Firefighters were called to a report of a chemical substance leaking from a backpack at North Greenwich Underground station.
Crews worked to isolate the leak and make the scene safe. Around 300 people left the station before firefighters arrived. Four people were treated at the scene by London Ambulance Service crews.
The Brigade was called at 1750 and the incident was over for firefighters by 2039. Crews from East Greenwich, Poplar, Bethnal Green and Bexley fire stations attended the scene.