Ozone pollution this year highest since 2021 in Delhi: Report

Hindustan Times

17-06-2025

Delhi has recorded its highest ozone (O3) levels this summer in the last five years, with readings breaching the safe mark for 14.2 hours on an average in a day, a report shows.
This eight-hour rolling average has gone up from 12 hours in a day from the last two summers, 11.7 hours in 2022 and 12.2 hours in 2021, according to the report released by Centre for Science and Environment (CSE) on Tuesday.
The eight-hour safe standard for the pollutant is 100 μg/m3.
The analysis also shows that Delhi recorded its widest geographical spread of ozone exceedance in the past five years this summer, with south Delhi and northwest Delhi being the worst impacted.
CSE said ground-level ozone levels exceeded the national safety standards on all 92 days in the summer period – considered as March 1 till May 31, while it exceeded the safe standards on 83 out of the 92 days in the summer.
'Ground-level ozone pollution remained consistently high across the region, with the maximum concentration exceeding 100 µg/m³ on each of the 92 days between March 1 and May 31, highlighting the persistent and widespread nature of the problem. The worst day in terms of spatial spread was April 28, when 32 out of 58 monitoring stations across the region reported ozone levels above the safe limit. The highest regional intensity was recorded on April 13, with the Delhi-NCR average reaching 135 µg/m³..' said the analysis.
Unlike primary pollutants (emitted directly from sources), ozone is not emitted directly from any source, but is formed through chemical reactions involving nitrogen oxides (NOx), volatile organic compounds (VOCs), and carbon monoxide — pollutants 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 near the ground.
CSE took into account summer trends from 2021 to 2025 (up to May 31) available on the Central Pollution Control Board's official portal.
Data showed that Nehru Nagar was the worst impacted station, exceeding standards on all 92 days. This was followed by Najafgarh (82 days) and Okhla Phase-II (81 days). In comparison, six locations – Anand Vihar, IHBAS, ITO, Shadipur, Narela and Knowledge Park in Greater Noida did not exceed the standards even on a single day this summer.
'This summer, several days have recorded ozone — instead of particulate matter — as the lead pollutant in the daily Air Quality Index (AQI). Out of 18 days between May 25 and June 11, ozone has been the lead pollutant on 12,' said Anumita Roychowdhury, executive director and in-charge of clean air programme at CSE.
'But there is no system to address this problem under the current Graded Response Action Plan for emergency action to reduce exposure or to address its sustained mitigation with longer term clean air action plan. While the policy attention is nearly fully focused on particulate pollution, co-control of toxic gases from vehicles, industry and combustion sources that contribute to ozone formation in the air is neglected,' she said, calling for focused action against the highly reactive gas.