Recently, a study by WHO also pointed out that Delhi has the worst air quality among cities in India.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/pm25-your-lungs-pollutants-delhi-air-twice-permitted-level-been-so-two-decades
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/financial-express-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/pm-25
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/who
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/indian-institute-technology-iit
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/odd-even-numbered-cars
[9] http://admin.indiaenvironmentportal.org.in/category/thesaurus/vehicular-pollution