Fire Retardant — Evolving to Meet the Challenges Presented by Longer, More Intense Wildfire Seasons

Melissa Kim, Director of Research & Development, Perimeter Solutions

Wildfire season has increased in length and intensity over recent decades, burning more structures and acres of land than ever before. In fact, Congressional Research Services reports1 that the five years with the most acreage burned since 1960 have all occurred over the past 15 years. The growth in wildfires has led many in the fire safety industry to say that there is no longer a wildfire season but a wildfire year. This proliferation of wildfires is becoming increasingly dangerous as property developers continue to build homes and businesses in the wildland-urban interface (WUI) – the areas where wildland intersects with land occupied by human development. On top of that, wildfires are damaging the air that we breathe. According to the Copernicus Atmosphere Monitoring Service2 global wildfires emitted 1.76 billion tons of carbon dioxide into the air in 2021 alone, which is more than the entire annual carbon emissions from Germany.3 The wildfire landscape has changed dramatically over the last century, and in order to save lives and protect property, fire retardant technology has had to continually evolve to meet the challenge.

The Early History of Aerial Wildfire Attack

Firefighters have worked to take advantage of the benefits aerial firefighting has to offer almost since the dawn of flight. Back in the 1920s, containers of water were literally dropped from planes to fight active wildfires.4 Aside from being dangerous to firefighters on the ground, the practice proved to be ineffective. 

The negative results didn’t dissuade future experiments. Heading into the 1940s, airborne attacks continued, although the tools of the trade remained rudimentary.5 Small planes flew over active wildfires with water continuing to serve as the extinguishing agent. The accurate application was virtually impossible, and aerial attacks remained ineffective.

El Dorado sm
Today, the USFS uses 100% phosphate-based fire retardant for aerial attacks. Removing ammonium sulfate from the solution helped increase efficacy and improved its environmental profile.

Realizing the limitations of water, chemicals started being added to improve its stability and effectiveness in dousing fires. Aircraft was also improved. Following World War II,6 fire management agencies started using air tankers with installed tanks to drop fire retardants. The early forms of fire retardant weren’t like what is available today, as various chemicals were used to find the right solution, Including bentonite and borate. This, incidentally, has led some to refer to firefighting aircraft as Borate Bombers to this day, even though the use of borate was during a brief time in the 1950s. Its use was eliminated due to its toxicity to the soil.7 After borate, other chemicals were tried as potential replacements, including sodium silicate, magnesium chloride, ammonium sulfate, ammonium phosphate, and others.8

In an effort to determine the most effective chemical retardant, the US Forest Service (USFS) collaborated with CAL FIRE, Los Angeles County, the City of Los Angeles, the US Department of Defense, and other organizations to launch a major study called Operation Firestop.9

Ground based retardant red
PHOS-CHEK Ground based retardant demonstration, Shingletown, CA, 07-24-2019

Fire safety experts tested various chemicals that had been used over the previous decades, including many of those mentioned above, to determine their effect on the ignition time of wood, the fire intensity of burning wood, and the retardant’s ability to suppress flaming wood. The researchers found that ammonium phosphate was among the most effective chemicals in reducing fire intensity, while boric acid, magnesium chloride, and polyvinyl acetate were discarded and no longer considered viable options as active ingredients for fire long-term retardants.10

The Dawn of a New Age in Fire Retardant Technology

The impact of the findings from Operation Firestop was far-reaching, as phosphate became the chemical of choice for fire retardants moving forward. In 1963, phosphate-based PHOS-CHEK® long-term fire retardant became the first fire retardant approved for use by the USFS.11 This began the evolution of what is now known as the USFS Qualified Product List (QPL) for fire retardant products, which identifies all fire retardants that are eligible for use on United States Federal and State lands. As of this writing, the only fire retardants fully approved on the QPL are phosphate-based.12

In 1970, scientist Aylmer D. Blakely decided to conduct further research on fire retardant chemicals and published a paper to share his findings entitled, “A Laboratory Method for Evaluating Forest Fire Retardant Chemicals.” Blakely introduced what he called the “superiority factor method” to determine the overall effectiveness of different chemicals – including the rate of weight loss, the amount of heat radiation emitted, and the amount of residue left behind after all combustion had ended as the fuel burned. The overall ranking of chemicals showed that diammonium phosphate, monoammonium phosphate, phosphoric acid, and potassium carbonate consistently ranked higher than any other chemicals in all three parameters.13

This study confirmed the results from Operation Firestop, which found that phosphate chemistry offered the highest effectiveness for fire retardancy of any active ingredient.

