Emergency crews were called to a London train station after CS gas was released in the ticket hall during rush hour.
Witnesses said people were "coughing and spluttering"
One person was treated for an asthma attack after gas was set off at Ealing Broadway, a BTP spokesman said.
He said the gas could have been released by "any one of thousands" of commuters and no-one has been arrested.
It caused disruption during evening rush hour. The station, which serves both Tube and main line trains, was later reopened.
The incident happened around 6.45pm and led to District and Central Line trains terminating at the stops before Ealing Broadway.
Main line trains passing through the station did not stop.
Six fire appliances were sent to the scene after ambulance crews alerted them to a suspected "tear gas" incident, a spokeswoman for the London Fire Brigade said.
Witnesses described seeing people emerging from the station "coughing and spluttering".
However, the immediate area was not evacuated and other witnesses said there did not appear to be a sense of panic.