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Delhi's air pollution problem has traditionally been related to particulate matter (PM₂.₅ and PM₁₀), but recent evidence suggests that additionally, ground-level ozone (O₃) is rapidly emerging as an equally important environmental and public health concern. Unlike particulate matter, ozone is not emitted directly from any source. It is a secondary pollutant formed through photochemical reactions involving nitrogen oxides (NOx) and volatile organic compounds (VOCs) generated mostly from automobile exhausts that, in the presence of sunlight, generate ozone. Consequently, ozone pollution is most severe during hot, sunny weather, during the daytime and is expected to worsen with rising temperatures associated with climate change.
Recent analyses from Delhi-NCR indicate that ozone pollution has become both widespread and persistent. The Centre for Science and Environment (CSE), using data from the Continuous Ambient Air Quality Monitoring System (CAAQMS), reported that in many areas in Delhi-National Capital Region (NCR) the national eight-hour ozone standard of 100 µg/m³ exceeded on every day between March 1 and May 31, 2025. The number of monitoring stations exceeding the standard each day has increased in the past five years. Exceedances persisted for an average of 14.2 hours daily, indicating prolonged population exposure.
The spatial pattern of ozone pollution differs markedly from that of particulate matter. Areas such as Nehru Nagar, Najafgarh, Okhla Phase II, Ashok Vihar, and Aya Nagar experienced the highest number of ozone exceedance days, despite not being among Delhi's most polluted locations for PM₂.₅ or NO₂. This reflects the unique atmospheric chemistry of ozone. Fresh nitric oxide (NO) emissions from vehicles can destroy ozone locally, resulting in lower concentrations in areas with high-density traffic. Instead, ozone often accumulates in residential and peri-urban areas where these scavenging reactions are less intense.
Several stations recorded extraordinarily high ozone concentrations during the summer of 2025. The highest reported daily maximum eight-hour average was 472 µg/m³ at Central Road Research Institute (CRRI)-Mathura Road in Okhla, while other stations recorded values exceeding 300-400 µg/m³. Although these extreme values require independent validation, they indicate the potential for severe short-term exposure events across the city.
Ozone is a highly reactive oxidant gas that directly injures airway epithelium. Controlled human exposure studies and epidemiological investigations have consistently demonstrated that even short-term exposure can produce cough, throat irritation, chest tightness, wheezing, dyspnea and pain during deep inspiration. Ozone causes airway inflammation, increases airway permeability and impairs mucociliary clearance. These acute effects may produce symptoms even in persons not having any prior respiratory disease.
One of the best-established effects of ozone is its adverse impact on lung function. Exposure to elevated ozone concentrations can cause measurable reductions in FEV1-FVC (a standard test to measure lung function) and peak expiratory flow rates even in healthy individuals. These effects are particularly pronounced during physical activity because increased ventilation delivers larger quantities of ozone to the distal airways. Children, older adults and individuals with pre-existing respiratory disease are especially vulnerable.
Ozone exposure is strongly associated with asthma exacerbations. Higher daily ozone concentrations are linked to increased asthma symptoms, increased use of rescue medication, emergency department visits and hospitalisations. Ozone also increases bronchial hyperresponsiveness, making asthmatic individuals more sensitive to allergens and other environmental triggers.
Similarly, short-term ozone exposure increases the risk of COPD exacerbations and hospital admissions. Meta-analyses have demonstrated significant associations between ozone levels and COPD-related morbidity, particularly during peaks of ozone formation.
While the acute effects of ozone are well established, increasing evidence indicates that chronic exposure also has important long-term consequences. Longitudinal cohort studies have shown associations between long-term ozone exposure and accelerated decline in lung function, increased incidence of asthma, progression of emphysema and increased respiratory mortality. Chronic ozone exposure may contribute to airway remodeling and persistent inflammatory changes within the lungs. For a city such as Delhi, where ozone exceedances are now occurring throughout much of the summer, repeated seasonal exposure may contribute to cumulative respiratory injury and reduced lung function over time, particularly among children and individuals with pre-existing respiratory disease.
Historically, ozone was regarded primarily as a respiratory pollutant, but contemporary research demonstrates significant cardiovascular effects as well. Short-term increases in ozone concentration have been associated with higher risks of cardiovascular mortality, myocardial infarction, stroke, arrhythmias and hospital admissions for cardiovascular disease. Ozone-induced systemic oxidative stress and inflammation are believed to play key mechanistic roles.
Long-term exposure has also been linked to increased cardiovascular mortality in several large cohort studies. Recent evidence suggests that chronic ozone exposure contributes independently to cardiovascular disease risk, even after accounting for other air pollutants.
A growing body of evidence links both short-term and long-term ozone exposure to increased mortality. Meta-analyses of multiple studies have demonstrated increased risks of total, respiratory and cardiovascular mortality with rising ambient ozone concentrations. Importantly, adverse effects have been observed even at concentrations below many existing regulatory standards.
The persistence of ozone pollution across Delhi-NCR is therefore not merely an environmental concern but a major public health challenge. The combination of prolonged seasonal exposure, increasing geographic spread and high peak concentrations raises concerns regarding future burdens of asthma, COPD, cardiovascular disease, hospital admissions and premature mortality.
The emerging ozone problem signals a transition from a predominantly particulate pollution crisis to a broader multi-pollutant crisis. Traditional air quality interventions focused exclusively on particulate matter are unlikely to adequately address ozone pollution. Effective control will require reduction of NOx and VOC emissions from transport, industry, power generation, waste burning and solvent use. Because ozone is a regional pollutant that can travel considerable distances from its source, mitigation strategies must extend beyond Delhi itself and involve coordinated action across the entire NCR.
There is a dearth of data on health effects of ozone in Indian population. So far, the health studies have remained focused on the effects of particulate pollution and there is a need to widen the scope of such studies to include the potential adverse health effects of ozone.
The current evidence suggests that ozone is becoming one of the most important warm-season air pollutants in Delhi. Given its documented effects on lung function, respiratory symptoms, asthma, COPD, cardiovascular disease and mortality, ozone deserves recognition as a major public health threat and should become an integral component of future air quality management and health surveillance strategies.
(Part of this is column has been published in the cover story “Silent Spread”, in the June 16-30, 2026 issue of Down To Earth)
