'Ozone-climate penalty' adds to India's air pollution – Phys.org

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April 14, 2025
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by Ranjit Devraj, SciDev.Net
India’s cities are already ranked among the world’s most polluted, based on concentrations of fine particulate matter in the air. Now new research indicates they are battling rising levels of another life-threatening pollutant—surface ozone.
A study published in the journal Global Transitions says deaths from ozone in India exceeded 50,000 in 2022 and caused losses of around US$16.8 billion—about 1.5 times the government’s total health spending that year.
“Surface ozone is a toxic gas that not only affects public health but also impacts ecosystems and climate due to the greenhouse effect,” says Jayanarayanan Kuttippurath, professor at the Indian Institute of Technology, Kharagpur and corresponding author of the study.
Ozone, a variant of oxygen, occurs at ground level as well as in the upper atmosphere. Formed naturally, the ozone in the stratosphere helps filter out harmful ultraviolet rays that are a part of the sun’s radiation.
However, surface ozone, also called ground-level ozone, is generated by interactions between pollutants. For example, nitrogen oxides found in vehicular exhaust can react with volatile organic compounds released by industrial activity and waste dumps to produce ozone.
Surface ozone is the primary component of smog and can have negative effects on human health and the environment.
“Our study showed that many regions in India had ozone levels exceeding the WHO-recommended exposure threshold of 70 micrograms per cubic meter,” Kuttippurath told SciDev.Net.
He said short-term exposure to ozone increases the risk of death from heart disease, stroke, hypertension and respiratory issues, while long-term exposure may decrease lung capacity, induce oxidative stress, suppress immune response and cause lung inflammation.
Climate change, rising temperatures and altered weather patterns can raise surface ozone in a phenomenon described by experts as the “ozone-climate penalty”.
Factors that affect ozone generation include solar radiation, humidity, precipitation and the presence of precursors—substances that lead to the formation of a pollutant through a chemical reaction—such as methane, nitrogen oxides and volatile organic compounds, said Kuttippurath.
He explained that ozone pollution increases during the hot summer months and declines during the monsoon period from June to September as heavy rains wash out the pollutants, and reduced solar radiation limits photochemical reactions.
Critically, human exposure to fine particulate matter—known as PM_2.5—may worsen the health effects of ozone, Kuttippurath added. “The combined effect of ozone and PM_2.5 may result in increased respiratory issues and a higher probability of death,” he warned.
PM_2.5 refers to particles smaller than 2.5 microns, which can enter the bloodstream through the lungs.
According to the 2024 World Air Quality Report, 11 of the world’s 20 cities carrying the highest burden of PM_2.5 are in India. The report ranked Delhi as the most polluted capital city in the world.
A study published in The Lancet Planetary Health found that the whole of the population of India lives in areas where PM_2.5 levels exceed WHO guidelines.
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Besides harming health, high levels of surface ozone reduce photosynthesis by damaging photosystems, CO₂ fixation and pigments. This leads to a reduction in carbon assimilation that results in the decline of crop yields.
According to the ozone study, India’s rice yield loss due to ozone pollution rose from 7.39 million tons to 11.46 million tons between 2005 and 2020, costing around US$2.92 billion and impacting food security.
Even if precursor emissions remain at the current level, climate change alone could contribute to the increase in surface ozone in the highly polluted regions of South Asia by 2050 with the Indo-Gangetic Plains, one of the most fertile regions in the region, likely to face significant crop yield losses, said Kuttipurath.
Anand Kumar Sharma, visiting faculty at the National Academy of Administration, Mussoorie and former deputy director general of the India Meteorological Department, says high levels of surface ozone are a matter of increasing concern in India, though for now other pollutants take priority.
“As indicated in the study, 50,000 deaths annually from ozone exposure is less pressing than the millions of annual deaths from PM_2.5 pollution,” said Sharma.
“Further, during the extremely hot pre-monsoon season, attention is focused on deaths from heat stroke and other causes.”
Sharma is confident that the National Clean Air Program policies now in place will eventually make a difference.
“Much of the surface ozone generated is taken care of by nature, such as the monsoon rains,” said Sharma.
“Dealing with the reported increase in ozone levels is best done by reducing the precursors—nitrogen oxides, methane and PM_2.5—and the National Clean Air Program instituted in 2019 is already making efforts in this direction.”
More information: G.S. Gopikrishnan et al, Exposure to surface ozone and its associated health effects and economic burden in India, Global Transitions (2025). DOI: 10.1016/j.glt.2025.03.002

Journal information: The Lancet Planetary Health
Provided by SciDev.Net
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India faces significant air pollution challenges, with surface ozone levels rising alongside already high PM_2.5 concentrations. In 2022, ozone-related deaths exceeded 50,000, costing approximately $16.8 billion. Surface ozone, exacerbated by climate change, affects health and agriculture, reducing crop yields. Despite efforts like the National Clean Air Program, reducing ozone precursors remains crucial to mitigating these impacts.
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