They're trying to ventilate the site in Stratford - and are asking residents who're nearby to close their doors and windows.
A number of patients are being treated by paramedics.
EARLIER -
Emergency teams in London are dealing with a "major incident" thought to involve the release of a gas at the Olympic Park's aquatics centre
A number of people are being treated for breathing difficulties at the site.
It's not clear if chlorine's involved, which is typically used in swimming pools.
Paramedics and fire crews are at the scene, and the area's been cordoned off.
