Fire retardants work by interrupting one or more of the three things a fire needs to sustain itself: heat, fuel, and oxygen. Some absorb heat and release water vapor. Others interfere with the chemical chain reactions that keep flames going. Still others form a physical barrier between the fire and whatever is burning. Most real-world fire protection uses a combination of these approaches, and the specific method depends on whether you’re talking about treated fabric, a coated steel beam, or the red slurry dropped from aircraft onto wildfires.
Cooling the Fire With Water Release
The simplest category of fire retardant works by absorbing enormous amounts of heat. Metal hydroxides, particularly aluminum trihydroxide and magnesium dihydroxide, are mixed into plastics, foams, and building materials during manufacturing. When temperatures climb, these minerals decompose and release water. That water turns to steam, which absorbs even more heat and dilutes the oxygen concentration around the material. The net effect is that the material stays cooler for longer and is harder to ignite.
Zinc borate works on a similar principle. Its crystal structure holds water molecules tightly until temperatures exceed about 260°C (500°F), at which point it releases that water in a burst of heat absorption. These types of retardants are popular partly because they’re simple and relatively nontoxic, though they need to be added in large quantities to be effective.
Breaking the Chemical Chain Reaction
Fire isn’t just heat and fuel. At the molecular level, combustion is a self-sustaining chain reaction driven by highly reactive fragments of molecules called free radicals. Hydrogen and hydroxyl radicals are the key players: they react with oxygen and fuel molecules in a cycle that releases energy and produces more radicals, which keeps the fire going.
Halogenated fire retardants, which contain bromine or chlorine, work by releasing halogen atoms into the gas phase when heated. These atoms react with the hydrogen radicals in the flame, pulling them out of the combustion cycle and converting them into stable hydrogen halide molecules. This directly competes with the chain-branching reactions that sustain the flame. Early mass spectrometry studies of methane flames showed that introducing halogen species into the combustion zone inhibited the formation of key intermediates and boosted hydrogen gas production, confirming that hydrogen atoms were being scavenged before they could feed the fire.
This gas-phase mechanism is extremely effective at small concentrations, which is why brominated retardants dominated consumer products for decades. But as we’ll see, that effectiveness came with significant health trade-offs.
Building a Protective Char Layer
Intumescent coatings take a completely different approach. Instead of cooling the fire or disrupting its chemistry, they create a physical shield between the heat source and the material underneath. These coatings contain three functional ingredients: an acid source, a carbon source, and a blowing agent.
When temperatures rise above about 250°C, the acid source (often ammonium polyphosphate) releases phosphoric acid. That acid reacts with a carbon-rich compound, stripping water molecules from it through dehydration and forming a tough, charred residue called carbonaceous char. At the same time, a blowing agent like melamine releases gases (ammonia, carbon dioxide, and water vapor) that puff the char layer up into a thick foam. The result is a low-density, porous insulating barrier with very low thermal conductivity. It acts like a heat shield, preventing the underlying material from reaching ignition temperature.
You’ll find intumescent coatings on structural steel in commercial buildings, where they can buy critical time before a beam loses its load-bearing strength. They’re also used in cable coatings, doors, and other fire-rated assemblies.
How Wildfire Retardants Work
The red slurry dropped from aircraft ahead of a wildfire is a long-term fire retardant, and its active ingredient is phosphate. The most widely used product line, Phos-Chek, has relied on phosphate since its introduction. Unlike water, which evaporates and loses its protective effect, phosphate works chemically with the vegetation it coats. It changes the way cellulose (the main structural molecule in plants) decomposes when heated, steering the breakdown toward char formation rather than the release of flammable gases. This renders the treated vegetation non-flammable even after the water in the mixture has dried.
Phosphate has been tested against dozens of alternative chemicals, including ammonium chloride, potassium carbonate, sodium silicate, and boric acid. It consistently outperforms them at preventing fire spread. The red color, by the way, is an iron oxide dye added so pilots and ground crews can see where retardant has already been applied.
