Ground-Level Ozone Emerges as Major Public Health Threat Across Delhi NCR

New evidence from Delhi's air quality monitoring network shows that ground-level ozone is now exceeding the national 8-hour standard of 100 micrograms per cubic metre on every monitored day in many parts of the National Capital Region, with exceedances persisting for an average of 14.2 hours daily. Unlike particulate matter, ozone peaks in summer and is expected to worsen as temperatures rise with climate change.

Data from the Centre for Science and Environment, using the Continuous Ambient Air Quality Monitoring System, found that between March 1 and May 31, 2025, many stations in Delhi-NCR exceeded the national ozone standard every single day. The highest recorded daily maximum 8-hour average reached 472 micrograms per cubic metre at CRRI-Mathura Road in Okhla, with other stations recording 300–400 micrograms per cubic metre. Significantly, the worst-affected areas — including Nehru Nagar, Najafgarh, and Aya Nagar — are not Delhi's most polluted zones for PM2.5, reflecting ozone's distinct atmospheric chemistry.

Ground-level ozone is a secondary pollutant formed when nitrogen oxides and volatile organic compounds from vehicle exhausts react in sunlight. Fresh nitric oxide from high-density traffic can actually destroy ozone locally, meaning concentrations accumulate in residential and peri-urban areas rather than near major roads. Ozone causes measurable reductions in lung function even in healthy individuals, and is associated with asthma exacerbations, COPD hospitalisations, cardiovascular mortality, and long-term respiratory damage in children and older adults.

Pulmonologist SK Chhabra describes ground-level ozone as one of Delhi's least appreciated public health threats. Effective control will require reducing NOx and VOC emissions across transport, industry, power generation, waste burning, and solvent use — a far broader intervention than particulate matter controls focused on dust and combustion. Because ozone is a regional pollutant that travels considerable distances, mitigation strategies must extend beyond Delhi and involve coordinated action across the entire NCR.

Key figure — 472 µg/m³ — highest recorded 8-hour ozone average in Delhi in 2025