FIRE-FIGHTING AGENTS. —Many materials may be used as fire-fighting agents. The following are the fire-fighting agents used most often aboard naval ships: • Water. Water is a cooling agent, and aboard ship the sea provides an inexhaustible supply. If the surface temperature of a fire can be lowered below the fuels’ ignition temperature, the  fire  will  be  extinguished.  Water  is  most efficient when it absorbs enough heat to raise its temperature to 212°F (l00°C). At this temperature, the water will absorb still more heat until it changes to steam. The steam carries away the heat, which cools the surface temperature. A secondary method of water extinguishment is  caused  by  steam  smothering.  When  water changes into steam by absorbing heat, it expands about 1,700 times in volume. The large quantity of steam displaces the air from the fuel, which smothers the fire. Steam-smothering systems are installed in boiler casings and catapult troughs. • Aqueous Film-Forming Foam. Aqueous film-forming  foam  (AFFF)  is  composed  of synthetically produced materials similar to liquid detergents. These film-forming agents are capable of forming water solution films on the surface of flammable liquids. The Navy mixes AFFF by volume in the following proportion: 6 parts of AFFF concentrate to 94 parts water. AFFF concentrate is a clear to slightly amber- colored liquid concentrate. The AFFF solution of water and concentrate possesses a low viscosity and spreads quickly over a surface. AFFF concentrate  is  nontoxic  and  biodegradable  in diluted form. AFFF concentrate may be stored for long periods without losing its effectiveness. The  concentrate  will  freeze  when  exposed  to temperatures below 32°F (0°C) but can be reused when thawed. AFFF, when mixed with water, provides three fire-extinguishing advantages. First, it forms an aqueous film on the surface of the fuel, which prevents the escape of the hydrocarbon fuel vapors.   Second,  the  layer  of  foam  effectively excludes oxygen from the fuel surface. Third, the water content of the foam provides a cooling effect. Foam is used mainly to extinguish burning flammable  or  combustible  liquid  spill  fires (class  B).  AFFF  has  excellent  penetrating characteristics  and  is  superior  to  water  in extinguishing class A fires. • Carbon Dioxide (CO₂). CO₂ extinguishes fires by smothering. Co₂ is about 1.5 times heavier  than  air.  That  makes  CO₂ a  suitable extinguishing agent because it tends to settle and blanket the fire. CO₂ is a dry, noncorrosive gas that is inert when in contact with most substances and will not leave a residue that damages machinery or electrical equipment. In both the gaseous state and the finely divided solid (snow) state, it is a non- conductor of electricity y regardless of voltage. CO₂ can be safely used in fighting electrical fires. C O₂ extinguishes  the  fire  by  diluting  and displacing its oxygen supply. If gaseous CO₂ is directed into a fire so that sufficient oxygen- supporting combustion is no longer available, the flames will die out. Depending on what is fueling the fire, that action will take place when the 21-percent oxygen content, normally present in air, is diluted with CO₂ below 15 percent oxygen. Some ordinary combustible class A fires require that the oxygen content be reduced to less than 6  percent  to  extinguish  glowing  combustion (smoldering  fire).  CO₂ has  limited  cooling capabilities,  may  not  cool  the  fuel  below  its ignition temperature, and is more likely than other extinguishing agents to allow reflash. Therefore, the fire fighter must remember to stand by with additional backup extinguishers. The temperature of the burning substance and its surroundings must be below its ignition temperature if the fire is to remain extinguished. CO₂ is not an effective extinguishing agent for fires in materials that produce their own oxygen supply (as an example, aircraft parachute flares). Fires involving reactive metals, such as magnesium, sodium, potassium, or titanium, cannot be extinguished with CO₂. Because of the relatively high temperatures involved, these metal fuels  decompose  CO₂ and continue to burn. WARNING C O₂ can  produce  unconsciousness  and death when present in fire-extinguishing concentrations. The reaction in such cases is more closely related to suffocation. A concentration of 9 percent will cause most people to lose consciousness within a few minutes. Caution must be exercised when discharging CO₂ in confined spaces. 7-11