Fire Retardants in Everyday Products
In the United States, upholstered furniture must comply with federal flammability requirements based on California’s Technical Bulletin 117-2013 standard. This test evaluates individual components of furniture, including cover fabric, filling material, barrier layers, and decking, against criteria for char length, smoldering, and transition to open flaming. The ignition source is a standard smoldering cigarette, reflecting the most common cause of furniture fires.
Notably, the regulation does not require or prohibit flame-retardant chemicals. Manufacturers can pass the tests using inherently resistant materials, barrier fabrics that wrap the foam filling, or chemical treatments. This flexibility was a deliberate shift from earlier standards that practically forced manufacturers to load foam cushions with chemical retardants.
For treated fabrics like workwear and military uniforms, one longstanding approach is the Proban process, which bonds a phosphorus-based polymer directly into cotton fibers. The treatment uses phosphonium salts that are pre-polymerized with urea and then grafted onto the cotton. When exposed to flame, the phosphorus promotes char formation in the cotton rather than allowing it to burn freely. The treatment survives repeated washing because the polymer is chemically bonded to the cellulose, not just sitting on the surface.
Aircraft Cabin Materials
Commercial aircraft interiors face some of the strictest flammability requirements of any consumer environment. Under FAA regulations, every material in crew and passenger compartments must meet specific flammability test criteria, from wall panels and ceiling linings to galley structures and stowage compartments. Seat cushions face additional testing beyond the baseline requirements. For planes carrying 20 or more passengers, large interior panels must also pass heat release rate and smoke density tests.
These layered requirements exist because an aircraft cabin is an enclosed space at altitude where evacuation options are extremely limited. The materials used in modern aircraft cabins, from thermoplastic panels to treated textiles, are engineered to resist ignition, limit heat output, and produce minimal toxic smoke.
Health Concerns With Older Retardants
The effectiveness of brominated flame retardants came with a serious downside. Polybrominated diphenyl ethers (PBDEs), widely used in electronics, furniture foam, and textiles from the 1970s onward, turned out to be persistent environmental contaminants that accumulate in human tissue. These are highly fat-soluble compounds that build up in lipid-rich tissues over time. In industrial production areas in China, specific PBDE compounds have been detected in both maternal and cord blood, meaning they cross the placental barrier.
Human exposure to PBDEs has been linked to disrupted glucose metabolism, altered thyroid and ovarian function, and potentially increased cancer risk. One well-studied compound, BDE-47, shows a linear association between blood concentration and type 2 diabetes risk. Mixtures of PBDEs have been associated with liver toxicity, including abnormal cell growth and fatty changes in liver tissue. Most commercial PBDE formulations have been phased out or banned, but because these chemicals persist in the environment and in older products still in use, human exposure continues.
A related concern involves PFAS-containing firefighting foams (known as AFFF), used for decades on fuel fires at military bases and airports. The Department of Defense was required to stop using AFFF at installations by October 2024, with waivers extending the deadline to October 2026 for some facilities. Shipboard use remains exempt. The transition to PFAS-free alternatives has been slow, partly because the fluorinated foams are exceptionally effective at smothering liquid fuel fires.
Bio-Based Alternatives
Newer fire retardant research is moving toward plant-derived and mineral compounds that perform well without the persistence and toxicity problems of older chemicals. Phytic acid (found in seeds and grains), chitosan (from crustacean shells), and lignin (from wood) are all being developed as fire retardant additives. In laboratory testing with thermoset resins, some of these formulations have cut peak heat release rates by 50 to 75%. A lignin-based additive reduced peak heat release in unsaturated polyester resin by up to 75%. Composites using cobalt alginate (derived from seaweed) achieved a 56% reduction. Modified hemp fiber combined with melamine cyanurate cut both peak heat release and total heat output by roughly 50%.
These numbers are promising but come from controlled lab testing on specific resin systems, and the path from laboratory composite to commercial product involves durability, cost, and processing challenges. Still, the direction is clear: fire protection is shifting toward chemistries that break down in the environment rather than accumulating in it.