Ozone (O3) is an inorganic molecule with the chemical formula O3. It is an allotrope of oxygen that is much less stable than the diatomic allotrope O2. Ozone is a pale blue gas with a distinctively pungent smell. It is an odorless, colorless gas made up of three oxygen molecules and is a natural part of the environment. Ozone is continuously produced in the Earths atmosphere by the action of ultraviolet light on atmospheric oxygen. Ozone is mostly found in the stratosphere, where it protects us from the Suns harmful ultraviolet (UV) radiation. However, near the surface where we live and breathe, ozone is a harmful pollutant that causes damage to lung tissue and plants. Ozone in the troposphere is not emitted directly into the air as a gas, but is formed through the photochemical reaction of nitrogen oxides (NOx) and volatile organic compounds (VOCs) . Nitrogen oxides and volatile organic compounds break apart in the presence of intense sunlight and recombine into new structures, creating ozone. Nitrogen oxides and volatile organic compounds are emitted by cars and trucks, industrial facilities, refineries, power plants, household products and cleaning supplies, and paints and solvents.

Some chemical properties of ozone include:

• Ozone dissolves in water resulting in the formation of hydrogen peroxide.
• Ozone reacts with lead sulfide resulting in the formation of lead sulfate.

Ozone is used at water treatment plants without filtration systems.