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52.9 SMOGS

The word smog is obtained from the combination of the words smoke and fog, It is one of the best known example of air pollution that occurs in many cities throughout the world.
The word smog was initially used to describe the smok-fog condition. However, nowadays, the name depends upon the composition or the mode of its formation. There are two types
of smogs as described below:


It is also known as sulphurous smog or London smog. This type of smog was first noticed in London in 1952 and it resulted in the loss of many human lives. It occurs in cool and humid conditions and is generally formed in the early morning hours of winter months. Its formation is primarily initiated by ·a mixture of particulates, gaseous oxides of sulphur and moisture present in atmosphere. The source of classical smog is the combustion of industrial and household fuels (coal and petroleum). Because of the presence of SO2 and carbon (soot) particles, classical smog has a reducing character. Classical smog produces more dangerous effects and causes severe lung and throat irritation.


This type of smog is distinctly different from the classical smog. It is also called Los Angeles smog as it was first noticed in Los Angeles in 1950. It is formed in warm, dry and sunny climate. It results by the photochemical reactions between oxides of nitrogen and hydrocarbons produced by automobiles and factories. Hence it is known as photochemical smog. It is formed around mid day of summer months when intensity of solar radiation is very high. The principal constituents of the photochemical smog are ozone and oxides of nitrogen. This type of smog is oxidising in character because of the presence of O3, NO2 and some other photochemical oxidants.


Ozone, nitrogen oxide and organic compounds (PAN, CH2 = 0, CH2 = CHCHO) are main constituents of photochemical smog each of which produce hazardous effects.

  • It causes irritation to eyes and affect the respiratory tract of human beings.
  • Ozone causes cracks in rubber materials and is also harmful to fabric, crops and ornamental plants.
  • The toxic nature of photochemical smog cause coughing, wheezing, bronchial constrictions and irritation to respiratory mucous system.
  • It attacks newly grown leaves and causes bronzing and glazing of their surfaces.
  • The brownish colour of photochemical smog due to NO2 reduces visibility. NO2 also produces throat and eye irritation and leads to several chronic disease of throat, eyes, lungs and heart.