Delhi’s ground-level ozone, a harmful air pollutant, remained above safe limits throughout this summer, according to a study released Tuesday by the Centre for Science and Environment (CSE), a Delhi-based environment think tank.
Pollution
Delhi’s ground-level ozone, a harmful air pollutant, remained above safe limits throughout this summer, according to a study released Tuesday by the Centre for Science and Environment (CSE), a Delhi-based environment think tank.
Experts have pointed out that this summer, ozone emerged as the dominant pollutant on several days. While particulate matter remains the main focus of government efforts to combat air pollution, ozone levels topped the charts on 12 out of 18 days between May 25 and June 11.
Advertisement
Despite this spike and its known health risks, ozone has yet to be included in the Graded Response Action Plan (GRAP), which guides emergency measures to curb air pollution. This leaves a major gap in strategies to limit both short- and long-term exposure to the gas.
Advertisement
“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,” said Anumita Roychowdhury, executive director and head of the clean air programme at the CSE.
Citing the highly reactive nature of the gas, experts have warned that unchecked ozone levels could trigger a serious public health crisis.
According to the study, ozone exposure poses significant respiratory risks, especially for vulnerable groups such as children with underdeveloped lungs, the elderly, and people with pre-existing respiratory conditions. Experts noted that ozone can inflame and damage the airways, reduce resistance to infections, and aggravate diseases like asthma, chronic bronchitis, and emphysema. This can increase both the frequency and severity of asthma attacks, often resulting in higher hospitalisation rates.
Unlike particulate matter, which is monitored based on a 24-hour average, ozone standards are set for an eight-hour average due to the gas’s highly reactive nature. The national pollution monitoring agency also tracks hourly ozone levels across all monitoring stations.
As per CPCB norms, the permissible limit is 100 µg/m³ for an eight-hour average and 190 µg/m³ for a one-hour average—both of which were breached at several stations across the Delhi-NCR region during the recorded period.
The report, titled ‘Ground-level ozone – Delhi-NCR: An Invisible Threat’, found that from March 1 to May 31, ozone pollution remained consistently high across the region. The maximum concentration exceeded the safe limit of 100 µg/m³ on all 92 days, underscoring the persistent and widespread nature of the problem.
Meanwhile, the surrounding NCR areas reported surges on 83 days during the same period.
In Delhi, the highest ozone levels were recorded on April 13, with an average concentration of135 µg/m³. In the wider NCR region, the peak was slightly higher at 139 µg/m³ on April 25.
According to CSE’s analysis, the worst-affected areas are in northwest and south Delhi, with Nehru Nagar in South Delhi exceeding the safe ozone standard on all 92 days of monitoring. It was followed by Najafgarh (82 days), Okhla Phase II (81), Ashok Vihar and Aya Nagar (77 each), and Wazirpur (72).
The study also indicated significant changes in hourly ground-level ozone data this summer.
Ground-level ozone pollution has intensified this summer, with the regional hourly peak rising by 9% compared to lockdown-era levels and eight-hour averages crossing 450 µg/m³ due to the removal of CPCB’s earlier data cap. Delhi’s Mathura Road station logged a record 472 µg/m³ on May 19, while hotspots across NCR—like Alipur, Ashok Vihar, Gurugram’s Gwal Pahari, and Noida Sector 62—also reported dangerously high concentrations.
Ozone, unlike primary pollutants, is a secondary pollutant formed through sunlight-driven reactions involving nitrogen oxides (NOx), volatile organic compounds (VOCs), and carbon monoxide (CO) emitted from human-made sources such as vehicles, power plants, factories, and other combustion activities, as well as from natural sources like vegetation.
The CSE warns that poor monitoring and oversimplified averaging methods are masking localised ozone spikes, underscoring the need for hotspot-specific strategies to reduce short-term exposure and protect public health.
“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,” said Shranajeet Kaur, deputy programme manager at CSE’s Urban Lab, in a press statement.
In light of the recurring rise in ozone pollution, Roychowdhury urged that ozone be formally included in Delhi-NCR’s clean air action plan. She cautioned that, unlike developed economies, which shifted to a multi-pollutant approach only after controlling particulate pollution and then facing a surge in NOx and ozone levels, India should act early to avoid falling into the same trap.
To combat rising ozone pollution, experts recommend integrating ozone into GRAP and city clean air plans to enable emergency measures targeting precursor emissions from vehicles, industries, and waste burning, while promoting zero-emission vehicles and clean fuels.
They also call for a regional action plan, as ground-level ozone disperses beyond city limits, affecting rural areas, agricultural productivity, and food security, demanding coordinated local and regional strategies.
Advertisement
