Ozone is a gas composed of three atoms of oxygen. It is less stable than o2. Ozone occurs both in the Earth's upper atmosphere (stratosphere)and at ground level(trophosphere). Ozone can be good or bad, depending on where it is found: Gaseous ozone is bluish in colour and has a pungent distinctive smell. The name of ozone is derived from the greek word “ozein “ meaning “To smell”. The smell of ozone can often be experienced near electrical transformers or nearby lightening strikes. It is formed in these instances, when an electrical discharge breaks Standard way to express total ozone in atmosphere is by Dobson units. Average concentration at a point is meassured in Parts per bilion .(ppb) or ug/m3.