Delhi-NCR ozone levels higher and more widespread this summer: CSE

• Ground-level ozone in Delhi exceeded national safety standard on nearly all 92 days of summer (March-May 2025); NCR recorded exceedance on 83 days during the same period.
• North-west and South Delhi worst affected. Ghaziabad and Noida reportworst ozone pollution in other NCR cities
• Delhi recorded widest geographical spread of ozone exceedance in past five years
• In Delhi, rolling eight-hour average stayed above standard for 14.2 hours on an average in a day, up from 12hours observed last summer.In rest of NCR, ozonelevel higher at 15 hours on an average
• Gwal Pahari in Gurugramreports most frequent night-time ozone events
• Sector 62 in Noida emerges as hotspot, registering an alarming eight-hour average concentration of 223 µg/m³
• In Ghaziabad, Vasundhara most affected location, with 48 days of exceedance. Lonistands out for high concentrations of both NO₂ and ground-level ozone
• Urgent action needed -- immediate inclusion of ozone and its precursor gases in the clean air programme to make the action on vehicles, industry, power plants, and all other combustion sources more stringent 

Access the complete CSE study report here

New Delhi, June 17, 2025: A new analysis from Centre for Science and Environment (CSE) has found higher number of days exceeding the standard for ozone as well as wider geographical spread of high ozone levels in Delhi and the National Capital Region (NCR) 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. 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,” says Anumita Roychowdhury, executive director and in-charge of clean air programme at CSE. 

“If unchecked this can become a serious public health crisis, as ozone is a highly reactive gas,” she adds.

“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 only 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 localised exposures in the hotspots and to design mitigation strategies accordingly. Managing short term exposures is critical to protect public health,” says Sharanjeet Kaur, deputy programme manager at CSE’s Urban Lab.

 The CSE assessment highlights that unlike primary pollutants (emitted directly from sources), 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 -- 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. VOCs also have natural sources, such as vegetation, adding to the complexity.

 The CSE study further indicates that as a highly reactive gas, ground-level ozone 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. 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.

Given its highly reactive nature the ambient air quality standards are set for eight-hour average, instead of 24-hour average.  

Theinvestigation method

 This assessment has traced trends during summer (March-May) between 2021 to 2025 (up to May 31). The analysis is based on publicly available granular real time data (15-minute averages) from the CPCB’s official online portal Central Control Room for Air Quality Management. The data has been captured from 57 official stations under the Continuous Ambient Air Quality Monitoring System (CAAQMS) spread across Delhi-NCR: Delhi has the maximum number of these stations (39) – with the rest spread across Gurugram (four), Faridabad (four), Noida (four), Ghaziabad (four) and Greater Noida (two)

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.

As ozone formation depends on complex atmospheric chemistry and on photochemical reactions, its level varies across time and space horizons. Meteorological parameters such as sunny and warm weather, stagnant wind patterns etc have a bearing on its formation. This analysis tracks exceedances at each station in core NCR. Breach of the standard by even one station is considered as exceedance. Days with multiple stations exceeding the standard indicates the severity of the spatial spread and number of people exposed.

The study has considered the 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 levels. 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)

Key highlights of CSE’s analysis 

Ground-level ozone exceedance is reported on nearly all days of 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.

However, when looking at the individual cities, distinct local trends and variations in ozone levels begin to emerge.

• Delhi: The highest intensity of pollution was observed on April 13, when the average ozone level stood at 135 µg/m³, with 25 out of 39 monitoring stations reporting exceedance.
• NCR outside Delhi: In the surrounding NCR areas, exceedances were reported on 83 days during the same period. The NCR average peaked slightly higher at 139 µg/m³ on April 25. However, the maximum number of stations in NCR reporting exceedance were nine (out of 10 stations with sufficient data for this period). This has been observed on both 18 and 28 April. Among NCR towns, the highest exceedance was observed in Ghaziabad, followed by Noida. Over the last five years, it has been noted that ground-level ozone has been an all-season problem, but it is exceptionally high during the months of April and May with hot and sunny weather conditions.

Geographical spread of ground-level ozone pollution highest in past five years:  Ground-level ozone usually exceeds the safety standard on all days of summer in some location in Delhi-NCR every year. This summer, the regional spatial spread (number of stations exceeding the standard across the core NCR) averaged 18.8 stations, which is the highest in the last five years. In comparison, the daily average was 12.6 stations during the summer of 2021 and 17.6 stations last summer, marking a 6.8 per cent increase this year.

Besides the increase in spatial spread of ground-level ozone, its duration has also gone up. This summer, at stations which reported exceedance, the rolling eight-hour average stayed above standard for 14.2 hours on an average, which is up from 12 hours observed last summer.

• Delhi: Delhi recorded a notable rise in the spatial spread of ozone exceedance, averaging 15.1 stations this summer, compared to 13.2 last year. The average duration of exceedance also increased to 12 hours.
• NCR cities: The NCR citieshave recorded a decline in spatial spread, which dropped to 3.6 stations from 4.4 last summer. However, this trend should be interpreted with caution, as Faridabad lacked complete data from all its monitoring stations. Despite this limitation, available data also shows that the average duration of exceedance in NCR rose sharply to 15 hours, surpassing even the average for Delhi.The highest average duration was reported in Gwal Pahari in Gurugram. 

