Chlorofluorocarbons (CFCs) are hydrocarbons, such as freon, in which part or all of the hydrogen atoms have been replaced by fluorine atoms. These can be liquids or gases, are non-flammable and heat-stable, and are used as refrigerants, aerosol propellants, and solvents. When released into the air, they slowly rise into Earth’s upper atmosphere, where they are broken apart by ultraviolet rays from the sun. Some of the resultant molecular fragments react with the ozone in the atmosphere, reducing the amount of ozone. The CFC molecules’ chlorine atoms act as catalysts in a complex set of reactions that convert two molecules of ozone into three molecules of ordinary oxygen.

This is depleting the beneficial ozone layer faster than it can be recharged by natural processes. The resultant “hole” lets through more ultraviolet light to Earth’s surface and creates health problems for humans, such as cataracts and skin cancer, and disturbs delicate ecosystems (for example, making plants produce fewer seeds). In 1978 the U.S. government banned the use of fluorocarbon aerosols, and currently aerosol propellants have been changed from fluorocarbons to hydrocarbons, such as butane.