Class 7 Biology Forests: Our Lifeline Ozone depletion in the stratosphere


  • Upper part of the atmosphere is called the stratosphere.
  • Ozone is a layer in the earth’s stratosphere which acts as a shield absorbing ultraviolet radiation from the sun.


Fig. ozone layer 

  • UV rays are highly injurious toliving organisms as the chemical bonds within DNA and proteins of living organisms break by its high energy.
  • The thickness of the ozone in a column of air from the ground to the top of the atmosphere is measured in terms of Dobson units (DU).
  • Ozone gas is continuously formed by the action of UV rays on molecular oxygen, and also degraded into molecular oxygen in the stratosphere so there should be a balance between production and degradation of ozone in the stratosphere.
  • The balance has been disrupted due to enhancement of ozone degradation by chlorofluorocarbons (CFCs).
  • CFCs discharged in the lower part of atmosphere move upward and reach stratosphere.
  • In stratosphere, UV rays act on them releasing Cl atoms.
  • Cl degrades ozone releasing molecular oxygen, with these atoms acting merely as catalysts.
  • Ozone depletion resulted in formation of a large area of thinned ozone layer, commonly called as the ozone hole.
  • UV-B damages DNA and mutation may occur which causes aging of skin, damage to skin cells and various types of skin cancers.
  • In human eye, cornea absorbs UV-B radiation, and a high dose of UV-B causes inflammation of cornea, called snow-blindness cataract.
  • An international treaty Montreal Protocol, was signed at Montreal (Canada) to control the emission of ozone depleting substances.

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