Particulate matter such as PM 2.5, which are less then 2.5 microns in size, are considered especially dangerous as these can penetrate deep into the lungs and enter the bloodstream.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/pm-25-levels-have-crossed-permissible-limit-pune-last-five-years-safar
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/indian-express-new-delhi
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-pollution
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/maharashtra
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/pune-t
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/pm-25
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-quality-monitoring