Hospital visits spiked, roads were closed and flights cancelled Monday as China choked under a vast cloud of toxic smog, with forecasters warning worse was yet to come.

At least 23 cities in the world's most populous nation have issued red alerts for air pollution since Friday, according to the official Xinhua news agency.

A host of emergency measures have been implemented to protect the public's health from the smog, which is smothering almost a ninth of the entire country.

On Monday evening -- the fourth day of the alert which is scheduled to end on Wednesday -- Beijing's air quality was better than feared, with PM 2.5 levels hovering around 200, according to data maintained by the US embassy.

The ministry of environmental protection claimed anti-pollution measures, such as temporary factory closures and taking half of cars off the roads, accounted for the better-than-expected numbers, Xinhua reported late Sunday.

But the figure remained eight times the World Health Organization's daily recommended maximum exposure level to the microscopic particles that carry major health risks.

And the relatively low number was just a temporary reprieve, Beijing's meteorological authority told AFP, adding that the worst haze would hit the city Monday night and linger until Tuesday.

In neighbouring Shijiazhuang, the capital of Hebei province, PM 2.5 levels stood as high as 701 at noon, with levels of larger PM 10 particles even higher.

In the port city of Tianjin, where readings for PM 2.5 climbed over 400 early in the morning, more than 131 flights were cancelled and around 75 delayed Monday morning, according to Xinhua.

Highways in the city were also closed, it said.

Several large hospitals in Tianjin saw a surge in the number of patients with respiratory diseases such as asthma, according to the People's Daily.

A red alert, issued when severe smog is expected to last more than 72 hours, is the highest of Beijing's four-tiered, colour-coded warning system.

In December last year the capital issued its first ever red alert since the adoption of an emergency response programme for air pollution in 2013, despite frequent bouts of serious smog.

Most of China's smog is blamed on the burning of coal for electricity and heating, which spikes when demand peaks in winter.

The issue is a source of enduring public anger in China, where fast economic growth in recent decades has come at the cost of widespread environmental degradation.

Chinese censors, who tightly control online commentary, were busily removing remarks about the smog from the twitter-like Weibo social media platform, according to the web site Freeweibo.com, which tracks posts deleted from the service.

One of them showed a headline from a March 1999 copy of the Communist Party mouthpiece the People's Daily newspaper pledging to "absolutely not let polluted air enter the new century."

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