Major cities across northern China choked Monday under a blanket of smog so thick that industries were ordered shut down and air and ground traffic was disrupted.

At least 23 cities issued red alerts for a swath of pollution that has hovered over much of the nation since Friday, China's Xinhua news agency reported. Alerts are expected to remain in effect through Wednesday.

Hospitals set emergency procedures in motion to deal with an influx of breathing-related illnesses.

Large hospitals in the port city of Tianjin, less than 100 miles southeast of Beijing, saw a surge in asthma and other respiratory issues, China's People’s Daily reported. The pollution forced the city to close the highways and caused delays and cancellations for dozens of flights, Xinhua said.

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"Off-the-charts air pollution chokes many places in #Hebei province, w/ PM2.5 levels exceeding 1,000 in its capital city of #Shijiazhuang," the People's Daily tweeted. Some coal and other industrial plants in Hebei were ordered to shut down until the smog eases.

Beijing's meteorological authority told Agence France-Presse the worst haze would hit the city Monday night and linger until Tuesday. Under a contingency plan for severe air pollution, the city shut down more than 700 heavy industry plants and required 500 more to reduce production, the South China Morning Post reported.

Dangerous smog levels are a recurring theme for Beijing and other major Chinese cities struggling to balance desperately needed industrial growth with environmental concerns. Much of the pollution is blamed on coal burning for electricity, and smog levels often are worse in winter when cold weather drives up energy use.

Earlier this month, Beijing authorities caused a stir when a draft of the Beijing Meteorological Disasters Prevention and Control Regulations defined smog as a meteorological disaster mainly caused by haze, blizzards or unfavorable meteorological conditions.

Tianjin and other cities already list smog as a meteorological disaster in local regulations, but the State Council has yet to issue a similar national regulation.

“'Meteorological disasters' are caused by natural conditions and cannot be controlled by human activity," Zhang Zitai, a Fudan University professor, told Legal Daily. "Smog, on the other hand, is mainly caused by human activity. Thus the plan to list it as a meteorological disaster not only goes against science, it will also create an excuse for polluters to escape their culpability."