Next-Gen Fire Retardant

Technical Review

Fortress has introduced the world’s first magnesium chloride based long-term fire retardant, and the only new fire retardant to achieve placement on the United States Forest Service’s (USFS) Qualified Products List (QPL) in over two decades. This technical review focuses on empirical testing data directly from the Forest Service – the very same testing that’s been required to qualify all currently available fire retardant products. In official Forest Service testing, Fortress products have outperformed legacy fire retardants on virtually every dimension. The empirical data demonstrates that:

(1) Fortress products are less toxic than legacy fire retardants (lower aquatic toxicity),
(2) Fortress products are more effective at suppressing fires (higher burn reduction index), and
(3) Fortress products are less corrosive (less uniform corrosion).

Aquatic Toxicity

The harmful effects of ammonium phosphate-based fire retardants on fish are well-established, with concerns stemming from laboratory studies and real-world firefighting incidents. Controlled experiments have demonstrated that ammonium phosphate-based fire retardant exposure can cause gill pathology, avoidance behavior, stunted development, lowered fitness, and death in chinook salmon populations [Dietrich 2013, Dietrich 2014]. In one publication, the authors concluded that full-strength ammonium phosphate fire retardants would need to have been diluted 2,000 to 4,000 times to eliminate any fish mortality in their study [Dietrich 2013]. Indeed, there are a number of recorded incidents of real-world retardant drops resulting in population-level fish kills [Pilliod 2003, Dietrich 2013]. Rightfully so, this well-documented side effect of ammonium phosphates has raised serious concerns about the environmental impact of legacy fire retardants.

Fortress fire retardants correct this longstanding issue. The detrimental effects of ammonium phosphates on fish populations have not been observed for magnesium chloride, even with similar laboratory testing [Hintz 2017] and the widespread use of magnesium chloride-based deicers and dust suppressants near natural water systems [Kunz 2015, Lewis 1999]. These scientific findings are corroborated by the Forest Service’s qualification testing, which clearly establishes the far and away superior aquatic safety of Fortress fire retardants (Figure 1).

Review the Data:
USFS Fish Toxicity of Long-Term Fire Retardants
USFS Fish Toxicity of Water Enhancer Concentrates & Colors
USFS Fish Toxicity of Class A Foam Concentrates

Fortress Products Are Less Toxic Than Legacy Fire Retardants

Fortress aquatic toxicity data from USFS product qualification testing

Figure 1: Aquatic toxicity of fire fighting chemicals as determined by USFS product qualification testing. A higher LC50 is more favorable, indicating lower toxicity. Aquatic toxicity is determined in the lab by exposing rainbow trout (Oncorhynchus mykiss) to various amounts of retardant over 96 hours. LC50 is the chemical dose at which 50% of the trout die over the course of exposure.

Burn Reduction

It’s understood that ammonium phosphate-based fire retardants work by catalyzing early charring and releasing water vapor from decomposed cellulose in wildland fuels [Forest Service]. Our magnesium chloride formulations provide additional reactions that hydrate fuels, cool the flame front, delay ignition, and actively disrupt combustion; ultimately providing superior fire suppression capabilities. The fire suppression mechanisms of magnesium chloride have been observed in the lab [Huang 2011, Kawamoto 2008, Wu 2008], and the superior efficacy of Fortress fire retardants been demonstrated empirically via the Forest Service’s burn reduction product qualification testing (Figure 2).

Burn tests are done by igniting retardant-treated test beds of Aspen Excelsior and Ponderosa Pine needles. Each product treatment is matched with a reference treatment of 10.6% weight diammonium phosphate (DAP), which emulates the formulations used in commercial ammonium phosphate-based fire retardants. Test beds are burned on the same day along with an untreated bed of the same fuel. Reduction index is calculated by comparing the rate of flame spread and rate of weight loss of the retardant beds to the untreated beds, and the overall reduction index is a composite of the Aspen Excelsior (Analog Fuel) and Ponderosa Pine (Natural Fuel) results. The 10.6% DAP benchmark was born out of the fact that legacy ammonium phosphate-based retardants have been the industry standard for multiple decades now.

As the USFS product qualification testing data shows, many products in the the Fortress portfolio have drastically outperformed the DAP benchmark (Figure 2), suggesting that our unique chemistry does indeed translate to more effective fire suppression.

Review the Data:
The official burn test results of Fortress products are only documented in proprietary communications between Fortress and the Forest Service. We’ve reported the official numbers here, but you can get in touch with us if you would like to request to review the official results.

