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Exceeding Ozone Thresholds in Indian Megacities
Context:
A new report by the Centre for Science and Environment (CSE) flagged that ground-level ozone (O?) levels exceeded permissible limits on over one-third of summer days in major Indian cities during March–May 2025.
Ozone (O?) Pollution Classification:
- Ground-level ozone is a secondary pollutant, formed via photochemical reactions between nitrogen oxides (NO?) and volatile organic compounds (VOCs) in the presence of sunlight.
Monitoring Methodology:
- The CSE study utilized data from 80 continuous ambient air quality monitoring stations across Delhi, Mumbai, Kolkata, Bengaluru, Chennai, and Hyderabad.
Notably, 8-hour average concentrations were assessed (as per National Ambient Air Quality Standards – NAAQS).
City-wise Observations (March–May 2025):
- Delhi: O? levels breached norms on each summer day between March 1 and May 31.
- Bengaluru: 45 out of 92 summer days (29% increase from 2024).
- Mumbai: 32 out of 92 days (42% increase from 2024).
- Kolkata: 22 out of 92 days.
Scientific Concern:
- Ozone, being highly reactive, adversely impacts respiratory health even with short-term exposure. Unlike primary pollutants, its levels rise under intense solar radiation and high ambient temperatures — making summer months critical.
Regional Contrast:
- Northern and central Indian cities (warmer zones) recorded consistent breaches; meanwhile, cities in temperate/wet zones (like Chennai or coastal areas) showed fewer exceedances due to meteorological variation.
Health Impact & Standards:
- The NAAQS sets an 8-hour average safe ozone level at 100 µg/m³. Repeated exposure above this threshold is linked to respiratory distress, aggravated asthma, reduced lung function, and increased hospital admissions.
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