Published: · 15 min read · Reviewed by Ertuğrul Öz, Certified Fire Chief & Training Specialist
Foam is one of the most misused tools in the fire service. Not because firefighters do not care about getting it right — they do — but because foam equipment is rarely trained on seriously, the chemistry changes faster than department SOGs do, and the consequences of getting concentration wrong are invisible until the fire tells you. I have seen AFFF applied to a Class A fire because no one changed the around-the-pump proportioner setting from the last drill. I have seen Class A foam mixed into Class B containers because the cans looked similar. And I have been at petroleum fires where the eductor was three feet closer to the nozzle than the manufacturer specification allows, which means the whole operation was running at wrong concentration and nobody knew it. This piece covers what foam actually does, how to get concentration right, what is changing with AFFF, and where departments reliably get this wrong.
What we cover:
- Class A foam: what it actually does and when to use it
- Class B foam: types, rates, and petroleum fire application
- Proportioning systems: eductors, injection, CAFS
- AFFF phase-out and fluorine-free alternatives
- Foam application technique — what actually works
- The mistakes departments reliably make
- Foam training that sticks
- Pre-incident foam checklist
Class A Foam: What It Actually Does
Class A foam is a surfactant solution — it lowers the surface tension of water. That is the whole mechanism. Water without foam has a surface tension that makes it bead up and run off the surface of most solid fuels. A 0.1% Class A foam solution penetrates wood, insulation, and other cellulosic materials dramatically better than plain water because the surface tension drop lets the water absorb into the fuel rather than rolling off it. The result is faster knockdown, better penetration into deep-seated fires (attic, walls, mattresses), and better overhaul because you wet the fuel mass, not just the surface.
What Class A foam is not
Class A foam is not a vapor suppression agent. It does not form a vapor-excluding blanket the way Class B foam does. It does not work on flammable liquid fires unless that liquid is something like a fuel-soaked wood pile where the wood is the real fuel. Applying Class A foam to a petroleum product fire produces foam bubbles that the fuel immediately destroys — you get wet, useless froth and no suppression. I have seen this happen at incidents where the apparatus had Class A loaded and the crew did not know which fire required which agent. The foam label matters.
Concentration rates for Class A
Most Class A foam concentrates are designed to work at 0.1% to 1.0% solution. The right concentration depends on the application:
| Application | Concentration | Why This Rate |
|---|---|---|
| Structural attack (wet foam) | 0.1 – 0.2% | Maximum penetration, closest to water behavior, best knockdown |
| Exposure protection / pre-wetting | 0.3 – 0.5% | Thicker foam for longer retention on exposed surfaces |
| Dry foam / CAFS for overhaul | 0.5 – 1.0% | Stiff foam for injection into walls and voids, stays put longer |
| Wildland / brush suppression | 0.1 – 0.3% | Penetration into fuel bed, quick knockdown of running surface fire |
The critical point here is that more is not better. A 1% Class A solution on a structural attack line does not perform better than a 0.2% solution — it costs more concentrate, produces thicker foam that is harder to control, and reduces visibility without improving knockdown. Set your proportioner to match the application, not to maximize foam production.
Class B Foam: Types, Rates, and Petroleum Fire Application
Class B foam is a completely different chemistry doing a completely different job. Where Class A lowers surface tension to improve water penetration, Class B works by forming a physical barrier between the fuel and the atmosphere. No fuel vapor contact with oxygen, no combustion. The foam blanket is what does the work — which means how you apply it is as important as what you apply.
AFFF (Aqueous Film-Forming Foam)
AFFF has been the standard Class B agent in the U.S. fire service for decades. It works by forming both an aqueous film that floats on the fuel surface and a foam blanket above it — double suppression mechanism. It is highly effective on hydrocarbon fuels (gasoline, diesel, jet fuel, heating oil). It is the fastest-knockdown Class B agent available. It is also a source of PFAS (per- and polyfluoroalkyl substances) — the chemicals now subject to regulatory restriction and phase-out. More on that shortly.
AFFF-AR (Alcohol Resistant)
Standard AFFF is destroyed by alcohol and polar solvents (ethanol, methanol, acetone, many industrial solvents). When AFFF contacts an alcohol fuel, the alcohol immediately breaks down the foam blanket. For alcohol and polar solvent fires you need AFFF-AR, which contains a polymer that forms a protective membrane between the foam and the fuel. AFFF-AR is typically used at 3% on hydrocarbons and 6% on polar solvents — check the specific product data sheet because rates vary. Using standard AFFF on an ethanol fuel fire is like using water on a grease fire — it actively makes the situation worse.
