The vivid red slurry that airtankers drop on wildfires is mostly water mixed with ammonium phosphate or ammonium polyphosphate fertilizer salts, a gum-based thickener to help it stick to vegetation, and iron oxide pigment that gives it the characteristic crimson color. The formula sounds simple, but the details of how it works, what else it contains, and what it leaves behind in the landscape are more complicated than most people realize.
What Goes Into the Mix
The workhorse ingredient in long-term aerial fire retardant is an ammonium phosphate salt, usually diammonium phosphate or ammonium polyphosphate. These are the same nitrogen-and-phosphorus compounds found in common agricultural fertilizer, which is one reason the environmental story gets complicated. The salt is dissolved in water to form a concentrated slurry, and everything else in the formula exists to make that slurry perform better on the way down and after it lands.
A thickening agent, typically a natural gum like guar gum or a synthetic polymer, gives the liquid a gel-like consistency so it doesn’t atomize into a useless mist at high speeds. The thickener also helps the retardant cling to leaves, bark, and grass once it hits the canopy rather than sheeting off. Corrosion inhibitors protect the airtanker’s internal plumbing and the aluminum skin of the aircraft. Small amounts of wetting agents (surfactants) help the liquid spread and penetrate dry, waxy vegetation. And then there’s the iron oxide, which deserves its own discussion.
Why It’s Bright Red
The red color isn’t decorative. Iron oxide pigment is added so pilots, ground crews, and incident commanders can see exactly where the retardant has been laid down. During a chaotic fire operation, knowing precisely which ridgeline or fuel break already has a retardant line is critical for deciding where to drop the next load. The iron oxide fades over weeks as sunlight and weather break it down, but it stays visible long enough for the duration of most fire operations. Without it, the slurry would be a translucent brownish liquid almost invisible against dirt and dried vegetation from the air.
The color also serves a safety function on the ground. Firefighters working in retardant drop zones can see the coated area clearly, which helps them avoid being in the wrong place when a second pass comes through. Some formulations have shifted from deep red to a more pink or orange tone in recent years, but the purpose remains the same.
How the Chemistry Stops Fire
Fire retardant doesn’t just cool things down with water the way a hose does. The ammonium phosphate is doing the real work, and it keeps working long after the water has evaporated. When vegetation coated in the retardant is heated by an approaching fire, the ammonium phosphate breaks down and releases phosphoric acid. That acid reacts with cellulose in the plant material, promoting a chemical process called char formation instead of the normal release of flammable gases.
Normally, when wood or grass gets hot enough, it undergoes thermal decomposition and releases volatile organic compounds that ignite. The phosphoric acid changes that process. It catalyzes dehydration of the cellulose, pulling water out of the plant’s molecules and converting the material into a carbon char layer. Char doesn’t burn the same way as volatile gases; it smolders slowly if it burns at all, and it forms an insulating shell that protects the unburned material underneath. This is why the retardant is called “long-term”: even after the water has dried, the chemical coating remains active against fire for hours or days until rain washes it away.
This mechanism also explains why retardant is dropped ahead of the fire rather than directly on the flames. Firefighters lay it in lines across the fire’s projected path, coating unburned vegetation so that when the flames arrive, the fuel won’t ignite easily. A retardant line works like a chemical firebreak.
Getting It From the Plane to the Ground
Dropping thousands of gallons of thick liquid from an aircraft traveling at high speed is a surprisingly complex fluid dynamics problem. The retardant exits the plane as a coherent column that immediately begins breaking apart. How it breaks up depends on the aircraft’s speed, altitude, and the momentum of the falling liquid relative to the surrounding air. Low-altitude, low-speed drops tend to concentrate more retardant over a smaller area, producing a heavier coating. Higher and faster drops spread the retardant over a larger footprint but coat each square meter more thinly.1Fire. A Review of Airtanker Drop Characteristics, Effectiveness, and Future Research Directions
The thickener plays a key role here. The retardant behaves as a non-Newtonian fluid: it resists flowing at rest but thins under shear forces. This shear-thinning behavior means the liquid holds together better as it falls through the air, reducing how much of the load drifts away as fine mist before reaching the canopy.2Annual Review of Fluid Mechanics. Fluid Dynamics of Airtanker Firefighting Larger droplets are better at punching through intense heat to cool flames directly, while finer droplets coat vegetation more evenly for preventive treatment.1Fire. A Review of Airtanker Drop Characteristics, Effectiveness, and Future Research Directions Pilots and air attack officers adjust speed and altitude based on whether they’re trying to knock down active flame or build a retardant line ahead of the fire.
