New, independent research seen by the BBC suggests Heathrow airport could build a new runway without breaking European pollution laws.
The study 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.
However, the Aviation Environment Federation (AEF) said the research was "highly speculative" and there was no guarantee pollution levels would fall.
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.
Prof 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."
In other words, it comes down to traffic on the roads, rather than planes 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."
Airport expansion has been shoved under a carpet in Downing Street for decades, but now the Conservatives say they want to get on with it.
The current "hot date" for a decision is 18 October, but it could change.
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