Thick, grey smog has fallen over Beijing, choking China’s capital in a haze that spurred authorities to cancel flights and close some highways in emergency measures to cut down on air pollution.

Beijing and much of industrial northern China are in the midst of a “red alert”, the highest level in China’s four-tiered pollution warning system.

The red alert has affected 460 million people, according to Greenpeace East Asia, which calculated that about 200 million people were living in areas that had experienced levels of air pollution more than 10 times above the guideline set by the World Health Organisation.

Traffic in heavy smog in Beijing where authorities have declared a five-day pollution “red alert”. Photo: Getty.

Members of the public closely watch levels of PM2.5, particles measuring 2.5 microns across that are easily inhaled and damage lung tissue.

The World Health Organisation designates the safe level for the tiny, poisonous particles at 25 micrograms per cubic metre. On Tuesday morning, the PM2.5 reading in Beijing climbed above 300.

In many northern Chinese cities, the reading has exceeded 500 micrograms per cubic metre.

State media reported that 169 flights have been cancelled at Beijing Capital International Airport, where visibility fell at one point to 300 metres.

Beijing’s skyline shrouded by the smog. Photo: AAP.

Sections of Beijing’s sixth ring road, the outermost highway encircling the city of more than 20 million people, were shut down in a bid to keep cars off the roads.

Authorities have even removed charcoal grills from restaurants and banned spray painting in parts of the city, state media reported.

Since the red alert went into effect, more than 700 companies stopped production in Beijing and traffic police were restricting drivers by monitoring their licence plate numbers. Dozens of cities closed schools and took other emergency measures.

In nearby Tianjin, authorities cancelled 350 flights and closed all highways in the municipality. Public transportation services were increased as restrictions on cars were imposed.

Authorities in the northern province of Hebei ordered coal and cement plants to temporarily shut down or reduce production.

Elsewhere, hospitals prepared teams of doctors to handle an expected surge in cases of pollution-related illnesses.

The alert is expected to end on Wednesday.