7.2: Suppression Agents

Water

Water extinguishes fire primarily by cooling, although it can also smother or separate fuel and oxygen.^1,2,3 Water is the most commonly used extinguishing agent used by firefighters. Water is an excellent agent for Class A fires which is the primary fuel in structural and wildland firefighting.

Water is a superior cooling agent due to two properties:

1. Specific Heat – Specific heat refers to the amount of energy needed to raise a substance to its vaporization point.

2. Latent Heat of Vaporization – Latent Heat of Vaporization is similar to Specific heat and refers to the energy that is required to convert a substance from a liquid to a gas.

Both these concepts were discussed in Unit 4 – Liquid Fuel Fires. In Unit 4, it was discussed in relation to the amount of heat needed to vaporize fuel. In the case of water as an extinguishing agent, these concepts work in the opposite direction and refer to the ability of water to absorb heat produced by combustion. As stated in Unit 4, If we put a pint of water or 2 cups, (approximately a pound of water) on the stove to boil, and assume it comes out of the tap at 65°F, it would require 147 BTU’s to reach its boiling point, and then another 970 BTU’s to convert it to steam.

When we look at this as an extinguishing agent, a gallon of water (16 cups) would absorb 1176 BTUs as it heated to its vaporization point, and then another 7760 BTUs as the water is converted to steam. Wood on average has a heat of combustion of 6000 to 9000 BTU’s. This means it takes about a gallon of water to absorb the heat created by a pound of wood, assuming we can convert the entire gallon to steam.

Advantages of water:

1. Availability – Water is a readily available resource in most parts of the United States.
2. Cost – Water is an inexpensive extinguishing agent compared to other agents, and in some areas, water can be obtained by firefighters with no cost at all.
3. Effectiveness – Water is a very effective extinguishing agent because it can both smother or separate fuel and oxygen in addition to being an exceptional method of cooling.

Disadvantages of water:

1. Weight – Water weighs approximately 8.34 pounds (lbs.) per gallon. Considering a typical firefighting handline can flow anywhere from 150 to 400 gallons per minute. That weight can put an enormous load on a structure that may already be compromised due to fire.
2. Runoff – Water runoff can cause numerous problems on a fire scene. First, if contaminated with hazardous materials, water can create a substantial environmental impact and require additional cleanup efforts beyond fire suppression. Additionally, runoff within a structure can lead to extensive water damage to areas that may otherwise be unaffected by the fire itself. Finally, water can cause safety hazards and access issues on the fire scene.
3. Reactivity – Water reacts with certain materials such as combustible metals and liquids which makes water hazardous to use as an extinguishing agent in certain situations.
4. Conductivity – Water, or more specifically the minerals and other impurities in water, conduct electricity, which can present a hazard to firefighters when used on class C fires.
5. Volume Expansion – Water absorbs a large amount of heat when being converted to steam, however as it is turned to steam, water expands 1700 times. To put this in perspective, a gallon of water takes up .13368056 cubic feet of space (approximately 1/8 of a cubic foot). Once converted to steam, that steam occupies 227 cubic feet of space. A 10 x 10 room has 800 cubic feet, so the steam will fill up about a 4th of the room. This steam can push smoke and fire out of the room or down to the floor where firefighters are. This impact needs to be considered when firefighters apply water to fires.

Carbon Dioxide

Carbon Dioxide is a gas with a vapor density of 1.53 which means that it's weight will push oxygen out of the area where it is utilized, extinguishing fires by removing oxygen from the combustion reaction.^4,5 CO2 also has some cooling properties as well.

Advantages of Carbon Dioxide:

1. Non-reactive – CO2 exists in the atmosphere already, so it does not react with chemicals or the surroundings.
2. Clean – CO2 does not cause additional damage.

Disadvantages of Carbon Dioxide:

1. Limited effectiveness in wind or open spaces
2. Gas under high pressure, so tanks are bulky and heavy.
3. Expensive in relation to water.

Foam

Firefighting foam comes in two different categories. Class A foam, utilized in wildland and structural firefighting and Class B foam which is used to extinguish flammable liquid fires.^6,7

Class A Foam

Class A foam extinguishes fire primarily by cooling.

Advantages of Class A foam:

1. One key aspect of Class A foam is its surfactant properties. A surfactant reduces the surface tension of water, allowing it to soak into fuels such as leaf litter, piles of trash, or upholstered furniture.
2. Class A foam can also be designed to stick or hold in place. This is used to keep water on exposures of fire such as the siding on neighboring buildings.

Disadvantages of Class A foam:

1. Adding Class A foam to firefighting water takes additional labor and logistics, and increases the cost associated with extinguishment.
2. Mixing Class A foam effectively typically requires specific equipment such as foam eductors or proportioners.

Class B Foam

Class B foam extinguishes by separating fuel vapors from the oxygen in the air. The water in foam also provides some cooling to both the fuel and exposures, reducing the possibility of reignition. There are many types of Class B foams, but the most common is Aqueous Film Forming Foam (AFFF)

Figure \(\PageIndex{1}\): Firefighter foam.
Copyright:"Firefighter training in Djibouti, August 2011" by US Army Africa, licensed CC BY 2.0.

Advantages of Class B foam:

1. Provides superior firefighting capabilities for liquid fuel fires.

Disadvantages of Class B foam:

1. Class B foam is expensive and can require additional labor and logistics, especially in large-scale operations such as aircraft or fuel storage fires.
2. Applying Class B foam takes specialized training and equipment such as eductors, proportioners and aerating nozzles.
3. May need to be reapplied several times to maintain an adequate foam blanket.
4. May be difficult to apply or maintain in certain weather conditions such as high winds.
5. Class B foam blankets can be broken down by polar solvents such as alcohol based fuels like ethanol. There are alcohol resistant Class B foams that must be used in these situations but these foams are more expensive.
6. Many Class B foams contain PFOA/PFOS chemicals which are carcinogenic and environmental hazards. Foams that do not contain these chemicals are starting to be available.

