Ozone is present in the stratosphere layer of earth's atmosphere, which is about 6 to 30 miles from the earth's surface.
It is very helpful as it protects the life on earth from the harmful ultraviolet rays that come from the sun.
It is harmful as well because when it is inhaled by humans it can damage the lungs and can cause various problems such as chest pain and respiratory diseases.
The ozone layer is depleted by the CFCs, as the UV rays of the sun breaks down the CFCs into chlorine atoms. These chlorine atoms react with ozone and deplete the ozone layer.