We already know that urban trees can help deter crime and prompt us to smile a bit more. We know that they mitigate stormwater runoff, sequester carbon and provide vital habitats to city-dwelling critters while lending invaluable visual appeal to otherwise foliage-starved concrete jungles. No argument here; urban trees are pretty much the best.

We also know that the health benefits attached to urban trees extend well beyond their uncanny mood-improving abilities. Urban trees are air scrubbers nonpareil, dutifully sucking up the pollutants that city dwellers release. This, in turn, helps the denizens of major cities breathe a bit easier — or, in more stark terms, breathe at all.

A comprehensive new study recently released by the Nature Conservancy titled “Planting Healthy Air” takes an eye-opening deep dive into the relationship between urban trees — or lack thereof — and public health, particularly potentially fatal respiratory diseases linked to dirty city air. The takeaway of the study — at 136 pages, there’s a lot to digest — is this: the planting of trees in cities cannot and should not be underestimated as it serves as one of the most cost-effective methods of curbing urban air pollution levels and combating the urban heat island effect. We’ve all taken refuge under the shady canopy of a tree to escape from the sweltering heat at one time or another, looked up and thought to ourselves phew, what a lifesaver. As the Nature Conservancy details, this is one hell of an understatement.

Nature Conservancy, the impact of urban trees on air pollution graphic (Graphic: The Nature Conservancy)

The lead authors of “Planting Healthy Air” conclude that by investing just $4 per capita in tree-planting efforts, cities could have a lasting impact on the respiratory health of residents. Additional trees planted in cities could potentially help reverse a truly troubling reality: more than 3 million people across the globe perish each year from air pollution-related illnesses brought on by the inhalation of fine particulate matter released by human activities that involve the burning of fossil fuels. Transportation-borne particulate matter — that is, the deadly air pollution released when firing up the engine of a car — is a biggie here. Trees can remove particulate matter released within their immediate vicinity by as much as a quarter.

What’s more, tens of thousands of city dwellers die each year from devastating heat waves. Given that canopies do a bang-up job of effectively cooling urban environments, their role in preventing heatwave-related deaths is also critical.

“Trees can have a significant local impact on pollution levels and temperatures,” notes Rob McDonald, the study's primary author and a scientist for global cities at The Nature Conservancy, in a press statement. “Urban trees can save lives and are just as cost-effective as more traditional solutions like putting scrubbers on smokestacks or painting roofs white.”

Globally, a “conservative” investment of $10 million in urban tree planting activities could help 68 million people breathe cleaner, less deadly air and provide 77 million urbanites with the peace of mind that the next heat wave won’t be their last. As the study’s authors point out, trees are the only solution that can do both: cool and clean air.

Nature Conservancy, the impact of urban trees on urban temperatures graphic (Graphic: The Nature Conservancy)

Of course, certain cities would benefit more from per capita tree-planting efforts than others. Looking at 245 of the world’s largest cities, the study identifies which urban areas would reap the greatest return on investment (ROI) from more trees — and a lot of them. Obviously, densely populated cities that suffer from both high levels of air pollution and are often struck with deadly heat waves top the list.

A majority of the cities found to have the greatest ROI in terms of both cleaner air and cooling are (somewhat predictably) big, crowded, hot and located in South Asia: Delhi and Mumbai, India; Dhaka, Bangladesh; Karachi, Pakistan; Kathmandu, Nepal, and on. The African cities of Cairo, Dakar and Freetown, Sierra Leone, also make the study’s top-ROI list as does the Haitian capital of Port-au-Prince.

While the study doesn’t provide case studies for all 245 cities taken into consideration, 15 cities across the globe with a desperate and not-so-desperate need for major tree-planting investments are further examined.

Atlanta, for example, was found to have a low ROI thanks in part to one of the densest urban canopies in North America. With trees covering 47.9 percent of the sprawling southeastern metropolis (the national average for U.S. cities in 27 percent), Atlanta’s “city in the forest” nickname is more than well deserved. However, the study does point out that Atlanta’s densely populated — and only getting denser — downtown neighborhoods could benefit from additional street-side trees, particularly with regard to heat mitigation.

Urban TreesUrban trees aren't just easy on the eyes. In densely populated cities with high levels of air pollution, they're also a lifesaver. (Photo: Takyua ASADA/flickr)

Denver, touted as being a success story in combating rampant air pollution that once held the city in a sooty grip, is also noted as having an all-around low ROI that’s largely due to extensive sustainability efforts and a low population density. However, like Atlanta, Denver’s increasingly crowded downtown neighborhoods sport a high ROI.

And there’s Los Angeles. While drought-ravaged, car-dependent L.A.’s citywide ROI is moderate when compared to other major global cities, localized tree-planting action is suggested in denser neighborhoods of central L.A. along with the cities of Santa Monica and Long Beach. The study concludes that an annual investment of $6.4 million in new trees in targeted neighborhoods could bring temperature-decreasing relief (a 2.7-degree Fahrenheit drop) to more than 400,000 Los Angelenos during Southern California’s sweltering summers.

Click here to view "Planting Healthy Air" in full and to see how your city stacks up on the tree-planting ROI scale compared to other cities around the globe. While most North American cities do rank on the extreme low end of the ROI scale compared to let's say, Ho Chi Minh City, there is of course, always room for improvement. After all, a few more trees never hurt anyone.

Matt Hickman ( @mattyhick ) writes about design, architecture and the intersection between the natural world and the built environment.