Delhi has been termed the most polluted city. The key source of pollution is the number of cars, which is increasing every year.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/rise-cars-adds-pollution
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/hindu-new-delhi
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/delhi
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-pollution
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/cars
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/vehicular-pollution
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/centre-science-and-environment-cse
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/public-transport
[9] http://admin.indiaenvironmentportal.org.in/thesaurus/national-green-tribunal-ngt
[10] http://admin.indiaenvironmentportal.org.in/category/thesaurus/health-effects
[11] http://admin.indiaenvironmentportal.org.in/category/thesaurus/environmentalists