Fire class	Geometric symbol		Pictogram	Intended use	Mnemonic
A				Ordinary solid combustibles	A for “Ash”
B				Flammable liquids and gases	B for “Barrel”
C				Energized electrical equipment	C for “Current”
D				Combustible metals	D for “Dynamite”
K				Oils and fats	K for “Kitchen”

Fire extinguishing capacity is rated in accordance with ANSI/UL 711: Rating and Fire Testing of Fire Extinguishers. The ratings are described using numbers preceding the class letter, such as 1-A:10-B:C. The number preceding the A multiplied by 1.25 gives the equivalent extinguishing capability in gallons of water. The number preceding the B indicates the size of fire in square feet that an ordinary user should be able to extinguish. There is no additional rating for class C, as it only indicates that the extinguishing agent will not conduct electricity, and an extinguisher will never have a rating of just C.

• Air-pressurized water (APW) cools burning material by absorbing heat from burning material. Effective on class A fires, it has the advantage of being inexpensive, harmless, and relatively easy to clean up. In the United States, APW units contain 9.5 litres (2+1⁄2 US gal) of water in a tall, stainless steel cylinder. In Europe, they are typically mild steel, lined with polyethylene, painted red and contain 6–9 l (1.6–2.4 US gal) of water.
• Water mist (WM) uses a fine misting nozzle to break up a stream of de-ionized (distilled) water to the point of not conducting electricity back to the operator. Class A and C rated. It is used widely in hospitals and MRI facilities because it is both completely non-toxic and does not cause cardiac sensitization like some gaseous clean agents. These extinguishers come in 6.6-litre (1+3⁄4 US gal) and 9.5-litre (2+1⁄2 US gal) sizes, painted white in the United States. Models used in MRI facilities are non-magnetic and are safe for use inside the room that the MRI machine is operating. Models available in Europe come in smaller sizes as well, and some even carry a Class F rating for commercial kitchens, essentially using steam to smother the fire and the water content to cool the oil.

• 

  General 2.5 gal. pump-type water fire extinguisher, 1960s, US

• 

  Stored pressure water extinguisher

• 

  Stored pressure loaded stream fire extinguisher

• 

  2.5 gallon water mist fire extinguisher for medical and MRI facilities

• 

  6-liter wet chemical fire extinguisher for use in commercial kitchens

• 

  Indian 5-gal. backpack pump tank for wildland firefighting, US

Wet chemical and water additives

Wet chemical (potassium acetate, potassium carbonate, or potassium citrate) extinguishes the fire by forming an air-excluding soapy foam blanket over the burning oil through the chemical process of saponification (a base reacting with a fat to form a soap) and by the water content cooling the oil below its ignition temperature. Generally, class A and K (F in Europe) only, although older models also achieved class B and C fire-fighting capability in the past, current models are rated A:K (Amerex, Ansul, Buckeye and Strike First) or K only (Badger/Kidde).

• Wetting agents: Detergent based additives used to break the surface tension of water and improve penetration of class A fires.
• Antifreeze chemicals added to water to lower its freezing point to about −40 °C (−40 °F). Has no appreciable effect on extinguishing performance. Can be glycol based or loaded stream, see below.
• Loaded Stream: An alkali metal salt solution added to water to lower its freezing point to about −40 °C (−40 °F). Loaded stream is basically concentrated wet chemical, discharged through a straight stream nozzle, intended for class A fires. In addition to lowering the freezing point of the water, loaded stream also increases penetration into dense class A materials and will give a slight class B rating (rated 1-B in the past), though current^[when?] loaded stream extinguishers are rated only 2-A. Loaded Stream is very corrosive; extinguishers containing this agent must be recharged annually to check for corrosion.

Halons, Halon-replacement clean agents and carbon dioxide

Clean agents extinguish fire by displacing oxygen (CO₂ or inert gases), removing heat from the combustion zone (Halotron I, FE-36, Novec 1230) or inhibiting the chemical chain reaction (Halons, Halotron BrX). They are referred to as clean agents because they do not leave any residue after discharge, which is ideal for protecting sensitive electronics, aircraft, armored vehicles and archival storage, museums, and valuable documents.

