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Firefighters were called to a suspected chemical incident at a flat on Lady Margaret Road, Southall.
Fire crews using detection and monitoring equipment carried out a sweep of the first-floor flat and found elevated readings of hydrogen sulphate and ammonia, believed to have been caused by drain cleaner.
They ventilated the premises and isolated gas and electricity supplies.
A woman and three children were taken to hospital by London Ambulance Service crews as a precaution.
The Brigade was called at 0041 and the incident was over at 0314. Two fire engines, two fire rescue unit and firefighters from Southall, Wembley and Bethnal Green fire stations attended the scene.