An Air China flight from Hong Kong to Dalian was forced to make an emergency descent after the plane lost oxygen and cabin pressure due to a co-pilot error. According to the Telegraph, the navigator was reportedly trying to smoke an e-cigarette while flying the aircraft.

"In the preliminary investigation, the co-pilot was found to be smoking an e-cigarette," China News explained. The 737 aircraft had 153 passengers and nine crew members aboard the three-hour flight.

"Smoke diffused into the passenger cabin and relevant air conditioning components were wrongly shut off, without notifying the captain, which resulted in insufficient oxygen,” said Qiao Yibin, an official of the regulator's aviation safety office.

According to CNN, an official from the Civil Aviation Administration of China (CAAC) told reporters that the co-pilot tried to turn off the ventilation system to keep the smoke from spreading into the cabin.

The move set off an alarm, which released the oxygen masks, forcing the pilots to descend almost 20,000 feet in less than nine minutes. After the drop, the crew managed to restore the air conditioning and the cabin pressure, as well as to return to cruising altitude.

Rapid loss of cabin pressure can have deadly consequences. In 2005, a Helios Airways flight from Cyprus to Greece crashed into hills outside Athens, killing all 121 people on board after a loss of cabin pressure incapacitated the crew, which left the aircraft on autopilot until it ran out of fuel.

"I didn't think too much of it at the time — we didn't know what was going on, nor did the flight attendants it seemed," Hoby Sun, a passenger aboard the Air China, told CNN. "I'm not physically hurt, but the psychological impact lingers. When I close my eyes, I see the oxygen masks dangling in front of me.”

Air China has not yet released an official statement, though the Telegraph reports that they will have a "zero tolerance" approach to the investigation.

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Air China, a member of the Star Alliance global network, has a fleet of more than 600 planes. Last year, the airline transported over 102 million passengers across six continents. Its last and only fatal accident was in 2002 when a Boeing 767 jetliner crashed into a hill near Busan, South Korea, during bad weather. The crash resulted in the death of 129 of the 166 people on board.