Delhi most polluted city, Aizawl least during winter 2024-25: Report

Mumbai: Delhi was the most polluted city in India during winter 2024-25, with an average PM2.5 concentration of 159 μg/m³ throughout the season.

Aizawl in Mizoram recorded the lowest winter-average PM2.5 level at 7 μg/m³, making it the cleanest city in the analysis.

During the previous winter (2023-24), Delhi also topped the list with a higher average of 171 μg/m³.

The Winter Ambient Air Quality Snapshot of India, published by the Centre for Research on Energy and Clean Air (CREA), found that 173 out of 238 cities had winter-average PM2.5 levels exceeding India’s National Ambient Air Quality Standards (NAAQS) of 40 μg/m³. Moreover, none of the 238 cities analyzed met the World Health Organization’s (WHO) guideline of 5 μg/m³, emphasizing the scale of air quality challenges.

In India, winter refers to the period from October 1, 2024, to February 28, 2025 (winter 2024-25).

Byrnihat in Meghalaya followed closely in winter 2024-25, recording a PM2.5 concentration of 157 μg/m³. Other cities among the top 10 most polluted included Hajipur, Ghaziabad, Gurgaon, Noida, Patna, Asansol, Durgapur, and Charkhi Dadri.

The top 10 most polluted cities are located in Bihar, Uttar Pradesh, Haryana, West Bengal, and Assam, along with Delhi.

On a broader state level, Rajasthan (34 cities), Bihar (24 cities), and West Bengal (7 cities) had all monitored cities exceeding NAAQS. Similarly, Maharashtra (30 out of 31 cities), Odisha (15 out of 16 cities), and Uttar Pradesh (15 out of 20 cities) had a high proportion of cities with winter PM2.5 levels above the national standard.

Over the 151-day winter period, 100 cities featured in the top 10 most polluted cities at least once. Of these, 44 cities appeared at least 10 times. Byrnihat had the highest recurrence (111 days), followed by Delhi (105 days), Hajipur (80 days), Ghaziabad (52 days), and Bahadurgarh (47 days).

The top 10 least polluted cities included six cities from Karnataka, three from Tamil Nadu, and one from Mizoram.

Among the 98 NCAP cities with Continuous Ambient Air Quality Monitoring Stations (CAAQMS) data, 78 recorded winter-average PM2.5 levels above NAAQS, while all 98 exceeded the WHO’s recommended limit. Similarly, in 140 non-NCAP cities, 95 surpassed NAAQS, and all remained above WHO guidelines. This highlights that air pollution is not limited to NCAP cities but extends to a wider set of urban areas.

In a press statement, Manoj Kumar, analyst at CREA, noted that, “Reducing winter air pollution is essential to bringing down the annual average pollution levels in Indian cities. CREA’s winter air quality analysis highlights that both NCAP and non-NCAP cities face high pollution levels, and the issue extends far beyond NCR, affecting regions across India. The upcoming NCAP revision presents a crucial opportunity to include more cities under the programme and implement targeted mitigation measures that focus on cutting pollution at the source, such as enforcing stricter emission standards rather than only addressing its effects.”