India’s leading metropolitan areas, including Kolkata, Bengaluru, Mumbai, Hyderabad, and Chennai, have experienced significant spikes in ground-level ozone pollution during the summer of 2025. This pllution has been highlighted by a new analysis from the Centre for Science and Environment (CSE), which notes that concentrations have often exceeded eight-hour standards. This assessment excludes Delhi, for which CSE had previously released a separate ozone pollution report.
The analysis, conducted by CSE’s Urban Lab under its ‘air quality tracker’ initiative, underscores the complex nature of ozone. Unlike primary pollutants, ground-level ozone is not directly emitted. It forms through intricate chemical reactions involving nitrogen oxides (NOx), volatile organic compounds (VOCs), and carbon monoxide (CO).
These precursors are released by vehicles, power plants, factories, and other combustion sources. In the presence of sunlight, these substances undergo cyclic reactions, leading to ozone formation at ground level. VOCs also have natural sources like vegetation, adding to the complexity. Ground-level ozone can accumulate in urban areas and can travel long distances and become a regional pollutant. This impacts agricultural productivity and food security.
Ozone has a highly reactive nature. Due to this, ambient air quality standards for ozone are set for eight-hour averages. This reflects its potential for harm even with short-duration exposures.
SERIOUS PUBLIC HEALTH
Anumita Roychowdhury, Executive Director, CSE, warns that if unchecked, this could become a serious public health crisis. She notes that North Indian cities typically see high ozone levels due to summer temperatures and intense solar radiation. Other warm-climate cities are now experiencing consistent exceedances in other seasons as well. Roy chowdhury urges an expanded policy focus that should include improved monitoring and mitigation of this toxic gas. She emphasizes the need for drastic control over emissions from vehicles, industry, and all combustion sources.
Sharanjeet Kaur, Deputy Programme Manager at CSE’s Urban Lab, highlights inadequate monitoring and analysis methods. These shortcomings have weakened understanding of this growing hazard. She emphasizes capturing high local build-up and exposures in hotspots. It’s more effective than relying on city-wide averages. This is crucial to design effective mitigation strategies.
HEALTH IMPACTS OF OZONE EXPOSURE
CSE’s review indicates that exposure to ground-level ozone can inflame and damage airways. It can increase susceptibility to infections. It also worsens respiratory conditions such as asthma, chronic bronchitis, and 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 hospitalization.
METHODOLOGY OF THE INVESTIGATION
The assessment traced trends during summer (March-May) between 2022 and 2025 (up to May 31). It used 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. Data was collected from 80 official stations under the Continuous Ambient Air Quality Monitoring System (CAAQMS) across Mumbai (31), Kolkata-Howrah (12), Bengaluru (14), Hyderabad (14), and Chennai (9).
Kaur noted that the analysis considered station-level trends in terms of days exceeding the eight-hour standard, consistent with global good practice. She explained that ozone formation is influenced by complex atmospheric chemistry and meteorological parameters like sunny, warm weather and stagnant wind patterns. Exceedance by even one station constitutes an exceedance for the metro city, while multiple stations exceeding indicate spatial spread and exposure. The study adopted the USEPA approach of computing eight-hour averages and taking the maximum value for daily ozone levels, defining trends by the number of “exceedance days.”
CITY-WISE HIGHLIGHTS: SUMMER 2025 OZONE TRENDS
Mumbai
- Summer trends: 32 out of 92 days recorded ozone exceedance, a 42% decline from summer 2024. March 29 was the worst day with eight stations exceeding. Highest regional concentration: 90 µg/m³.
- Ozone hotspots: Chakala (29 days exceedance), followed by Byculla and Kherwadi.
- Locational variations: Inverse relationship with NO₂; high NO₂ suppresses ozone. Chakala, however, reports high levels of both. Ozone is persisting longer into the evening in May 2025 than in May 2024, but the average hourly peak is 35% lower than last year. NO₂ peaks in morning/evening neutralize ozone.
