Ozone pollution spreads across Indian cities at a pace that is reshaping India’s air quality landscape, according to a new six-year analysis (2021–2026) by the Centre for Science and Environment (CSE). Once regarded as a short-lived summertime issue, ground-level ozone (O₃) has evolved into a persistent, year-round pollutant affecting multiple regions across the country. The study identifies Delhi-NCR as India’s largest regional ozone hotspot, with pollution now lasting longer each day and increasingly persisting through the night. Of the 25 major Indian cities studied, 15 recorded summertime ozone levels above India’s National Ambient Air Quality Standard (NAAQS) of 100 μg/m³ for maximum eight-hour exposure.

Experts warn that rising temperatures, stronger sunlight, and growing emissions of ozone-forming gases are creating a dangerous cycle that threatens public health, agriculture, and climate resilience.

Ground-Level Ozone Is Becoming India’s New Air Pollution Challenge

Unlike particulate matter (PM2.5), ground-level ozone is not released directly into the atmosphere. It forms when pollutants such as nitrogen oxides (NOx), volatile organic compounds (VOCs), and carbon monoxide react under intense sunlight and high temperatures.

As climate change drives hotter summers and more frequent heatwaves, these chemical reactions become faster and more intense. The result is an invisible pollutant that can spread across cities and even entire regions, making ozone pollution much harder to control than localized particulate pollution.

According to the World Health Organization (WHO), ground-level ozone is among the world’s most harmful air pollutants because prolonged exposure damages human health and ecosystems alike.

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Six Years of Data Reveal a Worrying Trend

The CSE analysis shows that ozone pollution spreads across Indian cities much more extensively than previously understood.

• Between March 1 and May 10, 2026, researchers examined ozone levels in 25 major cities and found widespread violations of national standards.
• Chandigarh recorded the highest average summer ozone concentration at 173 μg/m³, followed by Jaipur (120 μg/m³), Ahmedabad (115 μg/m³), and Bhopal (109 μg/m³).

Delhi-NCR, however, stood apart because of the consistency and duration of pollution rather than peak concentrations alone. During the entire 71-day study period, the region exceeded the national eight-hour ozone standard every single day.

Delhi-NCR Emerges as India’s Largest Regional Hotspot

The report identifies Delhi-NCR as the country’s most persistent ozone pollution hotspot.

Researchers found that an average of 8.79 monitoring stations within Delhi exceeded safe ozone limits every day, while surrounding NCR towns averaged 3.2 stations in violation daily. Rather than appearing as isolated pockets, ozone formed a widespread layer across the region, indicating that pollution is moving freely across city boundaries.

Major hotspots identified by the study include:

• Pusa IMD, Delhi
• Greater Noida Knowledge Park-V
• Sector 125, Noida
• Vasundhara, Ghaziabad

This regional pattern suggests that ozone cannot be managed through city-level interventions alone. Instead, coordinated action across the entire airshed will be necessary.

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Ozone Pollution Is Lasting Longer Than Ever

One of the most significant findings is that ozone is no longer limited to a few afternoon hours. Cities are experiencing much longer daily exposure windows, increasing the cumulative health burden on residents.

• Bhopal recorded the highest average ozone exposure duration at 17 hours per day, followed by Lucknow at 16.3 hours.
• Mumbai and Bengaluru each experienced ozone exposure for nearly 16 hours daily.

Researchers emphasize that simply counting the number of polluted days no longer provides a complete picture. The length of exposure is equally important because prolonged ozone concentrations increase the amount of pollution inhaled by people throughout the day.

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Night-Time Ozone Is Becoming a Serious Concern

Ground-level ozone has traditionally been considered a daytime pollutant because sunlight is essential for its formation. However, the new findings reveal that ozone is increasingly persisting overnight.

