As the most reliable and balanced news aggregation service on the internet, DML News offers the following information published by DailyMail:

An Air China jet that plunged 25,000ft in an emergency descent did so after a co-pilot mistakenly turned off air-conditioning systems in a bid to conceal his e-cigarette smoke.

Smoking is not allowed aboard Chinese commercial passenger flights.

The article goes on to state the following:

The incident, which resulted in the deployment of passenger oxygen masks, occurred Tuesday on a flight by the Chinese flag carrier from Hong Kong to the city of Dalian in northeastern China.

The Civil Aviation Administration of China (CAAC) said in a press conference in Beijing that the drama was triggered when the co-pilot sought to turn off a ventilation system to prevent his smoke spreading into the main cabin, the respected news site Caixin said.

But he accidentally switched off air-conditioning instead, leading to a decrease in cabin oxygen levels.

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