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, 2013Reuters
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A pedestrian overpass on a hazy day in the Pudong financial area of ShanghaiReuters
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Models in masks on the catwalk at a jewellery fair on a hazy day in Nanjing, Jiangsu provinceReuters
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Buildings shrouded in smog in Lianyungang, Jiangsu provinceGetty
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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 ProvinceGetty
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Homes disappears into smog in Wujiaqu, Xinjiang Uighur autonomous regionReuters
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An electronic screen is the only clear thing in heavy smog in ShenyangReuters
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An elderly man exercises in the morning along the Songhua river in JilinReuters
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A traffic policeman signals to drivers in Harbin, Heilongjiang province after smog forces closure of schools and highwaysReuters
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A haze blankets Guangzhou, capital city of China's southern Guangdong provinceReuters
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A woman walks along a street during a smoggy day in Changchun, Jilin provinceReuters
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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 BeijingReuters
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Cyclists cover their faces on a street in Zhengzhou, Henan provinceReuters
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Residents ride bicycles along a street amid heavy haze in Xingtai, Hebei provinceReuters
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Rowing a boat on a river during a hazy morning in Shaoxing, Zhejiang provinceReuters
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Running on the Bund in Shanghai as heavy smog engulfs the city skylineGetty
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Buildings are dimly seen through thick haze in ShanghaiReuters
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People wearing masks do tai chi during a hazy day in Nanjing, Jiangsu province,Reuters