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. From a report: 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.