Ozone is an allotropic form of oxygen. It is
present in the upper atmosphere (about 20 km
above the surface of the earth). It is believed to be
formed in the upper atmosphere by the action of
UV-rays on oxygen as
$ \, \, \, \, \, \, 3O_2 \, + \, UV-rays \longrightarrow \, 2O_3,$
$ \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \Delta H = 142.7 kJ \, mol^{-1} $
Therefore, UV-rays, which are harmful to human
beings are absorbed by oxygen to form ozone. The
layer of ozone, thus formed also prevents the
remaining UV-rays to reach the earth- surface.