What is the red fire retardant dropped by planes to fight LA wildfires?

Authorities battling a series of large wildfires in the Los Angeles area have employed a bright red fire retardant to tamp the flames, leading to dramatic images of the material blanketing scores of homes, cars and residential streets.

As large tankers drop tons of the retardant over swaths of Southern California, questions about it have circulated on social media, with many asking what it's made of, how it works and whether it's harmful to the environment.

The long-term retardant is a common fire-suppression tool that has been used for more than half a century. It is approved for use by the U.S. Forest Service, however, citing environmental concerns, the agency banned its use on federal lands near waterways or habitats of endangered species – except when lives or critical infrastructure are threatened.

Here is what we know about the fire retardant:

What is the fire retardant made of?

The vibrant red fire retardant is known as Phos-Chek and has been used by firefighters battling wildland blazes for over 50 years, according to its manufacturer, Perimeter Solutions.

Between 2009 and 2021, over 440 million gallons of long-term fire retardant were dumped across federal, state and private land, most of it in the western U.S., according to federal estimates.

The components of long-term fire retardants are mainly salts – typically agricultural fertilizers – that "alter the way the fire burns, decreases the fire intensity, and slows the advance of the fire," according to the U.S. Department of Agriculture Forest Service.

The components are mixed with water and can be made available as wet or dry concentrates depending on the needs of fire response authorities.

Why is fire retardant red?

The red color of the fire retardant is caused by the chemical component iron oxide, commonly known as rust.

The retardant is colored with the component so it is easily seen from the air and by firefighters and other authorities battling the blaze on the ground.

According to the USDA, the iron oxide fades and loses its color "through weathering, rain, or other environmental factors." It is expected to naturally lose its color within months.

How does fire retardant work?

The long-term fire retardant, often used to steer wildfires and protect critical infrastructure or populated areas, triggers a series of events once the flames approach it.

As it is heated, the components in the retardant prevent the cellulose in plant matter from fueling the fire, according to Perimeter Solutions. It instead emits water vapor as it decomposes, which "cools the fire" and essentially starves it of fuel.

Left behind is a black carbon coating "that both insulates and restricts air flow to any residual fuels," according to a manufacturer's report on its use. "When this occurs, the intensity of the fuel-starved fire decreases and control is much easier to achieve."

Is fire retardant safe?

Phos-Chek has been approved by the USDA after passing tests by the federal agency that determine whether it meets environmental safety standards, according to the government and the retardant's manufacturer. It also does not include any substances the state of California has linked to "cancer, developmental and/or reproductive harm."

"They are the safest, most effective, and environmentally friendly products available," Perimeter Solutions says on its website.

The USDA, however, requires a 300-foot buffer between long-term fire retardant and the sides of waterways, citing particular risk to fish. The agency makes exceptions to these guidelines, including when a fire threatens "life or property" and "the risk of damage to natural resources outweighs the risk of impacts to aquatic life," according to an environmental impact report by the USDA.

It also requires that any "intrusion" is reported, assessed, monitored and "remediated as necessary."

A study by researchers at the University of California, Los Angeles, published last year claimed that long-term fire retardants contained heavy toxic metals. The manufacturer of Phos-Chek, which was one of the long-term retardants included in the study, has repeatedly disputed the study's findings.

This article originally appeared on USA TODAY: What is the red fire retardant used to combat LA fires? Phos-Chek