Ozone (O₃) is a pale blue gas. It is made up of three oxygen atoms, but it is very different from the oxygen we breathe, which only has two oxygen atoms.

That extra atom is the difference between a healthy gas (oxygen) that our bodies need and a toxic gas (ozone) that is highly reactive and unstable.

At –112 °C, it condenses to form a dark blue liquid.

It is dangerous to allow this liquid to warm to its boiling point, because both concentrated gaseous ozone and liquid ozone can detonate.

At temperatures below –193 °C, it forms a violet-black solid substance.

Ground-level ozone is a major component of smog.

Stratospheric ozone, on the other hand, comprises the ozone layer, which protects us from the dangerous ultraviolet light from the sun. The two should not be confused.

Ozone is produced by ultraviolet light from the sun hitting the earth's atmosphere, certain electric devices (such as air ionizers), and as a byproduct of other types of pollution.

Outdoor air used for ventilation may have sufficient ozone to react with common indoor pollutants as well as skin oils and other common indoor air chemicals.

Many industries utilize ozone indoors. In such cases, ozone monitoring is crucial. Exposure of 0.1 to 1 µ mol/mol produces headaches, burning eyes, and irritation to the respiratory passages.

Even low concentrations of ozone in the air are very destructive to organic materials such as latex, plastics, and animal lung tissue.