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https://www.theguardian.com/cities/datablog/2017/feb/13/most-polluted-cities-world-listed-region
https://www.theguardian.com/environment/2016/may/12/which-are-the-worlds-two-most-polluted-cities-and-why
http://www.dailymail.co.uk/news/article-3019536/Living-working-Chernobyl-Fascinating-insight-lives-work-live-exclusion-zone-nuclear-plant-nearly-30-years-disaster-shook-world.html
http://timesofindia.indiatimes.com/home/environment/pollution/Gwaliors-air-among-dirtiest-in-the-world/articleshow/34861518.cms
https://en.wikipedia.org/wiki/Delhi#Air_pollution
http://www.telegraph.co.uk/travel/destinations/asia/india/articles/delhi-most-polluted-city-in-the-world/
https://www.worldcrunch.com/culture-society/welcome-to-xingtai-the-most-polluted-city-in-china
http://www.irishtimes.com/news/world/asia-pacific/china-today-xingtai-one-of-the-world-s-most-polluted-cities-1.2024707
http://www.academicjournals.org/journal/AJEST/article-full-text/20FA8BC51607
https://en.wikipedia.org/wiki/Linfen
http://www.greenforwardnews.com/the-most-polluted-city-on-earth-linfen-china/
http://content.time.com/time/specials/2007/article/0,28804,1661031_1661028_1661020,00.html
http://content.time.com/time/specials/2007/article/0,28804,1661031_1661028_1661021,00.html
http://www.blacksmithinstitute.org/projects/display/129