Can plants help to make London's air safe to breathe?

Air pollution is fast becoming a major environmental health issue, and with initiatives to combat it - like the new ULEZ in London -  the wider public is beginning to wake up to the danger it poses. Globally, increasing urbanisation and industrialisation has lead to around 90% of people now living in areas that have air classified as hazardous by the World Health Organisation. This means that billions of people are suffering a reduction to their quality of life as a result of the multitude of ailments that air pollution causes - from less severe colds and headaches, to potentially fatal respiratory diseases, developmental disorders, and cancers.

Recently, green infrastructure - the integration of plants into urban areas - has gained traction as a potential mitigation method for air pollution.  As well as improving the balance of Carbon Dioxide and Oxygen in air, plants are capable of removing, and dispersing airborne pollutants, and so are a potentially useful tool for improving air quality.

Air pollution is an umbrella term to describe airborne substances that are damaging to human health. The sources of these pollutants are almost always linked to human activities such as industry and transport - cars, motorbikes, and trucks are by far the biggest culprit from their exhaust fumes and tyre and brake degradation. The structure of urban spaces then compounds the problem by trapping these emissions between buildings, creating ‘street canyons’ that cause highly polluted roads and roadside spaces. This means that cities are very vulnerable to the problem and now the air in urban spaces is very often hazardous to people.

Many cities worldwide are now greening their urban spaces to make them healthier for their inhabitants. Green infrastructure comes in a variety of forms, from larger scale urban woodlands, to specially built plant containing structures, like our green screens or Parklets.

This increase in appetite for improving air quality, as well as desires for other important ecosystem services of plants, has ushered in a new era of urban greening, where the functionality of a plant is now seen as equally as important as its aesthetic

It is not, however, as simple as just putting plants into an area. As a large diversity of compounds contribute to air pollution, there is no one plant type that can combat them all. Instead, species must be utilised for their anatomical and physiological features to combat plants are suited to removal of specific pollutants - and so plants must be used intelligently in an area to maximise the impact of green infrastructure on air pollution.

At Meristem we have been at the forefront of this, intelligently utilising plants to provide infrastructure that delivers a wide multitude of services, from ecological pollinator support, to stormwater management. However, possibly our main focus has been in air pollution mitigation and we have been one of the drivers in using plants to help make London’s air safer.

One the most effective and simple methods of this is vegetation barriers - walls of planting that act as a blockade to trap or disperse pollution. We utilise Green screens (pictured), often of English ivy (Hedera helix), which are a grid trellis that houses a dense climbing plant, to produce these barriers. We employ English ivy as it is exceptionally well suited to this role, this is due to its rough leaf texture that increases the capacity for capturing airborne particles, and its high leaf density that allows it to easily form an effective barrier. Through collaboration with the Mayor of London’s Greener City fund, we have installed them to protect the playgrounds of many Primary Schools that bordered busy roads

A study by Kings College found that the installation of green screens in a school playground bordering a busy road reduced the levels of two of the most prevalent and dangerous pollutants, Nitrogen dioxide and microscopic particulate matter, were reduced by 36% and 41%, respectively, inside the playground.

Larger plants can also play a large role in improving quality, trees estimated to remove 2000 tonnes of air pollution every year in London, roughly the same as is released by taxis in central London. Their impact is not just in removal too. Trees positioned by the side of roads combat the impact of pollutants concentrating in ‘urban canyons’ between buildings, by dispersing pollutants away from street level. Our Parklets are a stylish way of integrating planting of trees into urban spaces, and we plan our designs to maximise the benefits of the planting to the space, creating an area that is both more healthy and attractive.

However, green infrastructure cannot stand alone as the solution to improving urban air quality. Despite its impact, it is very much a mitigation method, and it needs to be applied in conjunction with reducing the sources of pollution. The new Ultra Low Emissions Zone (ULEZ) in London is a significant step in the main source of pollution in cities, by reducing the number of cars present, Pedestrianisation is also an important part of the fight, and the greening of pedestrianised streets using vegetation barriers and large planting can create a green and clean space in cities for people to enjoy without jeopardising their health.