While surface ozone — a pollutant harmful to the heart and lungs — already exceeds safe limits across much of India in the hot pre-monsoon months, a new study finds that heatwaves push it to still higher levels, thus adding several hundred deaths to a far larger toll that the study links to ozone across the season.

The peer-reviewed study, published in the Nature Portfolio journal npj Clean Air on June 12, reports that surface ozone reaches 85-110 micrograms per cubic metre (μg/m³) in northern India during heatwaves and exceeds the World Health Organization guideline of 70 μg/m³ in every region of the country. The levels fall back within three to four days of a heatwave ending.

Because ozone levels stay high for much of the season, the study attributes a large number of deaths to it even outside heatwaves. During the heatwave days of 2024, it links about 26,500 deaths from ischaemic heart disease and chronic obstructive pulmonary disease (COPD) to ozone exposure. However, such health conditions are present before and after the heat too. The heatwave’s contribution to the toll is the rise in it over the preceding days: about 490 additional heart-disease deaths and 342 from COPD, or roughly 830 in all.

The overall numbers are so large mainly because of how they are calculated. They are not directly counted. The study applies a small increase in an individual’s risk of dying from these two common diseases to India’s population of more than a billion; even a slight per-person risk, spread across so many people and two of the country’s leading causes of death, adds up to tens of thousands.

The authors note that the mortality figures had to be modelled,as continuous ground-level ozone measurements were not available for the specific heatwave days in many affected cities. Therefore, the estimates rely on the assumed relationship between ozone exposure and disease rather than direct observation.

Surface ozone is not released directly but forms when sunlight drives a reaction among other pollutants, a process that speeds up in heat. “Ozone is very harmful, while NO₂ (nitrogen dioxide) and HCHO (formaldehyde) directly damage the respiratory system,” the authors write, referring to two of the gases involved in forming ozone.

The authors, Parambat Sangeetha (Kerala University of Fisheries and Ocean Studies) and Jayanarayanan Kuttippurath of IIT Kharagpur, combined two decades of temperature records from the India Meteorological Department with satellite data and global weather datasets to track ozone levelsand the gases that produce it. Heatwaves between 2004 and 2024 were identified using standard temperature thresholds.

The authors describe the work as the first comprehensive, long-term, and country-wide assessment of surface ozone during heatwaves in India, noting that previous studies were largely limited to individual cities or regions. They counted 188 heatwave events over the two decades with most severe years — 2010, 2016, 2019, and 2024 — following strong El Niño episodes. The Western Himalayas recorded the steepest long-term rise in ozone level, exceeding the WHO guideline by 115% in 2024.

The authors state that “coupled heat–ozone extremes are intensifying, requiring urgent integrated climate–air quality policy action.”

The study follows a recommendation by the 16th Finance Commission, in November 2025, that heatwaves and lightning be added to India’s list of nationally notified disasters, which would allow States to draw on the State Disaster Response Fund for relief.