Trees in cities may be having an inadvertent negative impact on air pollution. According to a new report that outlines how local governments in the UK can reduce toxic fumes that have a significant effect on public health, the lush trees of urban areas may be trapping pollutants and concentrating them at street level. This does not, however, mean they should all be cut down.
The addition of trees to cities is often thought to be a good thing. There have been plenty of reports that discuss how they benefit those living in built-up environments via a range of ecosystem services, from taking up excess carbon dioxide to slowing water that enters the sewer systems.
“Some people believe that trees reduce air pollution but this is not always true, and it is important to address this misconception,” write the authors of the draft report. “Their effect is dependent on factors including street design, number and siting of trees, species and canopy density, time of year and wind direction relative to the street."
These factors can all influence the air currents on the roads, slowing them down and trapping the polluted air at street level, while protecting those in buildings above the trees. The plants can also act as "sinks" for particulates, such as volatile organic compounds, that may have direct and indirect impacts on air quality.
Air pollution has become a major issue in cities around the world. While the problem may be more noticeable in certain areas, such as Mumbai and Beijing when they get smothered in smog, most urban areas are still heavily impacted. Some sites in London, for example, surpassed the yearly safe limit on air pollution set by the European Union within just eight days of the New Year. It is thought that polluted air contributes to roughly 40,000 premature deaths a year in the UK alone.
This has spurred the National Institute for Health and Care Excellence (NICE) to draft a guidance for local councils in the UK to try and tackle the issue. The report stresses that “it is also important to recognize the benefits of properly selected, sited and managed trees,” and that the placement of plants should be taken into account.
They have also set out other recommendations, including how the addition of traffic safety measures such as speed bumps can alter the level of pollution at street level. The report claims that the increase in acceleration and deceleration of vehicles increases the level of emissions, and that redesigning such measures could help to cut down on the pollution that people are breathing in.