Up to 1.6 million people could be affected by noise from Heathrow's planned third runway, researchers have claimed.
At peak times up to 47 planes an hour could fly over residents living near the west London airport, according to Greenpeace analysis.
A spokesman for Heathrow dismissed the claims, saying final flight paths have yet to be decided.
He added that plans aim to reduce the number of people affected by noise.
The new runway is due to be operational by 2026.
Analysis of possible flight paths for the new runway showed that more than 11m people live within areas identified as at risk of flights at levels above 65 decibels, according to Greenpeace Unearthed.
That volume is comparable to the noise level in a busy office.
The 11m figure is expected to be significantly reduced when officials settle on flight paths.
But experts claimed that the 1.6m people living in sections of the proposed flight paths closest to the airport are likely experience noise levels at or above 65 decibels.
Last year the Commons Transport select committee reported that a third runway was likely to cause “significant annoyance” for 653,000 people by 2030.
A spokesman for Heathrow rejected the analysis, saying the final flight routes had yet to be decided.
He said: “As part of our extensive consultation process, we are proposing options that would reduce the airport’s noise impacts.
“Our plans are designed to ensure that fewer people will be affected by noise than were affected in 2013, thanks to quieter planes, quieter airport design, quieter operations and a 6.5 hour ban on scheduled night flights.”
Heathrow will start a consultation on the expansion next month, including the layout of terminal buildings.
The airport won the controversial bid to build an extra runway, beating out south London airport Gatwick, who sought to build a second runway.
The expansion plans have been the subject of much criticism, anti-climate change demonstrations and has already faced a High Court challenge at the Royal Courts of Justice.
Plans aim for a two-mile runway northwest of the existing airport to be operating by 2026.
The third runway will increase traffic to the airport from 480,000 flights a year to 740,000.
