Are Urban Trees a Help or a Hindrance to the Environment?

A new paper published by the National Institute for Health and Care Excellence (NICE) has suggested that an abundance of trees planted in urban areas could be having a detrimental effect on air quality.

Though trees have generally been seen as capable of removing pollutants from the air and thus beneficial to the environment, NICE has claimed that this is not always the case. By slowing down air currents and creating air ‘traps’, trees can actually exacerbate pollution in some areas.

Not necessarily a good thing

The idea that trees can disrupt air quality is not a new one. As far back as 2009, experts speculated that too many trees in an urban environment can spoil air quality. More recently, mobile mapping technology has shown that higher concentrations of benzene and other contaminants thrive in heavily-trafficked areas in which many trees are planted.

Now, the body which provides independent advice to individuals, businesses and governments on health-related issues has also warned against placing too much faith in urban trees.

“It is not always true that trees reduce air pollution. Their effect is dependent on factors including species, canopy density, time of year and wind direction. Ventilation [on streets] will vary according to the size, distribution and species of tree and their position,” says the report.

“Leaves and branches slow air currents, causing pollutants to settle. They may also act as sinks for particulates and chemicals that may have direct or indirect effects in air quality. Air quality [under trees] may deteriorate at street level near vehicles.”

Careful planning is imperative

While NICE stops short of recommending that trees be cut down in urban centres, it does advise that local councils and planning authorities think carefully about where they are planted in the future.

In particular, it suggested that trees could have a beneficial effect if they were positioned between busy roadways and cycle lanes, since they could act as a screening measure between emissions and cyclists. However, it stressed that these sorts of incentives should not be pursued without careful consideration of how the trees would affect the build-up of contaminants in the area.

Other studies have indicated that certain trees, such as pines, can actually be responsible for emitting terpenes themselves. Terpenes are a group of volatile organic carbons (VOCs) which, when they come into contact with other contaminants, can pose a problem for air quality. However, pines are not normally favoured in urban environments and the levels of terpenes they emit are negligible, compared to the other pollutants which they help to foster by reducing windflow.

A raft of measures are required

NICE doesn’t just recommend caution when it comes to urban trees, but also lists a number of other initiatives it believes councils should consider implementing to tackle air quality issues.

Among others, these include repositioning speed bumps to discourage drivers from speeding up and slowing down between them. This continual acceleration and deceleration can lead to increased exhaust pipe emissions, in turn contributing more pollutants to the atmosphere.

The report also called for the introduction of lower speed limits in residential zones, the prohibition of idling outside schools and hospitals and the encouragement of smoother driving techniques in the populace in general.