Study flags high summer ozone levels in big cities

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

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).