Heatwaves worsen ozone pollution in India and threaten public health

In India, intense heatwaves trigger a dangerous rise in surface ozone pollution, a toxic gas formed by chemical reactions between atmospheric pollutants and solar radiation. During these episodes, ozone concentrations reach peaks of 85 to 110 micrograms per cubic meter in the northern part of the country, far exceeding the recommended health thresholds. These levels typically drop after three to four days, but the effects on the population persist much longer.

Data collected between 2004 and 2024 reveals a steady increase in ozone levels across several regions, particularly in the western Himalayas, the northwest, the west coast, and the north-central areas. In these zones, the annual increase ranges from 0.07 to 0.98 micrograms per cubic meter. Heatwaves, which are becoming more frequent and intense, coincide with systematic exceedances of health standard limits, with frequencies ranging from 39% to 115% depending on the year and region.

Surface ozone is a secondary pollutant, produced by the interaction between nitrogen oxides, volatile organic compounds, carbon monoxide, and sunlight. Its accumulation is favored by stable meteorological conditions, such as persistent high-pressure systems that block winds and reduce pollutant dispersion. In India, these episodes occur mainly between March and June, before the arrival of the monsoon, a period when sunlight is at its peak and rainfall is rare.

The health consequences are severe. In 2024, nearly 26,500 additional deaths linked to chronic respiratory and cardiovascular diseases were attributed to combined exposure to heat and ozone during heatwaves. These diseases, such as chronic obstructive pulmonary disease and coronary heart disease, worsen under the combined effect of extreme heat and pollution. Heat accelerates ozone formation, while this gas irritates the respiratory tract and increases oxidative stress in the body.

The most affected regions include Rajasthan, Gujarat, Punjab, Haryana, Madhya Pradesh, Uttar Pradesh, and Delhi, where temperatures can exceed 45 degrees. In the western Himalayas, rising temperatures and reduced snow cover alter energy exchanges between the ground and the atmosphere, further amplifying ozone concentrations. Coastal areas, such as those in Gujarat and Maharashtra, also experience peaks, though less pronounced, due to the moderating influence of the sea.

Scientists emphasize that these phenomena are set to intensify with global warming. Projections indicate an increase in the frequency, intensity, and duration of heatwaves, particularly in already vulnerable regions. The interactions between weather conditions, pollutant emissions, and climate change create a vicious cycle where each element exacerbates the others.

Ozone formation also depends on the availability of its precursors, such as nitrogen oxides and volatile organic compounds, emitted by vehicles, industries, power plants, and biomass burning. In northwest India, for example, agricultural fires and industrial emissions play a major role in increasing ozone levels during heatwaves. Winds then transport these pollutants over long distances, affecting areas far from the emission sources.

The most exposed populations live in the Gangetic plains, where population density is high and pollution sources are numerous. The elderly, children, and those already suffering from respiratory or cardiac problems are particularly vulnerable. Studies show that every increase of 10 micrograms per cubic meter of ozone during a heatwave significantly increases the risk of premature death.

The mechanisms at play are complex. Extreme heat promotes the photochemical reactions that produce ozone, while stagnant air prevents its dispersion. In mountainous regions, such as the Himalayas, the topography enhances solar radiation and limits ventilation, trapping pollutants near the ground. In contrast, in coastal areas, high humidity and sea breezes can sometimes reduce ozone formation, but not enough to offset the impact of heatwaves.

Researchers insist on the need for rapid action to mitigate these risks. Air pollution control policies and climate change adaptation strategies must be integrated. Without strong measures, the combination of heat and ozone could become a major health challenge in the coming decades, especially in a country like India, where urban and industrial growth is accelerating.

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DOI: https://doi.org/10.1038/s44407-026-00082-5

Title: Heatwaves trigger severe surface ozone pollution in India: regional hotspots, trends and health effects

Journal: npj Clean Air

Publisher: Springer Science and Business Media LLC

Authors: Parambat Sangeetha; Jayanarayanan Kuttippurath