Bengaluru, Mumbai, Kolkata, Hyderabad, Chennai etcrecordseveral days when ozone levels exceeded the standards, says analysis by Centre for Science and Environment (CSE)
- Bengaluru records a big 29 per cent jump in number of days exceeding the ozone standard, compared to previous summer. Mumbai, Kolkata and Hyderabad also show exceedance, but with lesser number of days compared to previous summer
- While in north Indian cities ozone pollution typically associated with summer months, but other mega cities located in warmer climate recordconsistently high ground-level ozone concentrations during other seasons as well
- All mega cities show ozone hotspots where number of days exceeding the standards is higher than other locations in the city
- While there can be annual variations in ozone exceedance due to varying emissions and atmospheric changes, cities and regions need to take urgent action to control this highly reactive and harmful gas
- What is needed: More aggressive reduction of gases from vehicles, industries, waste and solid fuel burning -- that contribute towards ozone formation in the atmosphere
Find the complete CSE study report click here
New Delhi, July 16, 2025: Every leading metropolis and mega city in India –including Kolkata, Bengaluru, Mumbai, Hyderabadand Chennai–has been struck by high ozone levels in the summer of 2025. Ground-level ozone pollution has spiked and concentrations have exceeded the eight-hour standards on a number of days. This has emerged from an analysis done by CentreforScienceandEnvironment(CSE), which was released here today.
This analysis does not cover Delhi: CSE’s assessment of ozone pollution in Delhi has already been released some time back click here.
The analysis hasbeen done by the Urban Lab inCSE under its ‘air quality tracker’initiative. The assessment highlights that unlike primary pollutants, ozone is not emitted directly from any source. It forms through intricate chemical reactions involving nitrogen oxides (NOx), volatile organic compounds (VOCs), and carbon monoxide(CO) --pollutants that are released by vehicles, power plants, factoriesand other combustion sources. In the presence of sunlight, these substances undergo a series of cyclic reactions that result in the formation of ozone at groundlevel.
VOCs also have natural sourcessuch as vegetation, adding to the complexity.Ground-level ozone accumulates not only in urban environments but can also travel long distances, turning into a regional pollutant.It can impactagricultural productivityand threatenfood security.
Given ozone’s highly reactive nature,the ambient air quality standards for it are set for eight-hour averages,instead of 24-hour averages.
Anumita Roychowdhury, executive director, CSE, says: “If unchecked,this can become a serious public health crisis as ozone is a highly reactive gasand can be harmfuleven with short-durationexposures.In contrast to cities in northIndia where high summer temperatures and intense solar radiation can lead to ozone levels exceeding the standards, other cities in warmclimates are experiencing consistent ozone exceedance during other seasons as well.”
Roychowdhury adds: “Thecurrentpolicy focusmust be expanded to include improved monitoringand mitigation ofthis toxic gasand reduction in high local exposures.Ozone mitigation needs drastic control over a range of gases from vehicles, industry and all combustionsources.”
“Inadequate monitoring, limited data and inadequate methods of trend analysis have weakened the understanding of this growing public health hazard across cities of India.Instead of merely averaging out the levels for the city –which is the standard practice to estimate AQI--it is also important to capture adequately the high levels of local build-up and exposures in the hotspots and to design mitigation strategies accordingly,” says Sharanjeet Kaur, deputy programme manager at the Urban Labin CSE
The CSE review shows that exposure to ground-level ozone can inflame and damage the airways, increase susceptibility to infections, and worsen respiratory conditions such as asthma, chronic bronchitisand emphysema. Children with underdeveloped lungs, older adults, and individuals with existing respiratory conditions are particularly vulnerable. Ozone exposure increases the frequency and severity of asthma attacks, often leading to higher rates of hospitalisation.
Theinvestigation method: Thisassessmenthastracedtrendsduringsummer(March-May)between2022and 2025 (up to May 31). It is based on publicly available granular real time data (15-minute averages) from the Central Pollution Control Board’s official online portal –the Central Control Room for Air Quality Management. The data has been captured from80 official stations under the Continuous Ambient Air Quality Monitoring System(CAAQMS),spreada c r o s s Mumbai (31), Kolkata-Howrah (12), Bengaluru (14), Hyderabad (14) and Chennai (nine).
Says Kaur: “Given the volatile and highly localised nature of ground-level ozone pollution build-up and its variability across space, and consistent with the global good practice,this analysis has considered station-level trends in terms of number of days exceeding the eight-hour standard over time.”
