Search Google Earth for China and you’ll see this.

But an unedited satellite photo might lookmore like this. That gray smudge is air pollution and it’scoming from Chinese cars, factories, and power plants. But it’s not only here.

In cities around the world, air pollutionis a big problem. A majority of humans now live in cities andthat number is only going to rise, which means more cars, more factories, andmore power plants.

As officials explore options for fighting air pollution, there is one tool that is often overlooked:trees.

Cities are centers of industry, but the resulting pollution is filling our lungs and making us sick. One major culprit is particulate matter: airborne particles of dust, soot and smoke that are released when we burn fossil fuels or kickedup during construction and farming.

When we inhale them, they can cause asthmaand they can also enter our bloodstream to cause strokes and even death. Experts estimate that outdoor air pollutionkills over three million people a year andas cities grow, leaders are funding creative—andoften expensive—solutions for the problem.

In London, the mayor spent over a millionpounds spraying city streetswith an adhesive that was supposed to glue pollutants to the road. and in the Netherlands, designers have createda giant air purifier they call “The Smog Free Tower”, which is cool, but there is another,simpler solution…

A new report from The Nature Conservancy shows that planting trees can bea cost-effective way to improve public health, which they do in two ways:First, a tree removes particulate matter when polluted air blows through its branches.

The particulate matter settles on the leavesand when it rains, the dust is washed down the gutter so we don’t inhale it.

Second, trees cool temperatures by providing shade and releasing water through photosynthesis, which cools summer temperatures by about two to four degrees fahrenheit.

But there is a catch! Trees can only clean and cool the air withina close radius: about one hundred feet, so city officials need to be careful where they plant.

Officials can maximize pollution reductionby planting trees where population density and air pollution overlap. The Nature Conservancy report uses data from Washington D.C. to create a map showing where planting trees will have the highest returnon investment.

And some trees work better than others:trees with larger, stickier leaves, like maples and elmsare more effective, but they also need to be considered within the larger ecosystem.

Compared to DC, many cities around the world have even more to gain from planting trees:this map shows where return on investmentis highest for reducing particulate matter.

With proper targeting, planting trees can be just as cost-effective as other strategies like converting public transportation to use less diesel fuel. But there is one major limiting factor: wateraccess.

What might work in Boston, will be less feasiblein a city like Doha, Qatar,where water is a scarce resource.And on top of that, many mayors don’t yetthink of trees as a public health resource.

Trees might not look like giant air filters,but that’s exactly what they are,and the sooner we start thinking of them that way,the sooner the air we breathe might be cooler and cleaner in cities around the world.