Should arborists be pushing trees' ability to improve air quality?

The issue of air quality is becoming more contentious at both national and local level, which may provide an opportunity to press the merits of urban tree planting and maintenance as part of the solution.

Image: HW
Image: HW

In yesterday’s Budget (22 November), the chancellor confirmed details of a new £220m Clean Air Fund to support the implementation of local air-quality plans. The Government has already been found in breach of its own air-quality standards, set down in law in 2010, with pressure group ClientEarth leading a campaign to hold it to account — an issue also taken up in national media campaigns.

Air quality is increasingly a factor in determining development projects, most prominently the third runway proposal at Heathrow Airport, but also in many smaller local housing and other projects.

In light of this, the cross-sector Trees Design Action Group (TDAG) and the Birmingham Institute of Forest Research (BiFor) will shortly be publishing a pamphlet on the subject, First steps in urban air quality for built environment practitioners, with a full guidance document and website by the middle of next year, to help practitioners assess the likely impact of vegetation on air quality.

BiFor director Professor Rob MacKenzie, who specialises in how plants affect air composition, tells Horticulture Week: "Right now the national news is doing a lot of the work for us. The profile of the pollution issue makes the issue easier to get across."

Having just given a talk to the Chartered Institution of Building Services Engineers on the subject, he says: "Building services people care about indoor air quality, so they care about outdoor air quality because that’s where it comes from."

He adds: "In the building profession there are a lot of professions, each making up a link in the chain, but not many forums where they all get together. Even TDAG doesn’t reach right across the sector."

But he cautions: "People trying to get something through planning are looking for a particular set of answers, but some things are genuinely difficult and expensive to work out." This includes the often vexed question of how roadside trees affect vehicle emissions, he says – "and that’s the one they want answers to most often".

He adds that research here "appears more contradictory than it really is", explaining that in some settings, removal of air-borne pollutants onto trees’ leaves "wins", while in others, trees serve more to prevent their dispersal.

But MacKenzie says urban greening is "coming anyway" — driven forward more by growing awareness of the need for rainwater attenuation, which is "driving the agenda", he says. "Air quality is often mentioned but is harder to be categorical about."

Greening measures

Ken Scarlett is consulting arborist with New Zealand’s GeoArb and managing director of Brighton-based environmental consultancy Advertant, which has run workshops on the contribution of greening measures to urban air quality. "Ultimately the sticking point is that along roads, trees will not reduce the pollution in the actual tarmac road envelope, they only act as a barrier to air dispersal," he says.

"However, to protect people and buildings, and to retain pollution rather than pass it on, GI [green infrastructure] is the solution. It’s the application of it that is the skill, and is a business opportunity that arbs and horts should be jumping on. Now our industry knows how to combine trees with hard surfacing, whether it be the Stockholm method of tree planting or plastic Silva cells, it is time we sold this advice and contracting skill to clients worried about air quality."

With suitable training, arboricultural consultants "could measure and certificate individual properties, and offer GI remedies and mitigation", he suggests.

To further this, he says the tree and landscape industries, together with others who have an interest, should form "a lobby group which attends air-quality events, parliamentary dos, consults on white papers, changes MPs’ and public opinions, and steps in front of the camera at every opportunity".

He adds: "Economics is the driver that causes pollution and it will be the driver that fixes it. Commercial business property should be forced to have a yearly air-quality and GI audit, and councils should be enforcing levies on businesses that do not reach GI goals. The real fear of air quality degrading your property and prematurely killing you is powerful, and a remedy of GI will sell. The arb industry needs to step up or miss out."

Trees "do more good than harm" for asthmatics

The latest in the growing body of evidence on urban trees’ positive impact on air quality and public health comes from a new study by the University of Exeter’s medical school, which found that people living in polluted urban areas are far less likely to be admitted to hospital with asthma when there are many trees in their neighbourhood.

The study, published in the journal Environment International, looked at more than 650,000 serious asthma attacks in England over a 15-year period. It found that in the most polluted urban areas, trees had a particularly strong association with fewer emergency asthma cases, though their impact was smaller in less polluted neighbourhoods.

Research leader Dr Ian Alcock says: "We know that trees remove the air pollutants which can bring on asthma attacks, but in some situations they can also cause localised build-ups of particulates by preventing their dispersion. We found that on balance, urban vegetation appears to do significantly more good than harm."

More than 5.4 million people receive treatment for asthma in the UK with an annual cost to the NHS of around £1bn, and it is the cause of more than a thousand deaths a year.

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