The national capital remained India’s most polluted megacity by a wide margin during the 2024-25 winter with an average PM2.5 concentration of 175 micrograms per cubic metre (µg/m³). However, a Centre for Science and Environment (CSE) analysis found that PM2.5 pollution in the national Capital declined in the 2024-25 winter (October 1 to January 31) compared to the 2023-24 winter (189 µg/m³). Kolkata was the second-most polluted megacity during the 2024-25 winter with an average PM2.5 concentration of 65 µg/m³.

Irrespective of their diverse geographic and climatic contexts, Delhi, Mumbai, Kolkata, Bengaluru, Hyderabad and Chennai -- have seen worsening PM2.5 levels during the winter of 2024-25 (October 1, 2024-January 31, 2025).

The overall level and the peaking points of pollution have, however, varied across cities depending on the meteorological conditions of different climatic zones.

The analysis showed that PM2.5 levels declined in Bengaluru, Mumbai and Kolkata compared to the previous winter but remained the same in Chennai and Hyderabad. The average PM2.5 levels during the 2024-25 winter were 52 µg/m³ in Hyderabad, 50 in Mumbai, 37 in Bengaluru and 36 in Chennai.

According to an analysis, Delhi with a winter average PM2.5 level of 175 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. Delhi’s average PM2.5 concentration during winter increased by 1 per cent compared to the average of previous three winters reaching 174 µg/m³, with peak pollution levels surging on November 18, 2024, to 602 µg/m³. In the other five megacities, the average winter PM2.5 level was 65 µg/m³ in Kolkata; 52 µg/m³ in Hyderabad; 50 µg/m³ in Mumbai; 36 µg/m³ in Chennai; and 37 µg/m³ in Bengaluru.

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. The megacities outside the IGP — Mumbai, Chennai, Hyderabad and Bengaluru — despite having more advantageous climatic conditions and natural ventilation, have also experienced increases in average PM2.5 concentrations.

“When compared to the three-year average, Delhi remained the worst performer, with average winter pollution levels continuing to rise, even though by just 1 per cent higher than the average of the last three years,” the analysis said.

Delhi recorded its highest daily PM2.5 level in four years, with pollution spiking to 602 µg/m³ on November 18, 2024 -- the steepest 24-hour average since 2021. This marks a 65 per cent increase from last winter’s peak and is 42 per cent higher than the city’s three-year average, highlighting a worsening pollution trend in the capital.

CRRI Mathura Road saw a 20 per cent increase in winter pollution concentration, while Ashok Vihar, Vivek Vihar and Gwal Pahari recorded 11-13 per cent rises in PM2.5 levels. Anand Vihar remained the most polluted location in Delhi, recording a winter PM2.5 average of 227 µg/m³. In the NCR region, Loni in Ghaziabad emerged as the worst affected area, with a PM2.5 average of 153 µg/m³, followed by Gwal Pahari in Gurugram at 149 µg/m³. NO ? concentrations spiked sharply, especially in November-December, with Anand Vihar recording a 71 per cent increase from October to December. ITO registered the highest NO ? levels at 167 µg/m³ in December.

 Compared to the winter of 2021-22, Mumbai, Kolkata and Hyderabad show improvement with their winter PM2.5 levels dropping by 16 per cent, 14 per cent and 16 per cent respectively. Delhi has recorded a 7 per cent reduction in pollution levels compared to the pervious winter, but 5 per cent higher than in 2021-22. Meanwhile, Bengaluru and Chennai have recorded an increase in PM2.5 levels compared to 2021-22, reflecting a worsening trend in air quality.

Kolkata and Chennai showed little improvement, with Kolkata’s maximum peak level of PM2.5 stagnating at 135 µg/m³, while Chennai saw a slight reduction from 147 µg/m³ last winter to 119 µg/m³ this year.

According to the analysis, in contrast, Hyderabad, Mumbai and Bengaluru recorded reductions in peak/maximum winter pollution levels. Hyderabad showed the most improvement in peak levels, with a 51 per cent drop in PM2.5 levels, down to 89 µg/m³ from 183 µg/m³ last year. Bengaluru followed with a 42 per cent decline, reducing peak levels to 67 µg/m³, while Chennai saw a 19 per cent improvement and Mumbai recorded a 12 per cent drop in its peak level, lowering its PM2.5 from 92 µg/m³ to 81 µg/m³.

This winter’s peak daily pollution levels in Kolkata, Chennai and Hyderabad were lower than their previous winter peaks, but still fell under the “poor” AQI category. Kolkata’s winter peak stood at 135 µg/m³ registered on November 2, 2024; for Chennai, it stood at 119 µg/m³ registered on October 31, 2024; and for Hyderabad it stood at 89 µg/m³ registered on November 25, 2024.

Air pollution trends differ across India’s megacities, with Delhi and Hyderabad experiencing their worst air quality in November, while other cities saw peak pollution in December and January. January emerged as the most polluted month for Kolkata, Bengaluru and Chennai, with air quality deteriorating further compared to previous months. Mumbai, on the other hand, recorded its worst pollution levels in December.

The analysis further stated that Mumbai recorded its lowest winter PM2.5 average (49 µg/m³) in four years, a 24 per cent decline compared to previous winters. With a winter PM2.5 average of 80 µg/m³, Deonar was the most polluted, followed by Shivaji Nagar (76 µg/m³), Malad West (75 µg/m³), Sewri (71 µg/m³) and Borivali and Kandivali (69 µg/m³ each).

“Kolkata recorded a seasonal PM2.5 average of 65 µg/m³ -- a 19 per cent decline compared to the previous three winters. Hyderabad’s seasonal PM2.5 average dropped to 52 µg/m³, marking a 5 per cent decline compared to previous winters. Bengaluru recorded a seasonal PM2.5 average of 37 µg/m³, marking a 7 per cent decline compared to the previous three winters. Chennai’s seasonal PM2.5 average stood at 36 µg/m³, showing 1.2 per cent increase compared to the previous three winters,” according to the analysis.