In response to several intense wildfires in the Los Angeles region, authorities have been using a bright red fire retardant to help control the flames. This has resulted in striking visuals of the retardant covering numerous homes, vehicles, and streets.

As large aircraft release substantial amounts of this retardant across Southern California, there have been many queries on social media about its composition, functionality, and environmental impact.

This long-term fire retardant has been a staple for fire suppression for over 50 years. It has received approval from the U.S. Forest Service, although due to environmental concerns, the agency prohibits its use on federal lands near water bodies or areas housing endangered species, except in emergencies where lives or vital infrastructure are at risk.

Here is what we know regarding the fire retardant:

What is the composition of the fire retardant?

The striking red fire retardant, called Phos-Chek, has been utilized by wildland firefighters for over five decades, according to its producer, Perimeter Solutions.

From 2009 to 2021, more than 440 million gallons of this long-lasting fire retardant have been dispersed across various lands—federal, state, and private—primarily in the western United States, as reported by federal estimates.

The primary ingredients of long-term fire retardants are mostly salts, generally derived from agricultural fertilizers, that “modify how the fire burns, reduce its intensity, and slow its spread,” as stated by the U.S. Department of Agriculture Forest Service.

These ingredients are mixed with water and can be delivered as either wet or dry concentrates based on the requirements of fire response teams.

Why is the fire retardant red?

The distinctive red hue of the fire retardant comes from iron oxide, a compound commonly recognized as rust.

This coloring makes it more visible from the air and to firefighters and other personnel working on the ground to manage the blaze.

The USDA notes that the red color from iron oxide fades over time due to exposure to rain, weather conditions, and other factors, and is expected to disappear within a few months.

How does the fire retardant function?

When the flames approach, the long-term fire retardant—often used to redirect wildfires and safeguard crucial infrastructure or populated regions—initiates a chain of reactions.

As the retardant heats up, its components hinder the combustion of cellulose in vegetation, as per Perimeter Solutions. Instead, it releases water vapor as it breaks down, which helps to “cool the fire,” effectively depriving it of fuel.

What remains is a black carbon layer that “insulates and limits air flow to any leftover fuel,” according to a report from the manufacturer. “This leads to a reduction in the intensity of the fuel-starved fire, making it much easier to control.”

Is the fire retardant safe to use?

Phos-Chek has received approval from the USDA after successfully passing evaluations that confirm it meets environmental safety criteria, as stated by both the government and the manufacturer. Furthermore, it does not contain substances identified by California as linked to “cancer, developmental and/or reproductive harm.”

“They are the safest, most effective, and environmentally responsible products available,” states Perimeter Solutions on its website.

However, the USDA mandates a 300-foot buffer zone between the long-term fire retardant and water sources due to potential risks to fish. The agency does allow exceptions, particularly when there is a threat to “life or property” and when “the risk of damage to natural resources surpasses the risk to aquatic life,” as outlined in an environmental impact report by the USDA.

The agency also requires any “intrusion” to be reported, evaluated, monitored, and addressed as necessary.

A study conducted by researchers at the University of California, Los Angeles, published last year suggested that long-term fire retardants contained hazardous heavy metals. The manufacturer of Phos-Chek has consistently challenged the findings of this study.