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• Ozone layer is found in the upper part of the atmosphere called the Stratosphere.
• Ozone layer surrounding earth acts as a shield for earth as it absorbs ultraviolet radiation coming from sun. Ozone gas (O3) in the lower stratosphere absorbs about 95% of the sun’s harmful incoming UV radiation.
• UV rays are highly injurious to living organisms since DNA and proteins of living organisms absorb UV rays and break the chemical bonds within these molecules.
• Ozone gas is continuously formed in atmosphere by the action of UV rays on molecular oxygen, and also degraded into molecular oxygen in the stratosphere.

O₂ + UV Light = O + O

O + O₂ = O₃ (Production)

O₃ + UV Light = O + O₂ (Destruction)

• There should be a balance between production and degradation of ozone in the stratosphere, but the balance is disturbed by the Chlorofluorocarbons (CFCs).
• CFCs are responsible for increase in thinning of ozone layer. CFCs are used as refrigerants. These are discharged in the lower part of atmosphere move upward and reach stratosphere.
• In stratosphere, UV rays act on CFcs, releasing Cl atoms. Cl degrades ozone molecule releasing molecular oxygen. This single Cl atom can react with millions of ozone molecules before being removed from the atmosphere.
• Cl atoms are not consumed in the reaction but they act as catalysts. Hence, whatever CFCs are added to the stratosphere, they have permanent and continuing affects on Ozone levels.
• Ozone hole: Although ozone depletion is occurring widely in the stratosphere, the depletion is particularly marked over the Antarctic region. This has resulted in formation of a large area of thinned ozone layer, commonly called as the ozone hole.
• Harmful effects of UV-B:  1. Damages DNA and cause mutation. 2. Causes aging of skin, damage to skin cells and various types of skin cancers
  3. In human eye, cornea absorbs UV-B radiation, but a high dose of UV-B causes inflammation of cornea, called Snow-blindness cataract, etc. Such exposure may permanently damage the cornea.
• Montreal Protocol: An international treaty signed at Montreal (Canada) in 1987 (effective in 1989) to control the emission of ozone depleting substances.
• Dobson units (DU): 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.