In an indication of the very poor health of the city's air, Mumbai has been declared by the World Health Organisation (WHO) as the fifth most polluted megacity in the world in terms of PM 10 (coarse pollution particles) levels. The city is 39 on the list of most polluted in terms of PM 2.5 (fine particulate matter) among 122 Indian cities monitored by the WHO. Navi Mumbai and Thane are 36th and 87th. WHO's urban air quality database 2016, released on Thursday, studied pollution in 3,000 cities and towns in 103 countries for 2008-15. Mumbai is 147th on the international scale of all cities (separate from the table on megacities). Gwalior and Allahabad are the second and third-most polluted cities in the world, while Delhi is at 11.

Both PM 10 and PM 2.5 are classified as carcinogenic by WHO. "Globally, in 2012, air pollution in urban areas is estimated to have caused 3.7 million deaths, more than one-third reported from South-East Asia," said Dr Poonam Khetrapal Singh, WHO regional director for SouthEast Asia.