The NGO has evaluated the data by the Central Board of Pollution Control (CPCB) and Environment Ministry.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/no-city-low-pollution-category-say-experts
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/asian-age-new-delhi
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/maharashtra
[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/centre-science-and-environment-cse
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/rspm-pm-10
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/central-pollution-control-board-cpcb