Wet Chemical

Wet chemical is used specifically for fires involving cooking oils.⁸ Wet chemical forms a barrier or crust over cooking oil through a reaction called saponification. This extinguishes by separating the fuel and oxygen, but also provides some cooling. Wet chemical is found in commercial kitchen hood systems, as well as in Class K extinguishers that protect those kitchens.

Figure \(\PageIndex{2}\): Extinguisher in a commercial kitchen.
Copyright: "Housing Improvement Project Goal 2: Canyon Mess Hall commercial kitchen (before)" by YellowstoneNPS. Public Domain.)

Dry Chemical

Advantages of dry chemical:⁹

1. Relatively inexpensive when compared to many other agents.
2. Most dry chemicals are easy to obtain and quickly refilled/recharged.
3. Dry chemical agents do not conduct electricity so they are safe for Class C fires.

Disadvantages of dry chemical:

1. Dry chemicals are very fine powders that are easily spread throughout the area when used and are difficult to clean up.
2. Dry chemical agents can cake, so extinguishers or systems require regular maintenance.

Figure \(\PageIndex{3}\): ABC Fire Extinguisher.
Copyright: "ABC fire extinguisher" by ProjectManhattan. Public Domain.

Dry Powder

Dry Powder is utilized on Class D fires.¹⁰ Dry powders can be in extinguishers, systems, or simply in a container ready to be scooped onto metal fires. Dry powders are designed specifically for a metal type or group of metal types and are placed specifically for the anticipated hazard.

Halogenated Agents

Halogenated Agents extinguish fire by inhibiting the chemical chain reaction of a fire.¹¹ However, these agents present a large environmental hazard due to their atmospheric ozone-depleting properties. At one time, Halon was very common in both installed systems and extinguishers, but due to their environmental impact, most of these have been replaced by other agents. While halons still exist, they are no longer being manufactured. They are primarily used in aerospace to protect extremely high-value equipment such as aircraft. The Department of Defense maintains a large stockpile of Halon for this purpose.

Clean Agents

Clean agents came about as a replacement for halogenated agents.¹² These agents leave no residue when used, and are environmentally friendly. Many manufacturers have patented their own types of clean agents made of various chemicals. These products work in various ways. Some are made of inert chemicals such as nitrogen or argon and simply displace oxygen to extinguish fires. Other chemicals inhibit the chemical chain reaction. These agents are used both in portable extinguishers as well as installed systems. Clean agents are expensive, so they are typically utilized to protect high-value or sensitive areas such as electronics, chemicals, or museum spaces.

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References

^1. McDonough, J. (2018, March 1). The science of water extinguishment. Firehouse. https://www.firehouse.com/operations-training/hoselines-water-appliances/article/12395735/the-science-of-water-extinguishment-international-innovations-john-mcdonough

^2. Lanjewar, M. (2026, February 14). Water extinguisher, when it works, when it fails, and how people misuse it. The Fire Manager. https://thefiremanager.com/water-extinguisher/

^3. O'Connor, B. (2023, October 4). Fire extinguisher types. National Fire Protection Agency. https://www.nfpa.org/news-blogs-and-articles/blogs/2023/08/01/fire-extinguisher-types

^4. National Center for Biotechnology Information (2026). PubChem Compound Summary for CID 280, Carbon Dioxide. https://pubchem.ncbi.nlm.nih.gov/compound/Carbon-Dioxide.

^5. Lanjewar, M. (2026, February 12). CO₂ fire extinguisher, where it works perfectly and where it becomes dangerous. The Fire Manager. https://thefiremanager.com/co2-fire-extinguisher/.

^6. "Firefighting foam" (n.d.) in Wikipedia. Retrieved August 20, 2026. https://en.wikipedia.org/wiki/Firefighting_foam

^7. United States Forest Service (2017, May). Class A foam fact sheet. https://www.fs.usda.gov/rm/fire/wfcs/documents/Foam_FactSheet_2017-0428.pdf

^8. McShane, M. (2026, May 12). Wet chemical fire extinguishers: The UK guide. Fire Safety Awareness Training. https://firesafetyawarenesstraining.co.uk/blog/wet-chemical-fire-extinguisher

^9. Cease Fire. (2020, November 13). Dry chemical fire suppression systems explained. https://ceasefire.com/FireSuppression/Article/Everything-You-Need-to-Know-About-Dry-Chemical-Fire-Suppression-Systems

^10. Impact Fire. (2025, November 18).The ins and outs of Class D fire extinguishers. https://resources.impactfireservices.com/class-d-fire-extinguisher

^11. A-Gas. (2026). Clean agent fire protection & Halon FAQs. https://www.agas.com/us/resources/clean-agent-fire-protection-halon/

^12. Welland-Fulford, M. (2023, July 17). The various fire extinguisher clean agents. MEP Fire. https://www.mepfire.com/understanding-the-various-fire-extinguisher-clean-agents

For additional information:

Ferguson, L.H., Janicak, C.A. (2015). Fundamentals of fire protection for the safety professional (2nd ed.). Bernan Press.

Occupational Safety and Health Administration (n.d.) Portable fire extinguishers. United States Department of Labor. https://www.osha.gov/etools/evacuation-plans-procedures/emergency-standards/portable-extinguishers/about. Public Domain.