But, it takes more than just the active ingredient to formulate a usable long-term fire retardant. To be effective, retardants must:

Be safe for people, animals, fish, and the environment

Cause minimal corrosion to protect aircraft and other equipment

Have higher recovery rates (meaning a high percentage of the solution hits its target)

Be stable

Be able to be stored for extended periods

Be visible when dropped and remain visible for pilots when it is on the ground to form a continuous fire line


Innovations have been made over the years to improve these retardant characteristics. Better thickeners were developed to improve drop characteristics, more sophisticated corrosion inhibitors were added, enhanced flow conditioners made mixing easier and more efficient, and new colorants enhanced visibility and environmental profile.

In 1971, better gum thickeners were added to fire retardant to create a low viscosity solution that also improved drop characteristics. Corrosion inhibitors were also added. This was a critical advancement, as it enabled fixed-tank helicopters, whose equipment is highly sensitive to corrosion, to be used for aerial wildfire attacks.

That same year, iron oxide was first used in fire retardants.14 This added red coloring to the solution, allowing pilots to see where retardant had previously been dropped so that they could create continuous fire lines and improve the containment of active wildfires. The drawback of iron oxide is that when applied, it would permanently stain surfaces that it contacted, including homes and other structures. An improvement was made in 1975, with the first “fugitive color” retardants.15 This advancement allowed fire management agencies to effectively fight fires in aesthetically sensitive areas, such as national parks and in the growing WUI areas in the Western United States.

Ten years later, in 1985, retardants with a synergistic blend of ammonium sulfate and ammonium phosphates were introduced – which delivered fire retardant performance similar to ammonium phosphate retardants at a considerably lower cost.16

The following year, the USFS introduced the first modern specifications for long-term fire retardants – Specification 5100-304, which covers a broad range of issues relating to efficacy and use of fire-retardant chemicals, including:

Uniform and intergranular corrosion

Product stability

Fire-retarding effectiveness

Physical parameters

Mammalian toxicity, including oral toxicity (ingestion), dermal toxicity (absorption through the skin), eye irritation, and skin irritation

Aquatic toxicity

Human health and ecological risk assessments using data from toxicity tests and safety data sheets (SDS)17

This specification has been updated multiple times, most recently in January 2020. Since its introduction, all fire retardants fully qualified on the QPL have had to meet the requirements established in this specification through testing conducted by the Wildland Fire Chemical System (WFCS).18 Part of the National Technology and Development Program (NTDP), the WCFS provides the USFS and other land management agencies with information that helps them to use fire suppression chemicals and aerial delivery systems safely and effectively.

Following the creation of established criteria by the USFS, fire retardant innovations continued to be made over subsequent years. 

Advanced gum-thickened retardants were developed, offering a range of viscosities. Previously, the only options for the fire manager were very low-viscosity (un-thickened-water-like) or high viscosity retardants. This innovation allows the agencies to use the retardant to optimize its characteristics based on the delivery system, topography, fuel, and specific fire situation. Liquid concentrate fire retardants were introduced in addition to the previous all powder concentrates. This eliminated the potential to generate dust particles but maintained virtually the same level of effectiveness as a powder concentrate.

In the 2000s, modern 100% phosphate-based fire retardant was developed. Removing ammonium sulfate from the solution helped to increase the efficacy of the retardant and reduced the amount of ammonia being used, improving its overall environmental profile.

Today, phosphate-based retardants continue to be recognized by the scientific community as the most effective wildland fire retardants. They offer significantly higher recovery rates than water or retardants based on other chemical solutions. In the presence of the fire retardant, the cellulose fuel decomposes to non-flammable, nearly pure carbon and water                                                               vapor. This process occurs prior to the flaming decomposition of cellulose alone and requires the heat energy of the advancing flame front to occur, thus                                                               removing the fuel and reducing the intensity of the advancing fire making it safer for frontline crews and easier to obtain control.. 

Incorporating Ground-Based attack

Firefighters have turned to ground-based fire retardant to complement or supplant aerial fire-retardant applications for years to help:

Reinforce conventional control lines

Create ingress and egress routes

Establish control lines

Limit spot fires

Control a fire within a burn

Protect assets

Stop fires before they start

With recent advancements that have made it more durable and given it greater adherence, long-term fire retardant can now be applied to flammable vegetation and cellulosic materials to provide extended protection from wildfires until a significant rain event occurs. Unlike the red retardant dropped from aircraft, ground-based retardant is uncolored, leaving only white residue after it dries.

