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Winter is usually seen as the battleground in the fight against air pollution. This is the season when cold air and low wind speeds trap particulate matter over cities, raising health risks. Summer, by contrast, is regarded as a period of relief, as stronger winds help disperse pollutants. But that seasonal respite is beginning to fray because of rising concentration of ground-level ozone.
Ground-level ozone is quite different from stratospheric ozone, which occurs naturally in the upper atmosphere and forms a protective shield against the sun’s harmful ultraviolet rays. At ground level, ozone is a pollutant. Colourless and invisible, it leaves no trail of soot. Instead, it triggers chemical reactions that turn surrounding gases into microscopic particles. These scatter sunlight, creating haze—and fuelling a growing but poorly understood atmospheric crisis.
Unlike particulate matter (PM2.5 and PM10), which can be traced to a tailpipe, an industrial smokestack or a construction site, ozone is not emitted directly by human activity. It is a secondary pollutant, baked into existence in the open air. When primary air pollutants — nitrogen oxides (NOx), carbon monoxide (CO) and volatile organic compounds (VOC) — are released by urban metabolism such as vehicles, factories, diesel generators and the open burning of waste, they mix together in the lower atmosphere. Under intense heat and bright ultraviolet sunlight, this chemical cocktail undergoes a series of photochemical reactions, and results in the formation of ozone.
Higher temperatures, in fact, amplify the problem. They accelerate the chemical reactions that produce ozone, while also prompting vegetation and human activities to release more vocs. “Warmer air causes ozone to form faster, so periods with higher temperature (such as during heatwaves) can result in more ozone production, resulting in poorer air quality,” said Pallavi Pant, head of global initiatives at the US-based Health Effect Institute, which publishes the annual State of Global Air Report.
The latest State of Global Air 2025 (SOGA 2025) report offers some stark warning for India, ranking the country third among those with highest exposure to ozone pollution globally. While particulate matter remains the leading cause of air pollution-related deaths, ambient surface ozone is rapidly emerging as a major respiratory threat.
In 2023, noted SOGA 2025, long-term exposure to ozone was linked to an estimated 470,000 deaths worldwide; roughly half occurred in India. The country’s combination of high precursor emissions, intense sunlight and rising temperatures makes it particularly vulnerable to ozone pollution.
To map India’s vulnerability to ground-level ozone pollution, the Centre for Science and Environment (CSE, Delhi, has analysed real-time data from the Central Pollution Control Board covering 25 major cities from 2021 to May 2026. The analysis suggests that ground-level ozone is altering the country’s air-quality profile.
Summer dataset for this year alone, from March 1 and May 10, showed that ozone exposure has expanded beyond isolated, short-lived spikes to become a widespread urban reality. Of the 25 major cities analysed, 15 recorded seasonal average concentrations well above 100 microgrammes per cubic metre (µg / m³), the limit set by cpcb under the National Ambient Air Quality Standards for average eight-hour exposure. Beyond this threshold, ozone can pose significant risks to human health. (Because ozone is a highly reactive, toxic gas that causes acute, immediate damage to the respiratory system and lungs, exposure to it is measured using the maximum eight-hour average concentration. Exposure to particulate matter, by contrast, is measured using a 24-hour average.)
Ground-level ozone poses a distinct health risk. When inhaled, it irritates the airways and can make it harder to breathe. “Even short-term exposure can cause cough, throat irritation, chest tightness, wheezing, dyspnea and pain during deep inspiration,” said SK Chhabra, Emeritus Consultant, Department of Pulmonary Medicine, Primus Superspeciality Hospital in Delhi.
Doctors are reporting trends such as unexpected respiratory flare-ups and asthma attacks during the hottest months of the year. They cannot tell from their day-to-day clinical experience alone whether these cases reflect a broader shift in respiratory disease patterns. Establishing a direct cause-and-effect link would require detailed, city-specific epidemiological studies that match real-time health data with measurements of ambient air chemistry. Without attributing the change to any single environmental factor, Barnali Bhattacharya, a Pune-based consultant paediatrician and specialist in asthma and sleep disorders, said the seasonal rulebook appears to be changing. “During the summer months, doctors usually step-down medication for asthma and other chronic respiratory conditions because it is considered a safe period,” she said. “But lately, I have been noticing sudden, severe flare-ups. In many cases, reducing the dosage is simply not possible.”
What makes these summer flare-ups puzzling is that the usual triggers have changed little. Regional dust loads and seasonal pollen cycles do not typically undergo massive, erratic shifts from one year to the next. If the baseline allergens remain broadly constant, this unusual clinical shift warrants an investigation to determine what else is contributing to the problem and whether rising summer heat and air pollution is playing a hidden role.
Neeraj Gupta, senior allergist and paediatrician at Sir Ganga Ram Hospital in New Delhi, said, “It would be interesting to examine whether unexpected summer exacerbations correlate with days of ozone exceedance. This may represent an under-recognised environmental trigger deserving greater attention in both clinical practice and research.”
He added that ozone is a potent, aggressive oxidant and it can induce airway inflammation, increase bronchial hyper responsiveness, impair overall lung function, and directly amplify a patient’s response to inhaled allergens like dust and pollen. Several global studies have linked short-term ozone exceedances with increased asthma symptoms, a heavy reliance on rescue medication, emergency room visits, and severe exacerbations — particularly in children and individuals with underlying allergic airway diseases.
(This article is from Down To Earth’s cover story, “Silent Spread”, published in the upcoming June 16-30, 2026 issue.)