Fortress Products Are More Effective Than Legacy Fire Retardants

Fortress burn reduction data from USFS product qualification testing

Figure 2: Overall burn reduction index of Fortress Fire Retardant products relative to 10.6% diammonium phosphate (DAP) reference as determined by official USFS product qualification testing. Higher values mean more effective burn reduction, which is more favorable.


The Forest Service’s required corrosion testing has thus far demonstrated that Fortress products are less corrosive than legacy fire retardants (Figure 3).

Uniform corrosion rates are determined by 90-day weight loss tests on metals relevant to chemical storage and application. Testing is done by preparing coupons of 2024-T3 aluminum, 4130 steel, and UNS C26000 yellow brass. The test coupons are weighed and then immersed in a test solution of the retardant for 90 days. At the end of the test duration, each coupon is cleaned, dried, and weighed, and the corrosion rates are calculated. Lower values are more favorable, as they indicate that there is less material loss to corrosion.

Intergranular corrosion is also tested on aluminum and magnesium to observe the potential for localized attack along the grain boundaries of the metals. In this test, the test coupons resulting from the uniform corrosion test are mounted, polished, and etched. The etched coupons are then examined microscopically at 500X magnification. No intergranular attack has been observed with Fortress products per official USFS product qualification testing.

Review the Data:
USFS Uniform & Intergranular Corrosion of Long-Term Fire Retardants

Fortress Products Are Less Corrosive Than Legacy Fire Retardants

Fortress uniform corrosion data from USFS product qualification testing in Missoula

Figure 3: Uniform corrosion data fire retardant products as determined by USFS product qualification testing in Missoula. Uniform corrosion rates are determined by 90-day weight loss tests on metals relevant to chemical storage and application. Values reported are the average of all replicates. Lower values are more favorable, indicating less corrosion. This same testing is done at the Forest Service's San Dimas facility. Follow this link to view corrosion data from San Dimas.


[Dietrich 2013] Dietrich, J. P., Myers, M. S., Strickland, S. A., Van Gaest, A., & Arkoosh, M. R. (2013). Toxicity of forest fire retardant chemicals to stream-type chinook salmon undergoing parr-smolt transformation. Environmental Toxicology and Chemistry, 32(1), 236–247.
[Dietrich 2014] Dietrich, J. P., Van Gaest, A. L., Strickland, S. A., Hutchinson, G. P., Krupkin, A. B., & Arkoosh, M. R. (2014). Toxicity of PHOS-CHEK LC-95A and 259F fire retardants to ocean- and stream-type Chinook salmon and their potential to recover before seawater entry. Science of the Total Environment,490, 610–621.
[Hintz 2017] Hintz, W. D., & Relyea, R. A. (2017). Impacts of road deicing salts on the early-life growth and development of a stream salmonid: Salt type matters.Environmental Pollution, 223, 409–415.
[Huang 2011] Huang, Q., Lu, G., Wang, J., & Yu, J. (2011). Thermal decomposition mechanisms of MgCl2·6H2O and MgCl2·H2O. Journal of Analytical and Applied Pyrolysis, 91, 159–164.
[Kawamoto 2008] Kawamoto, H.; Yamamoto, D.; Saka, S. Influence of neutral inorganic chlorides on primary and secondary char formation from cellulose. J. Wood Sci 2008, 54, 242–246.
[Kunz 2015] Kunz, B. K., & Little, E. E. (2015). Dust Control Products at Hagerman National Wildlife Refuge, Texas. Transportation Research Record: Journal of the Transportation Research Board, 2472(1), 64–71.
[Lewis 1999] Lewis, W. M. (1999). Studies of Environmental Effects of Magnesium Chloride Deicer in Colorado. Colorado Department of Transportation ResearchBranch, (November), 21–39. Retrieved from Accessed 03/08/2021.
[Pilliod 2003] Pilliod, D. S., Bury, R. B., Hyde, E. J., Pearl, C. A., & Corn, P. S. (2003). Fire and amphibians in North America. Forest Ecology and Management,178(1–2), 163–181.
[Wu 2008] Wu, Y.; Yao, C.; Hu, Y.; Zhu, X.; Qing, Y.; Wu, Q. Comparative Performance of Three Magnesium Compounds on Thermal Degradation Behavior of Red Gum Wood. Materials 2014, 7, 637-652.

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