It’s Raining Red

Why chemical fire retardants are on the rise

Kira Hoffman

Bomber drops fire retardant foam at Mammoth Lakes, California. Photo by jcookfisher , CC, cropped from original

If you drove through central British Columbia in the late summer of 2018, it would have been hard to miss the charred trees and blackened landscapes. BC experienced its worst wildfire season on record, with fires burning over 1.35 million hectares of forests throughout the province.

In some places, interface fires spread right through communities, and thousands of people were evacuated from their homes. Rightfully, the extreme fire season saw wall-to-wall media coverage – and as usual, images of airplanes dropping bright red fire retardant were commonly featured.

Wildland-urban interface fires in BC are on the rise, as is the use of chemical fire retardants in the form of foams, gels, and powders. The 2017 fire season saw approximately 12 million litres of Phos-Chek dropped to suppress fires threatening communities in BC – surpassing the amounts used in any single season to date.

How fire retardant works

The type of fire retardant most commonly used in BC is a concentrated liquid called Phos-Chek LC95A-MV. It contains ammonium phosphate, corrosion inhibitors, thickening agents, and iron oxide, which gives it a bright red colour and makes it visible when creating fire lines. The retardant is diluted in water to 15% concentration and dropped by aircraft to aid ground crews and suppress wildfires that are threatening communities or infrastructure.

Generally, fire retardant isn’t dropped directly on fires, but rather on vegetation surrounding a fire to aid in its containment or help slow its spread. Fire retardant chemicals render trees, grasses, and vegetation non-flammable by giving off water vapour and altering the combustion process of cellulose in plants. Fire retardant is more effective than water alone because it acts as a long-term fire suppressant until heavy rains wash it away.

Retardants and the environment

Fire retardants contain many of the same ingredients found in common garden fertilizers. Although they are United States Department of Agriculture (USDA) and United States Forest Service (USFS) approved and considered safe for humans and the environment, studies have demonstrated adverse effects on salmon and trout. Salmon at the smolt stage are vulnerable to chemical fire retardant when it’s accidently dropped over their habitat, particularly in stagnant pools and river eddies where it is not quickly dispersed downstream.

Ash, mud, and debris resulting from fires can also affect waterways, but the long-term risks of fire retardant in sensitive aquatic and terrestrial habitats likely outweigh those other adverse effects of a fire event. Although Phos-Chek is sold as an environmentally beneficial product because it acts as a fertilizer after the fire, it also encourages the spread of invasive plants, which often capitalize on nutrient-rich environments.

Over the years, the Environmental Protection Agency, US Department of Agriculture (USDA), and US Forest Service (USFS) have conducted several independent reviews to test the toxicity of Phos-Chek to terrestrial and marine environments. These reviews confirm that exposure to Phos-Chek under certain conditions is toxic to aquatic species, and have prompted laws prohibiting fire retardant drops within 100 m of waterways. Reporting is also required for all off-target drops. No independent safety approval testing in Canada exists due to funding constraints; therefore, Canada follows US guidelines.

As a former BC wildfire fighter, I’ve been covered in fire retardant several times after gusty winds caused a missed drop by tankers. Although it caused some mild skin and eye irritation, it mostly felt like I’d been sprayed with soap. The retardant washed easily out of my clothes, but my leather boots continued to have a distinct red tinge for years.

A brief history of fire retardants

The BC Wildfire Service has been using fire retardant since the 1950s. Phos-Chek was first named in 1962; once produced by Monsanto, the family of Phos-Chek retardants is now owned by Israel Chemicals Limited (ICL). Unlike flame-retardants applied to textiles and consumer goods, which contain chemicals such as bromine and chlorine, fire retardants applied to vegetation don’t bioaccumulate in the environment.

The amount of fire retardant dropped in a single aerial application depends on the size of the aircraft. Tanks range anywhere from 360 litres in a helicopter to 75,000 litres in a Boeing 747 SuperTanker. The SuperTanker made its first drops in the United States in 2017 on the Sonoma and Thomas fires in California, which were at that time, the largest documented fires in California history.

As aircraft capacity to carry fire retardant has increased, along with the size of fires and the length of fire seasons, we will likely see a continued increase in the use of chemical fire retardants. Phos-Chek is currently the only retardant approved by the USDA and USFS for aerial drops. Although more environmentally sound alternatives are not currently marketed, changes could be made to protect endangered species and sensitive habitat by increasing the width of the margins adjacent to waterways where retardant cannot be dropped or using only water in sensitive habitat areas.

When asked about the risk of fire retardants to homeowners and their property, the answer is often something like this: “Better red than black.” If your home or property is affected by a retardant, it’s best to take precautions and remove it as soon as possible with detergent and water. And if you notice that retardant has been accidently dropped near or in waterways, don’t hesitate to report it to fire authorities.


Kira Hoffman has a PhD in ecology and broad interests in terrestrial ecology, wildfire, conservation, climate change, and forest dynamics. Originally published on Science Borealis in 2017, this article has been updated to reflect the 2018 fire season

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