She points out that as ozone formation depends on complex atmospheric chemistry and on photochemical reactions, its level varies across the time and space horizon. Meteorological parameters such as sunny and warm weather, stagnant wind patterns etc have a bearing on its formation. Kaur says: “This analysis tracks exceedances at each station in metro cities -- breach of the standard by even one station is considered exceedance by the metro city in which the station is located. Days with multiple stations exceeding the standard indicates the severity of the spatial spread and number of people exposed.”
The study has considered global good practice and taken on board the USEPA approach of computing eight-hour averages for a day and then checking for the maximum value among them to capture the daily ozone pollution level. USEPA assesses city-wide or regional AQI based on the highest value recorded among all stations of the city or the region. Thus, trends have been calculated in terms of number of days when the daily level has exceeded the eight-hour standard (referred as exceedance days hereafter).
City-wise highlights
Mumbai
- Summer trends: Between March 1 and May 31 this summer, Mumbai recorded ozone exceedance on 32 out of 92 days across its air quality monitoring stations. This marks a 42 per cent decline compared to the same period last year. The worst day in terms of spatial spread was March 29, when eight out of 31 monitoring stations reported exceedance. The highest regional ground-level ozone concentration reached was 90 µg/m³.
- Ozone hotspots: Chakala isthe most chronically affected area in Mumbai. It exceeded the standard for 29 days between March and May. It is followed by Byculla and Kherwadi.
- Locational variations and relationship with other pollutants:The spatial distribution of ground-level ozone generally shows an inverse relationship with nitrogen dioxide. In areas with high NO₂ concentrations, particularly near traffic-dense corridors, ozone formation is typically suppressed as ozone reacts with nitric oxide (NO)and breaks down to dissipate.
Chakala consistently reports higher concentrations of both NO₂ and ozone. Such localised variations underscore the complex interplay between precursor pollutants, meteorological conditions and emission sources that shape ozone behavior across different parts of the city.
A comparison of May 2025 with May 2024 reveals that ground-level ozone is now persisting longer into the evening, even after sunset. However, the average hourly ozone peak this summer is 35 per cent lower than last year, indicating possible changes in atmospheric chemistry or precursor emissions.
Daily trends show that when NO₂ levels increase during morning and evening hours, it helps neutralisethe ozone concentrationand dissipate it. But ozone levels rise during off-peak hours when NO2 levels are comparatively lower.
- Ozone build-up during other seasons: Unlike most northern cities, Mumbai records higher ground-level ozone concentrations during winter months during December – February. During the winter of 2024-25, the city experienced 87 days of ozone exceedance—a 10 per cent rise from the previous winter’s exceedance of 78 days.The atmospheric conditions, photochemical reactions and emissions levels would have contributed to this trend.
Kolkata
- Summer trends: About 22 out of 92 days this summer (between 1 March and 31 May) have registered exceedance of ozone standards, as per the air quality monitoring stations of Kolkata. This marks a 45 per cent decline compared to previous summer. Kolkata has seen an overall improvement, with the average hourly ozone peak declining by 22 per cent this summer.
- Ozone hotspots: RabindraSarobar and Jadavpurhave seen more days exceeding the standards compared to other locations.
- Locational variations and relationship with other pollutants:The spatial distribution of ground-level ozone generally shows an inverse relationship with nitrogen dioxide. In areas with high NO₂ concentrations particularly near traffic-dense corridors, ozone formation is typically suppressed where ozone reacts with nitric oxide to break down and dissipate. It usually drifts and builds up in areas with less pollution.
- Ozone build-up during other seasons: While ozone pollution is typically associated with the summer months with strong sun in northern India, Kolkata has consistently recorded elevated ground-level ozone concentrations during the winter (November to February) and pre-monsoon seasons as well. The atmospheric conditions, relatively warmer climate, photochemical reactions and emissions levels would have contributed to this trend.
During the winter of 2024-25, Kolkata experienced 28 days of ozone exceedance, marking a 7 per cent decline from the 30 days recorded in the previous winter.
In the twin city of Howrah, 58 out of 92 days this summer (between 1 March and 31 May) have registered exceedance among its air quality monitoring stations. Dasnagar in Howrah has shown the maximum exceedance. During winter, Howrah has witnessed 81 days of ozone exceedance – increasing from 14 days the previous year.
Bengaluru
- Summer trends: Forty-five out of 92 days this summer (between 1 March and 31 May) have registered exceedance of ozone standards as per the data available from the air quality monitoring stations of Bengaluru. This marks a 29 per cent increase compared to last summer. The worst day in terms of spatial spread was March 31, when four out of 14 monitoring stations reported exceedance.
- Ozone hotspots: Hombegowda Nagar has seen the maximum exceedance. It exceeded the standard for 31 days this March-May. It is followed by Bapuji Nagar.
