When temperatures soar above 45°C across India’s plains, public attention naturally turns to heatstroke, dehydration, water shortages and power outages. Hospitals issue advisories, governments activate heat action plans and citizens seek refuge from the blistering sun. Yet, a far deadlier threat often goes unnoticed — one that cannot be seen, smelled or felt until the damage is done. As India experiences increasingly frequent and intense heatwaves, scientists are warning of an invisible health crisis unfolding in the atmosphere. A landmark study published in “npj Clean Air”, a Nature Portfolio journal, has found that heatwaves significantly amplify ground-level ozone pollution across India, exposing millions to heightened risks of respiratory and cardiovascular diseases. The research, led by Prof Jayanarayanan Kuttippurath of the Centre for Oceans, Rivers, Atmosphere and Land Sciences (CORAL) at IIT-Kharagpur, is the first comprehensive assessment of how heatwaves influence ozone pollution across India’s diverse climatic regions. The findings suggest that heatwaves are not merely weather events but compound disasters in which extreme temperatures and toxic air pollution combine to threaten public health. “People generally think heatwaves mean only extreme temperatures,” says Prof Kuttippurath. “What they do not realise is that heatwaves also create favourable conditions for ozone formation. This hidden pollution poses serious health risks and deserves much greater public attention.” An invisible pollutant Unlike smog or particulate matter, ozone is invisible. It is not emitted directly by vehicles or factories. Instead, it forms when pollutants such as nitrogen oxides and volatile organic compounds react under intense sunlight and high temperatures. While ozone in the stratosphere protects life by shielding Earth from harmful ultraviolet radiation, at ground level it becomes a dangerous pollutant capable of damaging lung tissue, triggering asthma attacks, worsening chronic respiratory illnesses and increasing cardiovascular stress. The researchers analysed heatwave events across seven temperature-homogeneous regions of India between 2004 and 2024. They identified 188 heatwave episodes during the study period and found a consistent rise in ozone concentrations whenever temperatures surged. During heatwave conditions, ozone levels frequently exceeded the World Health Organisation’s recommended safety threshold of 70 micrograms per cubic metre. In several regions, concentrations climbed to between 85 and 110 micrograms per cubic metre. Northwest India, North-Central India, the Indo-Gangetic Plain and the Western Himalayas emerged as major hotspots. Why heatwaves make pollution worse The relationship between heat and ozone is rooted in atmospheric chemistry. Heatwaves are characterised by intense sunlight, cloud-free skies and stagnant air. These conditions accelerate photochemical reactions that convert pollutants into ozone. At the same time, weak winds prevent pollutants from dispersing, allowing ozone to accumulate close to the surface where people breathe. The study found that ozone concentrations begin rising rapidly as heatwaves develop and often remain elevated for three to four days after temperatures start falling. “People assume the danger ends when the temperature drops,” says Prof Kuttippurath. “But ozone levels can remain high even after a heatwave officially ends, prolonging exposure and health risks.” The researchers also observed that high temperatures stimulate the release of both natural and human-made ozone precursors. Vehicle emissions, industrial pollution, thermal power plants and seasonal crop-residue burning further intensify the problem, particularly across northern India. A public health emergency Perhaps the study’s most significant contribution lies in quantifying the human cost of this phenomenon. Using established health-risk models, the researchers estimated that ozone exposure during heatwave periods substantially increased mortality associated with ischemic heart disease (IHD) and chronic obstructive pulmonary disease (COPD). Deaths linked to ischemic heart disease rose from about 15,125 before heatwave events to 15,615 during heatwaves. Similarly, COPD-related deaths increased from around 10,556 to nearly 10,900. While these figures may appear modest at first glance, they represent hundreds of additional lives lost during relatively short periods of extreme weather. The Indo-Gangetic Plain emerged as particularly vulnerable because of its dense population, industrial activity, vehicular emissions and recurring heatwaves. Already among the world’s most polluted regions, it now faces an additional health burden from rising ozone levels. Children, older adults, outdoor workers and people with pre-existing respiratory or cardiovascular conditions face the greatest risk. Climate change’s dangerous multiplier The findings come at a time when climate scientists are increasingly concerned about compound climate extremes. Heatwaves are becoming longer, hotter and more frequent due to global warming. According to climate projections cited in the study, large parts of India are expected to experience further increases in heatwave intensity and duration over the coming decades. This has important implications for ozone pollution. Higher temperatures accelerate ozone formation even if emissions remain unchanged. In other words, climate change is strengthening the atmospheric conditions that produce this harmful pollutant. Researchers warn that future summers could expose millions more Indians to the combined dangers of extreme heat and toxic air. “We often discuss heatwaves and air pollution separately,” says Prof Kuttippurath. “But our findings show they are deeply connected. Policies must address them together if we are serious about protecting public health.” Beyond temperature-based warnings India has made considerable progress in developing heat action plans. Cities such as Ahmedabad have been internationally recognised for their efforts to reduce heat-related mortality. However, most heatwave advisories continue to focus almost exclusively on temperature. The study argues that air-quality monitoring, ozone forecasting and public health alerts should become integral components of heatwave preparedness. Hospitals, local administrations and disaster-management agencies must prepare not only for heat stress but also for pollution-related illnesses that accompany extreme temperatures. For a country expected to face some of the world’s most severe climate impacts in the coming decades, recognising this hidden threat could save thousands of lives. The challenge before India is no longer simply surviving the heat — it is confronting the invisible dangers that rise with it. Key takeaways The study reveals that heatwaves in India are creating a hidden public health crisis by accelerating the formation of ground-level ozone, a harmful pollutant linked to respiratory and cardiovascular diseases. Researchers found that ozone levels routinely exceed WHO safety limits during heatwaves, particularly across Northwest India, the Indo-Gangetic Plain, North-Central India and the Western Himalayas. The research estimates that ozone exposure during heatwaves contributes to hundreds of additional deaths from ischemic heart disease and chronic obstructive pulmonary disease. With climate change expected to intensify both heatwaves and ozone formation, the researchers argue that India must integrate air-quality forecasting, ozone monitoring and health advisories into its heat action plans to better protect vulnerable populations.
