On October 15, IQAir, a Swiss company that sells air purifiers and runs a widely quoted air-quality monitoring platform, placed Mumbai fifth among 122 of the world’s most polluted cities and first in India, one place above Delhi. On the same day, the Central Pollution Control Board (CPCB), the government’s pollution watchdog, classified Mumbai’s air as ‘satisfactory’.

Mumbai’s air quality began deteriorating in the first week of October, after the monsoon withdrew early. Between October 1 and 23, the CPCB dashboard put the city’s air in the ‘unhealthy’ category on 11 days. But IQAir put it there on all 23. The disparity Mumbai’s air quality readings is also evident in the peak readings recorded by the two platforms.

IQAir’s peak reading of AQI was 188 on October 17. The CPCB’s was 127 on October 18.

The skyline from Dadar on Saturday, shrouded in haze (Express photo by Akash Patil)

The gap is not a case of one agency getting it right and the other getting it wrong.…