Fire retardant refers to two very different things depending on the context: the red slurry dropped from aircraft during wildfires, and the chemicals mixed into everyday products like furniture, electronics, and building materials. Both work by slowing or stopping combustion, but they use different ingredients and different strategies to get there.
What’s in Wildfire Retardant
The red liquid dropped from tanker planes onto wildfires is mostly water mixed with a fertilizer salt called ammonium polyphosphate, typically making up 80 to 100 percent of the concentrate. When the water evaporates, this salt reacts with the cellulose in wood and vegetation to form a protective char layer on the surface. That char acts as a heat shield, slowing fire spread and reducing intensity even long after the water is gone. This is why it’s called a “long-term” retardant, as opposed to plain water drops that only work while things stay wet.
Beyond the active ingredient, wildfire retardant contains a few supporting components. Performance additives (5 to 10 percent of the mix) help the liquid stick to vegetation and reduce drift when released from altitude. Attapulgus clay (1 to 5 percent) thickens the mixture. The distinctive red or orange color comes from iron oxide pigment (1 to 5 percent), added so pilots and ground crews can see exactly where it landed. Colorless versions also exist.
Most formulations also include a corrosion inhibitor to protect aircraft tanks and storage equipment. The exact chemicals used are typically kept as trade secrets, but lab analysis has found elevated levels of chromium and cadmium in retardants prepared for aerial use. Both metals are widely used as aluminum corrosion inhibitors in the aircraft industry, which likely explains their presence.
How Wildfire Retardant Affects Waterways
When aerial retardant lands in or near streams, the ammonium polyphosphate breaks down and releases ammonium and phosphate ions into the water. The phosphate acts like agricultural fertilizer runoff, feeding algae blooms and contributing to oxygen depletion. The ammonia is more immediately dangerous to aquatic life. Un-ionized ammonia is highly toxic to fish: rainbow trout exposed in lab conditions showed lethal effects at concentrations as low as 0.125 milligrams per liter. Salmon smolts exposed to current retardant formulations have shown gill damage and lower survival rates. This is why pilots are generally instructed to avoid dropping retardant directly into waterways.
Flame Retardants in Furniture and Electronics
The flame retardants built into couches, mattresses, circuit boards, and plastic casings are a completely different category. These are chemicals mixed directly into materials during manufacturing, designed to make those materials harder to ignite or slower to burn. The three main families are brominated compounds, organophosphorus compounds, and inorganic minerals.
Starting in the 1970s, brominated flame retardants became the industry standard. Polybrominated diphenyl ethers (PBDEs) were added to furniture foam and electronics casings. Circuit boards got tetrabromobisphenol A. Polystyrene building insulation used hexabromocyclododecane. These chemicals worked well at preventing ignition, but they turned out to be persistent in the environment and accumulate in human tissue. Health concerns led to regulatory action: the EPA prohibited the manufacture, import, and distribution of decabromodiphenyl ether (one of the most common PBDEs) in 2021.
As brominated retardants were phased out, organophosphorus flame retardants took their place. These are now among the most common flame retardant chemicals found in indoor environments. The compounds detected most frequently in household air and dust include triphenyl phosphate, tris(butoxyethyl) phosphate, tris(2-chloroethyl) phosphate, and tris(2-chloroisopropyl) phosphate. They come in three subtypes: chlorinated, alkyl-based, and aryl-based. The EPA has also started restricting some of these. Phenol, isopropylated phosphate (PIP 3:1), a widely used organophosphorus retardant, is being phased out, with distribution of products containing it prohibited after October 2026.
Inorganic Flame Retardants
The simplest flame retardants are inorganic minerals, primarily aluminum trihydroxide and magnesium hydroxide. These work through a straightforward mechanism: when heated, they decompose and absorb large amounts of energy in the process, pulling heat away from the material they’re protecting. They also release water vapor during decomposition, which dilutes flammable gases near the surface. These minerals are commonly used in plastics, rubber, and coatings where high loading levels (often 20 percent or more of the material by weight) are acceptable. They’re generally considered the least toxic option.
How All These Chemicals Actually Stop Fire
Fire retardants use several overlapping strategies to interrupt combustion. The most common is endothermic cooling. The retardant absorbs heat energy as it decomposes, lowering the surface temperature of the burning material enough to slow or stop the chemical reactions that sustain a flame.
The second strategy is char formation. Phosphorus-based retardants, whether in wildfire slurry or in product additives, break down under heat and produce phosphoric acid compounds. These acids strip water from the material’s surface through dehydration, leaving behind a dense carbon char. This char layer acts as a physical barrier, blocking heat transfer into the material below and trapping flammable gases so they can’t feed the flame. In some formulations, the phosphoric acid compounds melt into a glass-like layer on the surface for even better insulation.
The third strategy is radical scavenging. Flames sustain themselves through a chain reaction involving highly reactive molecular fragments. Certain retardants, especially phosphorus-based ones, release compounds into the gas phase that capture these reactive fragments and break the chain reaction. At the same time, the non-flammable gases produced during the retardant’s breakdown dilute the oxygen around the fire.
How Intumescent Coatings Work
Intumescent coatings are a specialized type of fire retardant used on structural steel, wood, and other building materials. Applied as a thin paint-like layer, they swell to many times their original thickness when exposed to fire, forming a thick, porous, insulating char that shields the material underneath.
These coatings contain three key ingredients that work together. An acid source, typically ammonium polyphosphate, breaks down under heat and releases acid. That acid reacts with a carbon source, often pentaerythritol or expandable graphite, stripping water from it and converting it into a stable char. Meanwhile, a blowing agent, commonly melamine, decomposes and releases gases like ammonia and carbon dioxide. These gases inflate the char into a foam-like structure, creating the thick protective layer. In testing, this combination has reduced peak heat release by nearly 60 percent and cut smoke production by roughly half compared to untreated materials.