Advances and Environmental Impact Assessment of Forest Fire Extinguishing Agents

Abstract

In the context of climate change, increasingly severe forest fires present a significant threat to lives, property, ecosystem functionality, and the sustainable development of forest resources. As a result, there is an urgent need for rapid, efficient, and environmentally friendly technologies for fire suppression and containment. This paper begins by reviewing the current research on forest fire extinguishing agents and materials that hold promise for effective fire suppression. Among these agents, gaseous and foam extinguishing materials exhibit drawbacks such as low efficiency or significant environmental hazards. In contrast, natural polymer hydrogels, which are high in water content, environmentally friendly, and biodegradable, show significant potential for developing clean and efficient extinguishing materials. Furthermore, this paper discusses existing environmental assessment standards for fire extinguishing agents, as well as the assessment systems proposed in various studies. It finds that, while universal assessment standards are fairly well-established, current research primarily focuses on enhancing fire suppression performance. However, the environmental performance assessment of forest fire extinguishing agents—often used in large quantities—remains inadequate. Therefore, there is an urgent need to establish a comprehensive and systematic environmental assessment system to address this theoretical and practical gap.

1. Introduction

Forests are a vital component of the Earth’s ecosystem, providing essential ecological services such as maintaining biodiversity, regulating the global climate, facilitating the carbon cycle, and ensuring water supplies [1]. However, as climate warming intensifies, the increasing frequency and severity of record-breaking forest fires pose a significant threat to the stability of forest ecosystems and the sustainable development of forest resources. From 2000 to 2021, the world lost 2.3 million square kilometres of forests, with fires being one of the leading causes of this loss [2,3]. The effects of forest fires are far-reaching. They lead to the degradation of ecosystem services and the loss of biodiversity, while also releasing vast amounts of greenhouse gases through combustion. This contributes to global warming, alters regional water cycles, and increases the likelihood of large, severe fires, creating significant risks for the entire Earth’s ecosystem [4,5]. For instance, carbon emissions from Canada’s forest fires in 2023 reached as high as 647 TgC, ranking just behind the fossil fuel emissions of India, China, and the United States [6]. Moreover, forest fires pose serious threats to human lives and often result in substantial economic losses [7,8]. For example, the 2018 California wildfires caused over 80 deaths and the destruction of nearly 19,000 buildings [9]. Similarly, the 2023 Hawaii wildfires resulted in 115 deaths, 850 people missing, and the destruction of 2207 buildings [10,11]. Delays in suppressing fires during their early stages and ineffective containment of their spread are key factors that contribute to the escalation of fire size and the resulting ecological and economic damage.

In recent decades, countries have implemented various measures to prevent and control forest fires. These strategies include creating firebreaks, digging fire ditches, and conducting planned management and burning of combustible materials [12,13,14]. However, these methods have limited effectiveness in both prevention and control. They can be challenging to manage, involve high economic costs, and may negatively impact ecosystems. Water is the most commonly used agent for suppressing forest fires, but its effectiveness is limited. It has a short retardation time to prevent the large-scale spread of fires. Traditional fire extinguishing agents such as diammonium phosphate, monoammonium phosphate, and ammonium sulphate are frequently used for forest fire suppression. Unfortunately, their flame-retardant efficiency is often inadequate, especially during high-intensity fires under extreme weather conditions, and their ecological compatibility is relatively poor [15,16,17]. Given these challenges, the research and development of highly effective and environmentally friendly forest fire extinguishing agents is crucial. A systematic review of the progress in this area, along with thorough environmental assessments, is essential for enhancing forest fire prevention and control technologies. This paper aims to review the research findings on extinguishing agents and materials that have potential for forest fire suppression, as well as the environmental assessment indicators associated with these agents. The goal is to provide theoretical references for the research, development, environmental assessment, and applications of them.

2. Advances in Fire Extinguishing Agent Research

2.1. Gaseous Extinguishing Agents

Gaseous fire extinguishing agents are commonly used in fire protection due to their advantages, including rapid extinguishing speed, broad application range, and minimal damage to protected objects. Currently, these agents are primarily utilized in specific scenarios such as electronic facilities, engine compartments, and archives, and they are rarely reported in the context of forest fires [18]. Gaseous fire extinguishing agents can be categorized into two types: natural and synthetic. Their fire-extinguishing effectiveness is based on either the asphyxiating properties of the gas itself or on interrupting the combustion chain reaction.

Natural gaseous fire extinguishing agents include carbon dioxide and inert gas extinguishing agents. Inert gas extinguishing agents consist of naturally occurring gases found in the atmosphere, such as nitrogen (N₂), argon (Ar), and their mixtures. These natural gaseous fire extinguishing agents offer several advantages, including relatively high density, stable properties, no depletion of the ozone layer (ODP), and low global warming potential (GWP) [19]. They primarily extinguish fires by asphyxiation. However, their application is limited by certain drawbacks, such as a low safety margin and poor fire-extinguishing efficiency [20].

