The fire service uses a variety of foam products, depending on the situation. The two most common categories of foam used are Class A and Class B. There appears to be some confusion as to when to use each one.

Class A Foam

Class A foam is designed to be used on Class A fires. The National Fire Protection Association (NFPA) states, “A Class A fire is a fire involving ordinary combustible materials, such as wood, cloth, paper, rubber, and many plastics. They burn with an ember and leave an ash. Extinguishment occurs by cooling the fuel to a temperature that is below the ignition temperature.” Class A fires are what we are dispatched to a vast majority of the time and include structure, dumpster, tire, prairie, and auto fires.

Water has historically been the fire service’s cooling agent of choice, but we can improve on that by adding Class A foam. On average, Class A foam is mixed with water at a ratio of 0.5% foam to 99.5% water. The National Institute of Standards and Technology (NIST) estimates that water treated with Class A foam will wet Class A combustibles up to 20 times more rapidly than water alone and states, “Water treated with a Class A foam is between three and five times more effective at fire extinguishment than water alone,” because the Class A foam is attracted to the carbon in the ash/by-products of combustion. So, while water runs off surfaces, the foam bubbles are attracted to and stick to the surfaces, which absorb heat much better. Less heat means less fire.

A company at a garage fire applies Class A foam for initial knockdown and overhaul.

(1) A company at a garage fire applies Class A foam for initial knockdown and overhaul. (Photos by Tim Olk.)

When used for initial attack, Class A foam knocks down the fire faster and limits extension and flashover. The water in the foam bubbles coming out of the hose absorbs heat much better than straight water alone. These same principles apply at auto fires. Class A foam decreases the amount of time and water needed to extinguish the fire and decreases the chance of a rekindle.

Throw Back to Basics: Foam Operations

Fire Apparatus Training: Basic Foam Operations


When used for exposure protection, Class A foam sticks to the surface of the exposures, which allows it to absorb heat transfer. In addition, the white Class A foam reflects radiant heat, which decreases the chances of an exposure catching fire.

Class A foam is better than water during attack, exposure protection, and auto fires, but it really shines during overhaul. Studies show that overhaul is one of the most dangerous times for firefighters. The by-products of combustion and incomplete combustion are highly toxic. To improve this toxic environment, we need to stop the burning process. While water runs off the burned material, the watery foam is attracted to and sticks to the burned material/ash. This means the watery foam stays there and absorbs heat. In addition, the chemical properties of the foam decrease the surface tension of the burned product, thus allowing the watery foam to penetrate deep into the burned product and absorb heat. These two actions stop the burning process, which means less toxic smoke plus decreased chances of a rekindle.

If your department has engines equipped with a Class A foam system, use it. Applying foam does not change your tactics, nor does it require any additional specialized equipment. When leading out at a structure fire, auto fire, dumpster fire, or tire fire, ask your engineer to “send the foam.” The engineer will activate the foam system by pressing a button on the pump panel, and you will have foam.

Some foam systems deliver a single preset foam concentration when activated (for our department, it is preset at 0.5%). A foam proportioning rate of 0.5% is a good general-purpose foam concentration, but if you are not achieving the desired results, the pump operator can easily adjust the proportioning rate with the control module on the pump panel.

Other Class A foam systems have multiple preset buttons that provide for specific proportioning rates for specific incidents; the newest engines in my department have individual presets for structure fires (0.5%), auto fires (0.3%), exposure protection (1.0%), and overhaul (0.2%). Remember, don’t wash the foam off; leave it on the burned material so it can continue to work during overhaul and after you leave the scene.

We need to put fires out as fast as possible and get out of the building. Class A foam will put out fires faster, which stops building degradation and stops combustion while reducing the hazardous gases of combustion. Class A foam makes the firefighters’ working environment safer. If you have Class A foam, use it.

