16 September 2011 - International Day for the Preservation of the Ozone Layer

Ozone molecule

Ozone or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. While the ozone in the lower atmosphere is an air pollutant with harmful effects on the respiratory systems of animals; the layer of ozone in the upper atmosphere is beneficial, preventing potentially damaging Ultraviolet (UV) light radiations from reaching the Earth's surface. Researchers over the past 40 years have monitored the status of the atmosphere and F. Sherwood Rowland and Mario Molina discovered in 1974 that chlorine, chemical contained in the chlorofluorocarbons (CFCs), gas used in spray cans, foam packaging, and refrigeration materials, was responsible for the depletion of the ozone layer and the creation of the hole in the Antarctic region.
Since 1987, more than 150 countries have signed an international agreement, the Montreal Protocol, which called for a phased reduction in the release of CFCs such that the yearly amount added to the atmosphere in 1999 would be half that of 1986. Modifications of that treaty called for a complete ban on CFCs which began in January 1996. Even with this ban in effect, chlorine from CFCs will continue to accumulate in the atmosphere for another decade. It may take until the middle of the next century for ozone levels in the Antarctic to return to 1970s levels.

More globally, ozone depletion is expected to remain a fact of life for several decades to come, but thanks to the research that led to early recognition of the problem and steps that have been taken to address it, the potential consequences are much less severe than they otherwise would have been.
On 19 December 1994, the United Nations General Assembly proclaimed 16 September the International Day for the Preservation of the Ozone Layer, commemorating the date, in 1987, on which the Montreal Protocol on Substances that Deplete the Ozone Layer was signed.

More information at:

Environmental Indicators: Ozone Depletion
NASA ozone watch
Ozone depletion phenomenon
UN news 23 August 2011