Police were forced to rescue a man who was locked inside his gas-filled South Harrow flat.
Officers broke down the door of the property in Grange Farm Close at 4.40pm and evacuated a 20-year-old, who ended up in hospital.
One firefighter, part of one of two crews called to shut off the gas supply, said of the deadly vapours: "It was quite potent."
An emergency services source suggested the man had turned on his gas cooker and then shut himself in his home on purpose.
Police called the ambulance service who took the man to Northwick Park Hospital in Watford Road, Harrow, for treatment.