A passenger jet with 162 people on board was forced to descend 25,000 feet (7,620 meters) in 10 minutes when the co-pilot apparently tried to hide the smoke from his e-cigarette, Chinese media quoted the country's civil aviation authority as saying on Friday.

The Air China Boeing 737 was flying from Hong Kong to the Chinese city of Dalian when the incident took place on Tuesday, according to the South China Morning Post.

Chinese officials said that the plane's co-pilot was vaping in the cockpit. Without telling the captain, the man apparently tried to turn off a circulation fan to prevent the vapors from his e-cigarette spreading to the main cabin.

Read more: E-cigarettes may harm unborn babies, immune system, say scientists

However, he "mistakenly switched off the air conditioning unit that was next to it, resulting in insufficient oxygen in the cabin and an altitude warning," said Qiao Yibin of China's air traffic regulatory body CAAC. The shutdown caused oxygen levels to drop, prompting the plane's emergency systems to instruct the pilots to start descending immediately.

Oxygen masks were also deployed as part of an emergency procedure. Pictures and videos posted on social media showed the masks hanging from the ceiling, but no noticeable panic among the passengers.

After the plane dropped to around 3,000 meters, the pilots realized the problem and switched the air conditioning back on, before returning the aircraft to its cruising altitude and completing the journey to Dalian. Nobody on board was injured.

Read more: Chinese pilot sucked halfway out of airplane makes successful emergency landing

Regulators said they had seized the flight data and audio from the cockpit voice recorder, and also questioned the plane's crew. If the preliminary information is confirmed, the authorities will "deal with it seriously," Qiao Yibin said.

Separately, Air China said there would be "zero tolerance" for pilots smoking in the cockpit.

The airline prohibits smoking, including e-cigarettes, on all its flights.

dj/nm (AFP, dpa, Reuters)

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