Ozone used to disappear when winter ended. Not anymore. A six-year study finds toxic ozone now blankets Indian cities through most of the year — and Delhi-NCR has it worst.
The Centre for Science and Environment (CSE) tracked ozone levels from 2021 to 2026. Heat, strong sunlight and rising emissions are pushing readings far above safe limits, even in cities once seen as only mildly polluted.
Ozone forms when nitrogen oxides (NOx) and volatile organic compounds (VOCs) react in sunlight and heat. Unlike smoke or dust, it cannot be seen. It damages lungs, crops and the climate.
“Rising ground-level ozone and prolonged exposure windows are transforming India’s localised, winter-time particulate problem into a year-round, transboundary crisis that peaks during the summer,” said Anumita Roychowdhury, executive director of CSE and the study’s lead author.
Ozone traps heat, which raises temperatures, which creates more ozone. “It’s a dangerous feedback loop,” Roychowdhury said.
For years, India’s clean air policy has centered on particulate matter — the dust and smoke that choke cities each winter, tracked under the National Clean Air Programme (NCAP). Ozone rarely entered the conversation. This study argues that omission can no longer hold.
Delhi-NCR Tops the List
CSE studied 25 major cities between March 1 and May 10, 2026. Fifteen recorded summer ozone averages above the national safety standard of 100 micrograms per cubic metre.
Delhi-NCR broke the national eight-hour standard on all 71 days of the study period. Nearly nine monitoring stations across the region exceeded the limit on an average day. NCR towns outside the capital averaged just over three stations in violation daily.
Four locations stood out as persistent hotspots: Pusa in Delhi, Greater Noida’s Knowledge Park-V, Sector 125 in Noida, and Vasundhara in Ghaziabad.
Chandigarh posted the highest reading of any city studied: 173 micrograms per cubic metre. Jaipur followed at 120, Ahmedabad at 115, and Bhopal at 109. Jaipur logged the second-highest number of exceedance days, at 62, behind only Delhi-NCR’s 71.
Mumbai broke the seasonal pattern entirely. The city saw ozone spikes from January through April, then again in November and December — evidence that the pollutant is no longer confined to summer. Chennai recorded the highest single-day ozone spike of any city in the study. Bengaluru showed both a wider geographic spread and longer exposure windows than in previous years.
Nights Are No Longer Safe
Ozone was once considered a daytime problem, one that faded after sunset. That assumption is breaking down.
Trapped air near the ground, combined with lower nitrogen oxide emissions at night, is letting ozone linger well past dark. Delhi logs 20 to 30 exceedance nights a month during heatwaves. Across the wider NCR, researchers counted 46 such nights — the highest of any region studied. Bengaluru followed with 14, Bhopal with 13, and Patna and Muzaffarpur with eight each.
Exposure durations are stretching longer, too. Bhopal residents breathed unsafe ozone levels for 17 hours a day on average, the longest stretch recorded anywhere in the study. Lucknow followed at 16.3 hours, with Mumbai and Bengaluru close behind at 15.9 hours each.
“Six years of data reveal that ground-level ozone is intensifying — more exceedance days, longer daily exposure, and persistence through the night due to atmospheric trapping,” said Sharanjeet Kaur, deputy programme manager at CSE’s Urban Lab and the study’s co-author.
CSE’s Prescription
CSE wants India’s next clean air programme, NCAP 2.0, to target ozone directly instead of focusing only on dust. “While the current national clean air programme focuses on particulate matter reduction, NCAP 2.0 must adopt a multi-pollutant strategy,” Roychowdhury said.
The group is pushing for combined controls on nitrogen oxides, VOCs and carbon monoxide — the gases that react together to form ozone. Controlling one without the other, CSE argues, will not stop the spikes.
CSE also wants regional planning that crosses state and city lines, replacing the current city-by-city approach with an integrated, airshed-wide strategy. Ozone, after all, does not stop at a border.
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