A person wears a mask while crossing a bridge in Beijing on Monday. Photo: AP

Large parts of northern China continued to choke under a vast cloud of toxic smog, disrupting life in dozens of cities.

At least 23 cities in the world's most populous country have issued red alerts for air pollution since Friday. Yesterday morning - the fourth day of the alert which is scheduled to end tomorrow - Beijing's air quality was better than feared, with PM2.5 levels hovering around 200, according to data maintained by the US Embassy.

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

But in Shijiazhuang, capital of northern China's Hebei province, levels of PM2.5 fine particulate matter soared to 1,000 micrograms per cubic meter, according to the Xinhua News Agency.

That compares with a WHO guideline of an annual average of no more than 10 micrograms.

Northeastern China's Liaoning province saw its most severe smog in seven years, with extremely poor visibility closing 18 expressways in the province.

In the port city of Tianjin, several large hospitals saw a surge in the number of patients with respiratory diseases like asthma, according to the People's Daily.

Red alerts - the highest of China's four-tiered, color-coded warning system - are issued when the Air Quality Index (AQI) is forecast to exceed 200 for more than four days in succession, 300 for more than two days or 500 for at least 24 hours. The AQI is a different measure from the PM2.5 gauge, reported Reuters.

Schools, factories shut down

Engulfed in choking smog, some northern Chinese cities have limited the number of cars on roads and temporarily shut down factories to reduce air pollution.

More than 700 companies stopped production in Beijing and traffic police were restricting drivers by monitoring their license plate numbers, state media reported.

Dozens of cities closed schools and took other emergency measures after a red alert was issued from Friday night to Wednesday for much of northern China.

"The smog has serious repercussions on the lungs and the respiratory system, and it also influences the health of future generations, so under a red alert, it is safer to stay at home rather than go to school," said Li Jingren, a 15-year-old high school student in Beijing.

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

China's air pollution is blamed on its reliance on coal and emissions from older cars.

The smog had earlier grounded flights in some cities.

Smog refugees

The situation is so dire that it's created a spate of "smog refugees" who are fleeing the smog-affected cities for cleaner air to the south.

Full-fare tickets to popular holiday destinations in the south - such as Kunming and Guilin - were still available, reported the South China Morning Post.

Searches made on the travel website Qunar.com for tickets on flights to inland areas in the west and coastal areas in the east of the country were three times higher than before the red alert was issued, the Beijing Evening News reported.

A report issued by Ctrip.com over the weekend estimated that 150,000 people in China would travel overseas to avoid the choking smog in December, and that each year, more than one million tourists travelled abroad for that reason.

Residents in the more affluent cities of Beijing, Shanghai, Chengdu, Guangzhou and Tianjin are those who travel the most to avoid the smog at home.

The report quoted a man saying that he had immediately booked tickets for a five-day trip to Hainan province's seaside resort of Sanya after learning that schools would be closed until Wednesday.

Netizens' anger burns

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.

Online, Chinese netizens shared images of themselves in face masks and footage of their neighborhoods engulfed in thick smog. Many Weibo posts poured scorn on the authorities' anti-pollution efforts.

The latest project of artist Liu Bolin, known as "the invisible man" for using painted-on camouflage to blend into the backdrops of his photographs, aims to put the spotlight on the pollution problem, according to Reuters.

He is walking around the capital wearing an orange vest with 24 smartphones attached on the front and back, live-streaming scenes of the smog, which he calls "a disaster".