Halons
Halons
Halons are chemicals that contain carbon , fluorine, and bromine . They are used in fire extinguishers and other firefighting equipment. Because of their bromine content, halons can destroy ozone molecules(O3) very effectively, thereby contributing to the depletion of ozone and the creation of holes in the ozone layer of the stratosphere . The ozone layer is located 10–28 mi (16–47 km) above the surface of the earth and it protects humans and the environment from the Sun's ultraviolet-B radiation. Halons account for approximately 20% of the ozone depletion.
Halons have been used since the 1940s, when they were discovered by U.S. Army researchers looking for a fire-extinguishing agent to replace carbon tetrachloride. Halons are very effective against most types of fires, are nonconductive, and dissipate without leaving a residue. They are also economical, very stable, and safe for human use.
Halons consist of carbon atom chains with attached hydrogen atoms that are replaced by the halogens fluorine (F)and bromine (Br). Some also contain chlorine (Cl). Halon-1211 ((bromochlorodifluoromethane, CF2ClBr), halon-1301 (bromotrifluoromethane, CF3Br), and halon-2402 (dibromotetrafluoroethane, C2F4Br2) are the major fire-suppressing halons. Halon-1211 is discharged as a liquid and vaporizes into a cloud within a few feet. Halon-1301 is stored as a liquid but discharges as a gas. Halons suppress fires because they bond with the free radicals and intermediates of the decomposing fuel molecules that propagate fire, thus rendering the fuel inert. They also lower the temperature the fire.
Halons may take up to seven years to drift up and distribute themselves throughout the stratosphere, with the highest concentrations over the poles. High-energy ultraviolet radiation breaks their bromine and chlorine bonds thus releasing these very reactive halogen molecules, which in turn break down the ozone molecules and react with free oxygen to interfere with ozone creation. Although chlorine is more abundant, bromine is more than 100 times more damaging to ozone.
Halons are categorized as class I ozone-depleting substances, along with chlorofluorocarbons (CFCs) and other substances with ozone-depleting potentials (ODPs) of 0.2 or greater. The Ozone Depletion Potential(ODP)is a number that refers to the amount of ozone depletion caused by a substance. Halon-1211 has an ODP of 3.0 and an atmospheric lifetime of 11 years. It also has a global warming potential (GWP) of 1300. Halon-1301 has an ODP of 10.0, an average lifetime of 65 years, and a GWP of 5600 to 6900. Halon-2402 has an ODP of 6.0. In contrast, CFC-11, a common refrigerant, has an ODP of 1. Halon-1211 and halon-1301 are the most common halons in the United States. Halon-2402 is widely used in Russia and the developing world. Although total halon production between 1986 and 1991 accounted for only about 2% of the total production of class I substances, it accounted for about 23% of the ozone depletion caused by class I substances.
Halon production in the United States ended on December 31, 1993 because they contribute to ozone depletion. Under the Montreal Protocol on Ozone Depleting Substances, first negotiated in 1987 and now including more than 172 countries, halons became the first ozone-depleting substances to be phased out in industrialized nations, with production stopped in 1994. Under the Clean Air Act , the United States banned the production and importation of halons as of January 1, 1994. The use of existing halons in fire protection systems continues and recycled halons can be purchased to recharge such systems. It is estimated that about 50% of all halons ever produced currently exist in portable fire extinguishers and firefighting equipment. The United States holds 40% of the world's supply of halon-1301. In 1997, approximately 1,080 tons (977 metric tons) of halon-1211 and 790 tons (717 metric tons) of halon-1301 were released in the United States. In 1998, the U.S. Environmental Protection Agency (EPA) prohibited the venting of halons during training, testing, repair, or disposal of equipment, and banned the blending of halons, to prevent the accumulation of nonrecyclable stocks.
The European Union has gone beyond the Montreal Protocol, banning the sale and non-critical use of halons after December 31, 2002, and requiring the decommissioning of non-critical halon systems by December 31, 2003.
Halon production and consumption continues in developing countries, especially China, the Republic of Korea, India, and Russia. Under the Montreal Protocol, developing countries were to freeze halon consumption by January 2002. A 50% reduction in halon consumption is required by January 2005 and the complete halt of halon production and use is slated for January 2010. However halon-1211 emissions increased by about 25% between 1988 and 1999. Since most of the increased manufacture and release of halon-1211 occurs in China, the United Nations Environment Programme is helping China to phase-out production by 2006.
Atmospheric levels of halon-1202, which is not covered by the Montreal Protocol, increased fivefold between the late 1970s and 1999, and by 17% annually in the late 1990s. It is not known whether this increase is a byproduct of the inefficient production of other halons in developing countries or whether some countries are manufacturing it for military applications. Halon-1202's ODP is about one-half that of the common CFCs.
Alternatives are now available for most halon applications. Existing halon supplies from fire suppression systems are being recycled for critical uses where no alternative exists.
[Margaret Alic Ph.D. ]
RESOURCES
BOOKS
Peterson, Eric. Standards and Codes of Practice to Eliminate Dependency on Halons: Handbook of Good Practices in the Halon Sector. Paris: United Nations Environment Programme, 2001.
PERIODICALS
Chang, Lisa. "Regulating Halon Emissions." NFPA Journal 93, no. 4 (July/August 1999): 90.
Poynter, Ronald J. "Halon Replacements: Chemistry and Applications." Professional Safety 44, no. 3 (March 1999): 46–50.
Zurer, Pamela. "Slow Road to Ozone Recovery." Chemical and Engineering News 77, no. 17 (April 26, 1999): 8–9.
OTHER
U.S. Environmental Protection Agency. Ozone Depletion. May 15, 2002 [cited May 19, 2002]. <http://www.epa.gov/ozone/index.html>.
ORGANIZATIONS
Halon Alternatives Research Corporation, Halon Recycling Corporation, 2111 Wilson Boulevard, Eighth Floor, Arlington, VA USA 22201 (703) 524-6636, Fax: (703) 243-2874, Toll Free: (800) 258-1283, Email: harc@harc.org, <http://www.harc.org>
Stratospheric Ozone Information Hotline, United States Environmental Protection Agency, 1200 Pennsylvania Avenue, NW, Washington, D.C. USA 20460 (202) 775-6677, Toll Free: (800) 296-1996, Email: publicaccess@epa.gov, <http://www.epa.gov/ozone>
United Nations Environment Programme, Division of Technology, Industry and Economics, Energy and OzonAction Programme, Tour Mirabeau, 39-43 quai André Citroën, 73759 Paris Cedex 15, France (33-1) 44 37 14 50, Fax: (33-1) 44 37 14 74, Email: ozonaction@unep.fr, <http://www.uneptie.org/ozonaction<