Pedestrians walk on an overpass amid heavy air pollution in Beijing on Dec. 18, 2016. (WANG ZHAO/AFP/Getty Images)

At a Glance

Children were sickened and hundreds of factories were ordered closed due to thick smog in northern China.Airlines were forced to cancel flights, and some cars were ordered off roads after a "red alert" was issued.

Children were sickened and hundreds of flights were canceled in northern China as thick, choking smog descended on the region, including the country's fourth-largest city, Beijing.

A total of 23 cities– Beijing included– announced emergency measures in a "red alert" declaration, according to the Associated Press. This included ordering factories and schools to close, and limiting the number of cars allowed on those cities' roads, the report added.

By Monday, the number of closed factories surpassed 700 and hospitals were treating an influx of young patients who had been sickened by the pollution, the AP also said. Most of the cases treated were respiratory issues worsened by the poor air quality.

"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," Li Jingren, a 15-year-old high school student in Beijing, told the AP.

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Reduced visibilities also disrupted air travel, , USA Today reported.

Dong Liansai, a climate campaigner with Greenpeace in Beijing, told the AP that this round of smog came from nearby provinces, where factories put out an excess of pollution that is pushed by the wind into cities like Beijing.

"If you are tracking back to the first day of this episode, you can see that the layer of the smog [in Beijing] is moving slowly from the south to the urban area in Beijing and then to the north," said Liansai. "You can easily find the large deployment [of smog] in the regions south of Beijing."

Beijing , according to China Daily. Officials have worked in recent years to slow the growth of the city and curb pollution by moving some factories to other provinces, but air quality remains extremely poor at times.