North-west and South Delhi neighborhoods are worst affected by ground-level ozone pollution

• Delhi: Nehru Nagar in South Delhi is the most chronically affected. It has exceeded the standard in this location for all 92 days over March to May. It is followed by Najafgarh, Okhla Phase II, Ashok Vihar, Aya Nagar and Wazirpur.
•  NCR cities: Ghaziabad, Noida, Gurugram and Greater Noida are also seriously affected by ground-level ozone pollution. Vasundhara in Ghaziabad is the most affected in the region where ground-level ozone exceeded on 48 days, followed by Sector 125 in Noida with 43 days. Faridabad does not have adequate data of ground level ozone.

Ground-level ozone hotspots are located in the areas with low levels of NO₂ and PM2.5

• Delhi: Spatial distribution of ground-level ozone is inverse to that of NO2 and PM2.5. However, Wazirpur stands out as a notable exception, recording high levels of both NO₂ and ground-level ozone. Hotspots for PM2.5 are completely distinct from the hotspots for ground-level ozone. This bears out the fact that while ozone is created in polluted areas with nitrogen oxide being the catalyst, it also gets mopped up in high NO2 areas as it further reacts. But the ozone that escapes to cleaner areas with less NO2 builds up faster as unavailability of NO2 hampers its dissipation.
• NCR cities: In NCR, a similar trend is observed, but with exceptions. Loni in Ghaziabad records high levels of both NO₂ and ground-level ozone. It also shows elevated PM2.5 levels, followed by Teri Gram in Gurugram. These exceptions suggest complex local dynamics that may involve high precursor emissions, limited dispersion and other meteorological factors contributing to elevated ozone levels despite the presence of NO₂ and PM2.5.

Regional hourly ozone peak level is up by 9 per cent compared to lockdown times

An indicative analysis of hourly ground-level ozone data, averaged across all monitoring stations and days during May shows significant changes in pollution patterns this summer. Compared to May 2024, ozone concentrations are no longer lingering in the air after sunset. However, the regional average hourly peak has risen by 9 per cent, highlighting an overall intensification of the ozone problem.

The resurgence of morning and evening rush-hour traffic is contributing to a temporary reduction in ozone levels around sunrise and sunset, as higher NO₂ concentrations from vehicle emissions react with and suppress ozone formation during these periods.

Notably, this year, unlike previous years, CPCB did not apply the 200 µg/m³ capping on ground-level ozone data. As a result, eight-hourly daily averages have been recorded exceeding 450 µg/m³, levels previously unreported due to earlier data truncation.

• Delhi: On May 19, 2025, the CRRI’s Mathura Road station in Delhi recorded a maximum eight-hour average of 472 µg/m³, the highest level ever observed at this location and the highest this summer across the core NCR network. In previous summers, the threshold of 190 µg/m³ was breached by several stations including Alipur, Rohini, Ashok Vihar and Dr KS Shooting Range. This summer, Alipur and Ashok Vihar recorded new highs of 383 µg/m³ and 241 µg/m³, respectively.
• NCR cities: Gwal Pahari in Gurugram emerged as a peak hotspot with eight-hour average of 315 µg/m³. It was followed by Sector 62 in Noida with 223 µg/m³.

Night-time ground-level ozone continues to persist

Ground-level ozone should ideally become negligible in the night air, but Delhi-NCR has been witnessing a phenomenon where ozone levels remain elevated even hours after sunset. This summer, regional night-time ozone was noted at six stations on an average every night; 2020 had reported night-time ozone in five stations per night. Night-time ozone has been considered when hourly concentration has exceeded the level 100 µg/m3 between 10 PM and 2 AM at any station.

• Delhi: Night-time ozone is most frequently reported in Najafgarh in South West Delhi – this summer, it was recorded on 55 nights.
• NCR cities: Outside Delhi, the most night-time ozone is reported in Gwal Pahari in Gurugram: 28 nights this summer.

The way forward

Says Roychowdhury: “The clean air action plan in Delhi-NCR needs to address the multi pollutant crisis urgently. It is important to learn from the advanced economies which, after controlling particulate pollution, have fallen into the grip of rising NOx and ozone crisis. India should prevent this trap.”

• Integrate ozone in the Graded Response Action Plan to take emergency action to target the emitter of precursor gases like vehicles and industry that form ozone and to reduce short term exposures.
• Clean air action plan for the city needs to integrate ozone mitigation to implement stringent measures to upscale zero emissions vehicles, clean industrial processes and fuels, eliminate waste burning with 100 per cent remediation of legacy waste, collection, segregation
• Develop regional action plan on ozone: Ground-level ozone gets created in polluted areas but drifts and accumulates in cleaner urban environments, urban peripheries and surrounding rural areas affecting agricultural productivity and food security. While it further reacts with pollutants in polluted areas and dissipates, it lives longer in cleaner environments. Ozone is thus a regional pollutant that requires effective control at both local and regional level

For interviews and more details, please contact Sukanya Nair of The CSE Media Resource Centre: sukanya.nair@cseindia.org, 8816818864