Class B Foam

Class B foam is for Class B fires, which are flammable/combustible liquid fires. The NFPA states, “Flammable liquids burn at normal room temperatures and combustible liquids need to be heated above normal room temperatures before they can burn.” Class B fires involve gasoline, diesel fuel, petroleum greases, tars, oils, oil-based paints, solvents, lacquers, alcohols, ethers, and the like. These products are high fire hazards that water will likely not extinguish. That’s where Class B foam comes in. Extinguishment occurs by creating a barrier between the fuel and the oxygen, the foam blanket.

The Class B foam used on our engines is alcohol resistant-aqueous film forming foam (AR-AFFF). The film blankets the fuel surface, which separates the product from oxygen, thus smothering the fire while separating the flames/ignition source from the fuel surface. In addition, the film suppresses the release of flammable vapors that can mix with air. The water within the foam cools the fuel and any adjacent metal surfaces, thus decreasing the chances of reignition if the foam blanket is disrupted.

Class B foam is “rolled” onto the spilled product. The foam produces a film over the product while the water within the foam cools the fuel.

(2) Class B foam is “rolled” onto the spilled product. The foam produces a film over the product while the water within the foam cools the fuel.

 A common Class B foam system: the buckets of foam concentrate ready to be used, the eductor in one of the buckets, and an air-aspirating nozzle on the end of the hoseline.

(3) A common Class B foam system: the buckets of foam concentrate ready to be used, the eductor in one of the buckets, and an air-aspirating nozzle on the end of the hoseline.

We use AR-AFFF on our engines because it is more practical. Although regular AFFF works well on some flammable liquids, it does not work well on those with an alcohol component. Many petroleum fuels today are blended with 10%, 15%, or greater alcohol concentrations. The nonalcohol resistant AFFF could be broken down by the alcohol even when the foam is used at 6% concentrations. Stock your fire apparatus with AR-AFFF, which can be used on all Class B fires and provides the safety and security desired.

Applying Class B foam is a bit complex and requires training. First, make sure you have the proper quantity of Class B foam concentrate on scene to cover the entire spill. Failing to cover the entire spill during the initial application could lead to burn back, which would consume the foam you already applied, creating an unsafe environment and wasting the foam.

Determining the quantity of foam concentrate needed can be a mathematical challenge. Here is a quick method I learned: First, determine the area of the spill (the length × width). Then divide the total area by 20 for hydrocarbon (3%) spills or divide by 5 for polar solvent (6%) spills. For example, a diesel spill fire is 10 feet × 30 feet = 300-square-foot area, divided by 20 (since diesel is a hydrocarbon) = 15 gallons of foam concentrate needed. Follow the application instructions noted with the Class B foam product you purchase; manufacturers may recommend different application concentrations based on the product that is spilled.

After you have determined you have the proper amount of foam concentrate on scene, set up the foam system. Follow your department’s standard operating procedures (SOPs) when assembling your eductor and laying out your hoselines and nozzles. Be sure your eductor is set to the correct finished foam concentration so your previous foam concentrate calculation matches your flow. Once your foam blanket is down, it is important to leave it undisturbed. Breaking a foam blanket could lead to reignition and burn back of the entire foam blanket, which would waste the foam and make the scene unsafe.

This crash rig is applying Class B foam at a tanker spill. Is the foam being applied appropriate for the spilled product? The Class B foam carried on many crash rigs is not alcohol resistant.

(4) This crash rig is applying Class B foam at a tanker spill. Is the foam being applied appropriate for the spilled product? The Class B foam carried on many crash rigs is not alcohol resistant.

AFFF (including AR-AFFF) is not without controversy. There are concerns that the fluorinated surfactants (PFAS=Per- and polyfluoroalkyl substances) used to make AFFF are an environmental hazard and a health hazard. PFAS are known as “forever chemicals” because they never break down. PFAS harm the environment and build up in our bodies, where they can stay for decades. PFAS chemical exposure is associated with cancer and harm to the reproductive, developmental, and immune systems. The federal government decided to phase out PFAS AFFF in the military by the year 2024. Many countries throughout the world and many U.S. states have already banned PFAS AFFF. There are PFAS-free/nonfluorinated AFFF and AR-AFFF options available. Make sure your department is proactively phasing out the old AFFF and replacing it with safer PFAS-free/nonfluorinated AFFF and AR-AFFF.

