Chlorine is a component of many other compounds. It is the second most abundant halogen and 21st most abundant chemical element in Earth's crust. The great oxidizing power of chlorine led it to its bleaching and disinfectant uses, as well as being an essential reagent in the chemical industry. As a common disinfectant, chlorine compounds are used in swimming pools to keep them clean and sanitary. In the upper atmosphere, chlorine-containing molecules such as chlorofluorocarbons have been implicated in ozone depletion.