Chlorine- or bromine-containing halon fire extinguishing agents were commonly used in the past due to their excellent fire-extinguishing performance. However, because of their harmful effects on the ozone layer, they have been phased out in accordance with the Montreal Protocol [21]. As substitutes for halon, hydrofluorocarbons (HFCs) such as heptafluoropropane, pentafluoroethane, and trifluoromethane have been employed. While these substances do not deplete the ozone layer, their high GWP has led to strict regulations regarding their use [22]. There is a growing focus on finding alternative fire extinguishing agents that exhibit superior fire-extinguishing efficiency while being environmentally friendly, specifically having no ozone depletion potential (ODP) and a low GWP. One such agent, perfluoro(2-methyl-3-pentanone) (C₆F₁₂O), has gained international attention due to its non-toxic and non-flammable properties, as well as its excellent electrical insulation and fire-extinguishing capabilities. This substance has been successfully utilized in various applications, including electrical and electronic equipment, aircraft, and marine vessels [23]. However, using extinguishing agents that contain per- and polyfluoroalkyl substances (PFAS) in large-scale forest fires presents two major challenges. First, these agents can produce highly toxic byproducts, such as hydrogen fluoride (HF) and carbonyl fluoride (COF₂), during the fire suppression process, which poses significant risks to human health and the ecological environment [24]. Second, the European Chemicals Agency (ECHA) has proposed a ban on the production and use of PFAS starting in 2023, leading to regulatory restrictions on their application [25]. Additionally, single gaseous extinguishing agents have inherent limitations in complex, open environments. For example, experimental studies on lithium-ion battery (LIB) fires have shown that although CO₂ and heptafluoropropane can suppress open flames, their limited cooling capacity increases the risk of reignition. In contrast, water mist has proven to be more effective in preventing reignition due to its superior cooling capabilities. Specific data indicate that compared to having no fire extinguishing agents, the peak average temperatures recorded before the depletion of CO₂, heptafluoropropane, and water mist were reduced by 43, 75, and 133 °C, respectively [26]. It is anticipated that the insufficiency of cooling efficacy observed in small-scale experiments will be further exacerbated in complex and fully open forest fire scenarios.

From a practical perspective in forest fire suppression, the synergistic use of different extinguishing agents—especially the combination of gas and liquid—holds great potential. When a forest fire occurs, water is often the simplest and most effective method for extinguishing the flames. However, the upward movement of the fire plume and the high temperatures generated during combustion can cause most of the water to evaporate before it reaches the base of the flame, significantly reducing its effectiveness [27]. Gaseous fire extinguishing agents can quickly snuff out the flame, allowing water to reach the base more easily and lowering the surface temperature of combustible materials [28,29]. For instance, when C₆F₁₂O (a gaseous agent) is used in combination with water mist to extinguish lithium-ion battery fires, the suppression time is just 1 s, which is markedly shorter than the 235 s required when using water mist alone, showcasing excellent fire-extinguishing effectiveness. Although the combination of CO₂ and water mist has a longer extinguishing time of 21 s, it is more cost-effective. In enclosed spaces, combining inert gases with water mist can significantly enhance the effectiveness of fire suppression [30]. For a 1.7 kW heptane pool fire, the synergistic combination of water mist and nitrogen (N₂) achieved an extinguishing time of approximately 102 s, which is 18.4% shorter than using N₂ alone and 83.4% shorter than using water mist alone [31]. However, in non-enclosed forest environments, the effectiveness of gas–liquid synergistic extinguishing is greatly influenced by environmental factors, necessitating further simulation studies and experimental verification regarding elements such as wind speed and terrain. Moving forward, the application of gas–liquid synergistic fire extinguishing technology in forest fire prevention and control should focus on optimizing the selection of gas types, precisely regulating the gas–liquid ratio, and designing efficient fire extinguishing systems tailored to specific forest terrains and fire characteristics. Additionally, prioritizing the use of environmentally friendly gases in combination with water can improve fire-extinguishing efficiency while minimizing potential harm to the ecological environment.

2.2. Foam Fire Extinguishing Agents

Foam fire extinguishing agents were first proposed by Johnson for use in firefighting as early as 1877 [32]. These agents can mix with water to create fire-extinguishing foam through either chemical reactions or physical methods. Based on the materials used in their formulation, foam fire extinguishing agents are categorized into two types: protein-based and synthetic foaming agents. The mechanism by which foam extinguishes fire can be summarized in three main effects: the cooling effect from water evaporation, the smothering effect of the foam covering the surfaces of combustible materials, and the heat radiation shielding effect, which reduces the decomposition rate of these materials [33].