Delhi-NCR recorded 46 night-time ozone exceedance events, while Bengaluru, Bhopal, Patna, and Muzaffarpur also experienced repeated overnight pollution episodes. Scientists attribute this to shallow night-time atmospheric boundary layers that trap pollutants close to the ground. At the same time, reduced atmospheric mixing and slower chemical breakdown allow ozone to remain in the air long after sunset.

This trend is especially concerning because continuous overnight exposure leaves the human body with little opportunity to recover from daytime pollution.

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Health and Environmental Risks Continue to Grow

Ground-level ozone is a powerful oxidant that affects nearly every organ involved in breathing.

• Medical research has linked ozone exposure to asthma attacks, reduced lung function, chronic respiratory diseases, cardiovascular complications, and increased hospital admissions.
• According to the Global Burden of Disease project, ozone pollution contributes to hundreds of thousands of premature deaths globally each year.
• Children, elderly individuals, outdoor workers, and people with existing respiratory illnesses remain the most vulnerable.

The environmental impacts are equally significant.

• Ozone damages plant tissues, lowers crop productivity, reduces forest growth, and weakens ecosystems.
• Studies estimate that ozone pollution reduces yields of major crops such as wheat, rice, soybean, and maize, contributing to billions of dollars in agricultural losses worldwide.
• As a short-lived climate pollutant, ozone also traps heat in the lower atmosphere, further accelerating warming.

Also Read: Ocean Health Declines As Climate Change And Pollution Accelerate: UN Report

Climate Change Is Accelerating Ozone Formation

Experts increasingly describe ozone as both an air pollution issue and a climate challenge.

According to CSE Executive Director Anumita Roychowdhury, rising temperatures are intensifying ozone formation while ozone itself contributes to additional atmospheric warming. This creates a dangerous feedback loop in which hotter weather produces more ozone, and more ozone further increases heat.

With India experiencing more frequent and intense heatwaves, this relationship is expected to become even stronger unless emissions of ozone-forming pollutants are reduced substantially.

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Experts Call for a Multi-Pollutant Strategy

The Centre for Science and Environment argues that India’s current National Clean Air Programme (NCAP) focuses too heavily on particulate matter and must evolve into a broader air quality strategy.

CSE recommends several priority actions:

• Simultaneously reduce emissions of nitrogen oxides (NOx) and volatile organic compounds (VOCs), the primary precursors of ozone.
• Expand NCAP 2.0 into a comprehensive multi-pollutant programme rather than focusing primarily on PM2.5 and dust.
• Shift from city-specific interventions to integrated regional airshed management across neighbouring states.
• Accelerate clean transport, renewable energy adoption, cleaner industrial technologies, and zero-emission pathways.
• Strengthen continuous ozone monitoring, forecasting, and public health advisories during heatwaves.

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The Road Ahead

The latest CSE findings make it clear that ozone pollution spreads across Indian cities is no longer a localized or seasonal problem. It has evolved into a regional, year-round air quality challenge driven by rising temperatures, urban emissions, and changing atmospheric conditions. As ozone pollution continues to expand beyond traditional hotspots and increasingly persists during both daytime and nighttime, experts believe India’s clean air policies must evolve accordingly.

Addressing ozone will require coordinated action across sectors, stronger emission controls, and an integrated airshed management approach that protects public health, agricultural productivity, and climate resilience simultaneously.

Also Read: Atmospheric Dust Linked To Rising Urban Air Quality Concerns

Frequently Asked Questions

1. What is ground-level ozone?

Ground-level ozone is a harmful pollutant formed when nitrogen oxides (NOx) and volatile organic compounds (VOCs) react in the presence of sunlight and heat.

2. Why is Delhi-NCR considered India’s biggest ozone hotspot?

The CSE study found that Delhi-NCR exceeded the national ozone standard every day during the 71-day summer study period, making it the country’s most persistent regional hotspot.

3. How does ozone affect human health?

Long-term exposure can reduce lung function, worsen asthma, increase respiratory illnesses, and contribute to cardiovascular disease and premature mortality.

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