Substances that break down stratospheric ozone, principally chlorofluorocarbons, freons and halons used as refrigerants, industrial solvents and propellants in aerosol spray cans. These substances are stable and long-lived in the lower atmosphere, but drift up to the stratosphere where they break down through the action of ultraviolet radiation. This releases highly reactive atoms (chlorine and bromine) that react with ozone molecules and break them apart.
See also ozone layer.