Heatwaves and Ozone Together Raise India's Cardiac Deaths, Study Finds

A new peer-reviewed study has found that heatwaves and surface ozone act together to raise the number of heart-related deaths in India. Surface ozone is a harmful pollutant for the heart and lungs, and it already crosses safe limits across much of the country during the hot months before the monsoon. The study, published in the journal npj Clean Air on 12 June 2026, shows that heatwaves push ozone even higher, adding several hundred extra deaths on top of a much larger seasonal toll.

During heatwaves, surface ozone in northern India reaches 85 to 110 micrograms per cubic metre, and it crosses the World Health Organization guideline of 70 micrograms per cubic metre in every region of the country. The good news is that levels fall back within three to four days after a heatwave ends. But because ozone stays high through much of the season, it is linked to deaths even outside heatwave days.

For the heatwave days of 2024, the study connects about 26,500 deaths from heart disease and chronic lung disease to ozone exposure. Of these, the heatwave's own added contribution, meaning the extra rise over the preceding days, was about 490 heart-disease deaths and 342 lung-disease deaths, or roughly 830 in total. The authors explain that these large figures are not counted directly. Instead, a small rise in each person's risk is applied across India's population of more than a billion. Even a tiny per-person risk, spread over so many people and two of the country's biggest killers, adds up to tens of thousands.

Surface ozone is not released directly from any source. It forms when sunlight drives a reaction among other pollutants, including nitrogen dioxide and formaldehyde, and this process speeds up in heat. The researchers, from a fisheries and ocean studies university in Kerala and from IIT Kharagpur, combined two decades of temperature records from the India Meteorological Department with satellite data and global weather datasets. They identified 188 heatwave events between 2004 and 2024, with the worst years, 2010, 2016, 2019 and 2024, all following strong El Nino episodes. The Western Himalayas saw the sharpest long-term rise in ozone, exceeding the WHO limit by 115 percent in 2024.

The authors describe this as the first long-term, country-wide study of surface ozone during heatwaves in India and call for combined climate and air-quality policy. The work follows a November 2025 recommendation by the 16th Finance Commission to add heatwaves and lightning to India's list of notified disasters, which would let states draw on the State Disaster Response Fund for relief.