• Halon (including Halon 1211 and Halon 1301), are gaseous agents that inhibit the chemical reaction of the fire. Classes B:C for 1301 and smaller 1211 fire extinguishers (2.3 kg; under 9 lbs) and A:B:C for larger units (9–17 lb or 4.1–7.7 kg). Halon gases are banned from new production under the Montreal Protocol, as of January 1, 1994 as its properties contribute to ozone depletion and long atmospheric lifetime, usually 400 years. Halon may be recycled and used to fill newly manufactured cylinders, however, only Amerex continues to do this. The rest of the industry has moved to halon alternatives, nevertheless, halon 1211 is still vital to certain military and industrial users, so there is a need for it.

Halon was completely banned in Europe and Australia except for critical users like law enforcement and aviation, resulting in stockpiles either being destroyed via high heat incineration or being sent to the United States for reuse. Halon 1301 and 1211 are being replaced with new halocarbon agents which have no ozone depletion properties and low atmospheric lifetimes, but are less effective. Halon 2402 is a liquid agent (dibromotetrafluoroethane) which has had limited use in the West due to its higher toxicity than 1211 or 1301. It is widely used in Russia and parts of Asia, and it was used by Kidde‘s Italian branch, marketed under the name “Fluobrene”.

• Halocarbon replacements, HCFC Blend B (Halotron I, American Pacific Corporation), HFC-227ea (FM-200, Great Lakes Chemicals Corporation), and HFC-236fa (FE-36, DuPont), have been approved by the FAA for use in aircraft cabins in 2010.^[27] Considerations for halon replacement include human toxicity when used in confined spaces, ozone depleting potential, and greenhouse warming potential. The three recommended agents meet minimum performance standards, but uptake has been slow because of disadvantages. Specifically, they require two to three times the concentration to extinguish a fire compared with Halon 1211.^[28] They are heavier than halon, require a larger bottle because they are less effective, and have greenhouse gas potential.^[29] Research continues to find better alternatives.
• CO₂, a clean gaseous agent which displaces oxygen. Highest rating for 20 lb (9.1 kg) portable CO₂ extinguishers is 10B:C. Not intended for class A fires, as the high-pressure cloud of gas can scatter burning materials. CO₂ is not suitable for use on fires containing their own oxygen source, metals or cooking media, and may cause frostbite and suffocation if used on human beings.
• Novec 1230 fluid (AKA dry water, or Saffire fluid), a fluorinated ketone that works by removing massive amounts of heat. Available in fixed systems and wheeled units in the US and in portables in Australia. Unlike other clean agents, this one has the advantage of being a liquid at atmospheric pressure and can be discharged as a stream or a rapidly vaporizing mist, depending on application.
• Potassium aerosol particle-generator, contains a form of solid potassium salts and other chemicals referred to as aerosol-forming compounds (AFC). The AFC is activated by an electric current or other thermodynamic exchange which causes the AFC to ignite. The majority of installed currently are fixed units due to the possibility of harm to the user from the heat generated by the AFC generator.
• E-36 Cryotec, a type of high concentration, high-pressure wet chemical (potassium acetate and water), it is being used by the U.S. Military in applications like the Abrams tank to replace the aging halon 1301 units previously installed.

• 

  Amerex 10lb. CO₂ Fire Extinguisher, Circa 1989, US

• 

  Halon 1211 Fire Extinguisher

• 

  Halon 1301 Fire Extinguisher

• 

  5lb. Halotron-1 fire extinguisher

• 

  FE-36 Cleanguard fire extinguisher

Class D dry powder and other agents for metal fires

There are several class D fire extinguisher agents available; some will handle multiple types of metals, others will not.

