Night pollution a major contributor.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/city-tops-lung-cancer-cases-experts
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/statesman-kolkata
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/calcutta-d
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/west-bengal
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/lung-cancer
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/cancer
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/state-pollution-control-board-spcb-0
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-pollution
[9] http://admin.indiaenvironmentportal.org.in/category/thesaurus/vehicular-pollution
[10] http://admin.indiaenvironmentportal.org.in/category/thesaurus/high-court
[11] http://admin.indiaenvironmentportal.org.in/category/thesaurus/public-interest-litigation-pil
[12] http://admin.indiaenvironmentportal.org.in/category/thesaurus/spm
[13] http://admin.indiaenvironmentportal.org.in/category/thesaurus/health-effects
[14] http://admin.indiaenvironmentportal.org.in/category/thesaurus/pollution-under-control-certification