Why ‘green’ infrastructure is critical for improving air quality

Dr James Levine and Dr Emma Ferranti of the Birmingham Institute of Forest Research discuss the importance of Green Infrastructure for air quality ahead of a seminar hosted by environmental charity City of Trees in Manchester.

Green infrastructure (GI) refers to all vegetation in urban areas, including: parks, gardens, green roofs and walls, grass verges and street trees. Just like our grey infrastructure, GI delivers environmental, health, social and economic benefits in our towns and cities that are critical to modern society.

Quantifiable benefits include sustainable drainage and microclimate regulation that offer increased resilience to more frequent high-rainfall events, and higher air temperatures, in our changing climate.

Many benefits of GI have a direct bearing on human health and translate into economic benefits through health-costs saved and working days gained.

We focus here on one public health benefit – improved urban air quality.

The World Health Organization (WHO) identifies air pollution as the greatest environmental risk to human health, with 90% of the world’s urban population living in cities exceeding its air quality guidelines. In the UK each year, it contributes to roughly 40,000 deaths and costs city-regions over £20bn.

We focus particularly on roadside air quality as road transport is the main source of outdoor air pollution in urban areas.

How does GI affect urban air quality, and how can we deliver reliable benefits?

GI can, in principle, affect both the deposition (removal) and dispersion (distribution) of air pollutants.

At regional and larger scales, the process of deposition, in which some particulate and gaseous air pollutants collide with and stick to leaves, is important. Nationally, this saves the UK over £1bn in health costs each year.

However, at the scale of urban planting, the latest independent review by DEFRA’s Air Quality Expert Group (AQEG) concluded that the benefits of deposition are very limited: it removes only a few per cent of particulate matter and perhaps an even smaller fraction of nitrogen dioxide.

In our streets, GI’s potential to improve air quality primarily lies in its ability, not to remove pollution by deposition, but to control its dispersion (i.e., distribution close to source) and thereby reduce public exposure.

First and foremost, we must reduce vehicle use, and thereby reduce road transport emissions; GI has a role to play in creating attractive environments that incentivise active travel, such as walking and cycling.

However, we can also use GI to create vegetation barriers that reduce the public’s exposure to what is emitted, and thereby further improve public health outcomes.

Note: the ban on sales of petrol and diesel vehicles from 2040 will reduce exhaust emissions of, for example, nitrogen dioxide.  It will not have the same impact, however, on the emissions of particulate matter, a significant fraction of which come from break, tyre and road wear.  These sources of road transport pollution will be undiminished by electrification, and could even increase due to the weight of electric vehicles.

Permeable barriers

Vegetation barriers are permeable (i.e., allow some air to pass through them) but they force a fraction of air to take a different path around them. This can improve local air quality in two ways. Firstly, by forcing polluted air to take a longer path from its source  (i.e. vehicles) to receptors (e.g. pedestrians and cyclists), we increase the extent to which it mixes with surrounding air en route, and reduce the concentrations to which those people are exposed.

On a busy road bounded by buildings, the surrounding air will still be polluted, but importantly less polluted than a parcel of air beginning its journey at the exhaust pipe or a point of friction between brakes and wheels, or wheels and road.

Under the right conditions, a second process comes into play.  When the wind blows directly and consistently from source to receptors, a dense vegetation barrier is introduced between the two, and there is sufficient open space beyond the barrier, major reductions in exposure to local sources of pollution can be achieved: AQEG reports that the concentrations of pollutants emitted immediately upwind of the barrier can be as much as halved in its immediate wake.  (Downwind of a 2m high hedge, for example, the reduction may extend only a few metres.)

Under these conditions, a small sheltered region is created immediately downwind of the barrier, like a windbreak, that is somewhat bypassed by pollution originating immediately upwind of it.

The trick is to create these small sheltered regions where people are predominantly exposed to pollution, whilst avoiding putting barriers in the wrong places where they could trap pollution and increase exposure.

There is no ‘one size fits all’ solution.  Exemplifying the ‘Right Tree, Right Place’ philosophy, it requires careful consideration of local conditions of wind, urban form and interactions between the two.  Get it right, however, and GI can deliver

reliable, localised improvements in air quality and make valuable contributions to sustainable urban drainage, thermal comfort and biodiversity, not to mention attractive, life-affirming environments.

How can I determine what GI is appropriate to improve air quality?

We are currently developing a prototype ‘GI4RAQ Platform’: free, web-based software to predict the impacts of GI on roadside air quality (RAQ) on a site by site basis.  Meanwhile, please refer to the guidance we recently wrote in partnership with the Greater London Authority. It is about the overarching principle of ‘Reduce, Extend, Protect’: first reduce emissions; secondly, extend the source-receptor pathway (see above); and, finally, protect the most vulnerable (i.e., the young, the old, and those with pre-existing medical conditions, such as Chronic Obstructive Pulmonary Disease).

Improving air quality, not only where the concentrations of pollutants are highest, but the greatest numbers of people – and most vulnerable people – are exposed for longest, will deliver the greatest benefits for public health and health equality.

Dr James Levine will be speaking at the seminar on October 17 from 8:30am – 3pm at The Lowry, MediaCityUK, Salford, and Greater Manchester.

Bookings are via Eventbrite only  – https://www.cityoftrees.org.uk/event/role-green-infrastructure-gi-create-healthy-resilient-cities