Even with all these adjustments, a lot of the dropped retardant never reaches the ground target. Wind shear, thermal updrafts from the fire, and canopy interception all reduce what actually lands on the intended fuel below. Research into how retardant is retained by tree canopies versus dripping through to the forest floor remains an active area of study, because the effectiveness of a drop depends on what ends up where.
What It Does to Fish and Waterways
The U.S. Forest Service has strict rules against dropping retardant within 300 feet of waterways, and the reason is straightforward: the stuff can kill fish. When ammonium phosphate retardant enters a stream or lake, the ammonium ions convert to un-ionized ammonia, which is directly toxic to aquatic life. Research on chinook salmon undergoing their transition from freshwater to saltwater found that ammonia levels from retardant were high enough to cause acute mortality.3PubMed. Toxicity of forest fire retardant chemicals to stream-type chinook salmon undergoing parr-smolt transformation Young salmon at that developmental stage are already physiologically stressed, making them especially vulnerable.
But direct chemical toxicity isn’t the only aquatic concern. The phosphorus and nitrogen in retardant are potent fertilizers, and when they enter low-nutrient waterways, particularly alpine lakes and headwater streams that normally receive very little nutrient input, they can trigger algal growth. Excessive nutrient loading in lakes has been linked to cyanobacteria blooms, which can produce toxins harmful to people and wildlife. This is a growing concern for drinking water reservoirs near fire-prone areas in the western United States.4PubMed Central. Wildfires in the western United States are mobilizing PM2.5-associated nutrients and may be contributing to downwind cyanobacteria blooms
Accidental drops into water bodies do happen. In the chaos of a fast-moving wildfire, pilots may misjudge distances, or wind may carry the retardant off target. These incidents are investigated and reported, but the immediate ecological damage from a single load of retardant hitting a small stream can be severe for the organisms living there.
How It Changes Soil and Plant Communities
Because the active ingredient is literally fertilizer, retardant doesn’t just disappear after the fire is out. It changes the soil chemistry of every area it’s applied to, and those changes can persist for years. A study that tracked the effects of ammonium polyphosphate application on soil and plants for a full decade found that treated plots had significantly elevated levels of available phosphorus, sodium, and aluminum compared to untreated burned areas. Pine trees in the treated plots actually grew taller and wider than in other burned plots, but many of them died or developed distorted trunks, and certain native shrub species showed reduced coverage.5PubMed. Effects of fire and three fire-fighting chemicals on main soil properties, plant nutrient content and vegetation growth and cover after 10 years
The phosphorus enrichment is particularly problematic for native plant communities. Many native plants in fire-adapted ecosystems have evolved to thrive in low-nutrient soils, often with the help of symbiotic soil fungi called mycorrhizae that help them scavenge scarce phosphorus. When retardant floods the soil with easily available phosphorus, those fungal partnerships break down: plants stop supporting their fungal symbionts because they no longer need them. Field studies have found that retardant-treated areas show reduced mycorrhizal colonization and persistent shifts in plant communities toward exotic annual grasses.6PubMed. Cascading effects of fire retardant on plant-microbe interactions, community composition, and invasion
Invasive species like cheatgrass, which thrives on nutrient pulses and doesn’t depend on mycorrhizal fungi, benefit the most from retardant application. In greenhouse experiments, cheatgrass showed the largest growth response to retardant of any species tested.6PubMed. Cascading effects of fire retardant on plant-microbe interactions, community composition, and invasion This is deeply ironic: cheatgrass is itself one of the primary drivers of increased fire frequency in the western U.S. because it dries out early and burns easily. A fire-fighting product that promotes cheatgrass invasion could, over time, contribute to more fires.
Similar results have been observed in Australian heathlands, where application of Phos-Chek (a widely used commercial retardant) caused death of several key native species, enhanced weed invasion particularly at higher concentrations, and generally fertilized shoot growth in surviving plants.7International Journal of Wildland Fire. Effects of the fire retardant Phos-Chek on vegetation in eastern Australian heathlands
Toxic Metals in the Formula
Beyond the intended active ingredients, recent chemical analysis has revealed something that caught even researchers off guard: commercial fire retardant products contain concentrations of toxic metals far exceeding safe drinking water thresholds. A laboratory study of long-term fire retardants found levels of vanadium, chromium, manganese, copper, arsenic, cadmium, antimony, barium, thallium, and lead that were anywhere from 4 to 2,880 times greater than drinking water regulatory limits.8PubMed Central. Metals in Wildfire Suppressants
The most alarming numbers came from one widely used product, Phos-Chek LC-95W. Its chromium content exceeded the safe drinking water limit by a factor of 727, and its cadmium concentration was 2,880 times above the regulatory threshold. Concentrations of chromium, cadmium, and vanadium in that product exceeded the thresholds that would classify a substance as hazardous waste under both federal law and California state regulations.9PubMed Central. Metals in Wildfire Suppressants – Section: Results and Discussion
These metals are contaminants in the raw phosphate ore used to manufacture ammonium phosphate, not ingredients that anyone adds on purpose. Phosphate rock naturally contains trace metals, and the industrial process of turning that ore into fertilizer-grade ammonium phosphate doesn’t fully remove them. When hundreds of thousands of gallons of retardant are dropped across a fire season, those trace metals add up. The environmental and regulatory implications of this are still being sorted out, because fire retardant has historically been evaluated for ammonia and phosphorus toxicity, not for heavy metal contamination.
