Is Magnesium Chloride an Effective Fire Retardant?

Get the Facts

Fortress has introduced the world’s first magnesium chloride based long term fire retardant, and the first new fire retardant to achieve placement on the United States Forest Service’s (USFS) Qualified Products List (QPL) in over two decades. As we continue to challenge the current paradigm of fertilizer-based fire retardants and offer a superior alternative, the current industry incumbent has launched a full-scale campaign to misinform the public about magnesium chloride fire retardant, often with blatant disregard for the evidence.

With the upward trend in annual fire retardant use, the technology that we use to fight wildfires is more important now than ever before. At Fortress, we’ve made it our mission to save lives, lands, property, and planet by advancing the products that protect us from wildfires. We value honesty and transparency. It’s important to us that our claims are reinforced by credible evidence, and that we’re providing an honest account of ourselves and our peers in the fire retardant industry.

Below, we respond to many of the inaccurate claims that have been made about Fortress fire retardants and magnesium chloride, citing official Forest Service testing and contemporary scientific literature.

(1) You may have heard that magnesium chloride is not an effective fire retardant.

More specifically, you may have heard that “studies in 1954 and 1970 show that magnesium is far less effective than phosphate based retardants,” or that “magnesium chloride is not suitable as a fire retardant according to the Council for Scientific Industrial Research.” (link)


All long term fire retardants on the Forest Service’s QPL, including Fortress, have exceeded the same burn retardation efficacy tests as demanded by rigorous Forest Service testing. Official testing has shown that Fortress products have up to a 30% higher burn reduction index compared to the control chemical, 10.6% diammonium phosphate (DAP). It’s important to note that DAP is used as the benchmark in qualification testing precisely because it reflects the performance of fertilizer-based retardants, so if Fortress retardants are significantly outperforming the benchmark, it’s likely that they’re outperforming the commercial products too.

Figure 1: USFS burn test results. Higher values are more favorable, indicating superior burn reduction.

Given these findings, it’s evident that reports from 50 and 70 years ago are not at all an accurate representation of Fortress products or modern magnesium chloride fire retardant technology.

While MgCl2 is not new, the performance of Fortress products is. At Fortress, we found a way to take a material that has been discounted for the last 50 years, and developed innovative formulations with it that are not only more effective than ammonium phosphate retardants, but are better for the environment too.

(2) You may have heard that magnesium chloride is corrosive to aircraft and infrastructure.

More specifically, you may have heard that “salts, including magnesium chloride, are not approved for use in aircraft operational areas because they are corrosive to aircraft, according to the EPA.” (link)


According to official USFS corrosion testing, Fortress products have best in class corrosion protection for all qualified metals. A sample of this data is summarized in Figure 2, and all corrosion testing data is publicly available on the USFS website.

Figure 2: USFS corrosion test results for 4130 Steel at 120°F. Lower values are more favorable, indicating less corrosion.

(3) You may have heard that water retention and rehydration do not make magnesium chloride a more effective fire retardant.

More specifically, you may have heard that “long term retardants do not rely on water to be effective. Retardants physically block the fire, lower its flammability, and trigger a chemical reaction to stop fire in its tracks.” (link)


All long term retardants are effective in the absence of water, however wet retardants are more effective than dehydrated retardants because the water will consume energy, cool the flame front, and dilute combustion gasses as it evaporates.

MgCl2-based retardants, unlike ammonium phosphate, can rehydrate and become wet during diurnal swings if the relative humidity is above 33%. This added water acts as an additional retardant that is difficult to capture in laboratory testing.  We believe ground crews will be welcoming wet retardant lines early in the morning, days after Fortress products have been used.

(4) You may have heard that magnesium chloride is toxic to vegetation.

More specifically, you may have heard that “Magnesium chloride is crucial for plant growth, but high concentrations in the soil may be toxic or prevent plant life from easily accumulating water or nutrients.” (link)


This is true. It is well-known that excessively high concentrations of MgCl2 can be detrimental to certain vegetation, but it is also true that excessive amounts of fertilizer can burn vegetation. Any acute detrimental effects of MgCl2 are primarily related to its hygroscopic properties, and the concentrations to reach toxic levels or to impede nutrient uptake are way beyond what would occur under normal use of Fortress products for wildland fire applications.

