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'''Ozone''' (O₃) is an [[allotrope]] of [[oxygen]], the [[molecule]] consisting of three oxygen atoms instead of the more stable diatomic O₂.

At [[standard temperature and pressure]] ozone is a blue [[gas]]. Ozone forms a dark blue [[liquid]], below -112 °C, and a dark blue solid, below -193 °C. Ozone is a powerful [[oxidizing]] agent, and is unstable, decaying to ordinary oxygen through the reaction:

2O₃ → 3O₂. This reaction proceeds more rapidly with increasing temperature and increasing pressure.

Ozone has a sharp, pungent odor. It is present in low concentrations throughout the [[Earth's atmosphere]]. It is also formed from O₂ by electrical discharges, e.g., [[lightning]], and by action of high energy [[electromagnetic radiation]].

Some kinds of electrical equipment generate levels of ozone that a human can easily smell. This is especially true of devices using [[Voltage sparks and danger|high voltage]], such as [[television]] sets and [[photocopier]]s. Electric motors using [[brush]]es can generate ozone from repeated [[spark gap|spark]]ing inside the unit. Large motors, such as those used by elevators or hydraulic pumps, will generate more ozone than smaller motors.

The highest levels of ozone in the atmosphere are in the [[stratosphere]], in a region also known as the [[ozone layer]]. Here it filters out much [[ultraviolet]] light from the [[Sun]] that would be harmful to most forms of [[life]]. The standard way to express ozone amounts in the atmosphere is by using [[Dobson unit]]s. Ozone used in industry is measured in [[Parts per million|ppm]] ([[OSHA]] exposure limits for example), and percent by mass or weight.

Ozone was discovered by [[Christian Friedrich Schonbein]] in [[1840]].

== Stratospheric ozone ==

Ozone is notable for its ability to absorb [[ultraviolet radiation|UV-B radiation]]. Ozone is created [[nature|naturally]] within the [[ozone layer]]. [[Ozone depletion]] and ozone holes occur because of [[chlorofluorocarbon]]s and other upper atmospheric contaminants.

Ozone in the earth's [[Earth's atmosphere|atmosphere]] is generally created by [[ultraviolet light]] which breaks apart O₂ molecules, creating [[atom]]ic [[oxygen]]. The atomic oxygen then combines with an unbroken molecule, to create O₃. Sometimes the individual oxygen atoms will combine with [[nitrogen|N₂]] to create a [[nitrogen oxide]]; which may be dissociated by [[visible light]] to re-create ozone.

When ultraviolet light hits ozone it splits into a molecule of O₂ and an atom of atomic oxygen, a continuing process called the [[ozone-oxygen cycle]]. This cycle can be disrupted by the presence of atomic [[chlorine]], [[fluorine]] or [[bromine]] in the atmosphere; these elements are found in certain stable compounds, especially [[chlorofluorocarbon]]s (CFCs) which may find their way to the [[stratosphere]] and there be liberated by the action of ultraviolet light on them.

The nitrogen oxide cycle for the formation of ozone can also be broken by the presence of atmospheric water vapor, reducing nitrogen oxides to a more stable form.

== Use in industry ==

Ozone can be used for [[Bleach (chemical)|bleach]]ing substances and for killing bacteria. Many municipal drinking water systems kill bacteria with ozone instead of the more common [[chlorine]]. Ozone does not form [[organochlorine]] compounds, but it also does not remain in the water after treatment.

Industrially, ozone or ozonated water is used to:

* disinfect water before it is bottled,

* kill bacteria on food-contact surfaces

* scrub yeast and mold spores from the air in food processing plants

* wash fresh fruits and vegetables to kill yeast, mold and bacteria

* chemically attack contaminants in water ([[iron]], [[arsenic]], [[hydrogen sulfide]], [[nitrite]]s, and complex organics lumped together as "color"),

* provide an aid to [[flocculation]] (a process of agglomeration of molecules, which aids in filtration... this is where the iron and arsenic get removed),

* clean and bleach fabrics (the latter use is patented),

* assist in processing plastics to allow adhesion of inks,

* age rubber samples to determine the useful life of a batch of rubber.

== Use in medicine ==

Ozone, along with hypochlorite ions, is naturally produced by white blood cells and the roots of marigolds as a means of destroying foreign bodies. When ozone breaks down it gives rise to oxygen [[free radical]]s, which are highly reactive and damage or destroy most organic molecules.

Ozone has a number of medical uses. It can be used to affect the body's [[antioxidant]]-prooxidant balance, since the body usually reacts to its presence by producing antioxidant enzymes.

[[Ozone therapy]] has blossomed into a thriving field of [[alternative medicine]], and there are a host of successful applications. Ozone treatments are very safe when used correctly.

In the USA [[ozone therapy]] is illegal, as the FDA has not approved its use on humans, although it has been used and still is used succesfully in many other countries. Air purifiers which produce "activated oxygen", i.e. ozone, are often sold in the US in spite of FDA regulations.

== External links ==

*[http://earthobservatory.nasa.gov/Library/Ozone/ozone.html NASA's Earth Observatory article on Ozone]

<!-- *[http://www.clair.org/HowdoNOx.htm NOx cycle demonstrations] (commented out while website is being redesigned ) -->

[[Category:Chemistry]][[Category:Greenhouse gases]]

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