It is important to note that the nonfluorinated foam products available today are not a direct replacement for the AFFF products we presently use. The nonfluorinated foams do not function exactly like present-day AFFF. Plus, the nonfluorinated foam products may require higher flow rates, different discharge devices, and different application methods. Phasing out the old foam will take time, so protect yourself in the interim. Wear proper personal protective equipment when using AFFF. Do not use AFFF for random training. Use it correctly to limit your exposure. Avoid all contact with the foam product. Perform proper decon of your gear and yourself after working with AFFF to remove the product.

Use the Right Foam

Some may confuse Class A foam and Class B AFFF/AR-AFFF. Firefighters have failed to discern between Class A foam concentrate and Class B AFFF foam concentrate canisters and mixed them or used them interchangeably. These products are very different and incompatible with each other. The Class A foam system on your engines may be severely damaged if you add Class B AFFF to it. Mixing the different foam concentrates may cause them to congeal/gel and clog the foam tank, pump, and piping. Often, the only way to fix this problem is to remove and clean the entire system, a costly and time-consuming job.

Remember, if the item on fire is three-dimensional, meaning you can walk up and touch a solid object, it is likely a Class A foam fire. Have your engineer/pump operator send the Class A foam. If the product on fire is a liquid in a puddle, drum, tote, or tanker, it is likely a Class B fire. For most of us, Class B foam applications require eductors. Set up your system following your department’s Class B foam SOP.

Over the years, I have seen aircraft rescue and firefighting (ARFF) crash rigs brought in to apply Class B foam at large-scale flammable liquid incidents. This seems like a great idea, but it is vitally important to note that the foam being applied may not be providing the appropriate suppression and safety properties needed for the spilled flammable liquid. Currently, all “major” airports (certified Part 139 airports) are required to use foams that meet military specifications MIL-PRF-24385. When I look at the list of the AFFF foams that meet the military specifications, none of them are AR-AFFF.

Since the Class B Foam on crash rigs is not alcohol resistant, it is likely not the appropriate Class B foam product for most fuel spills we encounter, since more than 98% of U.S. gasoline contains up to 10% ethanol and blended gasoline products contain anywhere from 10% to 83% ethanol. In addition, the Class B foam on crash rigs is absolutely not appropriate for use on any polar solvents like esters, ethers, acetones, alcohols, aldehydes, ketones, and the like. As you can see, in a vast majority of the cases, the foam carried on many crash rigs is not appropriate for the types of spills we encounter on the streets.

If a crash rig is called out to apply foam at an acetone spill, it will not create the vapor suppressing foam blanket that is expected. Sure, you will see foam on the ground, but it is not doing what it needs to do to suppress the fire or vapor cloud. The acetone will likely break down the foam blanket, which would allow the product to off-gas. The vapors could then flow along the surface to distant ignition sources and explode or cause a flash fire. This mistake can have catastrophic consequences.

Do not become complacent. If you have had an incident in which you used a crash rig to apply foam, don’t assume “it worked.” You may have just been lucky and your product did not find an ignition source. It is vitally important to know the product that is spilled. It is equally important to know the type of foam carried on the ARFF crash rigs you use. Finally, it is important to know where you can access the correct foam needed for the product spilled.


NFPA. Reporters Guide: All about Fires:

FAA Ethanol Safety Document; Special Airworthiness Information Bulletin:

Foams meeting military specifications (MIL-PRF-24385), listed on the Navy’s Quality Product Database (QPD) Web site:

Evaluation of the fire protection effectiveness of fluorine-free firefighting foams:

Patrick L. Brown is a lieutenant in the Chicago (IL) Fire Department (CFD). He is an engineer instructor at Chicago’s Quinn Fire Academy; a former engineer of CFD Engine 117; and a state certified fire apparatus engineer, fire officer, incident safety officer, instructor 2, and HAZMAT Tech B.