Key Findings: Summer 2025 Ozone (O₃) Crisis
- Ozone levels exceeded prescribed limits for over ⅓ of summer days (March–May) in major cities.
- Study by: Centre for Science and Environment (CSE)
- Data Source: 80 ambient AQ monitoring stations in Mumbai, Kolkata, Bengaluru, Chennai, Hyderabad, Delhi.
Relevance : GS 3(Environment and Climate Change )
City-wise Breaches in Ozone Standards
| City | Days O₃ exceeded standard (out of 92) | Change vs 2024 |
| Delhi | 33 days | — |
| Bengaluru | 45 days | ↑ 29% |
| Mumbai | 32 days | ↑ 42% |
| Chennai | 15 days | ↑ (from 0) |
| Kolkata | 22 days (↓ from 40) | ↓ 45% |
Technical Insight
- Standard Breach Definition: 8-hour average > 100 µg/m³, as per National Ambient Air Quality Standards (NAAQS).
- O₃ is not directly emitted, but forms via photochemical reaction (sunlight + NOx + VOCs).
- Not monitored over 24-hour avg (unlike PM2.5/PM10) due to high reactivity.
Public Health Implications
- Ground-level ozone:
- Triggers respiratory illnesses, asthma, infections.
- Reduces lung function, especially in children & elderly.
- High temperatures + solar radiation → higher O₃ spikes in summers.
Expert View
“If unchecked, ozone pollution could become a serious public health crisis.” – Anumita Roychowdhury, CSE
Broader Pattern
- Indian megacities now consistently exceed ozone norms in summers.
- Northern India (e.g., Delhi) most affected due to:
- Temperature inversion
- Vehicular + industrial pollution
Ozone: Basics
- Ozone (O₃) is a triatomic molecule made of three oxygen atoms.
- Found in two layers:
- Stratospheric ozone (good ozone): Forms the ozone layer, protects life by absorbing harmful UV-B radiation.
- Tropospheric ozone (bad ozone): Acts as a pollutant and greenhouse gas, harmful to health and crops.
Ozone Layer & Its Importance
- Located in the stratosphere (10–50 km altitude).
- Absorbs 97–99% of the Sun’s medium-frequency ultraviolet light.
- Prevents skin cancer, cataracts, and damage to plants/marine ecosystems.
Ozone Depletion
- Caused by Ozone-Depleting Substances (ODS) like:
- CFCs (chlorofluorocarbons)
- Halons, Carbon tetrachloride, Methyl bromide
- Major ozone hole detected over Antarctica.
Global Measures
- Vienna Convention (1985): Framework for international cooperation.
- Montreal Protocol (1987): Legally binding treaty to phase out ODS.
- Kigali Amendment (2016): Added phase-down of HFCs (climate pollutants, not ODS).
