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 TM 55-1915-200-SDC
b. C02. C02 is effective as an extinguishing agent primarily because it reduces the air to a point where it will no
longer be capable of supporting combustion. As a rule, one pound of C02 will make inert 14.5 cubic feet of space.
Prolonged breathing in an atmosphere of high C02 concentration, about 2 to 3 percent, can cause headaches and
nausea. High concentration will cause suffocation, very much the same as immersion in water does when a person
drowns. Since C02 is 1-1/2 times heavier than air, it does not rise, but remains close to the surface in a deep or shallow
pool, according to the area covered and the amount of C02 used. With a portable C02 extinguisher, there is little danger,
inasmuch as its 35 cubic feet of C02 in the average compartment lies in shallow pools, lower than the average breathing
height. C02 is a dry, noncorrosive gas that is inert when in contact with most substances. C02 does not cause damage to
equipment.
C02 is most suitable for combating Class B and Class C fires; however, its smothering action is a temporary one and
because little cooling is accomplished, the fire fighter must remember that the fire will rekindle if the oxygen is supplied
again in the presence of an ignition source. C02 extinguishers have an effective range of about 3 to 5 feet.
c. Dry potassium bicarbonate chemical (PKP). Dry potassium bicarbonate chemical, usually called PKP (Purple K
Powder), is nontoxic, noncorrosive, and nonabrasive. It can be used effectively on Class B fires and is four times more
effective than equal weights of C02 in extinguishing flammable liquid fire. Dry chemical is also effective on Class C fires.
Smothering and heat radiation shielding contribute to the extinguishing efficiency of potassium bicarbonate; although the
exact mechanism is not known, studies suggest that a chain breaking reaction in the flame may be the principal cause of
extinguishment. Potassium bicarbonate has little effect on lowering the temperature of the burning material below the
ignition point; therefore, repeated applications may be necessary to prevent reflash. PKP is the most effective hand held
extinguisher in combating an oil spray fire. The effective range of this agent is 13 to 22 feet in comparison to 3 to 5 feet
for C02 type extinguishers. Potassium bicarbonate also covers a much larger area; however, this agent leaves behind a
residue while the other agents do not. Long discharges will reduce visibility and render breathing difficult. When
approaching a fire, a quick squeeze on the nozzle grip will give some assurance that the chemical is not caked and will
also knock down some smoke. If two PKP extinguishers are spent and little or no progress is made in extinguishing the
fire, the fire fighter should proceed to use low-velocity fog.
d. Aqueous film forming foam (AFFF). Aqueous Film Forming Foam, more often called AFFF, is a 6% solution of
fluorinated surfactant (detergent) in 94% water. It is nontoxic but highly detergent. It is intended for use on Class B and
Class A fires. The extinguishing action of AFFF stems from its ability to make water float on liquid fuels which are lighter
than water. As the extinguisher is discharged and the agent covers the fuel, the aqueous film floats over the fuel
6-10
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