A six-year data analysis conducted by the Delhi-based think-tank Centre for Science and Environment (CSE) claims that in India, ground-level ozone has developed from being isolated, short-term spikes to a widespread, all-the-year-round phenomenon in different regions of the country.
The rising ozone pollution, triggered by intense, rising heat, bright sunlight and growing emissions of gases that make up ozone, this secondary pollutant is transforming the country’s air quality profile and associated risks, says CSE. Ground-level ozone is a toxic air pollutant that aggressively attacks respiratory and environmental systems.
The 2021-2026 study finds that the northern India has been most affected by the ozone pollution with Delhi-NCR (National Capital Region) being identified as the largest regional hotspot in the country.
The report argues, based on the findings, that the current national clean air programme (NCAP), in its new avatar, should adopt a multi-pollutant strategy shifting from the current strategy of only particulate matter reduction.
Vicious loop
“Our analysis reveals that 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 across all regions,” pointed out Anumita Roychowdhury, executive director, CSE, and lead researcher and author of the analysis to The Plurals.
“Ozone is damaging public health, agricultural productivity and regional climate systems. As a climate pollutant, ozone traps heat in the air, which raises temperatures and causes even more ozone to form, creating a dangerous feedback loop,” she adds.
Widening zone
Among 25 major Indian cities analysed between March 1 and May 10, 2026, for ozone pollution, 15 recorded summertime averages well above the National Ambient Air Quality Standard (NAAQS) of 100 microgramme per cubic metre (μg/m3) for a maximum eight-hour exposure.
Chandigarh, followed by Jaipur, recorded the highest average summer ozone concentrations among all cities with 71 and 62 exceedance days, respectively, followed by Bhopal (60), Bengaluru (55), Patna (24) and Muzaffarpur (21). Ahmedabad recorded the third-highest average summer ozone concentration.
Emerging centres
Ozone pollution has spread beyond traditional pollution zones, and is concentrating in northern and western India.
Chandigarh registered the highest summer-time average at 173 μg/m3 (micrograms per cubic meter), followed by Jaipur (120), Ahmedabad (115) and Bhopal (109), according to a CSE release. Mumbai recorded a near year-round ozone footprint. Chennai recorded the highest episodic ozone concentration among all cities. Bengaluru is increasing spatial spread and prolonged ozone exposure.
Delhi-NCR epicentre
“Delhi-NCR exceeded the eight-hour national standard every single day during the 71-day study period. Within the capital, a widespread layer of toxic ozone uniformly blankets the monitoring network, with an average of 8.79 stations in active violation daily. NCR towns averaged 3.2 stations exceeding the standard daily,” a CSE statement said. The ozone hotspots were Pusa IMD in Delhi, Greater Noida Knowledge Park-V, Sector 125 in Noida and Vasundhara (Ghaziabad).
Ozone is staying at high concentrations for longer intervals. Bhopal logged the highest sustained exposure, remaining above safe limits for an average of 17 hours per day, followed closely by Lucknow (16.3 hours) and Mumbai and Bengaluru (15.9 hours each), said the CSE statement.
Traditionally viewed as a day-time phenomenon that drops off after sunset, toxic ozone is now regularly persisting well into the night.
Kolkata not showing spike
Kolkata recorded an average summer ozone concentration of 60 µg/m³. Unlike several other cities, it did not record any daytime or night-time ozone exceedance during the summer of 2026.
“The city has shown a steady improvement in summer ozone pollution over the years, with summer exceedance days declining from 46 in 2021 to 22 in 2025, before dropping to zero in 2026,” states Roy Choudhury.
Along with the summer ozone season, Kolkata also experiences ozone pollution during the winter months. The city recorded 12 ozone exceedance days during November 2025–January 2026, down from 20 exceedance days during November 2024–January 2025.
The estimated duration of summer ozone episodes remained relatively stable between 2021 and 2025 (12.0–13.5 hours).
National air plan needs ozone focus
The current national clean air programme (NCAP) focuses particulate matter reduction. But NCAP 2.0 must adopt a multi-pollutant strategy, suggests the expert.
“By targeting precursor gases like nitrogen oxides and VOCs (volatile organic compounds), which form ozone in the air, the new framework can accelerate the shift toward clean combustion, zero-emission pathways and integrated regional air shed management,” Roychowdhury explains.
“Six years of data reveal that ground-level ozone is intensifying with higher number of days exceeding the standards, lengthening of daily exposure especially during summer, and persistence during nights due to atmospheric trapping. More ozone hotspots are proliferating in different regions of India,” adds Sharanjeet Kaur, deputy programme manager, Urban Lab, CSE, and co-author.