Mumbai Beats Delhi in Pollution Rankings, but Are Official AQI Readings Telling the Full Story?

Mumbai has temporarily overtaken Delhi among the world’s most polluted cities, raising fresh questions about India’s air quality monitoring systems. Despite alarming readings on private monitoring platforms, government data has described air quality in both cities as moderate. The apparent contradiction highlights a crucial problem: air classified as satisfactory or moderate under Indian standards can still pose serious health risks.

Mumbai usually benefits from sea breezes that help disperse pollutants. However, deficient monsoon rainfall and unusually hot, humid and stagnant weather have reduced this natural advantage, allowing pollution to accumulate.

Delhi, meanwhile, is heading towards its annual pollution peak ahead of Diwali, when firecrackers and seasonal agricultural fires can further deteriorate air quality.

Some readings suggest that Delhi’s air has improved compared with previous years. The city recorded its lowest average Air Quality Index (AQI) for September in five years, while its October 24 reading was the best for that date in a decade.

However, the later Diwali date this year complicates comparisons with 2025. Diwali falls on November 8 this year, compared with October 20 last year. A comparison of pollution levels around Dussehra offers a more meaningful assessment. The day after Dussehra, Delhi’s AQI reached 245, its worst post-festival reading in five years.

The difference between government and private pollution readings also stems from how air quality is measured. India’s air quality standards were established in 2009, and the national AQI system was introduced in 2014. The World Health Organization tightened its air quality guidelines in 2021, recommending a daily PM2.5 limit of 15 micrograms per cubic metre.

India’s satisfactory category allows PM2.5 concentrations of up to 60 micrograms per cubic metre. Government monitoring generally uses 24-hour averages, while private platforms such as IQAir can report hourly readings. Differences in monitoring methods, averaging periods and standards can therefore produce sharply different pollution rankings.

Even an official satisfactory rating does not necessarily mean the air is safe. Under India’s classification, satisfactory air can contain PM2.5 levels of 31–60 micrograms per cubic metre, while moderate air ranges from 61–90. Both exceed the WHO’s recommended daily limit.

Recent readings cited for Delhi and Mumbai put PM2.5 concentrations at around 92–95 micrograms per cubic metre, roughly six times the WHO guideline.

Local emissions remain a major contributor to Delhi’s pollution even before stubble burning and Diwali firecrackers reach their peak. On October 20 and 21, estimates attributed around 37% of Delhi’s pollution to sources within the city, while stubble burning contributed less than 4%. Its contribution subsequently rose to 16% by Friday.