officially the Protocol on Substances That Deplete the Ozone Layer, treaty signed on Sept. 16, 1987, at Montreal by 25 nations; 168 nations are now parties to the accord. The protocol set limits on the production of chlorofluorocarbons (CFCs), halons, and related substances that release chlorine or bromine to the ozone layer of the atmosphere. On the basis of increasing scientific knowledge about the effects of CFCs and halons on the ozone layer, the original protocol has been amended several times. At meetings in London (1990), Copenhagen (1992), Vienna (1995), and Montreal (1997) amendments were adopted that were designed to speed up the phasing out of ozone-depleting substances; not all parties to the main protocol are parties to these amendments. The production and consumption of halons was phased out by Jan. 1, 1994, and of CFCs, carbon tetrachloride, methyl chloroform, and hydrobromofluorocarbons by Jan. 1, 1996, subject to an exception for agreed essential users. Methyl bromide is to be phased out by 2005, and hydrochlorofluorocarbons are to be phased out by 2020. (Phaseout dates are later for developing countries.)

Under the protocol, the ozone-depleting potential, or ODP, of any substance is measured with respect to an equal mass of CCl ₃ F, or CFC-11, which is assigned a value of 1.0. Most other CFCs have ODPs that range from about 0.5 to about 1.3. Hydrochlorofluorocarbons, which are being used as transitional replacements (until 2020) for CFCs in refrigeration, have ODPs that are generally less than 0.5. Hydrofluorocarbons, which are also replacing CFCs as refrigerants, have ODPs of zero. Ozone-depleting potentials are based on existing scientific knowledge and are to be reviewed and revised periodically.

See D. E. Newton, The Ozone Dilemma(1995).

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