replaced by bromine, along with other halogens. They are referred to by a system of code numbers similar to (but simpler than) the system used for freons. The first digit specifies the number of carbon atoms in the molecule, the second is the number of fluorine atoms, the third is the chlorine atoms, and the fourth is the number of bromine atoms. If the number includes a fifth digit, the fifth number indicates the number of iodine atoms (though iodine in halon is rare). Any bonds not taken up by halogen atoms are then allocated to hydrogen atoms. For example, consider Halon 1211. This halon has number 1211 in its name, which tells it has 1 carbon atom, 2 fluorine atoms, 1 chlorine atom, and 1 bromine atom. A single carbon only has four bonds, all of which are taken by the halogen atoms, so there is no hydrogen. Thus its formula is , hence its IUPAC name is bromochlorodifluoromethane. ANSI/ASHRAE Standard 34-1992 The refrigerant naming system is mainly used for fluorinated and chlorinated short alkanes used as refrigerants. In the United States, the standard is specified in ANSI/ASHRAE Standard 34–1992, with additional annual supplements. The specified ANSI/ASHRAE prefixes were FC (fluorocarbon) or R (refrigerant), but today most are prefixed by a more specific classification: CFC—list of chlorofluorocarbons HCFC—list of hydrochlorofluorocarbons HFC—list of hydrofluorocarbons FC—list of fluorocarbons PFC—list of perfluorocarbons (completely fluorinated) The decoding system for CFC-01234a is: 0 = Number of double bonds (omitted if zero) 1 = Carbon atoms -1 (omitted if zero) 2 = Hydrogen atoms +1 3 = Fluorine atoms 4 = Replaced by Bromine ("B" prefix added) a = Letter added to identify isomers, the "normal" isomer in any number has the smallest mass difference on each carbon, and a, b, or c are added as the masses diverge from normal. Other coding systems are in use as well. Hydrofluoro compounds (HFC) Hydrofluorocarbons (HFCs) contain no chlorine. They are composed entirely of carbon, hydrogen, and fluorine. They have no known effects on the ozone layer; fluorine itself is not ozone-toxic. However, HFCs and perfluorocarbons (PFCs) are greenhouse gases, which cause global warming. Two groups of haloalkanes, hydrofluorocarbons (HFCs) and perfluorocarbons, are targets of the Kyoto Protocol. Allan Thornton, President of the Environmental Investigation Agency, a non-governmental, environmental watchdog, says that HFCs are up to 12,500 times as potent as carbon dioxide in global warming. The