Silent hazard: Delhi’s summer begins with spike in ground-level ozone
As Delhi transitions from spring to the peak of summer, a silent, invisible pollutant has begun to build up in the air: Ground-level ozone.
Unlike winter smog, which is often visible and heavy, ozone pollution strikes more subtly. It forms when heat and sunlight react with nitrogen oxides (NOx) and volatile organic compounds (VOC) from vehicles, industries and other combustion sources. The result is a powerful oxidant that peaks during the warmest hours of the day — when outdoor activity is at its highest.
In under two months — between March 1 and April 25, 2025 — Delhi has recorded 56 days of ozone exceedances. This means that on every single day during this period, at least one location in the city breached the 8-hour ozone standard of 100 microgrammes per cubic metre (µg / m³). On April 13, the situation reached a disturbing citywide scale: 25 out of 39 stations reported exceedances; Delhi’s regional average ozone concentration climbed to 135 µg / m³. This wasn’t a one-off peak, but part of a persistent and dangerous trend.
The most alarming finding came from Nehru Nagar, where ozone levels exceeded the safe limit on all the 56 days. It stood out as the most consistent and critical hotspot for ozone exposure.
Najafgarh followed closely behind with 54 exceedance days, while Major Dhyan Chand National Stadium and Sri Aurobindo Marg recorded 50 and 49 days, respectively. Areas like Ashok Vihar, Wazirpur and Okhla Phase II also showed disturbingly frequent exceedances, highlighting how vast portions of Delhi’s urban are grappling with chronic ozone pollution.
In contrast, few locations such as Anand Vihar, ITO, NSIT Dwarka, Shadipur, IHBAS, Chandni Chowk, East Arjun Nagar, DTU, Narela and Sirifort reported zero exceedance days.
What makes ground-level ozone particularly concerning is its silent nature. It’s invisible, odourless and peaks during daytime when most people are active outdoors. Unlike particulate matter, which causes immediate discomfort, ozone exposure can quietly inflame the lungs, trigger asthma, aggravate heart conditions and reduce overall lung function — especially in children, the elderly, and outdoor workers. The risk is even higher when the exposure is chronic, as we’re now witnessing in several parts of Delhi.
Daily variation in ground-level ozone exceedances in Delhi
(Based on exceedances recorded at the monitoring stations in Delhi. Exceedance is computed as daily maximum 8-hour average, crossing the ground-level ozone 8-hour standard or 100 µg/m³. Period of study is March 1 to April 25, 2025.)
Interestingly, the pattern of high-exceedance locations also reflects the city’s urban heat. Areas like Najafgarh and Wazirpur are also some of Delhi’s hottest zones, with limited green cover, barren land and dense built-up areas that trap heat and drive photochemical ozone formation. This strengthens the scientific linkage between urban heat and rising ozone levels, suggesting that tackling heat should be a crucial part of managing summer pollution.
As summers grow longer and hotter due to climate change, ozone is becoming a significant year-round public health issue. The city urgently needs to include ground-level ozone in its clean air planning frameworks. This means better public alerts during high-ozone days, cleaner transport and energy systems to cut down on NOx and VOCs and serious attention to cooling the city through trees, shaded streets and reflective infrastructure.
The data from early summer is a warning sign. Ozone is here, it’s rising and it’s dangerous. Delhi cannot afford to wait until it is visible or overwhelming — it must act while the damage is still reversible.
The beginning of ozone season is not just about pollution — it’s about public health, urban design and climate resilience. It’s about protecting the millions who live, breathe and work in this city every day.