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Ground-level ozone at 5-yr high; south, northwest Delhi worst hit
New Delhi: The city experienced its highest ozone levels in five years this summer, according to a study by the Centre for Science and Environment. The eight-hour rolling average of the gas exceeded safe standards for 14.2 hours daily, increasing from 12 hours in previous summers.
The analysis revealed that south and northwest Delhi were most affected, showing the broadest geographical spread of ozone exceedance since 2018.
The study indicated that ground-level ozone surpassed national safety standards throughout the 92-day summer period from March 1 to May 31, with breaches occurring on 83 days. The established eight-hour standard for O3 is 100 micrograms per cubic metre.
"Ground-level ozone pollution remained consistently high across the region, with the maximum concentration exceeding 100 microgram per cubic metre on each of the 92 days between March 1 and May 31, highlighting the persistent and widespread nature of the problem," the study said.
"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."
Ozone forms through chemical reactions involving nitrogen oxides, volatile organic compounds and carbon monoxide, which are released by vehicles, power plants and factories.
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These substances react in sunlight to create ground-level ozone. This highly reactive gas poses serious health risks. It can inflame and damage the airways, increase susceptibility to infections and worsen respiratory conditions such as asthma, chronic bronchitis and emphysema.
CSE analysed summer trends from 2021 to 2025 (until May 31) using CPCB's official online portal, Central Control Room for Air Quality Management.
The findings showed Nehru Nagar breached the safe level on all 92 days, followed by Najafgarh (82 days) and Okhla Phase-II (81 days). Only six locations, Anand Vihar, IHBAS, ITO, Shadipur, Narela and Knowledge Park in Greater Noida, maintained levels below the standard.
On several days this summer, therefore, ozone was the lead pollutant instead of particulate matter in the daily air quality index. "Out of 18 days between May 25 and June 11, ozone was the lead pollutant on 12," said Anumita Roychowdhury, executive director, CSE.
The study also said there was no system to address the ozone problem under the Graded Response 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," Roychowdhury pointed out, calling for measures to curb ozone.
