Cause

Gas heavier than air that is called freon gas is the main cause.
Because it was non-flammable non-toxic,it has been widely used
,such as sprays, coolant of refrigerators and air conditioners,
and had been called"Substance of dreams".
As described earlier, freon gas will continue to rise toward
the ozone layer slowly ,over about 20 years because it is heavier than air.
It is a mechanism that strong sunlight will decompose freon
and generate the chlorine that destroy the ozone layer
Where a portion of the ozone layer is thinner in this reason
is called ozone hall.