British Airways, which commissioned the tests, said the results strengthened the case for another runway and demonstrated that pollution from aircraft was not a problem at Heathrow.
But campaign groups opposed to the expansion of the airport immediately accused BA of "fiddling the figures", arguing that the test procedures it had used were inadequate and designed deliberately to mislead.
A government White Paper in 2003 turned down a third runway at Heathrow in favour of a second one at Stansted on the grounds that nitrogen dioxide (NO2) levels around the west London airport would exceed the limits set down in an EU directive, which comes into force in 2010. It said the earliest a third runway could be considered was between 2015 and 2020.
But BA said yesterday the latest tests, carried out by AEA Technology and analysed at its National Environmental Technology Centre, showed that the limits were not being breached. BA said that at five of the eight test sites around Heathrow, levels of NO2 were below the limit of 40 micrograms per cubic metre laid down in the directive. The test sites where the limit was exceeded were inside the airport, on the perimeter and next to the M4 motorway while those where it was within the limit were in residential areas.
At the time of the White Paper, consultants working for the Department for Transport calculated that 14,000 residents living close to Heathrow would suffer NO2 levels above the EU limit even without the building of a third runway. But BA said its tests showed that few, if any, residents would be affected by excessive pollution. Willie Walsh, BA's chief executive, said: "The notion that flying is a selfish, antisocial activity that single-handledly threatens planetary catastrophe bears no relation to the evidence."
Clive Soley, the Labour MP who chairs the Future Heathrow lobby group, said: "These test results do improve things, which is a relief because I am concerned about the continuing decline of Heathrow. It is clear that the problem is pollution from road transport, not aircraft." The onus, he added, was now on the airport authorities to achieve zero emissions from vehicles within the airport perimeter.
But Pete Lockley of the Aviation Environment Federation, said: "The claim by BA does not stand up. They have used data from diffusion tubes, a method that the EU says is inadequate. It's totally irrelevant whether the pollution comes from aircraft or cars - pollution levels have to come down below the limits, full stop. The real question is whether the levels would remain below the legal limit if a third runway was built and mixed mode introduced. If they're at or around the limits now, where will they be with a third more planes taking off?" John Stewart, the chairman of the pressure group Hacan Clearskies, said: "It is no exaggeration to say that BA has been caught fiddling the figures. They seem to be deliberately seeking to mislead."
The DfT is soon expected to announce arrangements for monitoring the air at Heathrow as part of a review of the 2003 White Paper due to be published at the end of this year. Whether to sanction a third runway at Heathrow will be an early test for Douglas Alexander, the new Transport Secretary, particularly as BAA admitted this week the second Stansted runway may not be ready until 2015-16 - some three years later than planned.