• The term smog was first used by Dr H A Des Voeux
  • Smog has been coined from a combination of the words fog and smoke. Smog is condition of fog that had soot or smoke in it. 


  • Photochemical smog is also known as “Los Angeles smog”.
  • Photochemical smog occurs most prominently in urban areas that have large numbers of automobiles (Nitrogen oxides are the primary emissions).
  • Photochemical (summer smog) forms when pollutants such as nitrogen oxides (primary pollutant) and organic compounds (primary pollutants) react together in the presence of SUNLIGHT. A gas called OZONE (Secondary pollutant) is formed.

Nitrogen Dioxide  +  Sunlight  +  Hydrocarbons  =  Ozone (Ozone in stratosphere,is beneficial, but near the earth’s surface it results in global warming as it is a greenhouse gas)

  • The resulting smog causes a light brownish coloration of the atmosphere, reduced visibility, plant damage, irritation of the eyes, and respiratory distress.
  • Photochemical smog is a term used to describe air pollution that is a result of the interaction of sunlight with certain chemical in the atmosphere.
  • One of the primary components of photochemical smog in ozone.
  • While ozone in the stratosphere protects earth from harmful UV radiation, ozone on the ground is hazardous to human health.
  • Ground level ozone is formed when vehicle emissions containing nitrogen oxides (primarily from vehicle exhaust )and volatile organic compounds (from paints, solvents, printing inks, petroleum products, vehicles etc.)interact in the presence of sunlight.
  • Smog refers to hazy air that causes difficult breathing conditions. It is a combination of various gases with water vapour and dust.
  • Its occurrences are often linked to heavy traffic, high temperatures, and calm wind. During the winter, wind speeds are low and cause the smoke and fog to stagnate near the ground ; hence pollution levels can increase near ground level .
  • Smoke particles trapped in the fog gives it a yellow/ black colour and this smog often settled over cities for many days.


  • It hampers visibility and harms the environment.
  • Respiratory problems 
  • Deaths relating to bronchial  diseases
  • Heavy smog greatly decreases ultraviolet radiation.
  • Heavy smog results in the decreases of natural vitamin D production leading to a rise in the cases of rickets.
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