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FIRE RETARDANT – EVOLVING TO MEET THE CHALLENGES PRESENTED BY LONGER, MORE INTENSE WILDFIRE SEASONS Melissa Kim, Director of Research & Development, Perimeter Solutions
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ildfire 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. Realizing the limitations of water, chemicals started being added to improve its stability and effectiveness in dousing
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
While a familiar site today, the first fire retardants dropped from aircraft were not red. That was added in 1971 so that pilots could see where retardant had previously been dropped to create continuous fire lines and improve the containment of active wildfires.
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.
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
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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 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,
THE DAWN OF A NEW AGE IN FIRE RETARDANT TECHNOLOGY The impact of the findings from Operation Firestop was farreaching, 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
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