The use of vegetation in urban areas is often discussed as a potential solution to poor air pollution. However, it is a contentious issue and, if poorly conceived, it can make matters worse. In April 2019, the Greater London Authority (GLA) published a welcome and comprehensive guidance document on this topic.
Findings from the GLA report: Using Green Infrastructure to Protect People from Air Pollution
The GLA’s report, Using Green Infrastructure to Protect People from Air Pollution, specifically focuses on the current best practice for how green infrastructure can reduce exposure to air pollution produced by vehicles in the urban environment. The report considers the elements of green infrastructure situated within a city’s streets - from street trees and hedges, to parks and green spaces.
Filtering of Air Pollution
The report tells us that:
“… even the thickest hedges will allow some air to flow through them. The deposition of pollutants in the air that passes through the hedge onto the leaf surfaces will have a small but beneficial effect. The levels of pollutants here may be as much as halved, but the benefit tails off with increasing distance from the hedge. This was demonstrated by one study by King’s College London that found levels of NO2 reduced by 23 per cent when a green wall was placed between a busy road and a school playground.”
Dispersion of Air Pollution
Altering the speed and distance that pollutants have to travel before reaching people is noted as being key. This process, known as dispersion, means that as pollution travels, it is mixed with - and diluted by - cleaner air. The further the distance, the better. Green infrastructure in an urban setting, at street level, can make a significant impact. Being highly localised, dispersion can greatly reduce the amount of emissions people are exposed to.
Types of Vegetation Barriers
It is important to use the correct green infrastructure in the right place for this to be effective. Infrastructure which inhibits dispersion can have a detrimental effect.
For roads with little or no traffic, air quality can be better at street level than it is above the surrounding buildings. In this case, a dense avenue of trees can form a canopy, resulting in a pocket of clean air at street level. The trees need to be packed closely together; widely spaced trees will have little effect on air quality.
Green corridors not only reduce exposure to air pollution, they also encourage healthier travel choices such as walking and cycling. This, in turn, will reduce the number of vehicles on the road, thereby cutting road transport emissions.
Green roofs have many benefits for the environment, such as increased biodiversity, urban drainage and cooling buildings (reducing energy used for air conditioning). However, green roofs have little impact on reducing exposure to road transport pollution.
Dense, well maintained green walls can be used as an alternative to hedges as barriers between busy roads and pedestrians. When mounted on building facades, green walls have the potential to reduce public exposure to road transport pollution. The impact will vary depending on the height and width of the street canyon.
By combining a hedge with a dense line of trees, a taller vegetation barrier can be created. As deciduous trees lose their leaves for some of the year, it is advised to use evergreen trees for this type of project.
Open Green Spaces
Green open space next to a road, such as a park, can greatly reduce exposure to traffic pollution. Pollutants are able to disperse, rapidly decreasing to background levels. The presence of trees in the open space helps the street level polluted air to mix with the cleaner air above. Additionally, by bordering the open green space with a hedge or green wall, a sheltered region of air can be created immediately behind it.
The Use of Tall Barriers as Green Infrastructure
Where the aim is to protect those in a larger area, such as a school, a tall barrier is required. This can be achieved with the combination of a dense line of trees and a green wall or hedge beneath. The taller the barrier, the larger the area protected.
Choosing the Right Vegetation for Your Project
It is important to consider what type of vegetation you are using for your urban green infrastructure project for maximum benefit. Details of the general planting considerations are contained within the guidance.
The Future Role of Green Infrastructure in Towns and Cities
Whilst our transport system is evolving, urban vegetation will be vital in reducing our exposure to road transport pollution.
The use of the right vegetation barrier between vehicles and pedestrians, or indeed between the road and a cycle lane, can offer crucial protection.
Using Green Infrastructure to Protect People from Air Pollution, published in April 2019, was produced by the GLA in consultation with:
- The Birmingham Institute of Forest Research (University of Birmingham)
- The Global Centre for Clean Air Research (University of Surrey)
- Transport for London.
For more information on, please contact our air quality expert Lucinda Pestana: firstname.lastname@example.org