The 1987 Montreal Protocol on Substances that Deplete the Ozone Layer, ratified by the United States in 1988, was an agreement to phase out production and consumption of chlorofluorocarbons (CFCs) and halons believed harmful to the atmosphere’s protective ozone layer. Parties to the Montreal Protocol have since adjusted and amended the treaty to accelerate phase-out of controlled chemicals, add new chemicals to the controlled list, and establish an assistance mechanism to help developing countries comply.