Protein-based foam fire extinguishing agents are among the earliest types used for extinguishing fires, primarily composed of hydrolyzed concentrates of animal or vegetable proteins. However, they come with inherent drawbacks, including the strong odour associated with animal proteins and the high extraction costs of vegetable proteins. Continuous efforts have been made to improve these agents. For instance, Yan et al. [34] employed ultrasonic alkaline hydrolysis to recover proteins from activated sludge for preparing foam fire extinguishing agents. This method not only prevented secondary pollution from activated sludge but also eliminated the issues of strong odours and high costs. Additionally, Zaggia et al. [35] introduced 0.5% of perfluoroalkyl quaternary ammonium salts to protein foam, resulting in fluoroprotein foam fire extinguishing agents. The incorporation of fluorocarbon surfactants enhanced the fluidity of these protein foams, increasing their fire-extinguishing effectiveness by three to four times. It is important to note that fluorosurfactants are part of the chemical family of PFAS. Due to their surface-active properties, they have long been essential components of fluorine-based Class B foams [36]. However, the degradation products of fluorosurfactants—such as perfluorooctane sulfonates (PFOS), perfluorooctanoic acid (PFOA), and their salts—present significant environmental hazards, which has limited the use of fluorine-containing fire extinguishing agents [37,38].

Synthetic foam fire extinguishing agents are created by combining various types of surfactants with auxiliary substances such as foam stabilizers, anti-burning agents, and anti-corrosion agents. These agents are primarily used for flammable liquid fires [39]. Among them, aqueous film-forming foam (AFFF) is particularly notable for its excellent fire-extinguishing performance. However, its use is limited because it contains fluorocarbon surfactants. Current research and development efforts concerning fluorinated AFFF focus on replacing long-chain surfactants with short-chain fluorocarbon surfactants. For instance, Jia et al. [40] synthesized a novel fluorocarbon surfactant based on perfluorobutyl groups. When this surfactant was mixed with hydrocarbon surfactants, the extinguishing times were under 65 s, while the burnback times exceeded 14.5 min, highlighting its significant potential for application in AFFF. Similarly, Yang et al. [39] developed a perfluorinated branched short-chain anionic fluorocarbon surfactant, which, when combined with cetyltrimethylammonium bromide in a fire-extinguishing formulation, extinguished flames in just 31 s. While short-chain PFAS compounds are believed to have lower bioaccumulation and toxicity, making them safer than their long-chain counterparts [41,42], concerns remain regarding their long-term persistence in the environment and potential environmental risks. Additionally, while short-chain fluorocarbon surfactants can rapidly extinguish most fires, they do have drawbacks, such as low thermal stability and an insufficient cooling effect [43].

With the restriction of PFAS-containing foam fire extinguishing agents, there has been significant interest in the development and application of fluorine-free foam (F3) fire extinguishing agents. Unlike AFFF, F3 agents do not create an aqueous film on the fuel surface. Instead, their fire-suppression performance primarily depends on their foam tension, foaming capacity, and foam diffusivity [44]. F3 fire extinguishing agents typically consist of proteins, hydrocarbon surfactants, or organosilicon surfactants. The stability of the foam and the effectiveness of fire extinguishing can be enhanced through a synergistic combination of siloxane polyoxyethylene ether and alkyl polyglycoside surfactants. This mixed surfactant system has demonstrated a foam degradation time that is 3.5 times longer and an extinction performance that is five times better compared to the individual components [45]. Additionally, incorporating nanocellulose as a synergistic stabilizer can improve the dispersibility of nano-silica, further enhancing foam stability and fire-extinguishing efficiency. In small-scale tests, the 1227/SiO₂/nanocellulose foam system successfully extinguished flames within 30 s, matching the efficiency of commercial AFFF [46]. Since F3 foam fire extinguishing agents do not contain PFAS compounds, they potentially reduce the hazards associated with fluorinated foam. However, previous studies indicate that, compared to C6 AFFF, most commercially available F3 agents exhibit equivalent or greater toxicity to aquatic organisms [47], higher toxicity to plants [48], and adverse effects on nematodes [49]. High concentrations of surfactants directly entering forest soil may disrupt the soil microenvironment, posing potential toxicity to soil microorganisms and plant roots; however, research in this area regarding forest fire scenarios is still in its early stages. Moreover, the rheological properties of extinguishing agents are vital for their distribution patterns on the ground [15]. In complex forest terrains, such as steep slopes, these agents may be susceptible to rapid runoff, which reduces retention time and effectiveness. Therefore, the large-scale application of these agents in forest fires requires careful evaluation.

2.3. Hydrogel Fire Extinguishing Agent

In recent years, the development of hydrogel fire-extinguishing materials and their products has garnered significant attention, leading many of these agents to move towards commercial development [50]. Hydrogels improve the adhesion between water and material surfaces, enhance entrapment, cooling, and barrier capabilities, thereby increasing the efficiency of water-based fire extinguishing methods [51]. Their inherent adhesive properties effectively minimize water loss during the fire extinguishing process. Research indicates that the amount of gel needed to extinguish tree fires is only 54% of the water typically required [52]. This advantage makes hydrogels particularly practical and efficient in water-scarce regions that are prone to frequent forest fires [53]. Common types of hydrogel fire extinguishants include thermosensitive hydrogel extinguishants, hydrogel foam extinguishants, hydrogel dry water extinguishants, and conventional hydrogel extinguishants.

Table 1. The formation principles and properties of different types of hydrogel fire extinguishants.

