Ozone depletion

Ozone is a form of oxygen that is continuously generated and broken down (photo-dissociated) in the stratosphere through the absorption of short-wavelength ultraviolet radiation, following the reactions O3 -> O2 + [O] and O2 + [O] --(UV)--> O3; under normal conditions, generation and breakdown stay in balance, maintaining a fairly steady concentration of ozone in the stratosphere, roughly 12 to 15 kilometres above the Earth's surface. This ozone layer acts as a protective shield, absorbing UV radiation from the sun before it reaches the surface, with the shield naturally thicker above the poles than at the equator; UV radiation is highly damaging to living organisms because it is readily absorbed by DNA and proteins, and its high energy can break the chemical bonds within these molecules. The overall thickness of ozone in a column of air from the ground to the top of the atmosphere is measured in Dobson Units (DU).

This natural balance has been disturbed by chlorofluorocarbons (CFCs), which drift upward into the stratosphere where UV radiation breaks them apart, releasing chlorine atoms; these chlorine atoms degrade ozone while acting as a catalyst, meaning they are not consumed in the reaction and remain in the stratosphere to keep breaking down ozone over and over. This has led to widespread ozone depletion, most dramatically visible as a large area of severely thinned ozone over the Antarctic region, popularly called the ozone hole. UV radiation shorter than the UV-B range (roughly 100-280 nm) is almost entirely absorbed by an intact ozone layer, but UV-B itself (about 280-322 nm) can still get through and cause real harm -- it damages DNA and can trigger mutations, accelerates skin ageing, damages skin cells and raises the risk of various skin cancers, and, in the human eye, is absorbed by the cornea, where a high dose can cause painful inflammation (snow blindness), cataracts and even permanent corneal …