of oxygen normally present in the air is 20.8 percent.
You breathe and work best with this amount of oxygen.
A fire produces carbon monoxide (CO) when there
When a space is suspected of having an insufficient
is not enough oxygen present for the complete
amount of oxygen, wear an OBA. Keep the OBA on
combustion of all of the carbon in the burning material.
until atmospheric tests show that oxygen content is at
CO is a colorless, odorless, tasteless, and nonirritating
least 20 percent and no more than 22 percent by volume.
gas. However, it can cause death even in small
concentrations. A person who is exposed to a
concentration of 1.28 percent CO in air will become
unconscious after two or three breaths. They will
A fire produces carbon monoxide (CO) when
probably die in 1 to 3 minutes if left in the area. CO also
there is not enough oxygen present for the
has a wide explosive range. If CO is mixed with air in
complete combustion of all of the carbon in
the amount of 12.5 to 74 percent by volume; an open
the burning material.
flame or even a spark will set off a violent explosion.
Carbon dioxide (CO2) is produced by a fire when
A fire produces carbon dioxide (CO2) when
there is complete combustion of all of the carbon in the
there is complete combustion of all of the
burning material. CO2 is a colorless and odorless gas.
carbon in the burning material.
Although CO2 is not poisonous, unconsciousness can
result from prolonged exposure at 10 percent volume
and higher. Above 11 percent volume, unconsciousness
can occur in 1 minute or less. In a sufficient quantity,
death could occur, since CO2 does not provide any
Under normal conditions, oxygen accounts
oxygen to breathe. The danger of asphyxiation should
for what percentage of the chemical
not be taken lightly; CO2 does not give any warning of
its presence, even when it is present in dangerous
amounts. It does not support combustion and it does not
form explosive mixtures with any substances. Because
of these characteristics, CO2 is very useful as a
fire-extinguishing agent. It is also used for inerting fuel
oil tanks, gasoline tanks, and similar spaces.
Hydrogen sulfide (H2S) is generated in some fires.
It is also produced by the rotting of foods, cloth,
leather, sewage, and other organic materials. H2S can
Learning Objective: Recall the fundamentals of fire
be produced within 6 to 12 hours. Use caution when
fighting fires around sewage systems and in spaces
where there has been a sewage spill. H2S is a colorless
In general, fires may be extinguished by removing
gas that smells like rotten eggs. Air that contains 4.3 to
one side of the fire triangle (fuel, heat, or oxygen) or by
46 percent H2S is violently explosive in the presence of
slowing down the rate of combustion. The method or
a flame. H2S is extremely poisonous if breathed, even
methods used in any specific instance will depend
in concentrations as low as 20 parts per million (ppm).
upon the classification of the fire (table 4-1) and the
You may rapidly become unconscious, stop breathing,
circumstances surrounding the fire.
and possibly die after one breath in an atmosphere that
contains 1,000 to 2,000 ppm of H2S.
Although it is not usually possible to remove the
fuel to extinguish a fire, there may be circumstances in
A fire in a closed compartment may cause an
which it is possible. If part of the fuel that is near or
inadequate supply of oxygen for breathing. An
actually on fire can safely be jettisoned over the side, do
enormous amount of oxygen is used by the fire itself,
so as soon as possible. Damage control parties must
leaving relatively little oxygen to breathe. The amount