In 2015, Jun Yang, an urban ecologist at the Center for Earth System Science, Tsinghua University, in Beijing, ranked the most frequently occurring species in cities based on their PM 2.5 absorption capacity. In contrast, nettles emerged as the least useful of the species studied, though they still captured a respectable 32%.Ĭonifers, like pines and cypresses, are also good natural purifiers. Silver birch, yew and elder trees were the most effective at capturing particles, and it was the hairs of their leaves that contributed to reduction rates of 79%, 71% and 70% respectively.

In one recent study, Barbara Maher and colleagues at the University of Lancaster tested the ability of nine tree species to capture PM in wind-tunnel experiments. Recent research suggests that tiny hairs on plant leaves in particular may play a big role in trapping the solid and liquid particles that make up PM. “Trees can help reduce PM in two main ways,” says Prashant Kumar, the founding director of the Global Centre for Clean Air Research at the University of Surrey. By one estimate, 8.9 million deaths a year globally could be attributable to exposure to outdoor fine particulate matter.įrom an urban planning perspective, plants act as a readily available set of PM purifiers. It has also been linked to inflammation and heart disease.

Then there are PM2.5s, measuring 2.5 micrometres across, and even smaller nanoparticle pollution.įine particulate matter can easily penetrate into human respiratory system, causing lung and cardiovascular diseases or exacerbating respiratory illness. The largest of these particles measure up to 10 micrometers across (known as PM10s), which is around a fifth of the width of a human hair. PM comes in the form of tiny particles of organic chemicals, acids, metals and dust, emitted from fossil-fuel-burning vehicles and factories, as well as construction sites. Trees are particularly effective at removing particulate matter (PM), Nowak adds. But they also act as an ecosystems “liver” too, filtering atmospheric pollutants like sulphur dioxide and nitrogen dioxide through their leaves. Plants are often seen as the “lungs” of an ecosystem because they absorb carbon dioxide and emit oxygen, says Rita Baraldi, a plant physiologist at the Institute of Bioeconomy of the Italian National Research Council. Plus, lower temperatures decrease risk of harmful pollutants like ground level ozone that commonly spike on hot days in urban areas.īut trees also play a vital role in directly removing pollutants from the air. If buildings are shaded by trees, it reduces the need for conventional air conditioning, and the emissions of greenhouse gases that come with it. Indirectly, they can help by shading surfaces and reducing temperatures. Trees can improve air quality in direct and indirect ways. “But trees can be of great help,” he says. To make the most difference in air quality in a street or city, it has to be the right tree for the job.Īnd, of course, trees are only a way to filter pollution better is to reduce emissions of pollutants in the first place, notes David Nowak, a senior scientist at the US Forest Service who has been studying plants’ contribution to air quality for 30 years.

Some trees are markedly more effective at filtering pollutants from the air than others. While trees are generally effective at reducing air pollution, it isn’t as simple as the more trees you have in an urban space, the better the air will be. And Paris is planning an urban forest that will encompass its most iconic landmarks in an effort to adapt to climate change, and also improve the city’s air quality. Meanwhile, China’s Hebei Province, home to Beijing, has been working on a “green necklace” of plants that could help reduce pollution from factories that surround the capital. In January 2019, the mayor of London announced that 7,000 trees would be planted before the end of the following year. Trees have a remarkable range of traits that can help reduce urban air pollution, and cities around the world are looking to harness them.

Vegetation helps cities become better habitats for wildlife and for people, and it helps to make city air safer. But increasingly, greenery has been edging its way back into modern urban landscapes, and for good reason. Since prehistoric times, humans have busily cleared forests to make way for settlements. Cities usually come at the price of green space.

Factory town trees