(a) Ozone is a poisonous gas. It is made up of three atoms of oxygen combined together. It is formed high up in the atmosphere by the action of ultraviolet radiation on oxygen gas. The high energy ultraviolet radiation (UV radiation) coming from the sun splits oxygen gas into free oxygen atoms.
O2 + UV radiation + (From sun) → O + O (Oxygen atom)
The free oxygen atoms thus produced are very reactive.
One oxygen atom reacts with an oxygen molecule to form an ozone molecule:
O2 + O → O3(Ozone molecule)
(b) Ozone layer protect us from harmful effects as it absorbs most of the ultraviolet radiations coming from the sun and prevents them from reaching the earth.
(c) UNEP - United Nation Environment Program. In 1987, in an attempt to protect ozone layer, the United Nations Environment Program (UNEP) forged an agreement among its member countries to freeze CFC production at 1986 levels.