Firefighters were called to a chemical incident at a leisure centre on Watford Road in Wembley.
Crews carried out a sweep of the property, where a chlorine leak was found in a learner pool on the ground floor of the three-storey building. Firefighters ventilated the area and monitored the levels of fumes until there were no elevated readings.
Around 150 people left the building before the arrival of the Brigade. Two adults and nine children were taken to hospital by London Ambulance Service.
The Brigade was called at 1342 and the incident was over for firefighters by 1543. Two fire engines, two Fire Rescue Units and around 20 firefighters from Northolt, Wembley and Euston fire stations attended the scene.