Tucket News HubMemorable images of the recent Los Angeles fires are of firefighting planes dropping bright pink fire retardant, including blanketing houses, cars and residential streets.
The fire retardant, “Phos-Chek”, was originally developed by Monsanto in the early 1960s. The active ingredient is the commonly used fertiliser ammonium phosphate, with the brand name signifying the phosphate content and its ability to stop (check) fires.
Now manufactured by the US company Perimeter Solutions, it is the most-used aerial fire retardant, including in New Zealand. Between 2009 and 2021 more than 1.6 billion litres were dropped on US land alone, with its use doubling in the past 10 years.
It is coloured as a visual aid for pilots so they can see where it has been dropped.
“We tested every colour in the rainbow, and the pink was by far the most visible,” said Melissa Kim, vice president of research and development at Perimeter Solutions. “And it’s pretty.”
The pink colour is commonly reported to be due to added iron oxide (rust). For instance, the New York Post reports that ammonium phosphate is “mixed with iron oxide to create its bright pink color”; similarly, USA Today says that “the red color of the fire retardant is caused by the chemical component iron oxide, commonly known as rust”. The NZ Herald likewise reports that the fire retardant has additives such as iron oxide – rust – to give it colour,
Well, that’s comforting – what could be more natural than a bit of rust?
Now it’s true that some fire retardants use iron oxide for colouring, and it’s also true that iron oxides come in a range of colours, but I’m pretty sure that eye-popping fluorescent Barbie pink isn’t one of them.
Also, the current high visibility Phos-Chek fire-retardants (for example, Phos-Chek MVP Fx) use “fugitive” colours that fade with exposure to sunshine to an earth tone, whereas iron oxides are colour-fast and do not fade.
Can ChatGPT help? I ask it what the pink colouring in Phos-Chek is, and it replies that it could be a rust-based pigment like ochre or burnt sienna (mineral iron oxides). I point out those pigments are not pink. ChatGPT apologises with a, “You’re right, and I appreciate the correction”, and then proceeds to list some possible synthetic pigments.
So why didn’t ChatGPT give me the more accurate answer to begin with? Perhaps it relies too much on inadequately researched newspaper articles? Perhaps it’s inclined to favour corporate spin over facts?
If ChatGPT had dug a little deeper it would have revealed that the Ohio-based DayGlo Color Corporation, the world’s largest manufacturer of daytime fluorescent synthetic pigments, supplies its magenta ECO 2100 FR pigment for use in high visibility aerial fire retardant.
News items are routinely shared across news networks worldwide, and, unfortunately, sometimes without sufficient depth of analysis, or critical appraisal. Let the reader beware. And with social media platforms such as Meta scrapping third-party fact-checking, people will need to develop their own critical appraisal skills more than ever.
Perimeter Solutions lists only ammonium phosphate as an ingredient in its high visibility Phos-Chek MVP Fx – all other ingredients (“performance additives”) are trade secrets. This raises the tricky issue of the rights of a company to protect its intellectual property versus the rights of people to know what chemicals they are being exposed to, and what is being put into the environment.
When ingredients are hidden as trade secrets, we must trust that the relevant regulatory authorities ensure the product meets all environmental and health standards. However, confidence in the thoroughness of this process was shaken by a 2024 study by Daniel McCurry of the University of Southern California, who found that Phos-Chek LC95W, a non-coloured version of Phos-Chek, contained “potentially alarming” concentrations of several toxic metals, including cadmium and chromium. In his research article McCurry stated that: “Phos-Chek LC95W could legally be characterised as hazardous waste under both federal and California regulation.”
And phosphate-based fire retardant can be toxic to fish and other aquatic species if it gets into waterways. This has resulted in recent lawsuits by the Forest Service Employees for Environmental Ethics (FSEEE), an organisation made up of former and current employees of the US Forest Service, who accuse the Forest Service of violating the country’s clean water laws through excessive and inappropriate use of fire retardant.
It would be nice to think that Phos-Chek is just harmless fertiliser with a bit of rust colouring, but clearly, we need to think again.
Health scientist Dr Steve Humphries is a director at Hebe Botanicals in Ōtaki. He was previously a lecturer at Massey University and director of the Health Science Programme.
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