Delhi was the most polluted city during 2024-25, followed by Patna: Study
According to a new analysis by Climate Trends, Delhi was the most polluted city during 2024-25, with the highest annual PM2.5 levels and prolonged periods of “poor” air quality in winter, while Patna was the second most polluted city.
Climate Trends is a research-based consulting and capacity-building initiative that aims to focus on environmental, climate change, and sustainable development issues.
Based on air quality monitoring data from the Central Pollution Control Board (CPCB), this report analyzes how weather conditions influence the persistence of PM2.5 pollution in six major Indian cities: Delhi, Patna, Kolkata, Mumbai, Chennai, and Bengaluru. By combining CPCB air quality data (2024-2025) with seasonal clustering, the study separated pollution from emissions from climate change.
“Delhi continues to have the worst pollution crisis in the country, with the highest annual average PM2.5 levels and the longest duration of ‘severe’ or ’emergency’ category air days, driven by local emissions and regional factors.
The report states, “Patna is the second most polluted city after Delhi, with persistently high PM2.5 levels, driven by intense atmospheric stagnation, reflecting a growing crisis in the eastern Indo-Gangetic Plain.” Despite previously being less polluted, Bengaluru and Chennai showed signs of deteriorating air quality during the winter months, a new weak trend.
Both Mumbai and Chennai recorded an increase in their annual average pollution levels in 2025, indicating a growing concern throughout the year rather than just seasonal increases.
Sagnik Dey, head of the Centre for Atmospheric Sciences at the Indian Institute of Technology (IIT), Delhi, said that persistently high PM2.5 levels in northern cities are a sign of worsening air quality. It is strongly associated with wind speeds less than 1 m/s and high relative humidity, where stagnation events lead to significantly higher exposure levels.
They stated, “Ventilation capacity is the main driver of variation between cities. However, current NCAP evaluation frameworks primarily assess changes in observed concentrations without accounting for weather-related variations, which may lead to misinterpretation of policy impact.”
The report proposes major improvements to NCAP Phase III, including separate winter targets, meteorologically adjusted metrics, and integrated airshed-based planning along with weather-related dynamic action plans.
