Trees could make urban pollution even worse

The balance of co-existing in an urban jungle.
The balance of co-existing in an urban jungle.
Image: Reuters/Toby Melville
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Contrary to common belief, city trees may actually worsen the air we breathe, a UK-based health watchdog warns.

According to the National Institute for Health and Care Excellence (NICE), air quality on leafy avenues can actually worsen at street level, where vehicle sources emit pollutants and ventilation is restricted due to the overhead canopy. “Street trees were unlikely to reduce air pollution in most street designs and could worsen it in some cases,” said NICE in the 60-page draft issued on Dec. 1. “Leaves and branches slow air currents, causing pollutants to settle out.”

A 2012 study by Belgian researchers also modeled a variety of real-life examples of roadside urban vegetation to see whether or not they improved air quality or increased pollution concentrations. They also concluded trees on city streets could reduce ventilation.

The UK is desperately trying to improve its air quality, which causes an estimated 40,000 premature deaths a year in the country. The country’s high court held the government liable for being complacent about tackling air pollution in November this year: the case revealed that the Treasury blocked plans to charge diesel cars a fee for entering towns and cities plagued by air pollution to avoid irking motorists.

The Royal Colleges of Physicians and of Paediatrics and Child Health have called for tougher regulations against polluters, more effective tracking and reporting of harmful pollutants in the air, as well as better individual practices such as swapping cars for public transport or cycling and being mindful about second-hand tobacco smoke, nitrogen dioxide from gas cooking, and solvents from plastics and paints.

Urban trees and plants can improve mental and physical health. A 2015 study in Canada found that an average of 10 extra trees per block made people living there feel like they would be much healthier—as much as a $10,000 raise or being seven years younger would. The study participants believed that trees reduce air pollution—and it’s true, trees do pull ozone, particulates, and other pollutants into their leaves and out of the air that humans breathe. However, the research also showed that there are psychological benefits of living amid greenery: being around trees reduces stress as well as increases one’s propensity to exercise.

In the UK, the country’s urban green spaces are at risk of rapid decline as parks are desperately strapped for cash for their upkeep, according to the Financial Times. A University of Leicester study found that between 2006 and 2012, over 54,000 acres of green space was eaten up to make room for “artificial spaces,” mostly for housing.

But the solution is not to just plant new vegetation everywhere. For one thing, not all trees are equal. The pungency of a cedar, eucalyptus, or pine woodland, to name a few examples, comes from a blend of volatile organic compounds (VOC) in these species. When these VOCs interact with the nitrogen oxide that is in car emissions, in the presence of sunlight, they produce ozone—at ground level, the pollutant can be harmful enough to cause heart disease.

In addition, street design, the number and placement of trees, canopy density, the different seasonal changes, wind speed, and wind direction can all impact the health value of planting in cities. NICE recommends that these factors should be researched on a case-by-case basis.

In 2015, New York City undertook an initiative called TreesCount where 2,300 volunteers mapped all of its trees. Each one was “assigned a unique ID number, as well a color indicating its species,” according to Arch Daily. The ongoing project logs every tree’s exact location accompanied by its corresponding image in Google Street View. Then, using figures from US Forest Service that estimate, the total ecological benefits a tree gives—retention of rainwater, conservation of electricity, reduction in air pollution—in dollars can be calculated.

That’s one way to monitor the trees that help and ones that don’t. If other cities could develop similar ways to manage their urban greenery, they too could mitigate the costs of planting vegetation.