Trees aren’t just nice to look at. They also provide key environmental services, from removing harmful gases from the air to cooling buildings in the summer. As urban investments, they pay for themselves easily, as this U.S. Forest Service tool quantifying their value shows.

And that’s before we get into the potential health benefits of trees, a subject that’s still under-explored by scientists. To date, there had been little field research looking at how trees reduce neighborhood pollution, according to Barbara Maher, a professor at the University of Lancaster in the U.K. Which is why she wanted to look into the question more closely.

Houses behind the tree line saw a 50% reduction in soot, metal, acid, dirt, and pollen.



Maher’s team set up a temporary row of silver birch trees along a nearby, high-traffic road that has 12,000 vehicles a day passing by. The researchers wanted to see how much particulate matter the saplings could remove before the dust settled on nearby homes. And the answer was a lot. Houses behind the tree line saw a 50% reduction in the sort of microscopic matter (soot, metal, acid, dirt, pollen, and so on) that people breath into their lungs, and that heightens the risk of cardiac arrest, strokes, and asthma attacks.

The researchers used two methods to measure particulate concentrations in and around eight homes behind the trees. First, they employed a dust-counting machine in a target house and a control house outside the line. Second, they took swaps from dust-collecting TV and computer monitors inside all the homes before and after the trees were installed. The methods produced similar results–a halving of levels of particulate matter.

Putting leaves from the trees under a microscope afterwards showed how the trees did it. There were substantial build-ups of “traffic-derived particulates” in their “uneven, hairy surface,” the paper reports.

The research suggests cities, or individuals, should consider tactical “urban greening,” Maher writes in an email. “It suggests a possible cost-effective means of air quality improvement,” she says. “If the reduction in PM is 50%, this would save significant numbers of lives and money.”

That doesn’t mean putting trees everywhere, but rather placing them where they can make a difference.