Chemicals such as chlorofluorocarbons, hydrochlorofluorocarbons and halons are extremely useful compounds, delivering applications in aerosols, refrigeration and industrial cleaning. However, their harmful effects on the ozone layer have led to the introduction of international legislation (notably the 1987 Montreal Protocol) to phase out and replace the main ozone-depleting substances, and to control those that are exempt, as this topic describes.

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