Major Indian cities are grappling with high levels of ground‑level ozone this summer, according to the Centre for Science and Environment (CSE). Bengaluru, Mumbai, Kolkata, Hyderabad, and Chennai recorded multiple days where ozone levels crossed the safe limit.
Bengaluru led the rise. The city saw a 29 percent jump in days exceeding the ozone standard compared to last year.
“The trend is worrying,” said Anumita Roychowdhury, Executive Director at CSE. “Ozone is a toxic gas and even short‑term exposure can trigger serious respiratory issues.”
Ozone is not directly emitted. It forms through chemical reactions involving nitrogen oxides, volatile organic compounds, and carbon monoxide. Sunlight drives this reaction, making summer particularly vulnerable.
Unlike pollutants such as PM2.5 or nitrogen dioxide, ozone forms through reactions in the atmosphere. Vehicles, power plants, factories, and other combustion sources emit the gases that trigger ozone. VOCs, one of the key components, also come from natural sources like plants. Once formed, ozone can travel far, affecting both cities and rural areas.
“Ground-level ozone can hurt crop yields and threaten food security,” Roychowdhury said.
CSE’s analysis used real‑time data from 80 monitoring stations across the five cities. Each station’s data was evaluated for the number of days when eight‑hour average ozone levels breached national standards.
The data came from the Central Pollution Control Board’s public platform. The study covered March to May from 2022 to 2025. It used the US Environmental Protection Agency’s method, measuring the highest eight-hour average each day from each station. If even one station exceeded the limit, it counted as a breach for the city.
“Ozone build‑up is highly localised and influenced by temperature, wind, and precursor gases,” explained Sharanjeet Kaur, Deputy Programme Manager at CSE. “One or two stations exceeding the limit signals local hotspots that need immediate attention.”
She added that average city-level data hides these spikes. “One part of a city might show high ozone while another doesn’t. Averaging hides the risk,” Kaur said.
Breakdown of ozone exceedance trends in major cities
| City | Summer Exceedance Days | Top Hotspot(s) | Winter Exceedance Days (Change) | Key Trends |
|---|---|---|---|---|
| Bengaluru | 45 out of 92 days | Hombegowda Nagar (31 days) | Not specified | Highest rise; early build-up seen in spring |
| Mumbai | 32 days (↓ from last year) | Chakala (29), Byculla, Kherwadi | 87 days (↑ from 78) | Ozone persisted into evenings; worse in winter |
| Kolkata | 22 days (↓ from last year) | Rabindra Sarobar, Jadavpur | Not specified | Improvement in summer; localized spikes remain |
| Howrah | 58 days | Dasnagar | Sharp rise (exact number not given) | Summer and winter both showed high exceedance |
| Hyderabad | 20 days (↓ from 43) | Bollaram | 9 days (↓ from 43) | Clear improvement in both seasons |
| Chennai | 15 days (↑ from 0) | Alandur | Rise noted (exact number not given) | First-time summer exceedance; winter also worsening |
CSE warned that high ozone levels damage the lungs. It inflames airways, worsens asthma, and increases the risk of infections. Children, older adults, and people with existing respiratory conditions face the greatest risk.
“Ozone attacks the lungs,” said Roychowdhury. “It increases asthma attacks and hospital visits.”
Experts note that ozone and nitrogen dioxide interact in complex ways. Traffic-heavy areas with more NO₂ can suppress ozone, while cleaner zones allow it to build up and persist.
“We can’t ignore ozone anymore,” said Roychowdhury. “It’s not just a summer problem. Warmer regions now face year‑round risk.”
She added that developed countries saw a rise in ozone after reducing particulate matter without cutting nitrogen oxide emissions. India could face the same pattern.
“This is a growing public health hazard,” said Kaur. “Without better monitoring and focused action, we risk long‑term damage to people and the environment.”
CSE called for immediate action across all combustion sources, vehicles, industries, waste burning, and household fuels. It also urged the government to include ozone in clean air action plans and emergency systems.
“Ozone pollution is not a future threat,” said Roychowdhury. “It’s already here.”
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