What’s In That Red Slurry Crews Are Dumping On Colorado Wildfires?

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Photo: Cold Springs Fire (AP Photo)
A plane drops retardant while battling the Cold Springs Fire near Nederland, as viewed from Sugarloaf, Colo., Sunday, July 10, 2016.

Firefighters have dropped more than 169,000 gallons of bright red fire retardant on the Cold Springs Fire in the foothills of Boulder County.

We wondered: What's in the slurry, and how can homeowners clean it up once the fire is out?

Shirley Zylstra, with the U.S. Forest Service's Wildland Fire Chemical Systems division based in Missoula, spoke with Colorado Matters host Ryan Warner about the retardant.

On why fertilizer is the active ingredient:

“Fertilizer is actually what the active ingredient is. So with the retardants, the water that they contain is more of the vehicle to get the fertilizer to the target. All that water can evaporate, but the fertilizer is actually what’s doing the work; it’s slowing down or stopping the fire. An actual chemical reaction is going on between the components of the fertilizer and the heat of the fire, that pulls the heat out of the fire and reduces that intensity.”

On why the retardant is red:

“The product that they’re dropping in Colorado is colored with something called iron oxide, and iron oxide is a natural compound; it’s rust basically. If you’ve ever driven along the road and you see those red clay-type soils and road cuts, that’s iron oxide. And so it’s a natural ingredient that’s put in there. We’ve colored the retardant so that the pilots, when they’re dropping it, they can see where the retardant is going. It’s important that they are able to connect those drops so that they build a continuous line of the retardant.”

On the toxicity of the ingredients used:

“It could be irritating. That fertilizer in there is a type of salt, and so if you can imagine getting table salt into a cut or into your eyes or something, it can kind of sting. One thing that we really like to emphasize before any product is used out on the national forests or any federal wild lands, is it goes through a pretty stringent qualification process. We look at things like erosion, we look at the stability, we look at how effective a product is. But we also look at its toxicity ... Before a product can be used, it has to meet the EPA’s rating of practically nontoxic for both aquatic and mammalian species. And that includes human health.”

The Boulder County Office of Emergency Management says while cleaning the red slurry may be difficult, it can be done. Here are some of their tips:

Structures (wood/metal)

  • The red color of retardants comes from iron oxide (rust), which can be difficult to remove.

  • Wash retardant off as soon as possible. Some of these products may discolor metal.

  • Dampening a stained surface with water, and then scrubbing it with a wet, stiff-bristled brush dipped in borax has been effective.

  • Power washers may drive the red colorant into the surface of the wood and should generally be avoided.

  • Restrict water use to prevent puddles that may be attractive to pets.

  • Avoid leaving standing puddles of water by using absorbent materials, such as sand, soil, or other materials.


  • Rinse retardant off of vegetation. Avoid leaving standing puddles of water by using absorbent materials, such as sand, soil, or other materials.

  • Leaf burn may occur since retardants contain levels of fertilizer higher than what is often sold at garden stores. This causes vegetation and plants to appear dead after contact; however, they will generally recover and grow back – usually within one to two months.

  • As with any garden produce, wash fruit and vegetables thoroughly before using.