Similar high readings of PM 2.5 – tiny airborne particles that get embedded in lungs – were reported from regions around Beijing, including the port city of Tianjin and Hebei province.

The Air Quality Index (AQI) touched the 500-mark, the maximum, for all monitoring stations in Beijing.

“Concentrations of PM 2.5, tiny airborne particles that embed deeply in the lungs, peaked at 900 micrograms per cubic metre in southern Beijing,” the municipal environment monitoring centre said.

The World Health Organisation’s recommended maximum is 25 microgram per cubic metre, the official Xinhua news agency reported.

It added this was the worst air pollution faced by Beijing and much of northern China.

An “orange” alert issued by the Beijing municipality remained in place on Tuesday. Schools remained open but students were advised to stay indoors; construction sites in and around the city were shut down.

Visibility rapidly reduced through the day and the capital’s roads were packed with crawling traffic, adding to the dangerous haze that has been hanging over Beijing since last week.

The Beijing Environmental Protection Bureau said the burning of coal for heating and vehicular emissions have increased significantly in northern China with the onset of winter.

“Complex climate conditions, where pollutants are pressed in the lower atmosphere by cold air, also contributed to the thickness of the smog, and the situation deteriorated as smog built up around Beijing was blown into the capital by wind,” the Xinhua report said.

Residents of Beijing, forced to breathe air with a pungent chemical odour, took to humour on social media to breathe a little easy.

“The smog is so heavy that a man with my height cannot see my feet anymore,” said one user on WeChat, a popular smartphone application like WhatsApp.

Another joke making the rounds went like this: “A journalist: Ms, do you have much trouble with the smog in Beijing? Ms: Indeed much trouble: To begin with, I am Mr not Ms.”