• The word “smog” is an amalgam of the words “smoke” and “fog.”
  • Smog is produced by a set of complex photochemical reactions involving volatile organic compounds (VOCs), nitrogen oxides and sunlight. Ground-level ozone is the main constituent of smog.
  • Ozone can be beneficial or harmful, depending on its location. Ozone in the stratosphere acts as a barrier that protects human health and the environment from excessive amounts of solar radiation.
  • On the other hand, ground-level ozone trapped near the ground by heat inversions or other weather conditions causes the respiratory distress and burning eyes associated with smog.
  • Ironically, smog is often more severe farther away from the sources of pollution, because the chemical reactions that cause smog take place in the atmosphere while pollutants are drifting on the wind.
  • Smog in Delhi is primarily due to high vehicular and industrial emissions, construction work and crop burning in adjoining states.




1. Coal: Coal fires used to heat individual buildings or in a power-producing plant, can emit significant clouds of smoke that contributes to smog. Coal was the main culprit of 1952 London smog that was responsible for morbidity and mortality in thousands of people.

2. Transportation emissions: Traffic emissions such as from trucks, buses, and automobiles also contribute.

  • The major culprits from transportation sources are carbon monoxide (CO), nitrogen oxides (NO and NOx), volatile organic compounds, sulfur dioxide and hydrocarbons.
  • These molecules react with sunlight, heat, ammonia, moisture, and other compounds to form the noxious vapors, ground level ozone and particles that comprise smog.

3. Photochemical smog: Photochemical smog is the chemical reaction of sunlight, nitrogen oxides and volatile organic compounds in the atmosphere which leaves airborne particles and ground-level ozone.

4. Natural causes: An erupting volcano can also emit high levels of sulphur dioxide along with a large quantity of particulate matter both of which are key components to the creation of smog. However, the smog created as a result of a volcanic eruption is known as “vog” to distinguish it as a natural occurrence.


Smog and Temperature Inversion

  • Temperature inversion layers (also called as thermal inversions or just inversion layers) are areas where the normal decrease in air temperature with increasing altitude is reversed and air above the ground is warmer than the air below it.
  • Inversion layers block atmospheric flow which causes the air over an area experiencing an inversion to become stable.
  • During an inversion episode, temperatures increase with increasing altitude. The warm inversion layer then acts as a cap and stops atmospheric mixing. This is why inversion layers are called stable air masses.
  • Areas with heavy pollution are prone to unhealthy air and an increase in smog when an inversion is present because they trap pollutants at ground level instead of circulating them away.
  • This happens because the warmer air layer sits over a city and prevents the normal mixing of cooler, denser air. The air instead becomes still and over time the lack of mixing causes pollutants to become trapped under the inversion, developing significant amounts of smog.



Health Hazards of Smog:

  • Burning of eyes, throat and irritation to mucous membranes.
  • Shortness of breath, wheezing and coughing.
  • Asthma attack, chest pain, increased risk of respiratory disease.
  • Increased risk of heart attacks.
  • Chronic exposure can cause malignancy.
Section : Environment & Ecology