• Sodium chloride (Super-D, Met-L-X, M28, Pyrene Pyromet*) contains sodium chloride salt, which melts to form an oxygen-excluding crust over the metal. A thermoplastic additive such as nylon is added to allow the salt to more readily form a cohesive crust over the burning metal. Useful on most alkali metals including sodium and potassium, and other metals including magnesium, titanium, aluminum, and zirconium.
• Copper-based (Copper Powder Navy 125S) developed by the U.S. Navy in the 1970s for hard-to-control lithium and lithium-alloy fires. The powder smothers and acts as a heat sink to dissipate heat, but also forms a copper-lithium alloy on the surface which is non-combustible and cuts off the oxygen supply. Will cling to a vertical surface. Lithium only.
• Graphite-based (G-Plus, G-1, Lith-X, Chubb Pyromet) contains dry graphite that smothers burning metals. The first type developed, designed for magnesium, works on other metals as well. Unlike sodium chloride powder extinguishers, the graphite powder fire extinguishers can be used on very hot burning metal fires such as lithium, but unlike copper powder extinguishers will not stick to and extinguish flowing or vertical lithium fires. Like copper extinguishers, the graphite powder acts as a heat sink as well as smothering the metal fire.
• Sodium carbonate-based (Na-X) is used where stainless steel piping and equipment could be damaged by sodium chloride-based agents to control sodium, potassium, and sodium-potassium alloy fires. Limited use on other metals. Smothers and forms a crust.
• Ternary eutectic chloride (T.E.C.) dry powder is a dry powder invented in 1959 by Lawrence H Cope,^[30][31] a research metallurgist working for the UK Atomic Energy Authority, and licensed to John Kerr Co. of England. It consists of a mixture of three powdered salts: sodium, potassium and barium chloride. T.E.C. forms an oxygen-excluding layer of molten salt on the metal’s surface. Along with Met-L-X (sodium chloride), T.E.C has been reported^[32] to be one of the most effective agents for use on sodium, potassium, and NaK fires, and is used specifically on atomic metals like uranium and plutonium as it will not contaminate the valuable metal unlike other agents. T.E.C. is quite toxic, due to the barium chloride content, and for this reason is no longer used in the UK, and was never used in the US aside from radioactive material handling glove boxes, where its toxicity was not an issue due their confined nature. T.E.C. is still widely used in India, despite toxicity, while the West uses chiefly sodium chloride, graphite, and copper types of powder and considers T.E.C. obsolete.^[33]
• Trimethoxyboroxine (TMB) liquid is a boron compound dissolved in methanol to give it proper fluidity and allow it to be discharged from a portable fire extinguisher. It was developed in the late 1950s by the U.S. Navy for use on magnesium fires, especially crashed aircraft and aircraft wheel fires from hard landings. It is unique as an extinguishing agent in that the agent itself is a flammable liquid. When TMB contacts the fire, the methanol ignites and burns with a greenish flame due to the boron. As the methanol burns off, a glassy coating of boric oxide is left on the surface of the metal, creating an air-excluding crust. These extinguishers were made by the Ansul Chemical Co. utilizing TMB agent manufactured by the Callery Chemical Company, and were modified 2.5-gallon water extinguishers (Ansul used re-branded Elkhart extinguishers at the time), with a variable-stream nozzle that could deliver a straight stream or spray at the squeeze of a lever. A 6-inch fluorescent orange band with the letters “TMB” stenciled in black identified TMB from other extinguishers. This agent was problematic in that it had a shelf life of only six months to a year once the extinguisher was filled, since the methanol is extremely hygroscopic (absorbs moisture from the air), which causes corrosion to the extinguisher and renders its use on fire dangerous. These extinguishers were used from the 1950s–1970s in various applications, such as the MB-1 and MB-5 crash trucks.^[34]

