All 74 of the biggest cities in China failed to reach air quality standards last month, the Ministry of Environmental Protection admitted. Air pollution limits were exceeded on more than 70% of days in December 2013.

The 10 cities with the worst air quality last month were Xingtai, Shijiazhuang, Handan, Baoding, Hengshui, Xi'an, Tangshan, Huai'an, Nanjing and Wuhan.

For many years, environmental protection has been sacrificed for the sake of economic development. Coal burning and car emissions are major sources of pollution. In recent years, China has beefed up regulations and pledged financial resources to fight pollution.

Beijing has implemented a four-level colour-coded alert system - blue, yellow, orange and red in ascending order of severity. Measures such as closing factories, banning fireworks and closing schools may come into affect depending on the colour of the alert.

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Air pollution levels in the sky over Tiananmen Square in Beijing shown on March 6, 7, 8, 9, 10, 11, 12, 14 and 15, 2013 Reuters
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A pedestrian overpass on a hazy day in the Pudong financial area of Shanghai Reuters
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Models in masks on the catwalk at a jewellery fair on a hazy day in Nanjing, Jiangsu province Reuters
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Buildings shrouded in smog in Lianyungang, Jiangsu province Getty
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College students in masks pose with a plastic bag during a performance to raise awareness of air pollution in Xi'an, Shaanxi Province Getty
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Homes disappears into smog in Wujiaqu, Xinjiang Uighur autonomous region Reuters
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An electronic screen is the only clear thing in heavy smog in Shenyang Reuters
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An elderly man exercises in the morning along the Songhua river in Jilin Reuters
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A traffic policeman signals to drivers in Harbin, Heilongjiang province after smog forces closure of schools and highways Reuters
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A haze blankets Guangzhou, capital city of China's southern Guangdong province Reuters
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A woman walks along a street during a smoggy day in Changchun, Jilin province Reuters
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China World Trade Centre Tower III, one of the tallest buildings in the city at 330m (1083ft), swamped in thick haze in Beijing Reuters
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Cyclists cover their faces on a street in Zhengzhou, Henan province Reuters
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Residents ride bicycles along a street amid heavy haze in Xingtai, Hebei province Reuters
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Rowing a boat on a river during a hazy morning in Shaoxing, Zhejiang province Reuters
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Running on the Bund in Shanghai as heavy smog engulfs the city skyline Getty
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Buildings are dimly seen through thick haze in Shanghai Reuters
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People wearing masks do tai chi during a hazy day in Nanjing, Jiangsu province, Reuters