A report by WHO last year had stated that Delhi was the most polluted city in the world when it came to air quality.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/delhi-pollution-govt-plans-ban-trucks-buses-above-15-years
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/indian-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/vehicular-pollution
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/heavy-duty-vehicle
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-quality-standards
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/ministry-shipping-road-transport-and-highways
[9] http://admin.indiaenvironmentportal.org.in/thesaurus/national-green-tribunal-ngt
[10] http://admin.indiaenvironmentportal.org.in/category/thesaurus/health-effects