Ozone layer: A region in the Earth's atmosphere, at approximately 20-30 km altitude, containing about 10 ppm ozone. This ozone absorbs solar high-energy ultraviolet light, thus protecting organisms at the Earth's surface from this light's damaging effects. Chlorofluorocarbons (CFCs) and other organic halides have been implicated in depletion of the ozone layer, resulting in unnaturally large annual ozone holes over the Earth's Antarctic and Arctic regions.

A false color image of the Antarctic ozone hole, as measured on September 24, 2006. Bluer color = less ozone; redder color = more ozone. At 11.4 million square miles, this was the largest ozone hole measured to date.