Magnesium chloride road treatments have been used for de-icing and dust suppression at 2-3x higher concentration than what we use in Fortress long term fire retardants. This claim is likely informed by a study that was looking at magnesium chloride dust suppressant use, and does not accurately reflect the formulation or utilization patterns of Fortress fire retardants.

This claim is also interesting in light of the fact that fertilizer-based fire retardants promote the growth of fire-prone invasive species like cheatgrass when applied to wildlands [Besaw 2011, Marshall 2016], rendering treated areas more susceptible to fires in the future. This is a major point against ammonium phosphates.

(5) You may have heard that magnesium chloride is detrimental to aquatic wildlife.

More specifically, you may have heard that “a 2017 study by Colorado State University found that magnesium chloride can have detrimental effects on aquatic macroinvertebrates, an important food source for fish, amphibians and other wildlife.” (link)


According to official US Forest Service testing, Fortress liquid concentrate FR-200 has the lowest aquatic toxicity of any aerial long-term product on the market. FR-600, our ultra-durable ground retardant, also set a new standard in aquatic toxicity testing with zero kill at the highest tested level (5,000 mg/L). This data is summarized in Figure 3 and publicly available on the USFS website.

On the other hand, the harmful effects of ammonium phosphate fire retardants on fish and natural water systems are well-established, with concerns stemming from both laboratory studies and real-world firefighting incidents. Controlled experiments have demonstrated that ammonium phosphate fire retardant exposure can cause gill pathology, avoidance behavior, stunted development, lowered fitness, and death in chinook salmon populations [Dietrich 2013, Dietrich 2014].

Nutrient pollution and eutrophication are also a major concern associated with fertilizer-based fire retardant use. Fire retardant studies have demonstrated that even a single pulse of ammonium (NH3) and phosphate (PO4) in natural water reservoirs can stimulate toxic algae blooms and spur a decline in water quality [Angeler 2006].

While it is true that magnesium chloride can adversely affect aquatic maroinvertebrates [Kotalik 2017], this detriment does not measure up to the adverse effects of ammonium phosphate use.

Figure 3: USFS aquatic toxicity results. Higher values are more favorable, indicating lower toxicity.

(6) You may have heard that magnesium chloride retardants are heavier than fertilizer-based retardants, limiting the amount of retardant that airtankers can carry.

More specifically, you may have heard that “magnesium chloride retardants weigh more than fertilizer-based retardants, taking aircraft away from flight more often to refill, losing precious time needed to save property and lives.” (link)


It is true that ready-to-use Fortress products are slightly more dense than fertilizer-based retardants, but when you do the math, Fortress products only reduce the volume per tanker load by 2-3%. This slight reduction in tanker load is more than made up for by the fact that Fortress products provide several benefits over current fertilizer-based retardants, including efficacy per gallon and the ability to rehydrate with diurnal humidity fluctuations.


[Angeler 2006] Angeler, D. G., Rodríguez, M., Martín, S., & Moreno, J. M. (2004). Assessment of application-rate dependent effects of along-term fire retardant chemical (Fire Trol 934®) on Typha domingensis germination. Environment International, 30(3), 375–381.
[Besaw 2011] Besaw, L. M., Thelen, G. C., Sutherland, S., Metlen, K., & Callaway, R. M. (2011). Disturbance, resource pulses and invasion: Short-term shifts in competitive effects, not growth responses, favour exotic annuals. Journal of Applied Ecology, 48(4),998–1006.
[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.
[Kotalik 2017] Kotalik, C. J., Clements, W. H., & Cadmus, P. (2017). Effects of magnesium chloride road deicer on montane stream benthic communities. Hydrobiologia, 799(1), 193–202.
[Marshall 2016] Marshall,A., Waller, L., & Lekberg, Y. (2016). Cascading effects of fire retardant on plant-microbe interactions, community composition, and invasion. Ecological Applications, 26(4), 996–1002.

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