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What is controversial is much of this is a result of the breakdown of [[chlorofluorocarbons]] and the release of elemental [[chlorine]], which acts as [[catalyst]]s to break down the O<sub>3</sub> molecules.<ref>Chlorofluorocarbons (CFCs) themselves are not involved in the catalytic process; upon reaching the stratosphere, they are subject to higher levels of ultraviolet radiation that decompose the CFC and release atomic chlorine. [http://www.ciesin.columbia.edu/TG/OZ/ozndplt.html Catalytic destruction of ozone] </ref>
What is controversial is much of this is a result of the breakdown of [[chlorofluorocarbons]] and the release of elemental [[chlorine]], which acts as [[catalyst]]s to break down the O<sub>3</sub> molecules.<ref>Chlorofluorocarbons (CFCs) themselves are not involved in the catalytic process; upon reaching the stratosphere, they are subject to higher levels of ultraviolet radiation that decompose the CFC and release atomic chlorine. [http://www.ciesin.columbia.edu/TG/OZ/ozndplt.html Catalytic destruction of ozone] </ref>
[[Image:Ozonemech.gif|framed|The mechanism by which ozone is broken down in the stratosphere by Cl atoms. Note that the propagation steps repeat thousands of time, such that one molecule of a CFC can destroy thousands of molecules of O<sub>3</sub>]]
*In April 1991, the Environmental Protection Agency announced that ozone levels in the Northern Hemisphere shrank as much as 5 percent over the United States in the past 10 years.
*In April 1991, the Environmental Protection Agency announced that ozone levels in the Northern Hemisphere shrank as much as 5 percent over the United States in the past 10 years.