Types of Hydrogel Extinguishants	Principle	Characteristics	Physical State
Thermosensitive type	Interaction between hydrophilic and hydrophobic groups	Temperature-responsive	Solution/gel
Foam type	Physical interaction of surfactant molecules	High thermal stability	Foam
Dry water type	Core–shell structure formation by silica and water	Dual physical-chemical inhibition	Powder
Common type	Natural polymer	Foam	Gel

compares the formation principles, characteristics, and physical states of these four key types of hydrogel fire extinguishing agents.

Thermosensitive hydrogel fire extinguishants can undergo a solution-to-gel phase transition at a specific temperature known as the lower critical solution temperature (LCST) [54]. When the temperature is below the LCST, these thermosensitive hydrogels exist in a solution state characterized by high fluidity and low viscosity, making them easy to transport. However, when the temperature rises above the LCST, hydrophobic groups within the hydrogel attract and adhere to one another, while hydrophilic groups orient outward to form micelles. This process leads to a volume phase transition, resulting in decreased surface activity and surface tension. Consequently, this allows the hydrogel solution to spread more effectively on the surfaces of combustibles during fire extinguishing [55]. Importantly, this phase transition is reversible. Once the fire is extinguished and the temperature decreases, the hydrogel reverts to its initial solution state. This makes thermosensitive hydrogel fire extinguishants a promising smart material in fire suppression applications [50]. For instance, a thermosensitive hydrosol composed of N-isopropylacrylamide and sodium acrylate (P(NIPA-co-SA)) has demonstrated a significant inhibitory effect on coal spontaneous combustion, achieving an inhibition efficiency of up to 77.6% against the total heat release from coke coal [56]. Additionally, a thermosensitive hydrogel synthesized from methyl cellulose, sodium acrylate, and magnesium chloride exhibits fire-extinguishing performance that significantly exceeds that of two commercial fire extinguishants, reducing the total extinguishing time by over 50% and effectively preventing re-ignition [55].

The key components of gel foam fire extinguishants include foam-producing surfactants, gelling agents, and crosslinking agents. Compared to traditional foam fire extinguishants, gel foam fire extinguishants have superior water retention and fire-extinguishing efficiency, which also enhances the stability of the foam. For instance, a novel foam gel made from modified alkyl glycoside, developed by Wang et al. [57] effectively reduces the oxidation reaction rate of combustibles. This is achieved by increasing the terminal temperature of the oxidation stage by 45.79 °C and achieving a suppression rate of 64.07% for carbon monoxide (CO) generation. Additionally, the heating rate for this gel is significantly lower than that of raw coal samples under high-temperature conditions. Li et al. [58] prepared a high water retention foam gel using sodium carboxymethyl cellulose as the matrix. Their gel exhibited a high water absorption ratio of 415 and a good water retention rate of 52% at 100 °C, achieved through the graft copolymerization of acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid. However, the gel products in these studies contain chemical components that are difficult to degrade and may pose a risk of contaminating soil and groundwater. In contrast, Tian et al. [59] developed a protein gel foam fire extinguishant that uses hydrolyzed protein and sodium alcohol ether sulfate as composite foaming agents, along with sodium alginate as the gelling agent. Both hydrolyzed protein and sodium alginate are non-polluting natural organic compounds. This material demonstrates significant fire-extinguishing properties, with a 90% burnout time that is 54% longer compared to commercial fluorinated foam fire extinguishants. However, the cost of this material is relatively high. Balancing its environmental friendliness with economic efficiency is crucial for its industrialization.

Dry water (DW) is a flowable powder created by mixing hydrophobic silica with an aqueous solution under high-speed stirring, resulting in an internal water content of over 90% [60]. The hydrogel dry water fire extinguishing agent combines the benefits of both dry powder and water mist. The addition of hydrogel significantly enhances the stability of the core–shell structure of dry water [61]. The hydrogel fire extinguishing agent features a large specific surface area and high water content, which improve its cooling effect, fire-extinguishing ability, and overall efficiency. In comparison to traditional dry water fire extinguishing agents, hydrogel dry water fire extinguishing agents demonstrate a notable 20% reduction in water loss rate. Additionally, their pressure resistance and stability are significantly enhanced [62]. Gel-based dry water fire extinguishing agents can overcome the limitation of dry water cracking under external pressure, withstanding pressures more than four times higher than those applicable to conventional dry water. This capability significantly boosts its stability. The performance of hydrogel dry water fire extinguishing agents surpasses that of ABC dry powder, achieving extinguishment rates that are 61% faster and cooling rates that are 94% higher [63]. This innovative formulation shows promising potential for application in forest fire management. However, the widespread use of silicon in these agents raises concerns about its impact on forest soils and the negative effects of silicon on aquatic environments [49].

