(a) In the upper layer of atmosphere.
(b) Various man-made compounds like CFCs reach the layer react with the ozone molecules.
Reduction in the layer of ozone molecules causes ozone hole. It is actually thinning of ozone layer.
© Ozone prevents the harmful radiations of sun from reaching the surface of the earth. Depletion of this layer may harm many life forms by causing skin cancer, cataract in eyes, and causing global warming.