It is unstable at standard temperatures and pressures, and a powerful oxidizer. At the earth's surface it occurs naturally as a result of electrical discharges, most prominently in the form of lightning. The majority of tropospheric ozone is created when volatile organic compounds (VOCs) and the products of combusted fossil fuel such as nitrogen oxide react in the presence of strong sunlight.
In the stratosphere it serves a very important function, by acting to filter out most of the sun's ultraviolet radiation.
Scientists have measured an annual cyclical thinning and thickening in the layer of ozone over the north and south poles. (Activists refer to this as an "ozone hole".)
(Above) The mechanism by which ozone is broken down in the stratosphere by chlorine atoms. Note that the propagation steps repeat thousands of times, such that one molecule of a CFC can destroy thousands of molecules of O3
- In April 1991, the Environmental Protection Agency announced that ozone levels in the Northern Hemisphere had decreased by as much as 5 percent over the United States in the past 10 years.
- A minority opinion holds that ozone depletion has little to do with CFCs and that the ozone hole may be more natural than man-made. S. Fred Singer, an environmental scientist at the University of Virginia, has argued that a rise in methane or changes in the sun's cycle may affect ozone. He also believes the lack of historical data about ozone depletion makes it difficult to determine if similar changes occurred centuries before hairspray cans were invented. 
Dangers and benefits
According to the EPA, "Ozone is a light bluish gas that is harmful to breathe. Nearly 90% of the Earth's ozone is in the stratosphere and is referred to as the ozone layer. Ozone absorbs a band of ultraviolet radiation called UVB that is particularly harmful to living organisms. The ozone layer prevents most UVB from reaching the ground."
Ozone in the lower atmosphere is a known cause of respiratory problems and currently causes up to 4,700 deaths per year.