Utilities, railroads, insurance companies, and homeowners have successfully started using ground-based retardants to protect their assets from wildfire. For example, in 2018, there were 37 fire starts along a four-mile stretch of road through an area called Rocky Peak in California. At the beginning of the following wildfire season, ground-based retardant was applied to that same area, and not one fire was recorded the following summer. In 2021, the first phosphate-based ground retardant was added to the USFS QPL, so it too can now also be used on all federal lands. 

The Continuing Threat of Wildfires

As we have all witnessed in recent years, wildfire season continues to expand, and it’s not going to get any better. According to a recent report from the United Nations, the number of global wildfires will increase by 50% by the year 2100.19 Over the years, more air tankers have been added to fight wildfires, and air tanker operators have worked to increase capacity, and we will need to keep doing that in order to successfully manage what lies ahead.

The retardant industry also needs to continue to innovate and discover new technologies that will help us to do our jobs better to provide firefighters with the tools that they need to fight these fires while continuing to save lives and protect property.

Melissa KimMelissa Kim is Director of Research & Development at Perimeter Solutions. Prior to joining Perimeter Solutions, she served as the Global R&D Director for ICL Performance. Melissa has developed fire safety products since 2004 and is responsible for worldwide product development at Perimeter Solutions. She earned her bachelor’s degree in Biological Sciences from the University of California, Irvine.


1 “Wildfire Statistics,” In Focus, Congressional Research Service, August 1, 2022,, Accessed August 10, 2022

2 “Wildfires Wreaked Havoc in 2021, CAMS Tracked Their Impact,” Copernicus Atmosphere Monitoring Service, Copernicus, 6 December 2021,, Accessed, August 2, 2022

3 Abnett, Kate. “This is how much carbon wildfires have emitted this year,” Reuters, December 10, 2021,, Accessed August 2, 2022

4 Wilkinson, Stephan, “Firebombers! Flying on the Edge to Fight Fires,” Historynet, Historynet LLC, January 24, 2020,, Accessed August 2, 2022

5 Wilkinson, Stephan, “Firebombers! Flying on the Edge to Fight Fires,” Historynet, Historynet LLC, January 24, 2020,, Accessed August 2, 2022

6 Richardson, S.D., “Operation Firestop.” Empire Forestry Review, vol. 38, no. 1 (95), Commonwealth Forestry Association, 1959, pp. 26–34,, Accessed August 2, 2022

7 Goldberg, Edward, “Myth vs. Reality: Understanding the Chemistry of Wildfire Suppression,” Perimeter Solutions,, Accessed August 2, 2022

8 Goldberg, Edward, “Myth vs. Reality: Understanding the Chemistry of Wildfire Suppression,” Perimeter Solutions,, Accessed August 2, 2022

9 Richardson, S.D., “Operation Firestop.” Empire Forestry Review, vol. 38, no. 1 (95), Commonwealth Forestry Association, 1959, pp. 26–34,, Accessed August 2, 2022

10 Richardson, S.D., “Operation Firestop.” Empire Forestry Review, vol. 38, no. 1 (95), Commonwealth Forestry Association, 1959, pp. 26–34,, Accessed August 2, 2022

11 Goldberg, Edward, “Myth vs. Reality: Understanding the Chemistry of Wildfire Suppression,” Perimeter Solutions,, Accessed August 2, 2022

12 “Long-Term Retardant for Wildland Fire Management,” U.S. Forest Service Washington Office Fire & Aviation Management, US Forest Service, July 5, 2022,, Accessed August 2, 2022 

13 Blakely, Aylmer D., “Laboratory method for Evaluating Forest Fire Retardant Chemicals,” ScholarWorks, University of Montana, 1970,, Accessed August 1, 2022

14 “PHOS-CHEK® Innovations In Fire Retardants & Class A Foam, ” Phos-Chek Australia, Perimeter Solutions,, Accessed August 1, 2022

15 “PHOS-CHEK® Innovations In Fire Retardants & Class A Foam,” Phos-Chek Australia, Perimeter Solutions,, Accessed August 1, 2022

16 “PHOS-CHEK® Innovations In Fire Retardants & Class A Foam,” Phos-Chek Australia, Perimeter Solutions,, Accessed August 1, 2022

17 “United States Department of Agriculture Forest Service Specification for Long Term Retardant, Wildland Fire, Aircraft or Ground Application,” US Forest Service,, Accessed August 2, 2022

18 “Long-Term Fire Retardants,” U.S. Forest Service, US Department of Agriculture,, Accessed August 2, 2022

19 “Number of Wildfires to Rise by 50% by 2100 and Governments are not Prepared, Experts Warn,” United Nations, United Nations Environment Programme, February 23, 2022,, Accessed 3 August 2022



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