But an unedited satellite photo might look more like this.

That gray smudge is air pollution and it's coming from Chinese cars, factories, and power plants.

But it's not only here.

In cities around the world, air pollution is a big problem.

A majority of humans now live in cities and that number is only going to rise,

which means more cars, more factories, and more 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 kicked up during construction and farming.

When we inhale them, they can cause asthma

and they can also enter our bloodstream to cause strokes and even death.

Experts estimate that outdoor air pollution kills over three million people a year and

as cities grow, leaders are funding creative—and often expensive—solutions for the problem.

In London, the mayor spent over a million pounds spraying city streets

with an adhesive that was supposed to glue pollutants to the road.

and in the Netherlands, designers have created a 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 be

a 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 leaves and 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 within a close radius: about one hundred feet,

so city officials need to be careful where they plant.

Officials can maximize pollution reduction

by 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 return on investment.

And some trees work better than others:

trees with larger, stickier leaves, like maples and elms

are 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 investment is 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: water access.

What might work in Boston, will be less feasible in a city like Doha, Qatar,

where water is a scarce resource.

And on top of that, many mayors don't yet think 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.