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.