What is ozone hole? What was the aim of the Montreal Protocol?

The vertical column thickness of the ozone layer above 220 DU is considered normal but when thickness of the ozone layer in the stratosphere falls below 220 DU, then it is termed as ozone hole.

The aim of the Montreal Protocol is to phase out the use of ozone-depleting substances and to find or develop non-toxic substances by adopting the following methods:

1. Safe disposal of air conditioners and old refrigerators at recycling yards.
2. Avoiding ozone depleting substances from being released into atmosphere.
3. Properly disposing ozone-depleting substances as environmentally hazardous waste.