TMB was used experimentally by the US Air Force, specifically with regard to B-52 engine assemblies, and was tested in modified 10-gallon wheeled CBM extinguishers. Other agents were added to suppress the methanol flare up, such as chlorobromomethane (CBM), Halon 2402, and Halon 1211, with varied success. Halon 1211 was the most successful, and the combined TMB pressurized with halon 1211 and nitrogen was called Boralon was used experimentally by the Los Alamos National Laboratory for use on atomic metals, using sealed cylinder extinguishers made by Metalcraft and Graviner which eliminated the moisture contamination problem. TMB/Boralon was abandoned in favor of more versatile agents, though it is still mentioned in most US firefighting literature.^[35]

• Buffalo M-X liquid was a short-lived oil-based extinguishing agent for magnesium fires, made by Buffalo in the 1950s. It was discovered by the Germans in WWII that a heavy oil could be applied to burning magnesium chips to cool and smother them, and was easy to apply from a pressurized extinguisher, which was made by the German firm Total. After the war, the technology was more generally disseminated.^[36]

Buffalo marketed a 2.5-gallon and 1-quart extinguisher using M-X liquid discharged through a low-velocity shower head-type nozzle, but it was met with limited success, as it was going up against Ansul’s Met-L-X, which could be used on more types of metals and was non-combustible. M-X had the advantage of being easy to recharge and non-corrosive since it was oil-based, but production did not last long due to its limited applications.

• Some water-based suppressants may be used on certain class D fires, such as burning titanium and magnesium. Examples include the Fire Blockade and FireAde brands of suppressant.^[37] Some metals, such as elemental lithium, will react explosively with water so water-based chemicals are not used on such fires.

Most class D extinguishers will have a special low-velocity nozzle or discharge wand to gently apply the agent in large volumes to avoid disrupting any finely divided burning materials. Agents are also available in bulk and can be applied with a scoop or shovel.

• Note. “Pyromet” is a trade name that refers to two separate agents. Invented by Pyrene Co. Ltd. (UK) in the 1960s, it was originally a sodium chloride formulation with monoammonium phosphate, protein, clay and waterproofing agents. Modern Pyromet made by Chubb Fire is a graphite formulation.^[38]

• 

  Ansul Met-L-X 30lb. cartridge-operated sodium chloride dry powder

• 

  Amerex 30lb. Stored Pressure Sodium Chloride Class D Dry Powder, 1990s, US

• 

  Ansul Lith-X Cartridge-Operated Fire Extinguisher, graphite-base for lithium fires and other alkali metals

• 

  Ansul 30lb. Na-X cartridge-operated sodium carbonate fire extinguisher for sodium fires using non-corrosive agent.

• 

  A TMB extinguisher for magnesium fires

• 

  Buffalo fire extinguishers for magnesium fires using M-X liquid

• 

  Ternary Eutectic Chloride fire extinguisher for metal fires, UK.

Fire extinguishing ball

Several modern “ball” or grenade-style extinguishers are available on the market. The modern version of the ball is a hard foam shell, wrapped in fuses that lead to a small black powder charge within. The ball bursts shortly after contact with flame, dispersing a cloud of ABC dry chemical powder which extinguishes the fire. The coverage area is about 5 m² (54 sq ft). One benefit of this type is that it may be used for passive suppression. The ball can be placed in a fire-prone area and will deploy automatically if a fire develops, being triggered by heat. They may also be manually operated by rolling or tossing into a fire. Most modern extinguishers of this type are designed to make a loud noise upon deployment.^[39]

This technology is not new, however. From about 1880 glass “fire grenades” filled with a weak solution of common salt and ammonium chloride in water were popular. The addition of the salts was to prevent freezing, with ammonium chloride thought to be more effective in extinguishing flame. They were deployed by hurling them at the base of the fire. Containing only about one imperial pint (0.57 l) they were of limited use. Some later brands, such as Red Comet, were designed for passive operation and included a special holder with a spring-loaded trigger that would break the glass ball when a fusible link melted, or were sealed with wax to melt in contact with flame and release the contents. As was typical of this era, some glass extinguishers contained the toxic (but effective) carbon tetrachloride. These glass fire grenade bottles are sought by collectors.^[40][41]