- Other seasons: Mumbai records higher ozone in winter (Dec-Feb). Winter 2024-25 saw 87 exceedance days, a 10% rise from previous winter.
Kolkata
- Summer trends: 22 out of 92 days recorded exceedance, a 45% decline from summer 2024. Average hourly ozone peak declined by 22%.
- Ozone hotspots: Rabindra Sarobar and Jadavpur.
- Locational variations: Inverse relationship with NO₂; ozone drifts to less polluted areas.
- Other seasons: Consistently elevated ozone in winter (Nov-Feb) and pre-monsoon. Winter 2024-25 saw 28 exceedance days, a 7% decline from previous winter. Howrah, a twin city, saw 58 exceedance days this summer (Dasnagar being worst) and 81 in winter 2024-25, a significant increase from 14 days the previous winter.
Bengaluru
- Summer trends: 45 out of 92 days recorded exceedance, a 29% increase from last summer. March 31 was the worst day with four stations exceeding.
- Ozone hotspots: Hombegowda Nagar (31 days exceedance), followed by Bapuji Nagar.
- Locational variations: May 2025 shows 28% decline in average hourly ozone peaks compared to May 2024; ozone no longer persists after sunset. NO₂ peaks in morning/evening neutralize ozone.
- Other seasons: Ozone forms in both winter and summer. This year shows a 31% increase in ozone concentrations in spring (Feb-Apr) compared to last year, indicating early-season formation.
Hyderabad
- Summer trends: 20 out of 92 days recorded exceedance, a 55% decline from last summer. Highest regional intensity: 51 µg/m³.
- Ozone hotspots: Bollaram (17 days exceedance). ICRISAT (2 days) and Ramachandrapuram (1 day) also saw exceedances.
- Locational variations: May 2025 shows ozone lingering after sunset, with average hourly ozone peak 3% higher than May 2024.
- Other seasons: Frequently experiences wintertime ozone exceedances. Winter 2024-25 saw 9 exceedance days, a sharp decline from 43 days in previous winter.
Chennai
- Summer trends: 15 out of 92 days recorded exceedance this summer. No exceedance in summer 2024, but 3 in 2023 and 19 in 2022. Highest regional intensity: 64 µg/m³.
- Ozone hotspots: Alandur (15 days exceedance), no other stations reported exceedances.
- Locational variations: May 2025 shows ozone lingering well after sunset. Average hourly ozone peak surged by 76% compared to May 2024.
- Other seasons: Typically experiences rising ozone levels in early summer. Winter (Dec-Feb) exceedance days increased from 7 last year to 10 this winter.
THE WAY FORWARD: URGENT ACTION FOR CLEAN AIR
Ground-level ozone is becoming a significant pollutant of concern. Cities are experiencing days exceeding eight-hour standards. While summer months with strong sunshine and heat naturally lead to higher exceedance, this is becoming a year-round problem in warmer climates.
Roychowdhury emphasizes that “Clean air action plan for cities and the states need to address this multi-pollutant challenge urgently.” She warns against repeating the mistake of advanced economies that faced rising NOx and ozone crises after controlling particulate pollution.
Clean air action plans must integrate ozone mitigation. This includes stringent measures for:
- Zero-emission vehicles: Upscaling adoption to reduce precursor emissions.
- Clean industrial processes and fuels: Transitioning industries away from polluting sources.
- Waste management: Eliminating waste burning with 100% remediation of legacy waste, improved collection, segregation, and material recovery.
- Clean household fuels: Replacing solid fuels with cleaner alternatives.
Additionally, ozone must be integrated into the Graded Response Action Plan (GRAP) to trigger emergency actions. These actions should target emitters of precursor gases, like vehicles and industry, to reduce short-term exposures.
DEVELOPING REGIONAL ACTION PLANS
Ground-level ozone is a regional pollutant. It forms in polluted areas but drifts and accumulates in cleaner urban environments, peripheries, and surrounding rural areas. This impacts agricultural productivity and food security. While ozone reacts and dissipates in highly polluted areas, it persists longer in cleaner environments. Therefore, effective control requires both local and regional action.