With only three of these 25 cities using the most advanced BS IV fuel emission norms in India, environmentalists said an aggressive plan to combat vehicular pollution was vital. The AAP data contains results of outdoor air pollution from almost 1,600 cities in 91 countries and finds that of the 20 cities with the worst air quality, 13 are in India.

However, the Indian Institute of Tropical Meteorology, which functions under the Ministry of Earth Sciences, rejected the WHO findings, which, incidentally, are based on data collected by the Central Pollution Board. The institute claimed that the WHO overestimated Delhi’s pollution levels and underestimated that in Beijing. Further, the Delhi government pointed out that there was ambiguity in the data available to WHO on pollution in China.

Nevertheless, environmentalists said that it was time to admit that there was a problem at hand, especially with Dehradun, Pune and Jammu — cities traditionally thought to be ‘cleaner’ cities — making it to the list of 20. According to WHO, all these three cities have recorded levels of fine particulate matter (PM) with diameter of 2.5 micrometres or that are lower than in Beijing.

Compared to Beijing’s 56 microgram per cubic meter (ug/m3), Dehradun, Pune and Jammu were found to have PM 2.5 concentrations of 77, 59 and 56 ug/m3, respectively. Delhi is leading the pack globally with PM 2.5 levels at 153 ug/m3.