Surface ozone compounds heatwave deaths in India – study

[NEW DELHI] Toxic ground-level ozone is surging during India’s increasingly fierce heatwaves and adding to the death toll from heart and respiratory disease, according to new analysis.

Over the past two decades, surface ozone has breached the safe limit set by the World Health Organization (WHO) across every region of the country during the pre-monsoon hot season, the study published in the Nature journal npj Clean Air explains. During heatwaves, levels become particularly intense.

India has been seeing a rising trend in the ferocity and frequency of heatwaves in recent years. The latest, which started in April, reached maximum temperatures of above 47 degrees Celsius in May.

Jayanarayanan Kuttipurath, study co-author and climate scientist at the Indian Institute of Technology’s Centre for Ocean, River, Atmosphere and Land Science, in Kanpur, told SciDev.Net: “In this study we present the first comprehensive long-term assessment of surface ozone changes during heatwaves in India using surface, satellite, and reanalysis data for 2004–2024 measured during the pre-monsoon months of March, April, May and June when heat waves peak.

“The pre-monsoon rise in ozone concentrations is attributable to intense solar radiation, which accelerates the chemical kinetics of ozone production, further intensified during heatwaves, which amplify ozone production and degrade air quality.”

Between 2004 and 2024, the seven regions covered by the study recorded 188 heatwave events during which surface ozone levels exceeded the WHO threshold of 70 micrograms per cubic metres. Northern India saw the highest concentration, peaking to 85—110 micrograms per cubic metre.

“With the frequency and intensity of heatwaves on the rise, so too are the concentrations of surface ozone and associated health issues,” Kuttipurath said.

In 2024, the researchers estimated that 15,615 deaths were linked to chronic obstructive pulmonary disease and 10,898 to ischemic heart disease during heatwaves. This is compared to 15,125 and 10,556 respectively before the heatwaves, a 3.2 per cent increase in both cases.

Growing threat

Ozone exposure contributes heavily to deaths caused by cardiovascular and chronic obstructive pulmonary disease, according to the State of Global Air 2025 report, which ranks India third among countries exposed to ozone pollution.

Particulate matter remains the leading cause of air pollution-related deaths but surface ozone is rapidly catching up.

The report attributes India’s vulnerability to ozone pollution to a combination of high emissions of precursor gases, plus intense sunlight and rising temperatures.

These precursor gases, including nitrogen oxides, volatile organic compounds, carbon monoxide and methane, generate ozone through reactions driven by solar radiation and atmospheric chemistry. They come from the burning of fossil fuels, industrial activity, and vehicular exhaust as well as natural processes by trees and plants.

In the stratosphere, ozone shields the Earth from harmful ultraviolet rays. At ground level, the same gas is a toxic, highly reactive pollutant. This tropospheric ozone forms mainly when sunlight breaks down nitrogen dioxide in the presence of volatile organic compounds, releasing an oxygen atom that combines with oxygen (O2) to form ozone (O3).

Policy responses

Bomidi Lakshmi Madhavan, a scientist at India’s National Atmospheric Research Laboratory, says the study is significant because it shifts attention away from particulate matter alone and onto ozone, which is gaining importance in a warming climate.

“By linking heatwave-enhanced ozone exposure with elevated mortality risks from respiratory and cardiovascular diseases, the authors underscore the need for integrated policy responses that combine climate adaptation, heat-action plans and ozone precursor emission controls,” Madhavan told SciDev.Net.

“As heatwaves become more frequent and intense across India, ozone pollution is likely to emerge as a major environmental health challenge.”

Ronita Bardhan, professor of Sustainable Built Environment and Health at the University of Cambridge says the study reveals an important route through which climate change affects human health.

The significance of the study “lies in showing that populations are often exposed simultaneously to multiple environmental hazards, rather than to heat or air pollution in isolation,” Bardhan told SciDev.Net.

She warned that: “From a risk and vulnerability perspective, the true burden is likely to be even more complex than current estimates suggest.

“Heat-related risk is shaped by a range of interacting factors, including housing quality, occupational exposure, access to cooling, healthcare availability, pre-existing disease burden, and the characteristics of the built environment.”

Roxy Mathew Koll, climate scientist at the Indian Institute of Tropical Meteorology, Pune, says that since 2000 the Indian subcontinent has seen sharp rises in temperatures, intensifying heatwaves and disrupting monsoons.

“Sustained investment—in people, policy, and long-term planning—is essential to reduce heatwave-related deaths and health impacts, especially since heatwaves are now annual affairs from February to June and expected to intensify further in the future,” he said.

This piece was produced by SciDev.Net’s Global desk.