Heatwaves and Surface Ozone Pollution in India: Rising Health Risks and Climate Concerns

A pioneering country-wide study (2004–2024) has found that intensifying heatwaves accelerate the formation of harmful surface ozone (O₃), significantly increasing cardiovascular and respiratory mortality risks across India.

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Key Findings of the Study

• The Double Burden of Heatwaves and Ozone Pollution: The study found that heatwaves significantly accelerate the formation of surface ozone (O₃), causing ozone concentrations to rise to 85–110 μg/m³ across parts of northern India.
  • These levels substantially exceed the World Health Organization (WHO) guideline value of 70 μg/m³, indicating heightened health risks during extreme heat events.
• Health Impact: The researchers estimated that exposure to elevated surface ozone levels contributes substantially to seasonal mortality associated with cardiovascular and respiratory diseases.
  • During the heatwave days of 2024, approximately 26,500 deaths from Ischaemic Heart Disease (IHD) and Chronic Obstructive Pulmonary Disease (COPD) were linked to ozone exposure.
  • The ozone surge associated with heatwave conditions was estimated to have caused nearly 830 additional deaths above normal baseline levels.
• Geographical Hotspot: The Western Himalayan region recorded the most rapid long-term increase in surface ozone concentrations.
  • Ozone levels in the region exceeded the WHO guideline value by about 115% in 2024, making it one of the most vulnerable regions in the country.
• Climate Linkages: The most severe heatwave–ozone episodes observed during the study period, including those in 2010, 2016, 2019 and 2024, occurred following strong El Niño events.
  • This suggests a close relationship between large-scale climate variability and extreme ozone pollution episodes in India.

About the Study

• Institutional Affiliation & Publication: The study was conducted by researchers from the Indian Institute of Technology (IIT) Kharagpur and Kerala University of Fisheries and Ocean Studies (KUFOS) and was published in the peer-reviewed journal npj Clean Air.
• Scope of the Study: It is the first comprehensive long-term, country-wide assessment of changes in surface ozone (O₃) during heatwave conditions in India, covering the period 2004–2024 and analysing 188 heatwave events.
• Methodology: In the absence of continuous ground-level ozone monitoring across many regions, the study used surface observations, satellite datasets, meteorological reanalysis products, and atmospheric modelling techniques to estimate surface ozone concentrations and their behaviour during heatwaves.
• Data Sources: The analysis integrated 20 years of India Meteorological Department (IMD) temperature records, satellite observations, and global atmospheric datasets to examine the relationship between heatwaves, ozone formation, and associated health risks.
• Significance: The study provides the first national-scale evidence that heatwaves significantly intensify surface ozone pollution, creating a compound heat–ozone hazard with serious implications for public health and climate adaptation policies in India.

About Ozone (O₃)

• Surface Ozone (Ground-Level Ozone) is a secondary air pollutant formed through chemical reactions between Nitrogen Oxides (NOₓ) and Volatile Organic Compounds (VOCs) in the presence of sunlight and high temperatures.

• • It is a major component of photochemical smog and is often referred to as “bad ozone.” (UPSC CSE Mains 2022)
• Dual Nature of Ozone:
  • Stratospheric Ozone (“Good Ozone”): Stratospheric ozone forms the ozone layer in the upper atmosphere and protects life on Earth by absorbing harmful ultraviolet (UV) radiation from the Sun.
  • Tropospheric or Surface Ozone (“Bad Ozone”): Surface ozone is a harmful air pollutant that acts as a short-lived climate pollutant and poses significant risks to human health, agriculture and ecosystems.

• Major Sources of Surface Ozone (O₃): Surface ozone is a secondary pollutant formed when Nitrogen Oxides (NOₓ) and Volatile Organic Compounds (VOCs) react in the presence of sunlight and high temperatures.
  • Vehicular emissions are a major source of NOₓ and VOCs, particularly in urban areas.
  • Thermal power plants release large quantities of NOₓ during the combustion of fossil fuels.
  • Industrial activities, including refineries, cement plants, steel industries, and chemical manufacturing units, emit significant ozone precursors.
  • Biomass burning, such as crop residue burning, forest fires, and household fuel combustion, contributes substantially to ozone formation.
  • Diesel generators and construction machinery add to NOₓ emissions, especially in urban and peri-urban areas.
  • Paints, solvents, adhesives, and petrochemical products release VOCs that facilitate ozone formation.
  • Natural vegetation emits biogenic VOCs, which can also contribute to ozone generation under suitable conditions.
  • Heatwaves, intense solar radiation, and stagnant atmospheric conditions accelerate ozone formation and accumulation.
• Impacts of Surface Ozone (O₃)
  • Human Health Impacts: Surface ozone irritates the respiratory system and reduces lung function.
    • Long-term exposure increases the risk of cardiovascular diseases, hospital admissions, and premature mortality.
  • Agricultural Impacts: Ozone damages plant tissues and reduces the efficiency of photosynthesis.
    • Sensitive crops such as wheat, rice, soybean, cotton, and pulses are particularly vulnerable..
  • Ecosystem Impacts: Surface ozone weakens forests and natural vegetation by reducing plant growth and productivity.
  • Climate Impacts: Surface ozone is a Short-Lived Climate Pollutant (SLCP) that contributes to global warming.
    • It exerts a warming effect by trapping heat in the atmosphere.
  • Economic Impacts: Ozone pollution increases healthcare costs and reduces labour productivity.

About Heatwaves

• Definition: According to the India Meteorological Department (IMD), a heatwave is a period of abnormally high temperatures determined using both absolute temperature thresholds and deviations from normal temperature conditions.
  • In general, the maximum temperature must reach at least 40°C in plains, 37°C in coastal regions, and 30°C in hilly regions, along with specified departures from normal temperatures, for a heatwave to be declared.

Why Do Heatwaves Increase Ozone Formation?

• Faster Photochemical Reactions: High temperatures accelerate the photochemical reactions between Nitrogen Oxides (NOₓ) and Volatile Organic Compounds (VOCs), resulting in increased formation of surface ozone (O₃).
• Enhanced Solar Radiation: Intense solar radiation during heatwave conditions increases atmospheric photochemical activity, thereby boosting the rate at which ozone is produced.
• Stagnant Atmospheric Conditions: Heatwaves are often associated with stagnant air masses and low wind speeds, which restrict the dispersion of pollutants and allow ozone concentrations to accumulate near the Earth’s surface.
• Reduced Natural Removal of Pollutants: Clear skies, limited cloud cover, and reduced rainfall decrease the natural cleansing of the atmosphere, enabling ozone and its precursor pollutants to persist for longer periods.
• Prolonged Ozone-Forming Conditions: Extended heatwave events sustain high-temperature and high-radiation conditions, leading to prolonged episodes of elevated surface ozone pollution.

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Conclusion

The study underscores the growing link between climate change, heatwaves, and surface ozone pollution in India. Strengthening air-quality monitoring, heat-action plans, and climate-resilient public health systems will be crucial to reducing the risks from heat–ozone extremes. 

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