Photos: Chinese cities choke on smog Visitors, some wearing masks to protect themselves from pollutants, share a light moment as they take a selfie at the Jingshan Park on a polluted day in Beijing on December 7, 2015, the day Beijing's city government issued its first red alert for pollution, the highest level of warning. Hide Caption 1 of 14

Photos: Chinese cities choke on smog A woman walks along a path at a park in Beijing on December 7. Smog is blurring the view of the buildings in the background. Hide Caption 2 of 14

Photos: Chinese cities choke on smog A mask-wearing vendor awaits customers at the Jingshan Park on a polluted day in Beijing on December 7. Hide Caption 3 of 14

Photos: Chinese cities choke on smog Tiananmen Gate stands shrouded with heavy pollution and fog in Beijing on December 1, 2015. Previously under an orange alert, the second-highest warning level, the Chinese capital enters yet another week choked by toxic smog under the newly issued red alert -- due to be in force until noon of December 10 local time. Hide Caption 4 of 14

Photos: Chinese cities choke on smog A Chinese woman protects herself with a mask as she walks past Tiananmen Square in smog-hit Beijing on November 30, 2015. Levels of PM 2.5, considered the most hazardous pollutant, crossed 600 units in Beijing, nearly 25 times the acceptable standard set by the World Health Organization. Hide Caption 5 of 14

Photos: Chinese cities choke on smog Buildings are shrouded by heavily polluted haze in Beijing on November 30. China's capital and neighboring regions have seen the worst smog of the year this week. Hide Caption 6 of 14

Photos: Chinese cities choke on smog Roads and buildings in Baoding, China's most polluted city, are cloaked in thick smog on November 30, where the air quality index (AQI) has reached a 'hazardous' level. Hide Caption 7 of 14

Photos: Chinese cities choke on smog A woman protects herself from pollutants with a piece of cloth as she walks past a construction site on November 30 in Beijing. Hide Caption 8 of 14

Photos: Chinese cities choke on smog Residential blocks are cloaked in smog in Lianyungang, in eastern China's Jiangsu province, on November 30. Hide Caption 9 of 14

Photos: Chinese cities choke on smog Vehicles drive along a highway in Beijing with a traffic sign that reads "Slow down, low visibility" on November 30. Hide Caption 10 of 14

Photos: Chinese cities choke on smog A Genghis Khan statue is obscured by a cloak of orange-tinged smog on November 29, 2015 in Hohhot, capital of Inner Mongolia, China. Hide Caption 11 of 14

Photos: Chinese cities choke on smog The pollution cast an apocalyptic glow over the pagodas on the streets of Hohhot, Inner Mongolia on November 29. Hide Caption 12 of 14

Photos: Chinese cities choke on smog People wearing face masks walk across Tiananmen Square in Beijing on November 28. Hide Caption 13 of 14