Halons
Halons are gaseous compounds composed of carbon, bromine, and another halogen, such as hydrogen, chlorine, or fluorine. Known as bromofluorocarbons, they belong to the family of chlorofluorocarbons (CFCs) and are classified by the U.S. Environmental Protection Agency as Group II of Class I ozone-depleting substances. Initially used during World War II for fire suppression, halons are particularly effective for extinguishing electrical fires due to their non-reactivity with electrical equipment. However, their use was banned in 1994 under the Montreal Protocol because of their significant ozone-depleting potential.
Halons are estimated to be three to ten times more harmful to the ozone layer compared to other substances in their class, primarily because of the presence of bromine. When halons are released into the atmosphere, bromine radicals react with ozone particles, leading to their destruction. This process is exacerbated by the long atmospheric lifetime of halons, resulting in a rising concentration despite historically lower usage compared to CFCs. In response to environmental concerns, less harmful alternatives to halons have been developed, including halocarbon-based agents and inert gases that suppress fires without threatening the ozone layer.
Halons
Definition
Halons are gaseous compounds of carbon, bromine, and another halogen (such as hydrogen, chlorine, or fluorine). These gases, also known as bromofluorocarbons, include the chlorofluorocarbon (CFC) family. They are classified by the US Environmental Protection Agency (EPA) as Group II of the Class I ozone-depleting substances.
Halons were first used during World War II as fire-extinguishing agents. They are particularly useful in extinguishing electrical fires, because they are covalently bonded and thus do not react with electrical equipment. They continued to be used through 1994, when they were banned by the Montreal Protocol as ozone-depleting substances.
Significance for Climate Change
Halons are members of the class of halogenated hydrocarbons, which have been shown to deplete the ozone layer. By some estimates, halons are three to ten times more destructive of the than are other substances in this family, including CFCs, because they contain bromine. The bromine radicals in halons react with ozone particles and remove them from the stratosphere. They sometimes react with atmospheric chlorine as well, magnifying their effects on ozone. The bromine-containing halons are much more destructive of atmospheric ozone than are other compounds, such as those containing chlorine, because bromine compounds are much more likely to disassociate in ultraviolet light, so many more ozone molecules are destroyed before the bromine molecules diffuse.
Halon use worldwide has historically been less than CFC use, so halons are not as widely known as ozone-destruction agents. However, halons are more destructive than some other ozone-depleting compounds, because their concentration in the atmosphere is still rising as a result of their long lifetimes.
Less environmentally destructive fire-extinguishing agents have been found to replace halons. These include halocarbon-based agents that absorb heat and inert gas agents that deplete oxygen, smothering fires. Both of these agents are less threatening to the ozone layer than are halons.
Bibliography
"Halon in Aviation." UN Environment Programme, 21 Feb. 2023, ozone.unep.org/halon-aviation. Accessed 19 Dec. 2024.
"Halons Program." US Environmental Protection Agency (EPA), 29 Apr. 2024, www.epa.gov/ozone-layer-protection/halons-program. Accessed 29 Apr. 2024.
Miziolek, Andrzej W., and Wing Tsang, eds. Halon Replacements: Technology and Science. Washington, D.C.: American Chemical Society, 1995.
Schwarzenbach, René P., Philip M. Gschwend, and Dieter M. Imboden. Environmental Organic Chemistry. 2d ed. Hoboken, N.J.: Wiley, 2003.
Taylor, Gary. Eliminating Dependency on Halons: Case Studies. Paris: United Nations Environment Programme, 2000.