chemical compound formerly used in firefighting (fire fighting). A halon may be any of a group of organohalogen compounds containing bromine and fluorine and one or two carbons (carbon). The effectiveness of halons in extinguishing fires arises from their action in interrupting chain reactions that propagate the combustion process. Halons are nonconductors of electricity and can be used in fighting fires in flammable liquids and most solid combustible materials, including those in electrical equipment; they are ineffective on fuels containing their own oxidizing agent or highly reactive metals (metal), such as sodium or potassium. Halon 1301 (bromotrifluoromethane) is especially favoured for extinguishing fires involving electronic equipment because it leaves no residue and does not cause electrical short circuits or damaging corrosion of the equipment.

Halons are both atmospheric ozone depleters and greenhouse gases. In accordance with the Montreal Protocol, their manufacture and consumption were phased out in industrialized nations by Jan. 1, 2000.