Planting Trees Can Clean Air, Cool Cities

At a time when pollution is putting more children at risk, trees can improve air quality and human health.

The simple act of planting more trees can improve the quality of life in urban areas by cooling temperatures and reducing pollution, according to a new report. In its study, "Planting Healthy Air," the Nature Conservancy (TNC) argues that, "an investment in tree planting of just US $4 per resident in some of the world's largest cities could improve the health of tens of millions of people."

The 21st century seems set to become the "urban century," says the report, with more than half of humanity expected to live in cities by 2050. But that development poses challenges, including keeping the air clean.

Specifically, particulate matter (created primarily by the burning of agricultural residues, fuel wood, and fossil fuels) can be inhaled into the lungs and is estimated to cause 3.2 million deaths per year - a figure that, by mid-century, could almost double.

In fact, a UNICEF study published on the same day as TNC's report found that almost one in seven of the world's children, 300 million, live in areas with the most toxic levels of outdoor air pollution - six or more times higher than international guidelines.

"Air pollution is a major contributing factor in the deaths of around 600,000 children under five every year - and it threatens the lives and futures of millions more every day," said UNICEF Executive Director Anthony Lake in a press release. "Pollutants don't only harm children's developing lungs - they can actually cross the blood-brain barrier and permanently damage their developing brains - and, thus, their futures."

RELATED: Tress Transformed Into Living, Breathing Buildings

Tackling this issue requires a multi-faceted approach including, for example, the use of cleaner-burning fuels. But one that is relatively simple, cheap and effective is planting trees.

There are, of course, multiple reasons why planting trees is a good thing for urban environment, and "Planting Healthy Air" recites many of them: "Whether planted along a city street or growing in a park or residential yard, [trees and other vegetation] provide many benefits to people, such as aesthetic beauty, enhancement of property values, erosion prevention, storm water management, and noise reduction. Trees also sequester carbon, helping mitigate climate change."

But a handful of recent studies have shown that trees are also able to scrub the air of particulate pollution. For example, research published in 2013 found that silver birch trees were able to reduce metallic particles in the air by 50 percent, apparently by trapping them on the hairy surfaces of their leaves. By scouring through an array of such published research, the TNC study found that trees can remove as much as a quarter of the particulate matter pollution within a few hundred yards.

RELATED: Drones Could Plant 1 Billion Trees a Year

Tree cover can also have a cooling effect - by as much as 3.5 degrees Fahrenheit within trees' immediate vicinity, according to TNC. That's an issue of considerable importance, given that urban heat is already the deadliest type of weather-related disaster facing the world and is only going to worsen as global temperatures continue to rise.

An annual global investment of US $100 million in tree planting could provide 77 million people with cooler cities and 68 million people with measurable reductions in particulate matter pollution, the study concluded. The impact is greater than some places than others: cities with high population density, high levels pollution and heat, combined with a low cost of planting trees, showed the highest return on investment.

As a result, India, Pakistan and Bangladesh topped the global country rankings, with Delhi, Karachi and Dhaka constituting the top three cities, of 245 studied, in terms of benefits from reductions in both heat and particulate pollution. But the data also showed benefits to tree planting for neighborhoods in every city that was studied.

WATCH: The Crazy Plan to Capture and Store CO2 Under the Ocean