National capital’s air dirtiest among megacities during winter: Report

The megacities of India have witnessed rising pollution levels during 2024-25 winter, according to a new report.

In an analysis by the Centre for Science and Environment (CSE), it is revealed that PM 2.5, a major pollutant, has peaked in Delhi, Mumbai, Kolkata, Bengaluru, Hyderabad and Chennai from October 1, 2024, and January 31, 2025.

“While Delhi, located in the land-locked Indo-Gangetic Plains (IGP) with adverse meteorology, has recorded the highest level of pollution during winter, Kolkata, also at the tip of the IGP, ranks second. Mumbai, Chennai, Hyderabad and Bengaluru, despite having more advantageous climatic conditions and natural ventilation, have also experienced increases in average PM2.5 concentrations,” the report said.

Delhi’s average PM 2.5 concentration during winter increased by 1 per cent as compared to the average of previous three winters, reaching 174 micrograms per cubic metre, with peak pollution levels surging on November 18, 2024, to 602 micrograms per cubic metre.

“The peaking of pollution during winter in any climatic zone is a sign of the underlying problem of persistent air pollution in these rapidly urbanising and motorising cities. The growing impact of local sources of pollution shows up in the pollution hotspots across these cities, increasing local exposures and risks,” says Anumita Roychowdhury, executive director, research and advocacy, CSE.

During this winter, Kolkata and Hyderabad were most polluted after Delhi, while Mumbai, Chennai and Bengaluru saw the fastest worsening of localised pollution levels.

Delhi, with a winter average PM 2.5 level of 175 micrograms per cubic metre (µg/m³), remains the most polluted megacity by a large margin during winter, even though the winter average has shown improvement over the previous winter.

Kolkata was the second-most polluted megacity during the 2024-25 winter, with an average PM 2.5 concentration of 65 micrograms per cubic metre.

In the other five megacities, the average winter PM 2.5 level was 52 micrograms per cubic metre in Hyderabad, 50 micrograms per cubic metre in Mumbai; 36 micrograms per cubic metre in Chennai; and 37 micrograms per cubic metre in Bengaluru.

In terms of cleaner air, Mumbai had only 19 days of ‘good’ AQI, followed by Kolkata with just 11 - the lowest among megacities (excluding Delhi). In contrast, Hyderabad (21 days), Bengaluru (44 days) and Chennai (55 days) recorded higher numbers of ‘good’ AQI days.

The report said though winter was a problematic season for all megacities, the intensity of the problem varies.

“The days with bad air quality occurred in clusters during the winter season in the megacities. The clustering of bad air days was longer in Delhi, Kolkata and Mumbai, but of shorter duration in Hyderabad, Bengaluru and Chennai. The intensity and duration of these bad air days were long enough in Delhi to get classified as a smog episode,” the report said.