Health Risks for People on the Ground
For the general public, brief contact with fire retardant during or after a drop is generally considered low-risk. The ammonium phosphate salts are similar to what’s in garden fertilizer, and the iron oxide is an inert pigment used in cosmetics. If retardant lands on your property, washing it off skin with soap and water and hosing down structures is the standard advice.
The story is different for the people who work with retardant regularly. At retardant mixing bases, workers called mixmasters handle dry retardant powder, pouring it into mixing tanks to create the slurry loaded onto tankers. Occupational sampling of these workers found that their inhalable dust exposure during mixing was high enough to raise concern about non-carcinogenic health effects, with hazard quotients above the safe threshold at both bases studied. Researchers recommended the use of respirators, eye protection, gloves, and coveralls for anyone doing this work regularly.10International Journal of Wildland Fire. Mixmaster exposure to dust during mixing of wildland fire retardant chemicals
Firefighters on the ground can also be caught in retardant drops, sometimes deliberately if they’re sheltering in place. The physical impact of several hundred gallons of liquid hitting at speed is itself dangerous. And with the recent findings about heavy metals in certain products, questions about chronic exposure in firefighters who spend entire careers working in retardant-treated environments are starting to get more attention, though long-term health studies specific to retardant exposure are still limited.
The Difference Between Retardant and Foam
Not everything dropped from aircraft on fires is the red stuff. There are two broad categories of aerial fire-fighting products that often get lumped together but work differently. Long-term retardants are the ammonium phosphate slurries described above: they chemically alter fuel so it resists combustion, and they keep working after the water evaporates. Short-term suppressants include plain water, water enhanced with surfactants (wetting agents), and firefighting foams. These work primarily by cooling and smothering: the water absorbs heat, the foam blankets fuel to cut off oxygen, and the surfactants help water penetrate into dry material that would otherwise repel it.
Short-term suppressants lose their effectiveness once the water dries or the foam breaks down, usually within minutes to hours. They’re most useful for direct attack on active flames. Long-term retardants are the tool of choice when the goal is to build a line ahead of a fire and have it hold even if it takes hours for the fire front to arrive. The two products serve different tactical purposes, and most large fire operations use both.
Greener Alternatives in Development
The environmental downsides of phosphate-based retardant have pushed researchers to look for alternatives. The overall trajectory has been from simple inorganic salt solutions toward more engineered materials that aim to suppress fire while doing less ecological damage.11Advanced Engineering Materials. Advancing Wildfire‐Retardant Materials: Engineering Strategies for Direct and Indirect Suppression
One promising direction involves hydrogels: water-absorbing polymer networks that can hold many times their weight in water and release it slowly when exposed to heat. Some hydrogels can be applied to vegetation as a preventive treatment days or even weeks before fire season, creating a reservoir of water and a physical barrier against flames. Recent work on cellulose-based hydrogels embedded with kaolin (a natural clay mineral) has shown strong fire retardant performance while using biodegradable, non-toxic materials. In laboratory tests, cotton fabric coated with these hydrogels showed dramatically reduced heat release compared to untreated fabric.12PubMed. Kaolin-embedded cellulose hydrogel with tunable properties as a green fire retardant
Other research is exploring water-enhancing polymers that can be mixed into conventional water drops to increase their effectiveness, essentially making each gallon of water do more cooling and wetting work so less total liquid is needed. The appeal of these approaches is that they could reduce or eliminate the phosphorus and nitrogen loading that drives so much of the ecological damage from current retardants.
None of these alternatives have replaced ammonium phosphate retardant at operational scale yet. The challenge is that the current product is cheap, effective, and backed by decades of operational experience. Any replacement needs to match that performance in the extreme conditions of a wildfire while also being safe to manufacture, load, and drop from existing aircraft. The gap between a promising lab result on coated cotton fabric and a product that works when 3,000 gallons of it are falling through smoke and wind onto a mountainside canopy is enormous. But given growing pressure over water contamination and invasive species promotion, the search for a cleaner formula is intensifying across multiple research groups worldwide.