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A new study by Delhi-based think tank Centre for Science and Environment (CSE) has revealed a disturbing trend: Ground-level ozone pollution is on the rise across India’s major cities. This invisible gas, unlike the more familiar fine particulate matter, also known as PM2.5, poses a serious health threat, particularly to those with respiratory problems.
The report, Air Quality Tracker: An invisible threat, was released August 6, 2024. The researchers analysed metropolitan areas of Bengaluru (Karnataka), Chennai (Tamil Nadu), Kolkata (West Bengal), Mumbai and Pune (Maharashtra). They also looked at data for Delhi-National Capital Region, Greater Ahmedabad (Gujarat), Greater Hyderabad (Telangana), Greater Jaipur (Rajasthan) and Greater Lucknow (Uttar Pradesh).
All 10 areas studied witnessed exceedances of the national ozone standard, with Delhi being the most affected. Smaller cities like Ahmedabad and Pune are experiencing a particularly rapid increase in ozone pollution, the report further found.
Between April and July this year, Delhi-NCR recorded 176 days of ground-level ozone exceedances, the highest among the ten metropolitan areas studied. Mumbai and Pune both had 138 days, followed by Jaipur with 126 days and Hyderabad with 86 days. Kolkata had 63 exceedance days, Bengaluru 59, Lucknow 49 and Ahmedabad 41. Chennai had the fewest exceedances, with just nine days.
Contrary to expectations, ozone levels were elevated even at night, with Mumbai recording the most instances of night-time exceedances. The duration of ozone exposure is concerning, lasting an average of 12-15 hours across most cities, the researchers found.
High-end and green neighbourhoods, with lower levels of other pollutants, are ironically more susceptible to ozone build-up. While summer is the peak season for ozone, the problem persists year-round in many areas, particularly in sunnier southern cities.
Ground-level ozone, a highly reactive gas, has serious health consequences. Those with respiratory conditions, asthma and chronic obstructive pulmonary disease — as well as children with premature lungs and older adults — are at serious risk. This can inflame and damage airways, make lungs susceptible to infection, aggravate asthma, emphysema and chronic bronchitis and increase the frequency of asthma attacks leading to increased hospitalisation.
Anumita Roychowdhury, executive director, research and advocacy, CSE
National Clean Air Programme’s reform agenda has to address the multi-pollutant crisis and the combined threat from PM2.5, ozone, nitrogen oxides and other gases, Roychowdhury added. The programme currently focuses more on controlling PM10 or coarse dust.
The researchers tracked trends from April 1 to July 18, covering the years 2020 to 2024. The analysis is based on granular, real-time data (15-minute averages) from Central Pollution Control Board.
Global experience indicates a common trade-off: as particulate pollution is reduced, issues with nitrogen oxides (NOx) and ground-level ozone increase, the report pointed out. This necessitates significant tightening of regulatory benchmarks for ozone to address toxic emissions from industry, vehicles, households and open burning.
Furthermore, ozone accumulates not only in metropolitan areas but also travels long distances, creating a regional pollutant that demands both local and regional action.
Currently, insufficient monitoring, limited data, and ineffective trend analysis methods have hampered understanding of this growing public health risk, the report added. The complex chemistry of ground-level ozone makes it a difficult pollutant to track and mitigate.
Due to its highly toxic nature, the national ambient air quality standard for ozone is set only for short-term exposures (one-hour and eight-hour averages), with compliance measured by the number of days that exceed these standards. This necessitates early action, the researchers underlined.
Stringent regulations are needed to curb nitrogen oxide emissions from vehicles, industries and other sources, the paper said. It also highlighted the need for a multi-pronged approach to air pollution control in India. “Immediately, refine the action strategy for combined control of particulate pollution, ozone and its precursor gases like NOx to maximise the cobenefits of the action plan,” it stated.
