Halons are low-toxicity, chemically-stable compounds that have been used for fire and explosion protection from the turn of the last century. Carbon tetrachloride (Halon 104) was used prior to 1900, but there was a catch: The combustion by-products were deadly. Given the number of fatalities caused by the combustion by-products, other compounds (including other halons) were tried. In 1947, investigation by the Purdue Research Foundation and the U.S. Army resulted in the discovery of two effective, low toxicity halons: 1211 and 1301. When used properly, these halons have an excellent fire fighting record with little, if any, risk. Today, Halon 1211 (defined as a liquid streaming agent) is used primarily in hand-held fire extinguishers. Meanwhile, Halon 1301 (defined as a gaseous agent) is used primarily in total flooding systems. These halons have proven to be extremely effective fire suppressants, which are clean (leave no residue), colorless, odorless, electrically nonconductive, and prove to be remarkably safe for human exposure.
A halon blend (or 1301) is far superior to the 1211 Halon propelled by nitrogen because it generates its own pressure-performance does not degrade significantly as the extinguisher is emptied. Extensive toxicity evaluations have been compiled on Halon 1301 and 1211. These evaluations have consistently shown that Halon 1301 is the safest extinguishing agent available (although not safer than pure water), and that Halon 1211 is the second safest. Halon concentrations of about 5 percent by volume in air are adequate to extinguish fires of most combustible materials.
How do they work?
According to the experts, three things must take place simultaneously to start a fire. The first ingredient is fuel (anything that can burn), the second is oxygen (sufficient air for human breathing is ample), and the last is an ignition source (high heat can cause a fire even if there is no spark or open flame). In order to stop the fire you need to remove one ingredient in the mix-the ignition source, the fuel source, or the oxygen source. Halon is so effective because it adds a fourth dimension to fighting the fire-it breaks the chain reaction. It works this way by stopping the fuel source, ignition source, and oxygen source from working together by way of chemical reaction. The most common extinguishing agents, such as water, carbon dioxide, dry chemical, and foams attack the fire physically, depriving the fire of one or more of the three critical elements needed for propagation. Halon differs from all other extinguishing agents in the way it puts out the fire. It offers some of water's cooling effect and some of carbon dioxide's smothering action, but the essential extinguishing methodology lies in its capacity to chemically react with the components of the fire.
A common fallacy about halon is that it displaces the air from the area it's dispensed into. That's absolutely incorrect. According to folks in the know, even for the toughest fire, less than an 8-percent concentration by volume is necessary. This means there is still plenty of air to use in the evacuation process.