Universal hydrogel fire extinguishants are characterized by their high water content, excellent adhesion, lack of foaming, insensitivity to temperature, and absence of a core–shell structure. These agents have demonstrated notable effectiveness in firefighting within enclosed spaces, such as coal mines. For instance, a composite material made from kaolinite micropowder-based superabsorbent polymer and ascorbic acid can significantly reduce the risk of spontaneous coal combustion by achieving a 94% reduction in low-temperature oxygen consumption [64]. The physical inhibitors within this material can form an oxygen-impermeable film on the surface of coal at low temperatures, while chemical inhibitors react with free radicals at higher temperatures. Additionally, superabsorbent hydrogel materials created from acrylamide and bentonite can diminish combustion and heat release in oxygen-containing bio-based materials by scavenging free radicals [65]. However, improvements are still needed in their adhesion and fire-extinguishing efficiency at high temperatures. The phosphoric acid-modified methyl cellulose/silica hydrogel requires 23% less gel than standard methyl cellulose hydrogel and has a 38% shorter fire-extinguishing time [66]. Nevertheless, its overall fire-extinguishing performance is still considered insufficient.

Currently, most research on hydrogel fire extinguishants is concentrated in industrial sectors, particularly coal mining. However, their application in forest fire prevention, containment, and suppression is still in its early stages. This area faces two significant challenges: performance and environmental impact. Firstly, there is a lack of assessment regarding their fire-blocking efficiency, specifically their ability to slow the spread of fire and resist reignition in real forest environments, which often include complex terrains and various vegetation types. This raises uncertainty about whether the impressive results seen in laboratory settings can be replicated in the field [15]. Secondly, the ecological impact of hydrogel fire extinguishants has not been systematically evaluated. This includes their effects on ecosystems, such as aquatic organisms, soil properties, microbial community structures, and plant growth and development. To advance the practical use of hydrogels in forest fire suppression, it is urgent to establish a comprehensive evaluation system that encompasses “performance optimization, environmental friendliness, and cost controllability.”

3. Evaluation System of Fire Extinguishants

3.1. Fire Extinguishant Standards

China’s current national standard system for fire extinguishants includes four main categories: gas, foam, water-based, and dry powder. Table 2 provides a summary of the national standards for these four types of fire extinguishing agents. The standard system for gas fire extinguishants is quite comprehensive. It features common agents such as carbon dioxide [67] and inert fire extinguishing agents [68] as well as specialized agents like heptafluoropropane (HFC227ea) [69] hexafluoropropane (HFC236fa) [70], bromotrifluoromethane [71], and bromochlorodifluoromethane [72]. Revised national standards for carbon dioxide fire extinguishing agents [73] and inert fire extinguishing agents [74] will take effect on 1 December 2025. These updated standards further tighten requirements for technical performance, standardize testing methods, and enhance quality control for gas fire extinguishants.

In the field of foam fire extinguishants, the existing standards—namely the Foam extinguishing agent [75] and the Class A foam extinguishing agent [76]—establish performance evaluation indicators for both general-purpose and Class A foam fire extinguishants. However, neither standard includes an assessment of environmental performance. Additionally, the Powder extinguishing agent standard [77] specifies more than ten performance indicators, but it also lacks indicators for evaluating environmental performance.

The current standard for water-based fire extinguishants, known as Water-based extinguishing agent [78], is set to be replaced by a new one [79]. This new standard enhances the original physicochemical properties and fire-extinguishing indicators by adding technical requirements such as the freezing point of the stock solution, the pH level, the corrosion rate, stability, acute oral toxicity, eye irritation, and permeability. These additions significantly improve the comprehensiveness of the evaluation system. Although China’s environmental evaluation system for fire extinguishants has made notable progress in recent years compared to Europe and the United States, it still has structural shortcomings. Surprisingly, there are very few studies that conduct comprehensive evaluation tests in line with national standards. Most of the existing research focuses primarily on fire-extinguishing performance and the physical and chemical properties, while often neglecting important aspects such as corrosivity, toxicity, and environmental and ecological impacts [52,80,81]. For instance, when examining hydrogel fire extinguishants, the evaluation mainly centers on microscopic characterization and fire-extinguishing performance tests [50]. While most natural polymer-based materials are known to be low-toxic or even non-toxic, their direct and indirect effects on forest ecosystems—particularly when used in forest fires—have not been systematically investigated.

Table 2. Current standards for Chinese fire extinguishants.

Types of Fire Extinguishants		Current Standards
Gaseous fire extinguishants	Fire extinguishing agent—carbon dioxide	[67]
	Inert fire extinguishing agent	[68]
	HFCs fire extinguishing agents	[82]
Foam extinguishing agent	Foam extinguishing agent	[75]
	Class A foam fire extinguishants	[76]
Powder extinguishing agent	-	[77]
Water based extinguishing agent	-	[78]

Table 2. Current standards for Chinese fire extinguishants.

