Thoothukudi is termed one of the Non-attainment cities by Ministry of Environment primarily on the basis of excess PM10 under the National Ambient Air Quality Monitoring Program.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/thoothukudi-sole-city-tn-have-high-levels-air-pollution
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/new-indian-express-chennai
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/tamil-nadu
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-pollution
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-quality-monitoring
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/rspm-pm-10
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/thermal-power-plants
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/industrial-pollution
[9] http://admin.indiaenvironmentportal.org.in/category/thesaurus/environmentalists