CFCs (chlorofluorocarbons) are a family of artificial chemical compounds containing chlorine, fluorine and carbon. They were formerly used widely in industry, for example as refrigerants, propellants and cleaning solvents. Their use has been generally prohibited by the Montreal Protocol, because of fears of their possible destructive effects on the ozone layer (see ozone depletion).
CFCs were developed by the American engineer Thomas Midgley in 1928 as a replacement for ammonia (then a common refrigerant). The new compound developed had to have a low boiling point, a lack of toxicity and be generally non-reactive. In a demonstration for the American Chemical Association, Midgley flamboyantly demonstrated all these properties by inhaling a breath of the gas and using it to blow out a candle.
Midgley specifically developed CCl2F2 (CFC-12). However one of the attractive features of CFCs is that there exists a whole family of the compounds, each having a unique boiling point which can suit different applications. As well as refrigerants, CFCs have been used as propellants in aerosol cans, cleaning solvents for circuit boards, and as blowing agents for making expanded plastics (such as those once used to store fast-foods.)
There has been a movement since the late 1970's to ban CFCs because of their destructive effect on the ozone layer. This damage was discovered due mainly to the work of scientists Sherry Rowland and Mario Molina, first published in 1974. It turns out that one of CFCs most attractive features - its unreactivity - has been instrumental in making it one of our worst pollutants. CFC's lack of reactivity gives it a lifespan long enough to diffuse up in to the upper stratosphere. Here the sun's UV radiation is strong enough to break off the chlorine atom, which on its own is a highly reactive free radical. This catalyses the break up of ozone in to oxygen:
Cl + O3 -> ClO + O2CFCs are a problem because the chlorine is regenerated at the end of these reactions, making it able to keep on reacting with millions of other ozone molecules. The ozone hole produced is able to let through UV light, which causes cancer in humans.
ClO + O -> Cl + O2
One major use of CFCs has been as propellants in aerosol inhalers for drugs used to treat asthma. The conversion of these devices and treatments from CFC to halocarbons that do not have the same effect on the ozone layer is well under way. There are some differences between asthma inhalers using CFCs and the newer propellants, but in general conversion is easy.