The story of fire retardant and why it works

A thick stream of red mist falling from an airplane is one of the most common and dramatic images of Montana’s wildfire season. But while the photos may seem self-explanatory, that red mist — also called long-term fire retardant — has its own complicated story and history.

There are three distinct types of fire retardant commonly deployed for firefighting, each with its own chemical makeup and criteria for use. Fire managers calculate a variety of factors, including the size of the fire and whether structures are at risk, to determine when to use them.

The U.S. Forest Service’s Wildland Fire Chemical Systems program in Missoula tests three types of fire retardants — long-term retardants, foam suppressants, and water enhancers — for effectiveness and health and ecological risk before deeming them ready for use.

Long-term retardant is most familiar as the red plume behind fire-fighting planes, and is designed to slow, not stop, wildfires. Kassidy Kern, fire communications specialist with the Forest Service, says long-term retardant is typically applied in front of advancing wildfires to reduce the fire’s intensity and rate of spread so firefighters can construct containment lines ahead of it. Kern said studies conducted by WFCS found that long-term fire retardant remains effective up to a week after its component water has evaporated.

Long-term retardant is composed of 85% water, 10% fertilizer or salt, and 5% added color or thickener. The water is the medium of delivery for distributing the fertilizer or salt. A chemical reaction between the heat of the fire and ammonium phosphate, the active fertilizer ingredient, pulls heat out of the fire, WFCS program leader Shirley Zylstra says in a YouTube demonstration video.

In the video, Zylstra applies long-term retardant to one bed of shredded aspen a week in advance, and leaves another untreated. After both are ignited, the retardant-treated bed emits a lot of smoke, but not a lot of flame. It takes about 11 minutes for the treated bed to burn itself out. It burns less intensely, and the charred wood it leaves behind won’t ignite again, Zylstra explains.

The untreated bed burns itself out completely in about five minutes.

Kern said foam suppressants are helpful for “mopping up” a fire by saturating the layers of duff — grass, twigs and leaves — on the forest floor. Foam suppressants are 99% water and the added chemicals help the foam cling to fuels.

Water enhancers, which are 97% water, change the consistency of water, allowing it to better stick to fuels and structures. The enhancers are either wet or dry, and are mixed with water to improve its fire extinguishing ability.

The environmental impact of fire retardant has a litigious history. In 2011 the Forest Service changed its fire retardant practices to better support endangered species, low-flow waterways and aquatic species. Retardant drops in “avoidance areas” that may include listed species weren’t allowed. Three-hundred-foot buffers were established adjacent to waterways.

WFCS evaluates all fire retardant products before they can be used by federal agencies. Environmental impact, health and safety and the composition of retardant products are tested before they’re added to the Forest Service’s Qualified Product List.

Until 2020, Phos-Chek, manufactured by a company called Perimeter Solutions headquartered in Missouri, was the only long-term fire retardant used by the Forest Service, Kern said.

“We’ve put a tremendous amount of investment and innovation into looking at formulations that can be applied preventatively,” said Jeff Emery, president of global fire safety at Perimeter. Another Perimeter product called Fortify has been used around utility lines, roads and homes to establish protection before a fire is on the horizon.

Fortress, another fire retardant manufacturer with two production facilities in the western U.S. and a distributor in Stevensville uses magnesium chloride, the same compound used to salt roads in the winter, as an active ingredient.

Fortress was established in 2014 and two of its products are awaiting final verification from the Forest Service before being added to the QPL list.

The added color is intended to make the retardant visible. If the vegetation in a drop area is thick, iron oxide may be added to intensify the color. If the vegetation is light, an additive called fugitive colorant will fade with exposure to sunlight. It’s important that pilots be able to see and connect lines of retardant, Kern said, because gaps can allow unimpeded fire to break through. Retardant visibility also helps firefighters on the ground determine good spots to dig firelines.

“It’s way more effective when you have ground people backing up the retardant we put out,” said Hon Schlapfer, fixed wing program manager at the Forest Service’s Aerial Fire Depot in Missoula. Schlapfer said retardant is just another tool used for fighting fires and is most effective when fires are small.

“In general, fire managers want to put aerial resources and retardant in areas where they will be the most effective without wasting effort, time, resources and taxpayer dollars,” Kern said in an email. Schlapfer said decisions to use fire retardant are based on an assessment of what resources are threatened by the fire. Comparing a fire deep in wilderness to one burning on Mount Sentinel in Missoula, he said, Sentinel would get the fire retardant first due to the prevalence of structures at risk.

At the Aerial Fire Depot’s air tanker base, large air tankers, or LATs, that can carry up to 4,000 gallons of retardant are available for fire response. When an incident commander determines fire retardant is needed, an air attack strategy is developed.

For example, an LAT might fly at about 1,500 feet to drop retardant; a helicopter might fly at 500 feet to scoop up water; the air tactical control group supervisor, who makes sure all the flying vehicles stay safe, flies above at 2,500 feet. Sometimes a lead plane will be required to guide the LAT through more treacherous areas.

The air tanker base in Missoula orders dry retardant from Perimeter and it arrives by semi-truck. Depending on the demand, that can mean two or three semi-trucks per day. The retardant is mixed with water at the base and then gets loaded into a specialized air tanker. LATs, very large air tankers (VLATs) and single-engine air tankers (SEATs) can be used to drop water, but are generally used for retardant, which is stored in an on-board tank.

In 2017, a record year for acreage burned in the state, a million gallons of retardant were dropped on Montana wildfires, according to the Missoula air tanker base. As of Aug. 1, the base reports 100,000 gallons used in Montana so far this fire season.

In 2017, the Forest Service dropped 18.9 million gallons of retardant on Forest Service lands nationwide, and in 2020 that jumped to 21.3 million gallons. Nationwide, the Forest Service’s 10-year average expense on fire retardant is more than $60 million, according to Kern.