Ozone Levels Exceeding Permitted Levels

Recently, a Centre for Science and Environment study has found that ozone levels are exceeding the permitted levels even during winter in Delhi-NCR, making the smog more “toxic”.

Main Points:

  • Year-round menace: Ozone is now a round-the-year problem requiring urgent action and the situation demands refinement of clean air action plan to add strategies for ozone mitigation, with stronger action on vehicles, industry and waste burning.
  • Ozone, a highly reactive gas and dangerous for those suffering from respiratory conditions and asthma, is becoming more widespread in Delhi and NCR across all seasons.
  • Contrary to the notion that ozone, produced photochemically from interaction between gases in the presence of sunlight, is predominantly a summer phenomenon, the analysis finds the gas has emerged as an equally strong concern during winter as well.
  • Occasional exceedance: The city-wide average largely remains within the standard with just occasional exceedance. But the ‘good’ category days have fallen to 115 days in 2020, which is 24 days less than in 2019.
  • A location-wise analysis shows that exceedance of the eight-hour average standard is quite widely distributed in the city.
  • At the moment, it is not possible to compute the peak pollution in the city as the data that is available from CPCB is capped at 200 ug/m3. This also makes daily Air Quality Index (AQI) reporting ineffective.


  • Ozone is an important gas in the atmosphere, which is actually a type of oxygen molecule consisting of three, instead of two, atoms.
  • It forms less than 0.00005% by volume of the atmosphere and is unevenly distributed.
  • It is between 20 km and 25 km altitude that the greatest concentrations of ozone are found. It is formed at higher altitudes and transported downwards.
  • It occurs naturally in the stratosphere and absorbs harmful ultraviolet rays of the sun.
  • However, at the ground level, it is a pollutant (Greenhouse gas) with highly toxic effects.
  • Vehicles and industries are the major sources of ground-level ozone emissions.
  • Carbon monoxide and Nitrogen dioxide play a major role in converting O2 to O3.
  • Ozone makes our eyes itchy, and watery. It lowers our resistance to cold and pneumonia.