The USDA Forest Service and other fire management agencies around the world have used phosphate-based fire retardants for more than 60 years to prevent the spread of wildfires. Over that time, fire retardant has proven to be an effective tool, helping firefighters to protect thousands of structures from destruction and to save countless lives.
It has also helped to protect the environment from the far-reaching and sometimes irreversible damages caused by wildfires.
In this article we will examine more than 80 years of scientific research that has determined that phosphate is the safest, most effective fire retardant available.
Melissa Kim
We will also explore the potential environmental consequences of not using fire retardants and why tools like PHOS-CHEK® are more important than ever as we face longer, more intense wildfire seasons.
One of the earliest tests of chemical fire retardants took place at the University of Idaho in 1940. Researchers evaluated seven chemicals for their ability to reduce combustibility in sawdust using a modified fire-tube apparatus. Diammonium phosphate proved most effective, followed by borax, zinc chloride and others.
In 1954, the U.S. Forest Service launched Operation Firestop, the first major study of chemical firefighting effectiveness. Conducted at Camp Pendleton, the study tested various chemicals for their impact on wood ignition time, fire intensity, and flame suppression.
Ammonium phosphate was again identified as among the most effective. The results from these tests led to phosphate-based retardants becoming the industry standard, and PHOS-CHEK eventually became the first fire retardant approved for aerial firefighting. Less effective chemicals were discarded for long-term use.
Sixteen years later, University of Montana scientist Aylmer D. Blakely expanded on Firestop’s findings in his 1970 paper A Laboratory Method for Evaluating Forest Fire Retardant Chemicals.
Using a “superiority factor” based on weight loss, burn residue, and heat radiation, Blakely ranked the effectiveness of additional chemicals. Diammonium and monoammonium phosphate, again, outperformed others across all parameters, reinforcing phosphate’s status as the most effective retardant chemistry and solidifying PHOS-CHEK’s position as the preferred solution.
A more recent study, conducted in 2022 and co-funded by Australia’s Department of Energy, Environment, and Climate Action (DEECA) and the Victorian Department of Jobs, Precincts and Regions took a different approach, comparing the effectiveness of wildfire retardants against water alone, and an untreated control using four different assessment methods:
The “indirect suppression test” was considered the most important in evaluating the effectiveness of retardant, because this is how retardants are typically used in battling wildfire. Two different retardants were compared to water and the untreated control, with effectiveness of each measured by its ability to stop or slow the fire spread, the percentage of the treatment area burned, and the change in fire forward spread rate within the treatment.
Retardant treatments outperformed water and untreated controls in all fire resistance measures, with significantly lower area burned and shorter fire penetration distances. Retardant A consistently resisted fire spread more effectively than Retardant B, and penetration distance emerged as the most reliable indicator of performance.
Organizations have recently expressed concerns about the use of fire retardant to battle wildfire citing the presence of heavy metals in fire retardants.
Melissa Kim
However, it is important to note that no heavy metals are intentionally added to PHOS-CHEK. The active ingredient in PHOS-CHEK is diammonium phosphate (DAP), which is used to make food products such as beer, wine, cheese, and bread. Exposure to PHOS-CHEK is minimal, as it is typically applied in remote, forested areas. Meanwhile, the consequences of uncontrolled wildfire are immediate, widespread, and well-documented.
To illustrate the scale of the environmental impact, the Copernicus Atmosphere Monitoring Service reported that global wildfires in 2021 emitted approximately 1.76 billion tons of carbon dioxide into the atmosphere. For perspective, the U.S. Environmental Protection Agency estimates that a typical passenger vehicle emits about 4.6 metric tons of CO₂ per year.
This is equivalent to the annual CO₂ emissions of more than 382 million passenger vehicles. Notably, 2021 wasn’t even considered a particularly active wildfire year. It didn’t rank among the top 15 years for total burned acreage in the United States over the past six decades.
Added to the environmental challenges presented with CO₂ emissions, air quality concerns are heightened with the release of other toxins from wildfires. With the expansion of the Wildland-Urban Interface (WUI), wildfires are now threatening more urban areas, evident in the destructive Los Angeles fires of January 2025.
The wildfire smoke from these urban fires is more dangerous than smoke from forest fires due to the variety of materials that burn, including vehicles, plastics, batteries, and building materials. When burned, these items release particulate matter and toxic chemicals, including lead, arsenic, cadmium, and other toxins that are dangerous to breathe.
One of the most concerning pollutants from wildfire is particulate matter (PM2.5), fine particles with a diameter of 2.5 micrometers or less are up to ten times more harmful to human health than PM2.5 from other sources.
These particles can travel great distances as witnessed during the 2023 and 2025 Canadian wildfires, which led to low air quality alerts across the United States. Among other health ailments, the Journal of the American Medical Association reported that the smoke from the 2023 Canadian wildfires led to a marked increase in the number of people being seen for asthma-related symptoms in New York City emergency departments.
Meanwhile, researchers from Kidney International Reports found that the air pollution created by the wildfires resulted in increased risk of mortality and hospitalization among hemodialysis patients in the Eastern and Midwestern United States.
Beyond the negative air quality impacts, wildfires also lead to large amounts of heavy metals being deposited in the soil and surrounding waterways, and these effects can last for years.
Melissa Kim
Researchers from University of California, Los Angeles (UCLA) compared water quality before and after a wildfire event occurred and found that wildfires could increase the presence of many pollutants in water by two orders of magnitude. Nutrients and suspended solids concentrations increased within a year after the fire, while peaks in heavy metal contamination didn’t occur until one-to-two years following the wildfire.
Wildfires also release polycyclic aromatic hydrocarbons (PAHs), which are semi-volatile organic compounds composed of two or more fused benzene rings bounded in linear, cluster, or angular arrangements. They are absorbed into soil following a fire, due to the soil’s lower vapor pressure and higher hydrophobicity (its ability to repel water). Burnt ash from wildfire contains high concentrations of PAHs, a known carcinogen, which leaches into burnt soil. Erosion carries the contaminated sediment to nearby soils, deteriorating the soil quality, and PAHs then enter into crops through plant uptake.
The toll of wildfire is becoming impossible to ignore. The damage caused to the environment lingers for years, affecting public health, biodiversity and climate resilience.
Melissa Kim
Phosphate-based fire retardants like PHOS-CHEK are not only scientifically proven to be the most effective tools for slowing or stopping wildfires, but they are also a critical line of defense in protecting the planet from the much greater environmental harm that wildfires cause.
In an era of increasingly extreme fire behavior, we cannot afford to leave our most effective tools unused.
