Ground-level ozone
Why in news?
Ground-level ozone pollution has surged in Indian cities, particularly Delhi-NCR, with elevated levels persisting through 2025 summers and even October, driven by precursors like NOx and VOCs from vehicles and industries reacting under sunlight.​
About
- Ground-level ozone (O₃) is a harmful air pollutant formed in the lower atmosphere (troposphere) when nitrogen oxides (NOₓ) and volatile organic compounds (VOCs) react in the presence of sunlight.
- Unlike the “good” ozone in the stratospheric ozone layer, ground-level ozone is considered “bad” ozone because it damages human health, crops, and ecosystems.​
- Ground-level ozone is a colorless gas and a major component of photochemical smog in cities and industrial regions.
- Typical background concentrations are a few tens of parts per billion, but levels can become much higher in polluted areas during sunny, warm conditions.​
How it forms
Ozone at ground level is not emitted directly; it forms through photochemical reactions. The key ingredients are:​
- Nitrogen oxides from vehicle exhaust, power plants, and industrial combustion.​
- VOCs from fuels, solvents, industry, and also natural sources like vegetation.​
Sunlight drives a chain of reactions that convert NOₓ and VOCs into ozone, making concentrations highest on hot, sunny afternoons in many regions.​
Health impacts
- Breathing ozone irritates the airways and can cause coughing, chest tightness, and shortness of breath, especially during outdoor exertion.
- It worsens asthma, increases hospital admissions for respiratory disease, and is linked with premature death in vulnerable groups.
Environmental impacts
- Ozone damages plant tissues, reducing growth and crop yields and causing visible leaf injury in sensitive species.
- It also contributes to regional haze, lowers visibility, and acts as a short-lived greenhouse gas that adds to global warming.​
Control and regulation
- Ground-level ozone in India is regulated primarily through the National Ambient Air Quality Standards (NAAQS) 2009, which set limits at 100 µg/m³ for an 8-hour average and 180 µg/m³ for a 1-hour average across industrial, residential, rural, and ecologically sensitive areas.
- The Central Pollution Control Board (CPCB) monitors compliance under the National Air Quality Monitoring Programme (NAMP), in collaboration with state boards and NEERI, though data capping at 200 µg/m³ has historically limited full exceedance assessment.​
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