What causes chlorofluorocarbons?

When a hydrocarbon undergoes halogenation, or halogen substitution (same thing,) one or more hydrogen atoms are replaced by fluorine, chlorine, bromine, or iodine. This is how, for example, drinking water can become contaminated by the chlorine used to disinfect it. Chlorofluorcarbons, or CFCs are dangerous gaseous chemicals which result from this type of process. CFCs catalyze (speed up) the chemical decomposition of ozone, which is an important UV-light absorber in our stratosphere.