Ozone pollution spreading across Indian cities, Delhi-NCR worst hit
A CSE analysis of 25 Indian cities has found ground-level ozone spreading across regions and lasting longer each day. The report links the shift to rising health risks, crop losses and a need for multi-pollutant action.
Based on six years of data from 25 cities, the study shows that ozone pollution is no longer confined to isolated summer spikes or a handful of northern urban centres. Instead, it has spread across regions, with prolonged exposure becoming increasingly common in both inland and coastal cities.
The analysis, covering the period from 2021 to 2026, identifies Delhi-NCR, which also sees high air quality index (AQI) nearly throughout the year mainly due to high concentration of fine particulate matter such as PM2.5 and PM10, as the country's largest regional ozone hotspot.
It has also revealed that cities such as Chandigarh, Jaipur, Ahmedabad, Mumbai, Chennai and Bengaluru, too, are witnessing alarming levels of exposure.
Researchers say rising temperatures, intense sunlight and increasing emissions of ozone-forming gases are reshaping India's air pollution profile, creating a threat that extends well beyond particulate matter.
HEALTH RISKS MOUNT
The findings raise serious concerns for public health, with experts warning that ground-level ozone is among the most aggressive air pollutants affecting the human body.
Unlike the protective ozone layer high in the atmosphere, ground-level ozone forms when nitrogen oxides and volatile organic compounds released from vehicles, industries, household fuels and waste burning react under strong sunlight.
According to the CSE, ozone damages lung tissues, triggers airway inflammation, worsens asthma and increases sensitivity to allergens such as dust and pollen. While short-term exposure can lead to respiratory distress and emergency hospital visits, prolonged exposure has been linked to heart attacks, strokes, cardiovascular deaths and irreversible progression of chronic obstructive pulmonary disease (COPD).
Medical practitioners have also observed unexpected spikes in asthma and respiratory illnesses during the summer months, although the report notes that dedicated epidemiological studies are needed to establish direct local links.
"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, trans-boundary crisis," said Anumita Roychowdhury, executive director at CSE and lead author of the study.
She said ozone not only harms public health but also damages agriculture and contributes to climate change by trapping heat, creating a feedback loop that further accelerates ozone formation.
Among the 25 cities analysed between March 1 and May 10 this year, 15 recorded average summer ozone levels above the National Ambient Air Quality Standard for eight-hour exposure.
Chandigarh recorded the highest average summer ozone concentration at 173 micrograms per cubic metre, followed by Jaipur at 120 micrograms and Ahmedabad at 115 micrograms. Bhopal also crossed the standard with an average of 109 micrograms per cubic metre.
Delhi-NCR emerged as the country's most persistent ozone hotspot. The region exceeded the national eight-hour standard on every day of the 71-day study period. Within Delhi, nearly nine monitoring stations on average recorded violations each day, while several NCR locations, including Greater Noida, Noida and Ghaziabad, consistently reported elevated ozone levels.
The report also points to a significant geographical shift in the pollution pattern. While ozone has long been associated with northern India, cities in southern and western India are now recording worrying trends.
Mumbai displayed a near year-round ozone footprint, with repeated episodes from January to April and again during November and December. Chennai recorded the highest episodic ozone concentration among all cities studied, while Bengaluru showed expanding geographical spread and longer periods of exposure.
CITIES FACE LONGER EXPOSURE
One of the study's most significant findings is that ozone is no longer peaking briefly before dispersing. Instead, it is lingering for much longer each day, increasing the cumulative health burden.
Bhopal recorded the longest average daily exposure, with ozone remaining above safe limits for nearly 17 hours. Lucknow followed with more than 16 hours of exposure, while Mumbai and Bengaluru each experienced elevated ozone for almost 16 hours daily.
Researchers argue that counting only the number of days exceeding standards fails to capture the true extent of health risks, as prolonged exposure substantially increases the toxic burden.
Another emerging concern is the persistence of ozone after sunset. Traditionally regarded as a daytime pollutant, ozone is now remaining at hazardous levels through the night in several cities.
Delhi-NCR recorded 46 nights with ozone exceedances during the study period, followed by Bengaluru, Bhopal, Patna and Muzaffarpur. Mumbai displayed an even stronger pattern of persistent nighttime ozone, a phenomenon linked to atmospheric conditions that trap pollutants close to the ground.
Beyond health, the report warns of broader environmental consequences. Ozone suppresses plant growth by interfering with photosynthesis, reducing crop productivity and threatening food security. CSE cited growing evidence suggesting that ambient ozone may be reducing India's wheat yields by around 14 to 15 per cent annually.
At a regional scale, ozone generated in urban and industrial areas can also drift towards the Himalayan region, contributing to warming and accelerating glacial melt.
Calling for urgent policy reforms, the CSE said the next phase of the National Clean Air Programme must move beyond its current emphasis on particulate matter and adopt a comprehensive multipollutant strategy.
