The foam in a fire extinguisher is a mixture of water, air, and a chemical concentrate that produces a thick, bubbly blanket designed to smother fires. The concentrate typically contains surfactants (soap-like compounds that lower water’s surface tension) and stabilizers that help the bubbles hold together long enough to cut off a fire’s oxygen supply. What makes firefighting foam unique compared to water alone is its ability to float on top of burning liquids, seal in flammable vapors, and prevent a fire from reigniting.
How Foam Puts Out a Fire
Foam attacks fire in three ways at once. First, the blanket of bubbles spreads across the surface of the burning material and physically blocks oxygen from reaching the flames. Second, the water content in the foam cools the fuel and surrounding area. Third, the foam layer acts as a vapor seal, trapping flammable gases beneath it so the fire can’t reflash after the flames go out.
This combination is why foam is especially effective on liquid fuel fires, like gasoline or jet fuel spills. Water alone would sink beneath these fuels or scatter them, potentially spreading the fire. Foam floats on top, forming a lid that stays in place.
What the Concentrate Contains
The specific chemistry depends on the type of foam, but most formulations fall into a few categories.
AFFF (Aqueous Film-Forming Foam) is the most well-known type. It combines hydrocarbon foaming agents with fluorinated surfactants. When mixed with water and discharged, the fluorinated compounds cause the foam solution to spread into an ultra-thin film across the fuel surface. This film moves quickly, sealing off vapors even ahead of the visible foam blanket. AFFF is rated for both Class A fires (ordinary combustibles like wood and paper) and Class B fires (flammable liquids).
Protein-based foams take a different approach. These older formulations use hydrolyzed animal protein (essentially processed keratin from hooves, horns, or feathers) combined with stabilizers and preservatives. They create a thicker, more viscous blanket that relies on strong bubble structure to smother the fire rather than forming a thin chemical film. They’re slower to spread but can be more heat-resistant.
Alcohol-resistant foams (AR-AFFF) are designed for fires involving polar solvents, liquids like methanol or acetone that would dissolve a standard foam blanket on contact. When AR-AFFF hits a polar solvent, it forms a polymeric membrane, a tough, gel-like barrier that resists being broken down by the fuel. On conventional hydrocarbon fuels like gasoline, it behaves like regular AFFF and forms the standard aqueous film.
Low, Medium, and High Expansion
Foam is also categorized by how much it expands when mixed with air. This expansion ratio determines where and how it’s used.
- Low expansion has a ratio under 20:1, meaning the foam volume is less than 20 times the volume of the original solution. It produces a dense, wet blanket ideal for large outdoor liquid fuel fires at refineries, airports, and storage facilities.
- Medium expansion ranges from 20:1 to 200:1. Municipal fire departments commonly use it for smaller incidents like traffic accidents, solvent spills, or chemical containment.
- High expansion exceeds 200:1, producing enormous volumes of lightweight, airy foam. It’s used to flood enclosed spaces like warehouses, ship engine rooms, and aircraft hangars where the goal is to fill the entire volume and displace oxygen.
The PFAS Problem
For decades, AFFF was considered the gold standard for fighting fuel fires. But the fluorinated surfactants that make it so effective belong to a chemical family called PFAS (per- and polyfluoroalkyl substances), sometimes called “forever chemicals” because they don’t break down naturally in the environment.
The two most studied PFAS compounds in firefighting foam are PFOA and PFOS. These chemicals are non-biodegradable, accumulate in the body, and have been linked to serious health problems. The International Agency for Research on Cancer has classified PFOA as a possible human carcinogen for kidney and testicular cancers. Studies have also found associations with elevated cholesterol, breast cancer, and effects on the liver and kidneys. Once in your body, PFOA takes 2 to 10 years to drop to half its concentration. PFOS can linger even longer, with an estimated half-life of 3 to 27 years.
Firefighters who have regularly used AFFF show elevated blood levels of these compounds. The environmental damage is also significant. When AFFF is discharged during training exercises or emergencies, the chemicals can seep into soil, contaminate groundwater, and reach drinking water supplies. Military bases and airports where AFFF was used extensively for decades are now among the most contaminated sites.
The Shift to Fluorine-Free Foam
Regulatory pressure is pushing the firefighting industry away from PFAS-based foams. In December 2022, Congress directed the FAA to prepare a transition plan for moving airport firefighting operations to fluorine-free foam (F3). In January 2023, the Department of Defense published a new military specification for F3 products, and manufacturers can now submit fluorine-free agents for qualification testing.
Fluorine-free foams work more like the older protein-based foams. They rely on the physical structure of the foam blanket to contain fuel vapors rather than producing a chemical film on the fuel surface. Early F3 products were considered less effective than AFFF on certain fuel fires, but formulations have improved significantly. Once a foam meets the new military specification and lands on the Department of Defense’s Qualified Product List, the FAA considers it acceptable for use at certified airports.
Cleaning PFAS out of existing fire equipment is itself a complex process. Washington State’s Department of Ecology, for example, recommends multiple rounds of hot water rinsing (110 to 160 degrees Fahrenheit) with glycol-based cleaning detergents, with each rinse sitting in the system for at least 30 minutes. The rinse water must be collected, tested for halogenated organic compounds, and disposed of as hazardous waste if contamination exceeds 100 parts per million. None of it can go down a drain, into a septic system, or onto the ground.
Where Foam Extinguishers Work Best
Portable foam fire extinguishers are rated for Class A and Class B fires. Their real advantage over other extinguisher types shows up on flammable liquid fires with some depth, like a fuel spill in a container, because the foam floats on the surface and prevents reignition. Foam agents are also used in Class K extinguishers designed for commercial kitchen cooking oil fires, where the agent forms a blanket that seals the oil’s surface.
Foam extinguishers have a few limitations. They are not suitable for use in freezing temperatures, since the water content can freeze and render the unit inoperable. They should not be used on electrical fires, because the water in the foam conducts electricity. And they’re not designed for fires involving combustible metals, which require specialized dry powder agents.
Properly stored foam concentrate has a long shelf life. Manufacturers have demonstrated effective firefighting performance from concentrates stored over 10 years in original packaging under proper conditions, kept away from direct sunlight and extreme temperatures.