Technical documents, not published at the time of the original proposals but prepared for the government by consultants Halcrow, show that the air traffic generated by a third runway would expose local people to levels of nitrogen dioxide that would be illegal under EU legislation.
Acknowledging it would be impossible to go ahead with the runway in the knowledge that it would be breaking the law by doing so, the government says: "Another runway at Heathrow could not be considered unless the government could be confident that levels of all relevant pollutants could be consistently contained within EU limits."
The Halcrow documents calculate how much of each of the pollutants from extra traffic and planes would be caused by proposed developments. In the case of nitrogen dioxide, a lung irritant and a problem for asthmatics, a large area round the airport would exceed maximum allowed levels.
Between 12,000 and 15,000 homes would have to be pulled down if some other method of reducing the pollution could not be found. The cost in compensation would be around £2bn at current house prices. The Harlington area and M4 corridor would be the worst affected.
The government consultation document says: "Our modelling shows that without very significant improvements in aircraft performance, an extra runway at Heathrow would lead to homes being exposed to exceedences in respect of the NO2 limit."
Although road traffic is one cause, the main rise results from extra aircraft, the report says. The privatised arm of the atomic energy authority, AEA Technology, has calculated that nitrogen dioxide emissions from aircraft in the UK will increase from 12.89 kilotonnes a year in 2000 to 22.49 kilotonnes a year in 2020.
The Halcrow report says it is possible to reduce the amount of nitrogen dioxide emitted at Heathrow by reducing thrust of aircraft at take-off, reducing reverse thrust at landing, improving engine performance, and using electric vehicles to tow aircraft to take-off point. This could reduce total emissions by half, but it suggests these mitigation methods are both "optimistic and demanding". It would also reduce the number of aircraft using the airport because take-off and landing procedures would be slowed.
Experts have already advised that these suggestions would be illegal under international air traffic rules because it would entail limiting the kind of aircraft that could take off and land at Heathrow.
The government has told the airlines and the British Airports Authority that unless they can bring suitable clean technology into use, or confirm that they will accept restrictions on access to airports by aircraft that do not meet the most exacting emission standards, they must "undertake fully to fund the purchase (and if necessary, demolition) of properties... and to properly compensate the owners."
The authority said it was preparing its comment on the issue, which would be included in its reply to the government's consultation on the future development of air transport in the UK by the November 30 deadline.
