Compounds obtained by replacing the hydrogen atoms of hydrocarbons by chlorine and fluorine atoms. Their high stability to temperature makes them suitable for a variety of uses, including aerosol propellants, oils, polymers, etc. They are often known as freons. Their former widespread use in aerosols and refrigerator coolants led to increased concentrations in the upper atmosphere, where photochemical reactions cause them to break down and react with ozone, which results in depletion of the ozone layer. They also contribute to the greenhouse effect. For this reason CFCs have now largely been replaced with less damaging alternatives, such as hydrofluorocarbons (HFCs). See also pollution.