Some cities in Assam and the northeast have reported a drastic drop in air quality over the past few years.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/northeast-air-pollution-sets-alarm-bells-ringing
[2] http://admin.indiaenvironmentportal.org.in/category/author/gaurav-das
[3] http://admin.indiaenvironmentportal.org.in/category/newspaper/times-india-guwahati
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/assam
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-pollution
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/guwahati
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/rspm-pm-10
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/vehicular-pollution
[9] http://admin.indiaenvironmentportal.org.in/category/thesaurus/health-effects