BEIJING • Large parts of northern China continued to choke under a vast cloud of toxic smog yesterday yesterday, worsening late in the evening and disrupting life in dozens of cities.

North-eastern China's Liaoning province saw its most severe smog in seven years, with extremely poor visibility closing 18 expressways in the province. All expressways in the northern municipality of Tianjin were also closed.

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 PM 2.5 levels hovering around 200, according to data maintained by the United States Embassy.

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

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

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

LIVING IN SMOG CITY Here's a sampling of comments from netizens bearing with the smog in northern China. BREATHLESS Even the act of breathing has become a luxury. I live in Xian. WEIBO USER RAININGDAY ALL CHOKED UP This kind of weather makes one feel desolate. It's been two days. Even breathing feels painful. WEIBO USER MESSIMILAN NEEDS NO TRANSLATION How do you say 'I want to poison you to death' in English? You say 'Welcome to Zhengzhou'. WEIBO USER BBBXER HUMAN COST I can't find my way home. The people in Tianjin are human air purifiers, contributing to the greening of industries in Tianjin. WEIBO USER MOMOMOMOMOXIAOBAI GET PRIORITIES RIGHT I beg my country to seriously tackle the smog problem. The environment is 100 times more important than the economy. Return the blue skies to me! WEIBO USER CHASER OF COMEDY BLIND SPOT I missed my (bus) stop because of the smog. You just cannot see anything. WEIBO USER BEIYU

In the port city of Tianjin, where readings for PM2.5 climbed over 400 early in the morning, more than 180 flights had been cancelled and around 60 delayed since the alert began, according to national broadcaster CCTV.

Several large hospitals in Tianjin 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, colour-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.

Those with means were heading south in pursuit of cleaner air, with tickets for flights to China's traditional resort areas in the south - such as Sanya, Dali, in Yunnan province, and Xiamen in Fujian province - almost all sold out.

Full-fare tickets to other popular holiday destinations in the south - such as Kunming, in Yunnan, and Guilin, in Guangxi province - were still available, reported 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.

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.

Online, Chinese netizens shared images of themselves in face masks and footage of their neighbourhoods 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".