'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 large area of thinned ozone layer, commonly called as the ozone hole.