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The best trees to reduce air pollution

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Trees have a remarkable range of traits that can help reduce urban air pollution, and cities around the world are looking to harness them. In January 2019, the mayor of London announced that 7,000 trees would be planted before the end of the following year. 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. 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.

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. Some trees are markedly more effective at filtering pollutants from the air than others. To make the most difference in air quality in a street or city, it has to be the right tree for the job.

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And, 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.

Trees can improve air quality in direct and indirect ways. Indirectly, they can help by shading surfaces and reducing temperatures. If buildings are shaded by trees, it reduces the need for conventional air conditioning, and the emissions of greenhouse gases that come with it. Plus, lower temperatures decrease risk of harmful pollutants like ground level ozone that commonly spike on hot days in urban areas.

But trees also play a vital role in directly removing pollutants from the air. 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. But they also act as an ecosystems “liver” too, filtering atmospheric pollutants like sulphur dioxide and nitrogen dioxide through their leaves.

Trees are particularly effective at removing particulate matter (PM), Nowak adds. 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. 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. Then there are PM2.5s, measuring 2.5 micrometres across, and even smaller nanoparticle pollution.

Fine particulate matter can easily penetrate into human respiratory system, causing lung and cardiovascular diseases or exacerbating respiratory illness. It has also been linked to inflammation and heart disease. By one estimate, 8.9 million deaths a year globally could be attributable to exposure to outdoor fine particulate matter.