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Ground-level ozone pollution is no longer confined to a few metro cities or a short summer window. A new analysis by the Centre for Science and Environment has found that ozone pollution is spreading across Indian cities and lasting longer through the day, turning what was once seen as a seasonal problem into a wider year-round public health concern. The study, which assessed air quality data from 25 cities between 2021 and 2026, found that Delhi-NCR remains the largest regional hotspot, while cities like Mumbai, Bengaluru, Chennai, Jaipur, Ahmedabad, Chandigarh, and Patna are also recording worrying ozone levels. Researchers warn that this invisible pollutant is now emerging as one of India’s fastest-growing air quality threats.
Ozone pollution is no longer limited to summers or big metros
Unlike particulate matter, ground-level ozone is not emitted directly. It forms when gases such as nitrogen oxides, volatile organic compounds and carbon monoxide react under sunlight. This makes it especially dangerous during hot summer afternoons, when rising temperatures speed up these chemical reactions. The Centre for Science and Environment says this creates a cycle in which hotter weather leads to more ozone, and ozone, in turn, traps more heat.
The report found that in several cities, ozone exceedances are becoming more prolonged. In Delhi NCR, ozone levels frequently remained above the national standard of 100 micrograms per cubic metre for 8 to 12 hours a day during peak summer. Similar patterns were recorded in Mumbai, Chennai and Bengaluru, suggesting that ozone is now becoming a widespread urban problem rather than a localised one.
Researchers also noted that ozone pollution is now appearing outside the usual summer months in some cities, indicating that changing weather patterns and growing precursor emissions are extending exposure windows.
Health risks are growing as exposure becomes longer
Ozone is a powerful oxidant that can inflame the lungs, worsen asthma, reduce lung function and increase the risk of chronic respiratory diseases. The US United States Environmental Protection Agency describes it as one of the most harmful air pollutants at ground level, especially for children, older adults and people with pre-existing heart or lung conditions.
The concern is not just urban health. Researchers say ozone also damages crops by affecting plant growth and reducing yields. This makes it both a public health and food security issue, especially in agricultural regions around polluted urban belts.
According to the CSE analysis, the growing spread of ozone means that cities can no longer focus only on reducing particulate pollution. Since ozone forms through chemical reactions, controlling it requires reducing emissions from vehicles, industries and combustion sources across wider regions.
India may need a multi-pollutant strategy to tackle ozone
Experts say India’s clean air policies remain heavily focused on PM2.5 and PM10, while ozone often receives less policy attention. The CSE has called for a broader multi-pollutant strategy under the National Clean Air Programme (NCAP), warning that tackling only particulate pollution will not be enough to improve air quality in the long run.
The report comes at a time when India is also seeing more frequent heatwaves, which scientists say can intensify ozone formation. A recent study by IIT Kharagpur estimated that heatwave-driven ozone exposure may have contributed to over 26,500 deaths in India in 2024, underlining the growing health burden of this lesser-noticed pollutant.
For policymakers, the message is becoming harder to ignore. As Indian cities enter another intense summer, ozone pollution is increasingly shaping up as the next major air quality challenge, one that is harder to see but potentially just as dangerous as winter smog.
References:
https://www.cseindia.org/ozone-pollution-spreading-across-indian-cities-says-new-cse-anaylsis-13169
Banner image: Photo by Yash Gupta on Unsplash
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