ozone+layer

The ozone layer is a thin blanket of ozone gas (O3) in the stratosphere.
 * The ozone molecule is a trioxygen molecule (O3) and is produced naturally in the upper atmosphere by ultraviolet radiation:
 * an oxygen molecule is broken down into two oxygen atoms by UV radiation: O2 (g) --> 2 O (g)
 * one oxygen atom then reacts with an oxygen molecule to form ozone: O (g) + O2 (g) --> O3 (g)
 * Ozone is also produced at ground level in smog, usually on hot, humid days with high exhaust pollution in urban areas.
 * Ozone is important to the planet's ecosystems because it blocks harmful UV radiation from the sun. This prevents UV from causing genetic damage to living things, from plankton to frogs to humans.
 * 85% of all skin cancer cases are caused by exposure to UV radiation
 * skin cancer is 'latent' - it may not appear for up to 50 years after exposure
 * it can be treated if detected early enough

The ozone layer has been thinned (drastically, at the south pole) by a class of man-made chemicals containing chlorine, called CFC's (chlorofluorocarbons).
 * Invented in the 1920's, CFCs were used for many years as refrigerants in air conditioners and as propellants in aerosol sprays.
 * One common class of CFCs was freon, used in air conditioners; another was that of the halons, used in fire extinguishers.
 * CFCs diffuse into the atmosphere slowly (for up to 15 years), and eventually make it to the stratosphere. Once there, they also react with UV light to produce chlorine atoms. These chlorine atoms react with ozone over and over again (one Cl atom can destroy 100,000 ozone molecules).

In order to prevent further destuction of this protective layer, many countries, including the US, signed on to the Montreal Protocol, a law designed to reduce the use of ozone-damaging CFCs. This law was signed into effect in 1987. The production of CFCs was frozen (not halted) at 1987 levels, and then reduced by 50% by 1999.

We will continue to see a reduction in the ozone layer for many decades to come, due to the slow diffusion of CFCs into the atmosphere, and due to the continued use of CFCs by nations not signed on to the Montreal Protocol.

See [|this link on ozone] for a comprehensive review of the topic.