Condensed aerosol fire suppression

Condensed aerosol fire suppression is a particle-based form of fire extinction similar to gaseous fire suppression or dry chemical fire extinction. As with gaseous fire suppressants, condensed aerosol suppressants use clean agents to suppress the fire. The agent can be delivered by means of mechanical operation, electric operation, or combined electro-mechanical operation. To the difference of gaseous suppressants, which emit only gas, and dry chemical extinguishers, which release powder-like particles of a large size (25–150 µm) condensed aerosols are defined by the National Fire Protection Association as releasing finely divided solid particles (generally <10 µm), usually in addition to gas.^[42]

Whereas dry chemical systems must be directly aimed at the flame, condensed aerosols are flooding agents and therefore effective regardless of the location and height of the fire. Wet chemical systems, such as the kind generally found in foam extinguishers, must, similarly to dry chemical systems, be sprayed directionally, onto the fire. Additionally, wet chemicals (such as potassium carbonate) are dissolved in water, whereas the agents used in condensed aerosols are microscopic solids.

Experimental techniques

In 2015, researchers from George Mason University announced that high volume sound with low bass frequencies in the 30 to 60 hertz range drives oxygen away from the combustion surface, extinguishing the fire, a principle was previously tested by the Defense Advanced Research Projects Agency (DARPA).^[43] One proposed application is to extinguish fires in outer space, with none of the clean-up required for mass-based systems.^[44]

Another proposed solution for fire extinguishers in space is a vacuum cleaner that extracts the combustible materials.^[45]

Maintenance

Most countries in the world require regular fire extinguisher maintenance by a competent person to operate safely and effectively, as part of fire safety legislation. Lack of maintenance can lead to an extinguisher not discharging when required, or rupturing when pressurized. Deaths have occurred, even in recent times, from corroded extinguishers exploding.

In the United States, state and local fire codes, as well as those established by federal agencies such as the Occupational Safety and Health Administration, are generally consistent with standards established by the National Fire Protection Association (NFPA).^[46] They commonly require, for fire extinguishers in all buildings other than single-family dwellings, inspections every 30 days to ensure the unit is pressurized and unobstructed (done by an employee of the facility) and an annual inspection and service by a qualified technician. Some jurisdictions require more frequent service. The servicer places a tag on the extinguisher to indicate the type of service performed (annual inspection, recharge, new fire extinguisher). Hydrostatic pressure testing for all types of extinguishers is also required, generally every five years for water and CO₂ models up to every 12 years for dry chemical models.

Recently the NFPA and ICC voted to allow for the elimination of the 30-day inspection requirement so long as the fire extinguisher is monitored electronically. According to NFPA, the system must provide record keeping in the form of an electronic event log at the control panel. The system must also constantly monitor an extinguisher’s physical presence, internal pressure and whether an obstruction exists that could prevent ready access. In the event that any of the above conditions are found, the system must send an alert to officials so they can immediately rectify the situation. Electronic monitoring can be wired or wireless.

In the UK, three types of maintenance are required:

• Basic service: All types of extinguisher require a basic inspection annually to check weight, externally validate the correct pressure, and find any signs of damage or corrosion. Cartridge extinguishers are to be opened up for internal inspection, and to have the weight of the cartridge tested. Labels must be inspected for legibility, and where possible, dip tubes, hoses and mechanisms must be tested for clear, free operation.
• Extended service: Water, wet chemical, foam, and powder extinguishers require a more detailed examination every five years, including a test discharge and recharge. On stored pressure extinguishers, this is the only opportunity to internally inspect for damage/corrosion.
• Overhaul: CO₂ extinguishers, due to their high operating pressure, are subject to pressure vessel safety legislation, and must be hydraulic pressure tested, inspected internally and externally, and date stamped every 10 years. As it cannot be pressure tested, a new valve is also fitted. If any part of the extinguisher is replaced with a part from another manufacturer, then the extinguisher will lose its fire rating.