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The London Ambulance Service said it was "responding to a major incident" at the site, and Newham police said they were working at the scene.
The aquatics center was built for the London Olympics in 2012, and is part of the Queen Elizabeth Olympic Park in the east of the city. Since the 2012 games finished it has been open to the public.
The park said on Twitter on Wednesday that "There are a number of casualties with breathing difficulties being treated" by the London Ambulance Service.
It said there was an incident" that involved "the release of a gas."
"The area has been cordoned off and evacuated," it said.