Fire foam is a firefighting agent made by mixing water, air, and a chemical concentrate to create a blanket of bubbles that smothers flames. It works where water alone falls short, particularly on liquid fuel fires like gasoline or jet fuel, where water would simply sink beneath the burning fuel and do nothing. Fire foam is also used on wildfires and structure fires to help water soak deeper into burning materials.
How Fire Foam Puts Out Fires
Foam suppresses fire through three overlapping mechanisms: smothering, cooling, and vapor suppression. When applied to a burning liquid, the foam spreads across the surface and forms a physical barrier that blocks flammable vapors from escaping into the air above. Without those vapors feeding the flames, the fire dies. The foam blanket also prevents the fire’s heat from radiating back down into the fuel, cutting off the cycle that keeps a liquid fire burning.
Cooling plays a bigger role than many people realize. When room-temperature foam contacts a hot fuel surface, it drops the temperature almost immediately. In laboratory models using heptane (a common test fuel), the surface temperature fell from near boiling at 98.6°C down to 54°C within seconds of foam application. That temperature drop alone cut the fuel’s vapor pressure by 75%, meaning far less flammable gas was escaping into the air. The foam also insulates the fuel from the flame above, letting the hot upper layer of fuel lose heat downward into the cooler liquid below.
Class A vs. Class B Foam
Not all fire foam is the same. The two main categories target very different kinds of fires.
Class A foam is designed for ordinary combustibles: wood, paper, brush, and structural materials. It works like a supercharged version of water. The concentrate contains a detergent-like surfactant that makes water “wetter,” allowing it to penetrate deeper into porous materials instead of running off the surface. This is why Class A foam has become standard equipment for wildfire crews and structural firefighters. It’s mixed at very low concentrations, typically between 0.1% and 1% foam concentrate to water depending on the situation. A crew fighting a brush fire might use just 0.1%, while an attack on a structure fire typically calls for 0.3%.
Class B foam is built for flammable liquid fires: petroleum, jet fuel, gasoline, and industrial solvents. These fires can’t be fought effectively with plain water because burning liquids spread when water hits them. Class B foam floats on top of the fuel and seals the surface. It’s mixed at higher concentrations, usually 1% to 3% for hydrocarbon fuels like gasoline, and 3% for polar solvents like alcohol-based fuels. The exact ratio depends on the fuel type and the foam manufacturer’s guidance.
What’s Inside the Concentrate
Fire foam concentrate is mostly water combined with surfactants (compounds that reduce surface tension so the foam spreads easily), organic solvents that improve the foam’s resistance to fuel breakdown, and stabilizers that keep the bubble structure intact long enough to do its job. A typical formulation includes both hydrocarbon surfactants, which help create the foam’s structure, and sometimes fluorocarbon surfactants, which give the foam its ability to spread across liquid fuels.
The organic solvent in many formulations serves a practical purpose: it makes the foam harder to destroy when it contacts fuel. Testing has shown that adding this solvent increases the time before the foam ignites by a factor of 1.2 to 3.7, depending on the surfactant combination. That extra durability matters when foam is sitting directly on top of a burning fuel spill.
AFFF and the PFAS Problem
The most effective Class B foam historically has been aqueous film-forming foam, or AFFF. Developed in the early 1960s by 3M and the U.S. Navy, AFFF contains fluorocarbon surfactants that do something remarkable: they lower the surface tension of the foam’s water layer so much that it spreads into a thin film on top of hydrocarbon fuels, even though water is denser than the fuel. This aqueous film acts as a vapor seal, preventing reignition even after the foam bubbles break down.
The problem is what makes that film possible. AFFF relies on per- and polyfluoroalkyl substances, commonly called PFAS or “forever chemicals,” because they essentially never break down in the environment. The two most studied PFAS compounds in foam, PFOA and PFOS, have been linked to serious health effects. Elevated exposure is associated with kidney and testicular cancers, with one meta-analysis finding that every 10 nanograms per milliliter increase in blood PFOA levels raises kidney cancer risk by 16% and testicular cancer risk by 3%. Research has also connected PFAS exposure to elevated cholesterol, liver damage, hormonal disruption, weakened immune function, low fetal birth weight, and increased cancer risk including breast cancer, prostate cancer, and non-Hodgkin’s lymphoma.
Firefighters face particular risk because they handle these foams directly. PFOA and PFOS have been banned or heavily regulated in many countries, and manufacturers have shifted toward shorter-chain PFAS alternatives or fully fluorine-free formulations.
Fluorine-Free Foam Alternatives
The firefighting industry is actively transitioning to fluorine-free foams, often called F3. These formulations replace fluorinated surfactants with plant-based or synthetic alternatives that don’t produce persistent environmental contamination. The U.S. Department of Defense, one of the world’s largest users of firefighting foam, has published military specifications for F3 products and is working to phase out legacy AFFF across its installations.
The trade-off, for now, is performance. Current fluorine-free foams take roughly 1.5 to 2 times longer than traditional AFFF to extinguish the same fire at the same application rate. They also show weaker burnback resistance (how well the foam prevents a fire from reigniting) and less effective vapor suppression. The core challenge is that without fluorinated surfactants, the foam can’t form that thin aqueous film on top of liquid fuel. Instead, it relies entirely on the foam blanket itself and surface cooling to do the work.
The DoD has acknowledged that F3 performance will likely improve over time, following the same trajectory AFFF did over decades of refinement. Manufacturers are working to improve extinguishment speed, fuel resistance, and compatibility with saltwater and premixed storage systems.
How Foam Is Mixed and Applied
Fire foam doesn’t arrive ready to use. The concentrate must be proportioned into the water stream at the correct percentage, then aerated to create the bubble structure. This happens through specialized equipment on fire apparatus.
- In-line eductors use the water stream’s velocity to draw concentrate into the flow at preset rates, typically offering settings at 0.25%, 0.5%, 1%, 3%, and 6%.
- Direct injection systems use an electronic pump to meter concentrate directly into the discharge piping, allowing more precise control over the mix ratio.
- Compressed air foam systems (CAFS) inject compressed air into the foam solution inside the pump’s discharge piping, producing a uniform, shaving-cream-like foam that sticks to vertical surfaces and travels farther from the nozzle. Operators can adjust the air-to-water ratio to produce anything from a wet, fluid foam for direct fire attack to a dry, clingy foam for long-term exposure protection.
At the nozzle end, foam expansion tubes or aspirating nozzles mix additional air into the stream to create the final bubble structure. Without proper aspiration, the foam comes out as a thin, watery solution that breaks down quickly.
Environmental Containment After Use
Because of PFAS contamination concerns, foam runoff is now treated as a hazardous spill in many jurisdictions. On U.S. military installations, any use or accidental release of AFFF triggers spill response protocols. Crews are required to contain and recover the foam to the extent possible, prevent it from reaching storm drains or groundwater, and coordinate disposal with environmental staff. The discharge must be reported, and a follow-up describing cleanup actions is due within 45 days.
This is a significant shift from past decades, when foam runoff from training exercises and fire suppression regularly entered the ground uncontrolled. Contamination from those historical releases is now one of the largest sources of PFAS in drinking water near military bases and airports worldwide. The cleanup costs and health consequences of that legacy contamination are a major reason the industry is moving toward fluorine-free alternatives as quickly as the technology allows.