London Ambulance Service treated patients at Tilston Bright Square in the Abbey Wood area of the capital.
The Metropolitan Police were also called to the scene where firefighters swept a nine-floor housing block.
London Fire Brigade did not disclose what they were looking for, saying only that they did not detect "any elevated readings".
Cops confirmed that there was some kind of chemical involved.
A Met spokesperson said: "Police attended Felixstowe Road at 0553hrs on Tuesday, 2 October where they assisted London Fire Brigade who had been called to a suspected chemical incident."
The police also confirmed that no one was arrested and that no offences had been committed, but it is still not clear what happened at the flats.
Reports about the event said that "green fumes" were involved at the residential site near a branch of Sainsbury's on Harrow Manorway.
A London Ambulance Service spokesperson told The Sun Online: "We sent two incident response officers and three ambulance crews. We also dispatched our hazardous area response team (HART).
"We treated 12 patients at the scene and took them to hospital."
More to follow…
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