I think halon will displace just as much oxygen as Co2.  I would like a
medical/chemical explanation why halon might be more survivable, all else
being equal.

Walter,

 

 

 

 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  Halon  is  a  halogenated  hydro- carbon  in  which  one  or  more  of  the  hydrogen atoms  have  been  replaced  by  atoms  from  the halogen  series  (fluorine,  chlorine,  bromine,  or iodine).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.