After the incident, several Chinese state-owned media outlets revealed that the sudden drop in altitude was believed to have been the result of pilots accidentally turning off the air-recycling fans while they were smoking in the cockpit mid-flight, reported CNN. During the Hong Kong to Dalian flight on Tuesday evening, the plane suddenly lost pressure and dropped around 25,000 feet, from 35,000 feet to 10,000, within ten minutes, according to flight data. Some passengers onboard the flight pulled out their phones as the oxygen masks dropped from the ceiling and posted the images to Chinese social media after the incident. Kai Tak: Hong Kong’s hair-raising airport closed 20 years ago 
 
 One, writing on Chinese microblogging platform Weibo, said: “The announcement from the cockpit said the ability to increase oxygen in the cabin malfunctioned so the plane lost pressure.” 
 
 According to tracking website FlightRadar24, the 159-seater Boeing 737 suddenly began descending at 7.40pm between Shantou and Xiamen, half an hour after it took off from Hong Kong International Airport. An announcement was heard over the cabin speakers saying the aircraft was carrying out an emergency descent due to cabin decompression. Hong Kong’s Civil Aviation Department said it had no information on CA106 from after it left the city’s airspace. “In the preliminary investigation, the co-pilot was found to be smoking an e-cigarette,” state-owned China News said, citing a news conference by the Civil Aviation Administration of China (CAAC) investigating Tuesday’s incident. "If the investigation discovers crew behaviors that have violated rules and regulations, we will adopt a zero-tolerance attitude and seriously punish those found responsible," the airline said in a statement. Some Wednesday flights operated by Air China were cancelled, including the 7.45am departure to Beijing, which is usually carried out by the plane serving flight CA106.
– "When I close my eyes, I see the oxygen masks dangling in front of me." That's the lingering anxiety of a passenger on a recent Air China flight that plummeted thousands of feet after oxygen levels dropped. A senior official from the Civil Aviation Administration of China tells CNN the reason for the descent stems from a co-pilot smoking an e-cigarette. The official says the pilot tried to shut off air-recycling fans so the vapor wouldn't reach the cabin, but "toggled the wrong switches," causing air pressure to drop. After the plane fell Tuesday during a three-hour flight from Hong Kong to the Chinese city of Dalian, pilots were able to bring the plane back to around 25,000 feet, and it landed safely. Various sources are listing different numbers for how far the plane actually dropped, but flight data cited by Newsweek shows an approximately 21,000-foot decrease over 18 minutes. Per the South China Morning Post, a couple of the passengers posted pics and video on social media showing people donning the oxygen masks that came down, though the mood was relatively calm. "The announcement from the cockpit said the ability to increase oxygen in the cabin malfunctioned so the plane lost pressure," one wrote on Weibo. Reuters notes that even though China's airlines have a pretty decent safety record, pilots are sometimes accused by passengers of smoking during flights. China bars flight crews from "smoking on all phases of operation," and passengers there haven't been able to use e-cigarettes on planes since 2006. Air China says it will "adopt a zero-tolerance attitude and seriously punish those found responsible," depending on the results of the CAAC's probe.