Heathrow could build a new runway without breaking European pollution laws, new independent research suggests.
The study seen by the BBC measured poisonous nitrogen dioxide (NO2) levels using 40 sensors in and around the airport. It then used modelling to predict what would happen in the future.
Ministers will decide within weeks whether to enlarge Heathrow or rival Gatwick and the environmental impact will play a big part in that decision. The work was led by the University of Cambridge and has no formal links to any airport or the government.
Professor Rod Jones from the University of Cambridge told the BBC: "If there is the development of a third runway, we expect there to be a marginal increase in NO2 coming from the airport itself, but that would be against the background of reduced NO2 from other traffic, because of Euro 6 engines and electrification of the traffic fleet."
This means that it comes down to traffic on the roads, rather than aircraft in the air, because that is where the bulk of the poisonous nitrogen dioxide gases come from.
As new, cleaner car, lorry and bus engines become more common, pollution levels should decline, wiping out any increase from a bigger Heathrow.
Prof Jones said using lots of smaller sensors, dotted in hard to reach places, gave them a clearer picture of what was going on.
"By deploying a network of sensors we can tell directly from the measurements, what's been emitted locally from Heathrow airport and what's been blown in, mostly from central London. That's the real strength of the sensor network," he said.
"The major result from this study is that we have tested the models far more critically than you can from a single measurement site."
Currently, the air has climbed above European health limits at several sites near Heathrow. This work suggests that as cleaner engines kick in levels will fall again.
The latest, million-pound research was funded by the Natural Environment Research Council and alongside Cambridge, experts from the universities of Manchester and Hertfordshire, Imperial College London, CERC Limited and the National Physics lab where involved.
Heathrow helped them put the sensors up and British Airways provided some flight data, but neither handed over any money or were involved in the actual work.
Ministers still have three options for airport expansion - a third runway at Heathrow, a second at Gatwick, or a third scheme that would double the length of one of Heathrow's existing runways.
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