Types of Fire Extinguishants		Current Standards
Gaseous fire extinguishants	Fire extinguishing agent—carbon dioxide	[67]
	Inert fire extinguishing agent	[68]
	HFCs fire extinguishing agents	[82]
Foam extinguishing agent	Foam extinguishing agent	[75]
	Class A foam fire extinguishants	[76]
Powder extinguishing agent	-	[77]
Water based extinguishing agent	-	[78]

Starting in the 1920s, the National Fire Protection Association (NFPA) released the Standard on Carbon Dioxide Extinguishing Systems (NFPA 12-1929) [83], which was the world’s first systematic standard for gaseous fire extinguishants. This standard laid the groundwork for the development of subsequent standards in the field. Over the years, an extensive environmental evaluation standard system has been established, covering various types of fire extinguishants and application scenarios. The Standard for Low-, Medium-, and High-Expansion Foam (NFPA 11-2024) [84] imposes strict restrictions on the use of traditional fluorinated surfactants, such as AFFF, Alcohol-Resistant AFFF (AR-AFFF), Film-Forming Fluoroprotein (FFFP), and other PFAS. Additionally, the Standard on Clean Agent Fire Extinguishing Systems (NFPA 2001-2025) [85] introduces toxicological parameters, including key indicators such as the 4 h median lethal concentration (LC50) in rats, the No Observed Adverse Effect Level (NOAEL), and the Lowest Observed Adverse Effect Level (LOAEL). It also sets occupational exposure limits for hydrogen fluoride (HF), a decomposition product of fluorocarbon fire extinguishants. Furthermore, the Standard for Water Additives for Fire Fighting and Steam Mitigation (NFPA 18A-2022) [86] requires mammalian toxicity tests, which include assessments for acute oral toxicity, acute dermal toxicity, acute eye irritation, and acute skin irritation. It also mandates aquatic organism toxicity tests and biodegradability assessments. Specifically, for the determination of LC50 in aquatic toxicity tests, it specifies using 10 rainbow trout aged 60 ± 15 days post-hatching, kept under static conditions at 12 ± 1 °C for 96 h.

The European Union’s regulation on the registration, evaluation, authorization, and restriction of chemicals (EC No 1907/2006) [87], issued in 2006, defines persistent, bioaccumulative, and toxic (PBT) substances, as well as very persistent and very bioaccumulative (vPvB) substances. The regulation specifies these definitions based on measurable criteria, including persistence half-life, bioconcentration factor, and toxicity threshold. It also outlines ecotoxicological testing requirements for substances across different tonnage categories, addressing key areas such as aquatic toxicity, biodegradability, and the impacts on soil and sediment. In 2008, the Classification, Labeling, and Packaging of Chemicals regulation (EC No 1272/2008) [88] further strengthened these requirements. Fire extinguishants containing PBT substances must be labeled and are subject to usage restrictions. Toxicity is assessed through tests such as the 96 h LC₅₀ for rainbow trout and the EC₅₀ for daphnia. Additionally, Foam Concentrates (EN 1568-3:2018) [89], developed by the European Committee for Standardization (CEN), introduces innovative multi-dimensional toxicological tests. These include bacterial and mammalian tests for foam concentrates, which further establish a comprehensive environmental protection evaluation system.

Table 3. Environmental evaluation indicators of some standards.

Standard	Environmental Evaluation Indicators	Environmental Indicator Evaluation Tests
GB 17835-2024 [79]	1. Acute fish toxicity 2. Acute oral toxicity 3. Eye irritation 4. Corrosion rate (impact on environmental media)	1. Acute fish toxicity: Using zebrafish, exposed at (20 ± 2) °C for 96 h, observing mortality 2. Acute oral toxicity: Oral gavage in mice, observing mortality within 3 days 3. Eye irritation: Dropping samples into the right eye of rats, observing the eye opening time within 60 min 4. Corrosion rate: Measuring mass loss of Q235A steel sheets and 3A21 aluminum sheets after immersion in samples, and calculating the corrosion rate
NFPA 2001-2025 [85]	1. ODP 2. GWP 3. Mammalian toxicity (NOAEL, LOAEL) 4. Impact of a low-oxygen environment caused by inert gases 5. Toxicity of decomposition products (e.g., acid gases)	1. Determination of ODP/GWP: With reference to methods in IPCC and the Montreal Protocol, specifying GWP (100-year scale) and ODP values of various agents 2. Mammalian toxicity test: Halocarbons: Determining NOAEL and LOAEL through cardiac sensitization tests and acute exposure tests in accordance with Significant New Alternatives Policy procedures Inert gases: Assessing the impact of low-oxygen environment on personnel, and determining exposure limits through oxygen concentration monitoring 3. Low-oxygen environment test: Measuring oxygen concentration in the enclosure after inert gas system discharge to ensure compliance with personnel exposure time limits (e.g., for concentrations of 43–52% corresponding to oxygen concentrations of 12–10%, exposure time ≤ 3 min) 4. Decomposition product test: Detecting concentrations of acid gases such as HF produced by halocarbon combustion through high-temperature decomposition tests
EC No 1907/2006 (REACH) [87]	1. PBT 2.vPvB 3. Aquatic ecotoxicity (fish, invertebrates, algae) 4. Degradability (biodegradation, hydrolysis)	1. PBT/vPvB assessment: Combining data on degradability, bioaccumulation, and toxicity 2. Aquatic toxicity test: Short-term/long-term fish toxicity, acute toxicity of Daphnia magna, and algal growth inhibition test 3. Degradation test: Ready biodegradability test and hydrolysis stability test 4. Adsorption/desorption test: Evaluating adsorption potential in soil and sediment 5. Soil toxicity test: Toxicity test on soil invertebrates and test on the impact on soil microbial activity

presents the environmental evaluation indicators for various fire extinguishant standards.

