Halons are fire extinguishing agents which are gaseous when discharged in the aircraft environment. Halons have, until recently, been in almost universal use in aircraft fire extinguishers, both portable and fixed. They exist in two forms - as Halon 1211, BromoChlorodiFluoromethane (CBrClF2) also known as 'BCF', and as Halon 1301, Bromotrifluoromethane (CBrF3).