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

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.

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.

By crashing into trees and plants, concentrated clouds of minuscule particles get dispersed and so diluted by the air, decreasing the risk of inhalation by humans. PM can easily get trapped in the waxy, hairy leaves of trees and shrubs. When it rains, most of these particles are washed away by water into drains.

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. 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 contrast, nettles emerged as the least useful of the species studied, though they still captured a respectable 32%.

Conifers, like pines and cypresses, are also good natural purifiers. The reason for conifers’ success in reducing PM is partly down to their canopy structure – the dense canopy of needle-like leaves typical of conifers is very effective at trapping pollutants. And their seasonal biology helps too. Conifers offer the best PM reduction because they are an evergreen species. Unlike deciduous trees, who lose their leaves during winter, evergreen species act as year-round filters.