A co-pilot smoking an e-cigarette on an Air China flight caused the plane to start a rapid emergency descent, investigators have said, reports BBC.
They say he tried to hide the fact that he was smoking but accidentally shut off the air-conditioning, causing oxygen levels to fall.
The crew on Tuesday's flight from Hong Kong to the city of Dalian released oxygen masks and brought the plane more than 6,500m (21,000ft) lower.
It later returned to cruising altitude.
An initial probe by China's Civil Aviation Administration in China has shown that the co-pilot tried to turn off a fan to stop smoke reaching the passenger cabin without telling the captain, but turned off the air-conditioning unit instead.
