What Are the Best Anti-Pollution Trees?

The benefits of trees in an urban setting have long been touted. Not only can they help to increase biodiversity and enhance the mood of those living and working around them, but they can work to reduce ambient air pollution. In particular, they are adept at mitigating concentrations of particulate matter (PM), which the latest monitoring technologies have revealed is a significant problem in towns and cities across the globe.

However, not all trees are created equal. Some are more effective at battling air pollution due to the size of their canopies and their individual leaves, while others carry with them secondary side-effects that can cancel out any advantages they might bring. As such, city planners should take into account the specific circumstances and conditions with which they’re dealing when deciding which trees are most suitable for alleviating air pollution in their area.

Direct and indirect contributions

Trees make sense as a means of improving air quality and reducing transport-related pollution in an urban setting in two ways: directly and indirectly. The latter aspect is mostly concerned with temperature. The shade provided by tree canopies can reduce ambient temperatures, thus negating the need for air conditioning in buildings and minimising the risk of ground-level ozone (which is harmful to human health) from forming.

However, they are most desirable for the direct impact they have on our airways. A street which contains many trees can help to disperse pollutants, thus diluting them in the surrounding air and reducing the risk that they are inhaled or ingested by humans. Meanwhile, hanging foliage can trap dirty air particles in among the leaves, especially if those leaves are covered in tiny hairs. Then, when it rains, the pollutants are washed away down into the city’s sewerage system.

For that reason, species with large canopies and hairy leaves are most preferable. In a recent study conducted by the University of Lancaster, elder, yew and silver birch trees were most effective at reducing PM concentrations in a wind tunnel, removing 70%, 71% and 79% of pollution, respectively. Meanwhile, coniferous species like cypresses and pines are also excellent pollution filters, with the added advantage that they flower all year round.

No one-size-fits-all solution

Having said that, conifers are not an evergreen solution for every situation. For starters, they are excessively sensitive to soils with a high content of salt, which often occurs in towns and cities due to the fact that salt is used to de-ice roads in winter. What’s more, the berries, bark and needles of yew trees are actually poisonous to humans, so planting them in a location where they might come into contact with vulnerable people (like young children) is not advisable.

For that reason, it’s a more complicated process than simply planting the same tree in every city. Experts on the topic actually recommend that an urban forest should prioritise biodiversity above a single, most-effective type of tree, and that no one species should comprise more than 5% to 10% of the overall planted trees in an urban environment.

Finally, planting some trees in certain locations can actually be detrimental to air quality. In narrow streets surrounded by tall buildings, for example, planting rows of trees can prevent the pollution from dispersing properly, effectively creating a greenhouse for contaminants. In those scenarios, hedges or green walls are preferable. Bearing that in mind, it’s clear that town planners must carefully deliberate and weigh up all aspects of the issue when deciding where, how and which trees to plant.