New Delhi: Ground-level ozone, considered largely a summer pollutant, is now emerging as a year-round concern in the city. An analysis of Central Pollution Control Board (CPCB) data shows that ozone was the lead pollutant in the daily Air Quality Index (AQI) on 16 of the 28 days this Feb, eight days in March so far and twice this Jan.In comparison, it was the main pollutant on nine days in Feb 2025 and two days in Feb 2024, 2021 and 2020, and once in Feb 2022. It wasn’t the dominant pollutant in Feb 2023. While ozone typically peaks during the summer months, especially between April and June, this data suggests that elevated levels are now being recorded in other months as well. For example, in Oct 2025, which was the coolest in three years with a mean maximum temperature of 31.7 degrees Celsius, ozone was the lead pollutant for 11 days. Anumita Roychowdhury, executive director (research and advocacy) of Centre for Science and Environment, said ozone pollution is escalating into a year-round public health concern. “Once a seasonal problem triggered by summer sunlight, toxic ozone levels now persist across all seasons due to high and continuous emissions of gases. Without urgent action to drastically cut gaseous pollutants from vehicles and industries, this highly reactive gas will increase public health risk,” she said.A review of hourly ozone data for the month of Feb indicates a worrying trend. The levels of the gas exceeded 101 micrograms per cubic metre for 29 hours in Feb 2026 and 18 hours in Feb 2025. In contrast, the hourly concentration did not cross the 100 micrograms per cubic metre mark in any Feb between 2020 and 2024 even once, according to data analysed by Envirocatalysts, a think tank.Though the national hourly standard for ozone is 180 micrograms per cubic metre, experts said the number of hours showing elevated levels this year is significantly higher than in previous years. Sunil Dahiya, founder and lead analyst at Envirocatalysts, said the average temperatures in Feb in previous years were almost similar, but the ozone levels peaked this Feb.“It means that primary precursors like oxides of nitrogen (NOx) and volatile organic compounds (VOCs) are driving ozone levels, as there is no direct correlation between ozone and temperatures. We have also seen that the gas has started turning into a main pollutant during the winter months and pre-summer months, which was not the case a few years ago,” said Dahiya. He added that this needs to be tackled in a systematic manner, with a focus on reducing the emission load of all pollutants.While ground-level ozone pollution has started rising in other months, it continues to remain consistently high across the Delhi-NCR region during summer. The eight-hour concentration exceeded 100 micrograms per cubic metre on each of the 92 days between March 1 and May 31, 2025, revealed a study conducted by Centre for Science and Environment (CSE) last year. Unlike primary pollutants, ozone is not emitted directly from any source. Ground-level ozone forms through complex chemical reactions between NOx and VOCs, which are emitted from vehicles, power plants, factories and other combustion sources. In the presence of sunlight, these pollutants undergo cyclic reactions that generate ozone at the surface level. VOCs can also originate from natural sources, including vegetation, said CSE. The centre, in its study, stated that ozone is a highly reactive gas, and ground-level ozone poses serious health risks. “It can inflame and damage airways, increase susceptibility to infections, and worsen respiratory conditions, such as 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 study said. In Dec last year, the Union environment ministry informed National Green Tribunal (NGT) that it will constitute a four-member expert panel to study ground-level ozone pollution, mitigation strategies and associated health impacts.
