Fire extinguisher foam is mostly water, mixed with a small percentage of foam concentrate that contains surfactants (soap-like chemicals), solvents, corrosion inhibitors, and stabilizers. The concentrate typically makes up 1% to 6% of the final mixture, with the rest being water and air. What varies between foam types is the specific surfactant chemistry, which determines whether the foam is designed for fuel fires, wood fires, or something else entirely.
The Basic Ingredients
Every firefighting foam starts with the same core recipe: water, a foaming agent, and air. The foaming agent is where the chemistry gets interesting. A typical foam concentrate contains several categories of ingredients working together.
Surfactants are the active ingredient. These are compounds that lower the surface tension of water, allowing it to spread across burning surfaces and smother flames instead of beading up and rolling off. Think of how dish soap lets water sheet across a greasy pan. Firefighting surfactants do something similar, but at extreme temperatures.
Solvents keep the concentrate stable in storage and help it mix properly with water. Glycol ethers are the most common, particularly a compound called butyl carbitol, which can make up anywhere from 5% to 50% of the concentrate depending on the formulation. Hexylene glycol also appears in many formulas at lower concentrations, typically under 10%. These solvents double as freeze-protection agents, keeping the concentrate liquid in cold storage.
Rounding out the formula are inorganic salts for stability, corrosion inhibitors to protect metal tanks and piping, and water as the base solvent.
Class A Foam vs. Class B Foam
The type of fire determines which foam chemistry gets used, and the two main categories work in fundamentally different ways.
Class A foam is designed for ordinary combustibles like wood, paper, and brush. It uses hydrocarbon-based surfactants, which are essentially souped-up soaps. The surfactant reduces water’s surface tension so it penetrates deep into organic fuels faster, soaking into the material rather than just sitting on top. This is why Class A foam is widely used in wildland firefighting. It turns water into a much more effective wetting agent, letting firefighters do more with less.
Class B foam is built for flammable liquid fires: gasoline, jet fuel, oil. These foams historically relied on fluorocarbon-based surfactants, which create a thin aqueous film that floats on top of the burning liquid and seals off its vapors. This filming action is what gives AFFF (aqueous film-forming foam) its name. The film spreads rapidly across the fuel surface, cutting off the oxygen supply and suppressing flammable vapor release. Class B foams are poor at wetting solid materials, and Class A foams can’t form a vapor-sealing film on liquid fuels. They’re not interchangeable.
The PFAS Problem
For decades, the fluorocarbon surfactants in Class B foams were per- and polyfluoroalkyl substances, commonly known as PFAS. These “forever chemicals” are extraordinarily effective at creating vapor-sealing films on burning fuels, which is why they became the standard for military and airport firefighting. A typical AFFF concentrate contains fluorotelomer sulfonates as its key active fluorinated ingredient.
The problem is that PFAS don’t break down in the environment. Every training exercise, every foam test, and every emergency deployment left persistent contamination in soil and groundwater. This has triggered a major regulatory shift. The U.S. Department of Defense was required to stop using PFAS-based AFFF at land-based military installations by October 2024, though waivers can extend some use through October 2026. Shipboard use remains exempt for now.
What Fluorine-Free Foams Use Instead
The replacement formulas, called fluorine-free foams or F3, draw on four main classes of ingredients to replicate what PFAS did without the environmental persistence.
- Hydrocarbon surfactants: Fatty acids, sugars, alcohols, polyethylene glycol, and natural thickeners like xanthan gum.
- Detergent-type surfactants: Various soap-like compounds in nonionic, anionic, or zwitterionic (dual-charge) forms that help the foam spread and cling to surfaces.
- Silicone surfactants: Siloxane-based compounds, including carbohydrate siloxanes, that mimic some of the surface-spreading properties of fluorinated chemicals.
- Protein-based agents: Derived from animal sources like horn, hoof, or silk, or from hydrolyzed protein. Protein foams have actually been around longer than synthetic foams and produce extremely stable, heat-resistant blankets.
The transition hasn’t been seamless. Some fluorine-free foams can’t handle extreme temperatures or be pre-mixed with water the way AFFF could, which creates compatibility issues with existing firefighting equipment. Military and industrial users are still working through these challenges.
How Expansion Ratio Changes the Foam
Beyond chemistry, foams are also categorized by how much air gets whipped into them. This is called the expansion ratio: the volume of finished foam compared to the volume of liquid solution that created it.
Low-expansion foam has a ratio of 1:1 to 20:1. It’s dense, wet, and heavy, which makes it ideal for flowing across burning liquid surfaces. This is the type you’d see sprayed directly onto a fuel spill.
Medium-expansion foam ranges from 20:1 to 200:1. It’s lighter and used to fill enclosed spaces or cover larger areas where you need a thicker blanket.
High-expansion foam reaches 200:1 to 1,000:1. At these ratios, the foam is mostly air with a thin skin of liquid. It’s used to flood entire rooms, warehouses, or ship holds, displacing oxygen and insulating surfaces. A small amount of concentrate can generate an enormous volume of foam.
The expansion ratio is controlled by the nozzle or generator, not the concentrate itself. The same foam solution can produce different expansion levels depending on how much air is mechanically mixed in during application.
What’s in a Handheld Extinguisher
The foam in a portable fire extinguisher you’d find in an office or kitchen uses the same basic principles but in a self-contained package. The canister holds a pre-mixed foam concentrate and water solution under pressure. When you squeeze the handle, the pressurized solution passes through an aerating nozzle that mixes in air, producing the foam blanket. Most portable foam extinguishers today use AFFF or fluorine-free surfactant blends rated for both Class A and Class B fires. The concentrate percentages and surfactant types mirror their larger industrial counterparts, just scaled down into a single unit.