A landmark study published in npj Clean Air, a Nature Portfolio journal, has found that heatwaves in India significantly amplify ground-level ozone pollution, creating a hidden public health emergency. The research, led by Prof Jayanarayanan Kuttippurath of IIT-Kharagpur, is the first comprehensive assessment of how heatwaves influence ozone pollution across India's diverse climatic regions.
Invisible Pollutant Threatens Millions
Unlike smog or particulate matter, ozone is invisible. It forms when pollutants such as nitrogen oxides and volatile organic compounds react under intense sunlight and high temperatures. While stratospheric ozone protects life, ground-level ozone damages lung tissue, triggers asthma attacks, worsens chronic respiratory illnesses, and increases cardiovascular stress.
The researchers analysed 188 heatwave episodes across seven temperature-homogeneous regions of India between 2004 and 2024. They found a consistent rise in ozone concentrations whenever temperatures surged.
Ozone Levels Exceed Safety Thresholds
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.
"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."
Why Heatwaves Worsen Pollution
Heatwaves are characterised by intense sunlight, cloud-free skies, and stagnant air. These conditions accelerate photochemical reactions that convert pollutants into ozone. 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."
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.
Quantifying the Human Cost
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 IHD 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.
Climate Change Multiplies Threat
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. Higher temperatures accelerate ozone formation even if emissions remain unchanged, meaning climate change strengthens the atmospheric conditions that produce this harmful pollutant.
"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, with cities like Ahmedabad internationally recognised for reducing heat-related mortality. However, most heatwave advisories 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.
