A series of chemicals traditionally used in fire supression systems. The first of these is carbon tetrachloride, used in the early 1900s. Sadly, the combustion by-product of carbon tetrachloride, or Halon 104, is lethal to humans. Halon 1211 and 1301 were developed to relieve the poisonous aspect of this effective fire supression agent. However, since Halon displaces some air when released, sealed environments which use Halon fire supression systems, namely computer server rooms, must contain portable air supplies to allow for safe evacuation.

Halon is a chlorofluorocarbon (CFC) and is considered to be therefore a bad thing that should be replaced with other less environmentally hostile substitutes. I'm not an environmental activist but do on the whole consider the Ozone layer a good thing.

The reason that it is used in placed like computer system (rather than the more normal carbon dioxide used for electrical fires) is that carbon dioxide can scramble magnetic information (such as tapes and disks).

One alternative to halon that I have seen is, of all things, water. My local telephone company has "gone green", and has replaced their halon gas systems with high pressure water lines.

Trouble is, that they are so keen on quickly stopping fires in the cable vault of their main office, that the fire suppression system is still very harmful to humans. The vault, which is easily fifty feet long and fifteen wide, with a ten foot ceiling, is a very large room. This very large room has sprinklers in it. Except that, rather than sprinkle, they can flood the room floor to ceiling in about a minute or so. Apparently, this causes no trouble (no being defined as "not enough for them to worry too much about") for the equipment in the room, as it is mostly either outdoor wire (which is protected from water damage) or optical fiber (which couldn't short out). Except for the cable above the pothead splices. And any electronic test equipment that is left in the room when the flood hits.

I should have asked more questions on that tour, like "how water resistant is that fiber splice" or "what happens to that air pressure monitor when the water hits".
