A third runway at Heathrow would triple the number of early deaths from pollution linked to the airport, a new study claims.
In contrast, moving London's main airport to the Thames Estuary - a plan championed by London mayor Boris Johnson - would cut the number of deaths, the report by the Massachusetts Institute of Technology (MIT) and Cambridge University says.
They say a third runway at Heathrow would increase deaths from air pollution linked to the airport from the current 50 a year to 150 by 2030, with deaths more than doubling to 110 a year even if the extra runway is not built.
Nationwide, the number of deaths linked to aviation pollution at current capacity will rise from 110 annually today to 250 in 2030, the report says.
A third runway at Heathrow would add a further 10 deaths a year because of an expected reduction in pollution at other airports, which will lose flights to the new, expanded hub.
The scientists say that moving the UK's main airport to the Thames Estuary, the plan championed by the London mayor, could cut deaths by up to 70%.
"On a nationwide basis, early deaths due to UK airport emissions decrease by a quarter relative to an unexpanded Heathrow," they wrote in the report by MIT's Laboratory for Aviation and the Environment, which looked at pollution emissions at the top 20 UK airports.
"In other words, airport capacity would be expanded and health impacts reduced under the Thames Hub scenario."
The difference is explained by the south-westerly prevailing wind, which currently blows pollution north east from Heathrow into London, according to the report.
Pollution from a Thames Estuary airport would be carried out to sea.
MIT said a full academic paper on the subject had been accepted for publication in the journal Atmospheric Environment.
Expanding Heathrow during the current parliament was ruled out in the coalition agreement in 2010.
The Government has appointed former Financial Services Authority chief Sir Howard Davies to lead an independent commission into airport capacity needs.
But his final report is not due until summer 2015 - after the next general election.
Mr Johnson repeated his call for a new "Boris Island" airport to the south-east of the city at the Conservative Party Conference last week, adding: "We are not taking the decisive action that we need to make ourselves competitive with other European countries, with Dubai, with all these competitors of ours that are putting on new runways."
A mayoral spokeswoman said today: "The Mayor's team are keen to examine this latest research, which potentially adds further weight to why the Government should rule out now the possibility of expansion at Heathrow.
"There are clear environmental benefits from meeting the increasing demand for aviation at a hub airport located to the east of London, rather than the madness of expanding an airport that is already constrained by location, straining at the seams and in the heart of the western suburbs of London.
"The right thing for the Government to do is to return to the pledges it was elected on and rule out Heathrow expansion before the Davies Commission starts its work."
A Department for Transport spokesman said: "Maintaining the UK's status as a leading aviation hub is vital to our economy. This is why we have set up an independent commission to look at all the options, which will naturally include an assessment of the environmental and social impacts of any proposal."
A Heathrow spokesman said: "The issue of air pollution is one faced by all major cities across the world. Aviation is a far smaller contributor to air pollution than road traffic, however we are already taking significant steps to tackle the problem. For example, we subsidise local public transport so people can travel for free without the need for a car.
"We also charge airlines based on how green they are - so the cleanest aircraft are charged less to land at Heathrow."
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