ozone(O3)consists of three oxygen atom. It is colorless (its color is light blue in high density) gas at room temperature and atmospheric pressure and has its own unique odor. Since its cohesiveness is weak comparing with oxygen, ozone is broken down immediately into oxygen and oxygen atom.

Certain amount of ultraviolet rays from the sun harmful
to human body is blocked by the ozone layer surrounding the earth.
Ozone existing in the air cleans, sterilizes, deodorizes and decolorizes
the air and also cleans the natural system by promoting the disintegration of organic material.

Ozone is sometimes misunderstood as『substance harmful to the environment』. Photochemical oxidant being one of the causes of photochemical smog, in its high density, is considered to be harmful to human health and agricultural produce. However, in that case, the principal culprit is not ozone but the first contaminating materials including nitrogen oxides hydrocarbon. Since ozone is used as an observation index, ozone seems to be misunderstood as harmful substance. Naturally, ozone always exists in the air and plays an important role of cleaning nature.