In the UK, air pollution is the largest environmental public health risk and human-made pollution is estimated to cause up to 36,000 premature deaths. Urban areas bear the brunt of the public health risk due to population density, high pollution levels and importantly, high deprivation levels. Plants, trees and other green infrastructure can play a role in reducing exposure to poor air quality.

How do plants influence air quality?

Vegetation can influence air quality in several ways, both positively and negatively. Green infrastructure can act as a physical barrier that can block and redirect the flow of polluted air and it can act as a surface upon which particles and other gases can be deposited and absorbed. However, vegetation can be a source of pollution by releasing biogenic volatile organic compounds, which can promote the formation of more dangerous pollutants such as ozone and particulate matter. As such, there is no ‘one size fits all’ solution for using vegetation and green infrastructure to improve air quality in urban areas and thought is required to ensure that the right green infrastructure is put in the right place.

In urban areas, emissions from road vehicles often cause the greatest level of exposure to air pollution.  By planting green screens, hedges and trees along roads, creating a barrier between the source of pollution and where pedestrians walk, the flow of polluted air can be redirected and dispersed, reducing the concentration of pollutants in the air.

Research has shown that these types of barriers can as much as halve the levels of pollutants just behind the barrier, with the efficiency of any reduction affected by the density of the barrier, its height and the wind direction.

In many cases, the installation of a tree increases the ‘roughness’ of the urban environment. This stimulates greater levels of turbulence in the air, which promotes the dispersion of pollution. However, a dense canopy of trees can act to trap pollution at ground level and minimise the amount of mixing with cleaner air from above. This problem can be exacerbated in urban areas where tall buildings flank either side of the road, forming what is known as a ‘street canyon’. However, trees spaced more widely in these environments have little effect on air quality but deliver many other benefits.

In addition, trees, green screens and hedges can also help reduce air pollution by absorbing and filtering particulate matter and other pollutants through their leaves and roots. In doing so, they can reduce the concentration of airborne pollutants in urban areas. Evergreen plants play a disproportionately important role in removing pollution in this way, as they keep their leaves (and large surface area) year around. 

It should, however, be noted that this means of reducing pollution in urban areas is not a ‘silver bullet’ and research suggests that re-vegetation at the scale seen in urban areas can only absorb a few percent of total emissions.  Large scale re-forestation (at a scale not typically seen in urban areas) can, nonetheless, play an important role in driving down regional pollution levels.  

As previously mentioned, some plants can also release pollutants which can worsen air quality. Species of Oak and Willow tend to emit higher levels of volatile organic compounds, compared to species such as Alder and, Maple, Larch, Pine, and Birch. However, it is important to note that emissions from plants are generally a small fraction of the emissions caused by human activities in urban areas, such as from burning fossil fuels and industrial processes. So ripping up Oak trees is not recommended!

The Greater London Authority has released new planning guidance on how Major development across the capital can benefit air quality.  This “Air Quality Positive” approach recognises how vegetation alongside other methods, such as reducing emissions from pollution sources and promoting sustainable transportation, should also be considered in order to effectively address air pollution.

It must, however, be highlighted that there are no safe levels of some pollutants. As such, every reduction matters. Living walls and ivy screens can play a role in ‘greening’ otherwise unused spaces, improving air quality, whilst bringing additional benefits such as promoting well-being and increasing biodiversity and climate resilience.

Green infrastructure and plants can play an important role in minimising exposure to poor air quality in urban areas, whilst also benefiting mental health, biodiversity and climate resilience. However, there needs to be consideration of what plants are best at particular locations, to maximise any benefits and to ensure that issues are not worsened.

Author: Harley Parfitt, Greenavon Air Quality Consultants

About the Author

Harley Parfitt runs Greenavon, an air quality consultancy that specialises in providing air quality assessments and odour assessments, for planning applications across the UK.