s individual environment. While the best kind of shade trees are big, leafy ones, those also suck up the largest amount of water. For catching particulate matter, trees with big leaves and sticky hairs—like weeping figs—are best. Bushes and shrubs, which traditionally grow in hot dry climates, don’t catch as much particulate pollution. In water-strapped cities like Los Angeles or Phoenix, decisions will need to be made carefully about how best to invest in trees.

Most cities are actually losing their tree cover over time as trees die and cities fail to replace them, so the time to do targeted planting in the densest, most urban cities is now, McDonald argues. The study shows that there is still room for trees even in places with a large amount of people. In addition to the study, the Nature Conservancy has built an interactive website where people can zoom into their city to see how investments in trees could change their landscape. “Even in dense cities like Karachi or New York, it’s possible to have more trees in the cityscape,” he says. 

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