A typical discharge of liquid CO₂ has a white, cloudy appearance caused by finely divided dry ice particles carried along with the flash vapor. Some water will condense from the atmosphere creating additional fog, which will persist for a long time. Halon. Halon is a halogenated hydro- carbon in which one or more of the hydrogen atoms have been replaced by atoms from the halogen series (fluorine, chlorine, bromine, or iodine). This substitution provides nonflammability and flame extinguishing properties. A halon numbering system has been developed to describe the various halogenated hydrocarbons. The first digit in the number represents the number of carbon  atoms  in  the  compound  molecule;  the second digit, the number of fluorine atoms; the third digit, the number of chlorine atoms; the fourth digit, the number of bromine atoms; and the fifth digit, the number of iodine atoms, if any. In this system, terminal zero digits are not expressed. The two types of halon used aboard Navy ships are Halon 1301 (the most commonly used) and  Halon  1211,  introduced  for  twin-agent (AFFF/Halon 1211) applications on flight and hangar  deck  mobile  fire-fighting  apparatus. Portable  Halon  1211  extinguishers  are  also planned for backfit into vital electronics spaces of all surface combatant ships. Halon 1301 consists of one atom of carbon, three atoms of fluorine, no chlorine atoms, one bromine atom, and no iodine atoms. For shipboard  installation,  Halon  1301  is  super- pressurized with nitrogen and stored in compressed gas cylinders as a liquid. When released, it vaporizes into a colorless, odorless gas with a den- sity of approximately five times that of air. Halon 1211 consists of one atom of carbon, one atom of chlorine, two atoms of fluorine, and one atom of bromine. Halon 1211 is also color- less, but it has a sweet smell. Halon 1211 is stored and shipped as a liquid and pressurized with nitrogen gas. Pressurization is necessary since the vapor pressure is too low to convey it properly to the fire area. The mechanism by which halon extinguishes a fire is not thoroughly known. However, Halon (1211 and 1301) chemically inhibits the flame front; the halons act by removing the active chemical species involved in the flame’s chain reaction. Halon decomposes upon contact with flames or hot surfaces above 900°F (482°C). Decomposition products are principally hydrogen fluoride and hydrogen  bromide. The short discharge time of Halon 1301 (10 seconds maximum) keeps the thermal decomposi- tion products well below lethal concentrations. The real hazard lies not in the by-products of the halon, but rather in the products of combustion from the fire. Products such as CO, combined with the oxygen depletion, heat, and smoke, pose a greater hazard to personnel. Personnel should not remain in a space where Halon 1301 has been released to extinguish a fire unless an oxygen breathing apparatus (OBA) is worn. If Halon 1301 should inadvertently be released into a space where no fire exists, personnel can be exposed to 5-to-7 percent concentrations of Halon 1301 for up to 10 minutes (depending upon the individual) without danger to their health. Halon 1301 can be considered a nontoxic and nonsuffocating extinguishing agent in the normal 5-to-7-percent concentrations;   however,  spaces  should  be evacuated on halon system discharge. Exposure to Halon 1301 concentrations of up to about 7 percent by volume and Halon 1211 concentrations of 2 to 3 percent by volume has little noticeable effect on humans. Exposure to Halon 1301 concentrations of between 7 and 10 percent and Halon 1211 concentrations of between 3 and 4 percent may cause personnel to experience dizziness and tingling of the extremities. Those symptoms indicate mild anesthesia. At Halon 1301 concentrations above 10 percent and Halon 1211 concentrations above 4 and 5 percent, the dizziness becomes pronounced. Subjects feel as if they will lose consciousness (although none have), and physical and mental dexterity is reduced. No significant adverse health effects have been reported from the use of Halon 1301 or 1211 as a fire extinguisher since their introduction into the marketplace. Direct contact with vaporizing liquid being discharged from Halon 1301 and Halon 1211 has a strong chilling effect on objects and can cause frostbite and burns to the skin. The liquid phase vaporizes rapidly when mixed with air and there- fore limits this hazard to the immediate vicinity of the nozzle. Note In flammable gas cylinder storerooms, 20 percent Halon 1301 is required to extinguish a fire. Therefore, if the system is activated, personnel must leave the space immediately. 7-12