Mumbai’s air is ‘satisfactory’ and ‘unhealthy’. How can both be true?

On October 15, the Swiss company IQAir ranked Mumbai fifth among the world's most polluted cities and first in India, a position it held above Delhi. However, the Central Pollution Control Board (CPCB), India's pollution watchdog, classified Mumbai's air as 'satisfactory' on the same day. This contradiction has persisted throughout the month, with Mumbai's air quality deteriorating after the monsoon ended early in the first week of October.

Between October 1 and 23, the CPCB dashboard classified the city's air as 'unhealthy' on 11 days, while IQAir classified it as such on all 23 days. This discrepancy is not solely due to one agency being more accurate than the other. Both government agencies and private platforms rely on different monitoring networks and scales to measure air quality.

Mumbai, being a peninsula with the Arabian Sea on three sides and surrounding hills, further complicates the issue, as pollution varies significantly across different parts of the city. The Maharashtra Pollution Control Board (MPCB) operates 26 Continuous Ambient Air Quality Monitoring Stations (CAAQMS) to measure air quality in Mumbai.

These stations use various sensors to measure six primary pollutants, which are then averaged to produce a single AQI figure for the city. Private platforms, on the other hand, combine government-approved station readings with low-cost sensors installed in various locations. These sensors typically measure only PM2.5 and PM10 particles.

The difference in methodologies and the varying breakpoints used for categorizing air quality contribute to the observed disparity in Mumbai's air quality readings.