The US Embassy in Beijing regularly tweets its Air Quality Index (AQI) readings ( @Beijingair ), reputedly the most reliable indicators in the city. It describes them as a ‘yardstick that runs from 0 to 500’ but on Saturday, the reading reached 886.

Screenshot of the US Embassy in Beijing's tweeter feed @Beijingair

By Sunday afternoon, the readings were downgraded to the maroon-coloured ‘hazardous’.

Chart from the US Embassy in Beijing's website

In a rare move, Zhang Dawei, of the Beijing Municipal Environmental Protection Bureau, told a news conference on Monday that “the pollution has affected large areas, lasted for a long time and is of great density”. He blamed growth in heavy industry, vehicle ownership, reliance on coal power for causing the haze, and low pressure, non-windy meteorological conditions for trapping it.

Video posted on YouKu (Chinese version of YouTube)

In the past, Chinese authorities preferred to play down the dangers of air pollution or remain mysteriously quiet at times when the smog was particularly bad. In 2009, WikiLeaks published a cable suggesting Chinese officials asked the US Embassy to stop tweeting about pollution in Beijing on the grounds the information was ‘confusing’ and could have ‘social consequences’.

Cable published by WikiLeaks (October 10 2009)

The authorities are urging Beijing’s residents to stay indoors and leave their cars at home. Schools were closed in some of the worst-hit districts, like Tongzhou, Daxing and Fangshan, and the authorities banned many other schools from outdoor activities like sport and morning flag ceremonies. Factories have been told to scale back emissions (albeit temporarily), officials have been told not to get in their cars and building sites have received a dosing of water spray, to dampen down the dust particles.