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What's So Great About Red Slurry?

U.S. Air Force photo/Senior Master Sgt. Dennis Martin
US Forest Service/Public Domain

As of 6pm Wednesday, air tankers flying out of the JeffCo tanker base had dropped 58,557 gallons of retardant on the High Park Fire. So what is the red stuff falling out of the planes? How does it work?

The phosphorous mixture helps to create containment lines by coating vegetation in front of the fire. When the fire interacts with the retardant, a chemical reaction occurs which creates only carbon and water—preventing flammable combustion.


Bernie Post is a technical representative for Ontario, California based Phos-Chek. The company produces retardant, gel, and foam used in fighting fires. Post says the retardant mixture is safe, but remains a chemical product.

“It’s phosphorus, and if you get a lot of it on your hands, you need to wash if off and you should probably put some lotion on your hands because it’s a salt and it will dry our skin out.”

But why is it red? Is it because of the chemical make up of the slurry? Nope. Post says red dye is added to the mixture so fire fighters on the ground can see where the retardant has dropped. Without it, the mixture would be almost invisible.

Post says water is mixed with the retardant simply for use as a carrier. Once the slurry coats the vegetation, the water evaporates and the mixture stays. If a large rain storm occurs, the slurry washes away leaving behind fertilizer that actually helps restore the land around the fire.

But is it truly safe?

Two federal lawsuits filed in 2003 and 2008 claimed the mixture was toxic to animals and especially small fish in lakes where runoff of the slurry entered the water ecosystem.

Late last year, Forest Service Chief Tom Tidwell released a report which allowed the continued use of retardant to fight fires. However, he created no-drop zones around water ways or areas with threatened or endangered species. Those places can still receive a slurry drop, but only if structures or lives are atrisk [.pdf].

That's a lot of slurry.

Here's is a list of the large air tankers dropping slurry on the High Park Fire, and how much retardant they can hold. Thanks to Jennifer Jones of the Forest Service for providing these stats.

  • P2-Vs can carry about 2,082 gallons of retardant. These are on exclusive use contracts.
  • CV-580s carry about 2,100 gallons of retardant. These are on exclusive use contracts.

The US Forest Service has other larger tankers fighting fires in other parts of the country.

  • The BAe-146 can carry about 2,800 gallons of retardant and is on an exclusive use contract – it is considered a “next generation” air tanker
  • The DC-10 can carry about 11,800 gallons of retardant and is on a temporary “call when needed” contract with the Forest Service.

ED: Here's a safety sheet published by the US Forest Service about fire retardant. They say it's safe, but some precautions are necessary if you come in contact with the product [pdf].