BEIJING, CHINA - DECEMBER 18: Tourists wearing masks visit the Tiananmen Square on December 18, 2016 in Beijing, China. At least 24 cities in North China issued red alerts on Friday as heavy smog will shroud the country's northern regions in the following two days. (Photo by VCG/VCG via Getty Images)
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Heavy smog causes chaos in China
A red alert for air pollution was issued in 23 cities in northern China, affecting severely schools and factories in Beijing.
00:48 - Source: CNN
Smog in China 12 videos
BEIJING, CHINA - DECEMBER 18: Tourists wearing masks visit the Tiananmen Square on December 18, 2016 in Beijing, China. At least 24 cities in North China issued red alerts on Friday as heavy smog will shroud the country's northern regions in the following two days. (Photo by VCG/VCG via Getty Images)
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Heavy smog causes chaos in China
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China's smog seen from above
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Timelapse: Smog envelops Beijing in minutes
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TOPSHOT - This overhead photo shows the sun rising above the skyline of Shanghai on a polluted day on February 23, 2018. / AFP PHOTO / Johannes EISELE (Photo credit should read JOHANNES EISELE/AFP/Getty Images)
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What is in our air
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Students take test surrounded by smog
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Why is Beijing's smog so bad?
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A plane grounded at the Chengdu Shuangliu International Airport in China by thick fog and smog from pollution.
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Smog strands over 20,000 at airport
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Chinese consumers rush to buy anti-smog products
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A woman wearing a face mask walks in heavy smog in Harbin, northeast China's Heilongjiang province, on October 21, 2013. Choking clouds of pollution blanketed Harbin, a Chinese city famed for its annual ice festival on October 21, reports said, cutting visibility to 10 metres (33 feet) and underscoring the nation's environmental challenges. CHINA OUT AFP PHOTO (Photo credit should read STR/AFP/Getty Images)
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China seizes faulty smog masks
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Smog cause hazardous air quality in Beijing
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Time-lapse video shows smog-stricken Beijing
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Is Beijing's smog 'red alert' helping air quality?
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