Protein and fluoroprotein foams
Protein-based foams (protein, fluoroprotein, film-forming fluoroprotein/FFFP) are less common in structural fire service but still used in industrial and airport firefighting. They are generally slower to knock down than AFFF but more resistant to fuel pickup (the foam blanket survives re-contamination better). Airport crash fire rescue (CFR) operations still use FFFP extensively. If your department has mutual aid agreements with airports, know what they are running.
Concentration rates for Class B
| Agent | Fuel Type | Rate | Notes |
|---|---|---|---|
| AFFF 3% | Hydrocarbons (gasoline, diesel, jet fuel) | 3% | Standard rate; some AFFF formulations are 6% — verify label |
| AFFF 6% | Hydrocarbons (older formulations) | 6% | Do not use at 3% — under-concentration fails suppression |
| AFFF-AR 3×3 | Hydrocarbons at 3%; polar solvents at 3% | 3% | Newer formulations work at same rate on both fuel types |
| AFFF-AR 3×6 | Hydrocarbons at 3%; polar solvents at 6% | 3% or 6% | Proportioner must be switchable between rates |
| F3 / FFF (fluorine-free) | Hydrocarbons; check sheet for polars | 3% or 6% | Rates vary significantly by brand — do not assume |
The rate printed on the foam concentrate container is not a suggestion. Running AFFF at 1.5% because your proportioner is set wrong or your eductor is out of spec produces foam that fails. The water film does not form properly. The blanket is thin and fragile. You get brief knockdown followed by re-ignition that looks like the fire is winning. Because it is.
Proportioning Systems: How Foam Gets Mixed
Getting the right amount of foam concentrate into the water stream is where most operational failures occur. There are three main proportioning systems in the fire service, and each has specific limitations that create specific failure modes.
The inline eductor
The inline (in-line) eductor is the most common portable proportioning method. It works on the Venturi principle: water flowing through a constriction creates a low-pressure zone that draws foam concentrate from a container through a pickup tube. Simple, no moving parts, no power required. Also the most failure-prone in field conditions.
The distance problem: Every eductor has a maximum rated distance between the eductor and the nozzle. For most common eductors (Elkhart, Task Force Tips, Akron), this is 200 feet of 1¾-inch hose. Beyond 200 feet, the back pressure at the eductor exceeds the concentrate-drawing capability and the system under-proportions — you get less foam than you think. This is the single most common eductor mistake in field operations. The nozzle ends up 250 feet from the eductor because the hose was already laid and nobody measured. The foam looks fine. The concentration is wrong.
The elevation problem: Eductors are also rated for a maximum elevation change between the eductor and the nozzle. Typically 10 feet. A nozzle operator working on a second floor 15 feet above the eductor position is operating outside spec. Again, the foam looks normal. The concentration is wrong.
The pickup tube depth problem: The foam concentrate pickup tube must be fully submerged in the concentrate container. If the container is nearly empty and the pickup tube is only partially submerged, you get air entrainment and variable concentration. Maintain concentrate level above the pickup tube bottom at all times during operations.
Around-the-pump proportioner
The around-the-pump (ATP) proportioner is a fixed metering system that bypasses a small amount of discharge water back through a venturi that draws concentrate from an on-board tank. It mixes concentrate into the pump suction, so all discharge outlets run foam at the same time. Consistent concentration, no field measurement required, works at any hose length.
The problem: the metering valve on most ATP systems is set manually, often during apparatus spec, and rarely rechecked. I have personally found apparatus where the ATP was set to 3% Class B from a previous use and the crew was running Class A structural operations — at 3% concentration instead of 0.2%. The foam production looks roughly similar. The cost in concentrate, the foam behavior on the fire, and the cleanup afterward are all wrong. Check your ATP setting every single time you transition between agents or between Class A and Class B operations. Make it part of your morning apparatus check. Write the current agent and setting on a piece of tape on the proportioner valve.
Compressed Air Foam Systems (CAFS)
CAFS injects compressed air into the foam solution before it exits the nozzle, producing foam with a much higher air-to-water ratio than an eductor or ATP system. The result is a stiff, shaving-cream consistency foam that clings to surfaces, penetrates voids more effectively, and uses dramatically less water per square foot of coverage than conventional foam or plain water.
CAFS advantages in structural firefighting: faster knockdown on room contents fires, excellent deep-seated penetration for overhaul, reduced water use (important for rural tanker operations), and lighter hose loads for nozzle crews. A charged 1¾-inch CAFS hose weighs roughly 40% less per foot than a conventionally charged line because you are delivering mostly air.
CAFS limitations: the system requires more complex apparatus maintenance, the air compressor adds a mechanical system that can fail, the nozzle technique is different from conventional handlines (CAFS streams do not reach as far; application technique is more sweeping and close-range), and Class B CAFS use requires specific agent compatibility verification. Not all Class B concentrates work in CAFS systems — the air injection can break down the foam too quickly before application.
