The NGT had divided air pollution into four categories — Category I (Average), II (Severe), III (Critical) and IV (Environmental Eme-rgency).
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/ngt-refuses-review-order-action-plan-pollution
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/asian-age-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/national-green-trbunal-ngt
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/central-pollution-control-board-cpcb
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/environment-pollution-control-authority-epca