• ‘Bad’ ozone formed in the lower atmosphere (troposphere) that harms plants and animals.
  • There is ‘good’ ozone also; this ozone is found in the upper part of the atmosphere called stratosphere, and it acts as a shield absorbing ultraviolet radiation from the sun.
  • The thickness of ozone layer is measured in terms of  Dobson units (DU)
  • Ozone (O3) gas is continuously formed by the action of UV rays on molecular oxygen, and also degraded into molecular oxygen in the stratosphere.
  • There should be proper balance of formation and degradation of ozone.

Ozone depletion:

  • Balance of ozone in stratosphere is disrupted due to enhancement of ozone degradation by chlorofluorocarbons (CFCs).
  • CFCs find wide use as refrigerants.
  • CFCs discharged in the lower part of atmosphere move upward and reach stratosphere.
  • In stratosphere, UV rays acts on CFCs and release active Cl atoms.
  • Cl degrades ozone releasing molecular oxygen.
  • Cl acts as catalysts and not consumed during reaction.
  • Whatever CFCs are added to the stratosphere, they have permanent and continuing affects one Ozone levels.
  • The depletion is marked particularly over the Antarctic region. This has resulted in formation of a large area of thinned ozone layer, commonly called as the ozone hole.
  •  Cultural or Accelerated Eutrophication:

o    Pollutants from man’s activities like effluents from the industries and homes can radically accelerate the aging process. This phenomenon is called Cultural or Accelerated Eutrophication.



·         Sewage and agricultural and industrial wastes.

·         Prime contaminants are nitrates and phosphates.

Effects on Environment:

·         Unsightly scum and unpleasant odors.

·         Robbing the dissolved oxygen form water.

·         Pollutant inflow kills the fish.

·         Decomposition of dead fish causes further depletion of DO.

·         Finally a lake can literally choke to death.

 Effects of UV rays:

  • UV radiations shorter than UV-B are almost completely absorbed by Earth’s atmosphere, if the ozone layer is intact.
  • DNA and proteins of living organisms are damaged by UV rays as they potentially absorb it.
  • The high energy of UV rays breaks the chemical bond in these molecules.
  • UV – B damages DNA and mutation may occur.
  • It causes ageing of skin.
  • Damage skin cells and causes skin cancers.
  • In human eye cornea absorb UV – B radiation and high dose of UV – B causes inflammation of cornea called snow-blindness, cataract etc.
  • Such exposes may damage cornea.


  • Montreal Protocol was signed at Montreal (Canada) in 1987 to control emission of ozone depleting substances.
  • Many efforts are being made to reduce emission of ozone depleting substances.

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