Heatwaves in India may have intensified ground-level ozone exposure linked to more than 26,500 deaths in 2024, an IIT Kharagpur-led study says.
The study linked ozone exposure during heatwaves to 15,615 deaths from ischaemic heart disease and 10,898 deaths from chronic obstructive pulmonary disease.
Surface ozone levels during heatwaves frequently rose to 85-110 micrograms per cubic metre, exceeding WHO guidelines across all Indian regions.
The worst-affected hotspots included north-west India, the Indo-Gangetic Plain, north-central India, the north-east and the western Himalaya.
Researchers say India’s Heat Action Plans must include ozone forecasting, air-quality health advisories and combined climate–air pollution response strategies.
Heatwaves in India may have intensified toxic ground-level ozone exposure linked to more than 26,500 deaths in 2024, according to a new study by researchers at the Indian Institute of Technology, Kharagpur.
The study published in the journal npj Clean Air on June 12, 2026 analysed the relationship between heatwaves and surface ozone levels across India between 2004 and 2024.
It found that heatwaves, long associated with heat stress, dehydration, heat exhaustion, heatstroke and cardiovascular strain, can also drive sharp increases in ground-level ozone, a harmful air pollutant linked to lung and heart disease.
“In 2024 alone, ozone exposure during heatwaves was associated with 15,615 deaths from ischaemic heart disease and another 10,898 deaths from chronic obstructive pulmonary disease,” said P Sangeetha of the Kerala University of Fisheries and Ocean Studies, Kochi, one of the authors of the paper.
The combined mortality burden was more than 26,500 deaths, the study said. By comparison, deaths attributed to ozone exposure before heatwave conditions were much lower, at 490 deaths from ischaemic heart disease and 342 deaths from chronic obstructive pulmonary disease.
Ground-level ozone is not emitted directly. It forms when pollutants such as nitrogen oxides, volatile organic compounds, carbon monoxide and methane react in sunlight.
Unlike ozone in the stratosphere, which shields the Earth from harmful ultraviolet radiation, surface ozone is a toxic pollutant. It can damage the lungs, aggravate respiratory illness and increase cardiovascular risks.
The study found that surface ozone concentrations during heatwaves frequently rose to 85-110 micrograms per cubic metre, exceeding World Health Organization guidelines across all Indian regions.
“The most severe hotspots include north-west India, the Indo-Gangetic Plain, north-central India, the north-east, and the western Himalaya,” said the other author of the paper, Jayanarayanan Kuttippurath of CORAL, IIT Kharagpur.
Ozone levels in these regions exceeded WHO limits by 39-45 per cent, peaking at 115 per cent above the limit in the western Himalaya, he said.
The findings show that heatwave impacts go beyond direct heat-related illness, Sangeetha said. “Elevated ozone levels during these events significantly worsen respiratory and cardiovascular outcomes, creating a compound heat-air pollution crisis,” she said.
The study found some of the most extreme and prolonged heatwave conditions in north-west and north-central India in 2024. In the north-west, two intense heatwave episodes occurred between May 16-23 and May 25-31 May. Peak temperatures reached 42.89 degrees Celsius (°C) and 44.12°C, and the events lasted eight and seven days respectively.
North-central India recorded two heatwave events between May 26-31 and June 14-18, with maximum temperatures reaching 44.33°C and 42.57°C. The study linked the severity partly to the absence of active western disturbances, which can otherwise bring relief from extreme heat in parts of northern India. No heatwave conditions were observed over north and central India during April 2024, owing to the frequent passage of active western disturbances from the far west, the researchers wrote.
Heatwave to severe heatwave conditions affected Jammu, Rajasthan, Gujarat, Madhya Pradesh, Punjab, Haryana, Chandigarh, Uttar Pradesh, Delhi, Himachal Pradesh, Vidarbha and Maharashtra, before extending to Uttarakhand, Odisha, Chhattisgarh and Bihar.
The highest temperature, 50.5°C, was recorded at Churu in western Rajasthan on 28 May 2024, the study said.
In peninsular India, a prolonged heatwave occurred from 29 April to 6 May, with the highest temperature reaching 41.34°C on 4 May. Severe heatwaves were also reported in south-eastern India, including Tamil Nadu, interior Karnataka and Rayalaseema. The duration of heatwave events across central and eastern India varied from four to eight days, indicating both regional heterogeneity and a broader expansion of extreme heat conditions across the subcontinent.
The study said chronic obstructive pulmonary disease is highly sensitive to changes in temperature, humidity and air quality.
During heatwaves, the body increases blood flow to the skin to release heat, raising blood pressure and respiratory rates. Ozone exposure can add oxidative stress and worsen respiratory and cardiovascular outcomes. The study noted that exposure to ozone during heatwaves can raise COPD mortality risk, with earlier research finding a relative risk increase of 1.0173 for every 10 micrograms per cubic metre rise in ozone.
Heat stress and ozone exposure can also independently increase the risk of ischaemic heart disease by contributing to vascular inflammation, changes in blood viscosity and higher risk of myocardial infarction, the study said.
Researchers warned that, as climate change intensifies extreme heat, compound events involving heatwaves and air pollution are likely to become more common. Current public health frameworks often treat heatwaves and air pollution as separate problems, the authors said.
Kuttippurath said most Heat Action Plans focus on heat stress and heatstroke, while overlooking ozone pollution. He said India urgently needed to integrate ozone forecasting systems, include air-quality health advisories and develop combined climate-air pollution response strategies. The study said such integration would be essential to protect public health in a warming climate.