Haze over the Forbidden City in Beijing, on Dec. 19.
Pedestrians wearing masks walk along a street engulfed by heavy smog in Dalian, on Dec. 19.
A Chinese girl wears a mask at Jingshan Park in Beijing, Dec. 19. The haze covered an area of about 1.42 million square kilometers the day before.
A woman sits on the back of a motorcycle in Liaocheng, Shandong province, China, on Dec. 19.
Chinese paramilitary policemen wear masks for protection as they march in the capital city in Beijing, on Dec. 19.
Chinese cities are also limiting the number of cars on roads.
A duck swims past the Old Summer Palace on a polluted day in Beijing, on Dec. 17.
A pedestrian crosses a smog-shrouded street in Lianyungang, eastern China's Jiangsu province, on Dec. 19.
Hospital visits spiked, roads were closed and flights cancelled on Dec.19 as China choked under a vast cloud of toxic smog, with forecasters warning the worst was yet to come.
Chinese people wearing masks for protection against pollution walk at Ritan Park in Beijing, on Dec. 19.
Pedestrians walk on an overpass in Beijing, on Dec. 18.
Beijing issued its first air pollution red alert for 2016 on December 15, with choking smog expected to cover the city and surrounding areas in north China until December 21.
A woman wearing a mask walks along a polluted bridge in Tianjin, on Dec. 19.
A woman takes photos of lion sculptures that have their mouth covered by masks in Xi'an Fine Art College in Xi'an, Shaanxi province, on December 12. Several art students spent the night putting masks on more than 800 stone hitching posts to raise awareness of air pollution.
"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.