It is important to highlight that existing international standard systems primarily focus on assessing water-related environmental risks while insufficiently addressing soil ecosystems. This issue is particularly evident in scenarios involving fire extinguishants used for forest fires. When these fires are extinguished, a significant amount of fire extinguishant penetrates the soil. However, there are no systematic evaluation criteria in place to assess the effects of fire extinguishants on soil’s physical and chemical properties, soil microbial communities, or the germination and growth of seeds in the soil. Moreover, existing standards typically overlook the risk of gaseous pollutants forming under high-temperature fire-extinguishing conditions. During a fire, both fire extinguishants and combustible materials can undergo complex chemical reactions that may release toxic gases. Unfortunately, current domestic and international standards generally lack the necessary evaluation indicators to assess this risk.

3.2. Research Status of Environmental Performance

3.2.1. Aquatic Ecotoxicity

The application of fire extinguishing agents, whether through ground-based or aerial methods, eventually enters aquatic environments through both direct and indirect pathways [90]. Fire extinguishing agents that enter aquatic ecosystems can have a range of negative effects, including fish mortality [91] and harm to algae, zooplankton, and the overall structure of aquatic communities [92,93]. Therefore, assessing the toxicity of these agents on aquatic species has become a crucial aspect of environmental evaluations. Aquatic organisms tend to be more sensitive than terrestrial species and mammals, particularly to aqueous film-forming foam extinguishants [94], highlighting the urgency for research on their toxicity to aquatic life. Studies show that eight types of aqueous film-forming foam extinguishants demonstrate consistent toxicity results on 14 different aquatic organisms. Furthermore, extinguishants that do not contain PFAS are found to be more toxic than those that do [47]. Even when diluted to concentrations as low as 0.1% to 0.01% of the recommended application levels, fire extinguishing agents can significantly inhibit the survival of freshwater microcrustaceans like Ceriodaphnia dubia and Daphnia magna [95]. Additionally, the chemical substances in forest fire extinguishing agents can also reach aquatic ecosystems through various pathways, posing potential risks. It is important to note that most existing studies primarily focus on acute toxicity (for example, 96 h LC₅₀), while there is insufficient assessment of chronic effects, such as bioaccumulation and reproductive impairment.

3.2.2. Terrestrial Plant Toxicity

Seed germination and plant growth tests are essential for evaluating the ecological safety of fire extinguishants used in forest fires. From a physiological standpoint, toxic substances in fire extinguishants can alter the internal environmental conditions necessary for tissue development, potentially impacting seed growth. Montagnolli et al. [96] found that commercially available perfluorinated compound-based AFFF extinguishants exhibit concentration-dependent toxicity affecting the seed development of lettuce (Lactuca sativa) and arugula (Eruca sativa). Both fluorine-free formulations and short-chain fluorinated AFFF types show phytotoxicity towards field mustard (Brassica rapa) [48]. Notably, most fluorine-free formulations demonstrate higher toxicity than their short-chain fluorinated counterparts, and they can still be toxic even at concentrations much lower than those typically applied in practice. Different plant species also exhibit varying levels of sensitivity to fire extinguishants. Anderson et al. [97] conducted toxicity tests with terrestrial plants such as radish (Raphanus sativus), wheatgrass (Agropyron cristatum), and spruce (Picea glauca) and found no adverse effects. However, this does not imply the absence of unknown ecological risks. Long-term studies have confirmed that the ammonium phosphate component in fire retardants impacts post-combustion soil nitrogen and phosphorus levels, as well as plant growth and coverage, for over 10 years [98]. This can lead to increased mortality in pine trees [98], leaf loss in eucalyptus and angophora forests, and reduced coverage of 19 species [99]. Additionally, the addition of phosphorus to burned wastelands affects plant communities for over 22 years [100]. These findings underscore the importance of conducting phytotoxicity tests that encompass multiple plant species and span various time frames.

