ON JANUARY 10th 1946 the cabinet approved plans for a third runway at Heathrow airport, west of London. Some 72 years later, and after more than a dozen commissions, reports and white papers on where to put it, the third runway remains unbuilt. The project moved a step closer on June 25th, when Parliament voted by 415 to 119 to build a new runway to the north-west of Heathrow. But a big problem could delay it still further: air pollution.

This topic matters, as the issue of emissions resulted in the High Court overturning a previous decision to build a third runway in 2010. In response to this week’s vote, four west London councils, the city’s mayor and Greenpeace, an environmental group, said they would apply for judicial review within six weeks. Ray Puddifoot, the leader of Hillingdon council, where the airport is located, thinks their case is even stronger than in 2010. “I’ve got no doubt that we will succeed,” he says.

The first issue is carbon emissions. Britain has a target to cut these by 80% from 1990 levels by 2050. To be on course to meet this goal, the Committee on Climate Change, an independent advisory body, says that Britain’s aviation sector needs to cap its emissions at 37.5m tonnes of the gas a year. But with an expanded Heathrow, it could produce over 40m by the 2030s. And the north-western site for the new runway will produce more carbon emissions than two alternative sites that were considered.

The second problem is emissions of harmful nitrogen oxides and particulates from vehicles. Heathrow is already the most polluted area of London outside the centre (see map). Nitrogen dioxide levels are rising and several spots near Heathrow already break the EU’s limits for the gas.

The airport (whose chairman, Paul Deighton, is also on the board of The Economist Group) has been slashing its own emissions, for instance by charging gassier planes more to land. But it is not on the airport site itself but at nearby major roads that EU limits are being broken, by people driving to and from it. Heathrow wants to raise the share of its passengers using public transport from 39% to 50% by 2030, by using congestion charging on local roads. But the airport lacks the power to impose such a scheme. It may also annoy locals if drivers clogged up backstreets while seeking to avoid charges on main roads.

Matthew Coogan, a transport expert, says people might be nudged out of their cars if the airport moved its car parks and drop-off areas far from the terminals, and made life easier for those arriving by train. Heathrow must hope such explanations of how it could meet its targets hold water in court. Otherwise it may be decades more before London gets its new runway.