The organisation has recommended simultaneous control of nitrogen oxides, volatile organic compounds and other combustion-related gases from transport, industry, household fuels and waste burning, along with regional airshed-based management that addresses pollution across state boundaries rather than within individual cities.
Based on six years of data from 25 cities, the study shows that ozone pollution is no longer confined to isolated summer spikes or a handful of northern urban centres. Instead, it has spread across regions, with prolonged exposure becoming increasingly common in both inland and coastal cities.
The analysis, covering the period from 2021 to 2026, identifies Delhi-NCR, which also sees high air quality index (AQI) nearly throughout the year mainly due to high concentration of fine particulate matter such as PM2.5 and PM10, as the country's largest regional ozone hotspot.
It has also revealed that cities such as Chandigarh, Jaipur, Ahmedabad, Mumbai, Chennai and Bengaluru, too, are witnessing alarming levels of exposure.
Researchers say rising temperatures, intense sunlight and increasing emissions of ozone-forming gases are reshaping India's air pollution profile, creating a threat that extends well beyond particulate matter.
HEALTH RISKS MOUNT
The findings raise serious concerns for public health, with experts warning that ground-level ozone is among the most aggressive air pollutants affecting the human body.
Unlike the protective ozone layer high in the atmosphere, ground-level ozone forms when nitrogen oxides and volatile organic compounds released from vehicles, industries, household fuels and waste burning react under strong sunlight.
According to the CSE, ozone damages lung tissues, triggers airway inflammation, worsens asthma and increases sensitivity to allergens such as dust and pollen. While short-term exposure can lead to respiratory distress and emergency hospital visits, prolonged exposure has been linked to heart attacks, strokes, cardiovascular deaths and irreversible progression of chronic obstructive pulmonary disease (COPD).
Medical practitioners have also observed unexpected spikes in asthma and respiratory illnesses during the summer months, although the report notes that dedicated epidemiological studies are needed to establish direct local links.
"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, trans-boundary crisis," said Anumita Roychowdhury, executive director at CSE and lead author of the study.
She said ozone not only harms public health but also damages agriculture and contributes to climate change by trapping heat, creating a feedback loop that further accelerates ozone formation.
Among the 25 cities analysed between March 1 and May 10 this year, 15 recorded average summer ozone levels above the National Ambient Air Quality Standard for eight-hour exposure.
Chandigarh recorded the highest average summer ozone concentration at 173 micrograms per cubic metre, followed by Jaipur at 120 micrograms and Ahmedabad at 115 micrograms. Bhopal also crossed the standard with an average of 109 micrograms per cubic metre.
Delhi-NCR emerged as the country's most persistent ozone hotspot. The region exceeded the national eight-hour standard on every day of the 71-day study period. Within Delhi, nearly nine monitoring stations on average recorded violations each day, while several NCR locations, including Greater Noida, Noida and Ghaziabad, consistently reported elevated ozone levels.
The report also points to a significant geographical shift in the pollution pattern. While ozone has long been associated with northern India, cities in southern and western India are now recording worrying trends.
Mumbai displayed a near year-round ozone footprint, with repeated episodes from January to April and again during November and December. Chennai recorded the highest episodic ozone concentration among all cities studied, while Bengaluru showed expanding geographical spread and longer periods of exposure.
CITIES FACE LONGER EXPOSURE
One of the study's most significant findings is that ozone is no longer peaking briefly before dispersing. Instead, it is lingering for much longer each day, increasing the cumulative health burden.
Bhopal recorded the longest average daily exposure, with ozone remaining above safe limits for nearly 17 hours. Lucknow followed with more than 16 hours of exposure, while Mumbai and Bengaluru each experienced elevated ozone for almost 16 hours daily.
Researchers argue that counting only the number of days exceeding standards fails to capture the true extent of health risks, as prolonged exposure substantially increases the toxic burden.
Another emerging concern is the persistence of ozone after sunset. Traditionally regarded as a daytime pollutant, ozone is now remaining at hazardous levels through the night in several cities.
Delhi-NCR recorded 46 nights with ozone exceedances during the study period, followed by Bengaluru, Bhopal, Patna and Muzaffarpur. Mumbai displayed an even stronger pattern of persistent nighttime ozone, a phenomenon linked to atmospheric conditions that trap pollutants close to the ground.
Beyond health, the report warns of broader environmental consequences. Ozone suppresses plant growth by interfering with photosynthesis, reducing crop productivity and threatening food security. CSE cited growing evidence suggesting that ambient ozone may be reducing India's wheat yields by around 14 to 15 per cent annually.
At a regional scale, ozone generated in urban and industrial areas can also drift towards the Himalayan region, contributing to warming and accelerating glacial melt.
Calling for urgent policy reforms, the CSE said the next phase of the National Clean Air Programme must move beyond its current emphasis on particulate matter and adopt a comprehensive multipollutant strategy.
The organisation has recommended simultaneous control of nitrogen oxides, volatile organic compounds and other combustion-related gases from transport, industry, household fuels and waste burning, along with regional airshed-based management that addresses pollution across state boundaries rather than within individual cities.