- Locational variations and relationship with other pollutants: A comparison of May 2025 with May 2024 shows a 28 per cent decline in average hourly ozone peaks -- ozone is no longer persisting in the atmosphere after sunset, as was observed last year. Daily trends show that when NO₂ levels increase during morning and evening hours, it helps neutralise the ozone concentration and dissipate it. But ozone levels rise during off-peak hours when NO2 levels are comparatively lower.
- Ozone build-up during other seasons: In Bengaluru, ozone formation occurs in both winter and summer. However, this year’s data reveals a significant shift of ozone concentrations to the spring season (February to April): a 31 per cent increase is visible, compared to the same period last year. This points to a growing trend of earlyseason ozone formation, driven by warmer temperatures, intensified solar radiation, and possibly changing precursor emission patterns.
Hyderabad
- Summer trends: There have been 20 days this summer (between 1 March and 31 May) that have registered exceedance among the air quality monitoring stations of Hyderabad. This marks a 55 per cent decline compared to last summer. The highest regional intensity was 51 µg/m³.
- Ozone hotspots: Bollaram is most chronically affected by ground-level ozone pollution. It exceeded the standard for 17 days this March-May. There have been no exceedances at other stations in the city barring ICRISAT (exceeding the standard for two days) and Ramachandrapuram (one day).
- Locational variations and relationship with other pollutants: A comparison of May 2025 with May 2024 reveals that ground-level ozone is now lingering in the atmosphere even after sunset, and the average hourly ozone peak is 3 per cent higher than last year.
- Ozone build-up during other seasons: Hyderabad frequently experiences wintertime ozone exceedances, driven by cold, stagnant conditions and poor vertical mixing. Urban traffic and industrial emissions, when combined with low dispersion, lead to ozone accumulation even under weaker winter sun. However, this winter season (December to January), the city witnessed a substantial improvement, recording just nine days of ozone exceedance, a sharp decline from 43 days during the previous winter.
Chennai
- Summer trends: About 15 out of 92 days this summer (between 1 March and 31 May) have registered exceedance as per the data available from the air quality monitoring stations of Chennai. However, no exceedance was reported during the same period last year. In comparison, the summer of 2023 saw three exceedance days, while 2022 recorded 19. The highest regional intensity this summer was 64 µg/m³.
- Ozone hotspots: Alandur is most chronically affected by ground-level ozone pollution. It exceeded the standard for 15 days this March-May. There have been no exceedances at other stations in the city.
- Locational variations and relationship with other pollutants: Data from May 2025 reveals that ground-level ozone is now lingering in the atmosphere well after sunset, and the average hourly ozone peak has surged by 76 per cent compared to May 2024.
- Ozone build-up during other seasons: Chennai typically experiences rising ozone levels in the early summer months, when drier air, clear skies and reduced wind speeds delay pollutant dispersion. This allows for increased photochemical activity, especially during bright, sunny days with minimal atmospheric mixing. This summer (March to May 2025), Chennai recorded 15 days of ozone exceedance, marking a significant shift from zero exceedance days during the same period last year. Additionally, during the winter months (December–February), the number of ozone exceedance days has also risen from seven days last year to 10 days this winter.
The way forward
Ground-level ozone is beginning to emerge as a pollutant of concern with cities experiencing days exceeding the eight-hour standards. While the level of exceedance is expected to be higher during the summer months with strong sun-shine days and heat, this is emerging as a round the year problem in the warmer climate. Says Roychowdhury: “Clean air action plan for cities and the states need to address this multi-pollutant challenge urgently. It is important to learn from the advanced economies that after controlling particulate pollution have fallen into the grip of rising NOx and ozone crisis. It is important to prevent this trap.”
- Clean air action plan for the city needs to integrate ozone mitigation to implement stringent measures to upscale zero emissions vehicles, clean industrial processes and fuels, eliminate waste burning with hundred percent remediation of legacy waste, collection, segregation and material recovery, and replace solid fuels with clean fuels in households. Integrate ozone in the Graded Response Action Plan to take emergency action to target the emitter of precursor gases like vehicles and industry that form ozone and to reduce short term exposures.
- Develop regional action plan on ozone: Ground-level ozone gets created in polluted areas but drifts and accumulates in cleaner urban environments, urban peripheries and surrounding rural areas affecting agricultural productivity and food security. While in polluted areas ozone further reacts with pollutants to dissipate, in cleaner environment it lives longer. Ozone is thus a regional pollutant that requires effective control at both local and regional level.
For more details, interviews etc, contact Sukanya Nair at sukanya.nair@cseindia.org, 8816818864.
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