CSE Analysis: Every leading metropolis and mega city in India – including Kolkata, Bengaluru, Mumbai, Hyderabad and Chennai – has been struck by high ozone levels in the summer of 2025. Ground-level ozone pollution has spiked and concentrations have exceeded the eight-hour standards on a number of days. This has emerged from an analysis done by Centre for Science and Environment (CSE), which was released here today.
The analysis has been done by the Urban Lab in CSE under its ‘air quality tracker’ initiative. The assessment highlights that unlike primary pollutants, ozone is not emitted directly from any source. It forms through intricate chemical reactions involving nitrogen oxides (NOx), volatile organic compounds (VOCs), and carbon monoxide (CO) — pollutants that are released by vehicles, power plants, factories and other combustion sources. In the presence of sunlight, these substances undergo a series of cyclic reactions that result in the formation of ozone at ground level.
VOCs also have natural sources such as vegetation, adding to the complexity. Ground-level ozone accumulates not only in urban environments but can also travel long distances, turning into a regional pollutant. It can impact agricultural productivity and threaten food security.
Given ozone’s highly reactive nature, the ambient air quality standards for it are set for eight-hour averages, instead of 24-hour averages.
Anumita Roychowdhury, executive director, CSE, says: “If unchecked, this can become a serious public health crisis as ozone is a highly reactive gas and can be harmful even with short-duration exposures. In contrast to cities in north India where high summer temperatures and intense solar radiation can lead to ozone levels exceeding the standards, other cities in warm climates are experiencing consistent ozone exceedance during other seasons as well.”
Roychowdhury adds: “The current policy focus must be expanded to include improved monitoring and mitigation of this toxic gas and reduction in high local exposures. Ozone mitigation needs drastic control over a range of gases from vehicles, industry and all combustion sources.”
“Inadequate monitoring, limited data and inadequate methods of trend analysis have weakened the understanding of this growing public health hazard across cities of India. Instead of merely averaging out the levels for the city – which is the standard practice to estimate AQI — it is also important to capture adequately the high levels of local build-up and exposures in the hotspots and to design mitigation strategies accordingly ,” says Sharanjeet Kaur, deputy programme manager at the Urban Lab in CSE.
The CSE review shows that exposure to ground-level ozone can inflame and damage the airways, increase susceptibility to infections, and worsen respiratory conditions such as asthma, chronic bronchitis and emphysema. Children with underdeveloped lungs, older adults, and individuals with existing respiratory conditions are particularly vulnerable. Ozone exposure increases the frequency and severity of asthma attacks, often leading to higher rates of hospitalisation.
The investigation method: This assessment has traced trends during summer (March-May) between 2022 and 2025 (up to May 31). It is based on publicly available granular real time data (15-minute averages) from the Central Pollution Control Board’s official online portal – the Central Control Room for Air Quality Management. The data has been captured from 80 official stations under the Continuous Ambient Air Quality Monitoring System (CAAQMS), spread across Mumbai (31), Kolkata-Howrah (12), Bengaluru (14), Hyderabad (14) and Chennai (nine).
Says Kaur: “Given the volatile and highly localised nature of ground-level ozone pollution build-up and its variability across space, and consistent with the global good practice, this analysis has considered station-level trends in terms of number of days exceeding the eight-hour standard over time.”
She points out that as ozone formation depends on complex atmospheric chemistry and on photochemical reactions, its level varies across the time and space horizon. Meteorological parameters such as sunny and warm weather, stagnant wind patterns etc have a bearing on its formation. Kaur says: “This analysis tracks exceedances at each station in metro cities — breach of the standard by even one station is considered exceedance by the metro city in which the station is located. Days with multiple stations exceeding the standard indicates the severity of the spatial spread and number of people exposed.”
The study has considered global good practice and taken on board the USEPA approach of computing eight-hour averages for a day and then checking for the maximum value among them to capture the daily ozone pollution level. USEPA assesses city-wide or regional AQI based on the highest value recorded among all stations of the city or the region. Thus, trends have been calculated in terms of number of days when the daily level has exceeded the eight-hour standard (referred as exceedance days hereafter.
Find the complete CSE study report here: https://www.cseindia.org/APC-
