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Halons are a group of compounds obtained by replacing the hydrogen atoms of a hydrocarbon with halogen atoms, such as bromine or fluorine. Halon 1211 is bromochlorodifluoromethane (CF2BrCl) and Halon 1301 is bromotrifluoromethane (CF3Br). Halons are very stable and are widely used in fire extinguishers where water and other alternatives would be inneffective and dangerous (e.g. when dealing with oil fires) or cause unacceptable collateral damage (e.g. with electronic equipment.) At high temperatures, halons decompose to release halogen atoms that combine readily with active hydrogen atoms, depriving the fire of fuel.

Halon canisters used in a fire-suppression system

There is concern that they are being broken down in the atmosphere to bromine, which reacts with ozone, leading to depletion of the ozone layer, along with other chlorofluorocarbons such as freon. However, these fears are debatable as the kinds of fires that require halon extinguishers to be put out will typically cause more damage to the ozone layer than the halon itself.

Halons have been phased out and the posession of halon equipment is prohibited in countries like the Netherlands and Belgium from January 1, 2004, based on the Montreal Protocol and guidelines of the European Union.