3.2.3. Effects on Soil Microorganisms and Animals

During forest fire suppression, the movement and transformation of fire extinguishants are primarily influenced by the soil. Soil acts as a habitat for microorganisms, plants, and soil-dwelling animals, facilitating their interactions. Numerous studies have investigated the environmental impacts of fire extinguishants on soil microorganisms. For example, Basanta et al. [101] found that the acrylic-based polymer known as Firesorb has positive effects on enzyme activity and microbial biomass in the short term. However, it has minimal impact on microbial community composition and negative effects on nitrogen mineralization. Long-term studies by Barreiro et al. [102,103] have confirmed that Firesorb has residual effects on the soil biotic community composition for up to 5 years, with changes in microbial community composition still detectable after 10 years. They also observed that ammonium polyphosphate significantly impacts soil communities more than Firesorb. Soil invertebrates, like earthworms, are often among the first organisms exposed to fire extinguishants. Standardized tests that assess avoidance behavior, lethality, and reproduction in earthworms serve as indicators of soil habitat function [104]. Avoidance tests determine acute toxicity, while lethal and reproductive tests assess chronic toxicity. Earthworms are vital for nutrient cycling and improving soil structure, and their avoidance behaviors indicate potential ecological risks. Substances that repel these essential soil organisms could pose significant threats to ecosystems. Additionally, data on earthworm body weight serve as an important indicator. Although earthworms survived acute tests involving AFFF, weight loss was observed in chronic tests [105]. This suggests a risk of bioaccumulation even at sublethal exposure levels [106]. It is essential to note that commercial flame retardants significantly impact both soil microorganisms and aboveground plant communities after application [107]. Thus, the linked effects between belowground and aboveground organisms should be systematically evaluated through multitrophic-level combined tests.

Table 4. Key indicators for environmental evaluation of fire extinguishants.

Evaluation Dimensions	Core Indicators	References
Aquatic ecotoxicity	Aquatic organisms (daphnia, fish, etc.) LC50, algal growth rate, algal biomass	[47,108,109,110]
Terrestrial plant toxicity	Seed germination time, seed germination rate, plant survival rate, root length, plant height, biomass, flowering time, seed yield, etc.	[111,112,113,114]
Soil ecological effects	1. Soil pH, soil element content (total carbon, total nitrogen, NH4+–N, NO3—N, Al, Ca, Cu, Fe, K, Mg, Mn, Mo, Na, P, Zn, etc.) 2. Soil microbial community richness and diversity 3. Soil animals’ LC50, concentration causing 50% reduction in soil animals’ reproduction (EC50), soil animals’ avoidance tests, soil animals’ body weight tests	[101,102,103,104,105,111,112,113,114]

Note: The acute toxicity of compounds is usually expressed as lethal concentration 50 (LC50) or effective concentration 50 (EC50) values. LC50: The lethal concentration of a compound in water required to cause 50% of the test samples to die within a fixed time period; EC50: The effective concentration of a compound in water that exerts an effect on the behavior and growth of 50% of the test samples within a fixed time period [115].

presents common assessment indicators for the environmental evaluation of fire extinguishants.

4. Conclusions and Outlook

In the context of forest fire prevention and control, the effectiveness of fire extinguishing agents and their impact on the environment are crucial factors that cannot be overlooked. (1). Gaseous Fire Extinguishers: These agents face limitations due to their high diffusivity, which makes it difficult to maintain effective concentrations in complex forest fire environments. As a result, their ability to suppress fires is restricted. While inert fire extinguishers are clean and leave no residue, their large-scale use is limited by the need for specialized storage equipment and the high costs associated with transportation. (2). Fluorinated Foam Fire Extinguishers: These were previously popular due to their impressive fire-extinguishing capabilities. However, the environmental and health risks posed by the persistent chemicals (PFAS) they contain have severely limited their usage. Although progress has been made with fluorine-free foams in terms of environmental safety, their fire-extinguishing effectiveness still falls short compared to fluorinated options. (3). Hydrogel Fire Extinguishers: This newer type of fire extinguishing agent significantly improves the adhesiveness and water retention of water, addressing issues commonly associated with traditional water-based extinguishers, such as rapid evaporation and a high risk of re-ignition. However, most existing studies primarily focus on enhancing their fire-extinguishing performance, with insufficient attention given to assessing their environmental impact.

In light of the unique challenges posed by wildland fire scenarios, future research must prioritize three key areas to enhance firefighting effectiveness while minimizing ecological impact: (1). Develop Forest-Specific Fire Extinguishing Agents: It is essential to create advanced smart-responsive materials that can endure the extreme conditions of forest fires. This includes designing materials that improve adhesion to leaf surfaces, particularly those of coniferous trees, incorporating nanostructures for the sustained release of active components, and developing temperature-responsive formulations suitable for extremely cold regions. Additionally, integrating these agents with remote sensing technology, unmanned aerial vehicles (UAVs), and intelligent algorithms can facilitate precise delivery, reducing the ecological risks associated with overdosing. (2). Establish a Life-Cycle Environmental Risk Assessment System: This assessment should go beyond traditional endpoints and include evaluations of acute and chronic aquatic toxicity, soil physicochemical properties, impacts on soil microbial communities, and indicators of plant development. Moreover, it is critical to address the potential generation of toxic gaseous pollutants resulting from the interaction of extinguishing agents with combustibles at high temperatures. Understanding the changes in soil microbial activity, particularly the expression of functional genes related to carbon, nitrogen, and phosphorus cycles, is vital for clarifying how extinguishing agents affect the environment. (3). Integrate Laboratory Simulations with Field Verification: Microcosm experiments should be utilized to simulate how extinguishing agents behave in ecological environments and to explore their transport and transformation patterns. Following this, long-term field monitoring should be employed to assess the real-world impacts of these agents on forest ecosystems. This comprehensive approach will provide a solid scientific foundation for developing and refining environmental evaluation standards.