Historically, air quality campaigners have found it difficult to ensure their cause is given serious consideration when it comes to the formation of local plans and planning decisions. As barrister Rose Grogan, of barristers' chambers 39 Essex Chambers, puts it: "Air quality tended to be something buried in an environmental statement that nobody paid much attention to."
Now, however, things are changing thanks to a combination of factors. Firstly, there is the government's failure to meet certain air quality standards, as required by the EU Ambient Air Quality Directive 2008 and since enshrined in UK law in the Air Quality Standards Regulations 2010.
The government is now trying to formulate a national air quality plan to tackle the issue in short order. However, the quality of the plan has come under repeated fire from campaign group ClientEarth, which this month announced that it was returning to the High Court to challenge the plan for the third time.
"The directive says that if you're above certain limits you were supposed to come below it by 2010," said Angus Walker, partner at law firm Bircham Dyson Bell. "If you didn't manage that you had to do it by 2015. And if you didn't manage that then you have to do it 'as soon as possible'. So, the government keeps coming up with plans and it keeps losing because they're not 'as soon as possible'. They don't meet that test."
In addition to a certain adeptness in using the courts to further its aims, ClientEarth clearly also has an effective PR machine. Since the campaign group's legal battles began, air quality has become big news, with The Guardian and the Evening Standard in particular campaigning on the issue. Add to that the emissions scandal that saw several car makers forced to admit that the low emissions claimed for their vehicles were far from accurate and air quality has risen up the agenda.
As a result, both planning lawyers and consultants believe that the issue is becoming a source of contention in planning decisions far more frequently. "Those factors explain why it's becoming more important - people have latched onto the issue and done a load of publicity about it," said Grogan. "The public are therefore more concerned about it."
Certainly, air quality has come to the fore on two major infrastructure projects. "At the national level, we have last week seen further delays to the decision for the Silvertown Tunnel because of air quality issues," said Angus Evers, partner at law firm Shoosmiths. "This is the second time that the decision has been delayed."
Higher profile still, air quality is at the heart of objectors' concerns about a third runway at Heathrow. "One of things that's being looked at is whether you can consent major infrastructure development if it increases the risk of non-compliance (with the EU directive)," said Grogan. "Can the directive make development itself unlawful?"
But air quality is also of increasing importance in more local disputes, with several cases ending up in the High Court to demonstrate the fact. In September, for instance, a judge dismissed a legal challenge against a 4,000-home development on the edge of Canterbury in Kent that had been launched by campaigners concerned about the impact on air quality.
Then there was the 2016 case involving a proposed new ferry terminal in Greenwich, in which objectors claimed the planning authority had failed to take account of the cumulative effects on air quality. The objectors failed, but elsewhere air quality arguments have met with more success. In March, Wealden District Council succeeded in its efforts to quash parts of a joint core strategy put together by two neighbouring local authorities. And last week, a judge backed an inspector's decision to dismiss plans for up to 330 homes in Kent over air quality concerns.
So, do objectors think they have found a rich new seam of argument against development? "It's difficult to understand whether air quality objections are genuinely around air quality," said David Deakin, a director at consultancy AECOM. "I'm sure that a great deal are, but you do wonder with the coverage of air quality in the media whether it is possible for some objections to be made where it is being used as a proxy."
"There are quite a lot of arguments that you can make about how the air quality directive translates over into the planning system," said Grogan. "A lot of that hasn't been worked out by the courts as to how the European-level obligation that binds the secretary of state impacts on the decision-making of local councils. It's a massive question mark."