Fire classification
Fire classification is a standardized system that categorizes fires based on the type of fuel or material burning, enabling the selection of appropriate extinguishing agents and methods to ensure effective suppression and safety.[1] This classification is critical in fire prevention and response, as using the wrong extinguisher can exacerbate the fire or pose additional hazards, and it forms the basis for regulations in building codes, occupational safety, and emergency training worldwide.[2] In the United States, the system—primarily governed by the National Fire Protection Association (NFPA) standards such as NFPA 10—divides fires into five primary classes.[1] Class A fires involve ordinary combustible solids like wood, paper, cloth, rubber, and certain plastics, which are typically extinguished with water or foam to cool and soak the material.[1][2] Class B fires stem from flammable or combustible liquids and gases, such as gasoline, oil, grease, or propane, requiring non-conductive agents like dry chemical or carbon dioxide to smother the flames without spreading the fuel.[1][2] Class C fires occur in energized electrical equipment, where non-conductive suppressants like dry chemical or CO2 are essential to avoid electrical shock.[1][2] Class D fires involve combustible metals such as magnesium, titanium, sodium, or potassium, necessitating specialized dry powder agents designed to form a crust that excludes oxygen.[1][2] Class K fires arise from cooking appliances with vegetable or animal oils and fats, suppressed using wet chemical agents that saponify the grease to prevent re-ignition.[1][2] These classifications influence fire extinguisher labeling, placement requirements, and training protocols under Occupational Safety and Health Administration (OSHA) guidelines, which mandate accessible extinguishers rated for specific classes in workplaces handling hazardous materials.[2] While the U.S. system is widely adopted, variations exist internationally; for instance, European standards under EN 2 align closely but designate cooking oil fires as Class F and may classify certain gases separately.[3] Overall, fire classification promotes proactive risk assessment in industries, construction, and commercial kitchens, reducing injury and property damage through tailored protection strategies.[2]Overview
Definition and Purpose
Fire classification refers to the systematic categorization of fires based on the type of fuel or heat source involved, which enables the selection of appropriate extinguishing agents and methods to effectively suppress them.[4] This approach recognizes that different fire types require specific interventions to interrupt the combustion process without exacerbating hazards, such as electrical shock or explosion risks.[1] The primary purpose of fire classification is to minimize risks to life, property, and the environment by aligning suppression techniques with the unique characteristics of each fire type, thereby reducing the potential for spread, toxic emissions, and re-ignition.[1] It plays a critical role in firefighter training, equipment labeling, and regulatory compliance, ensuring that portable extinguishers and suppression systems are deployed as a first line of defense against incipient fires.[5] By standardizing responses, classification systems enhance overall fire safety protocols across various settings, from residential to industrial environments.[4] Understanding fire classification is predicated on the fire tetrahedron model, which describes combustion as requiring four interdependent elements: fuel, oxygen, heat, and a self-sustaining chemical chain reaction.[4] Effective classification targets these elements selectively—for instance, by cooling (removing heat), smothering (limiting oxygen), or inhibiting the chain reaction—tailored to the fuel type, thereby underscoring why mismatched suppression can fail or create secondary dangers.[4] Note that lettering conventions for certain classes, such as Class C, may vary by region (e.g., denoting electrical fires in the U.S. versus gases in Europe).[1]Historical Development
The development of fire classification systems originated in the early 20th century, primarily driven by the need to standardize testing for portable fire extinguishers amid growing industrialization and urban fire risks in the United States. In the 1910s and 1920s, Underwriters Laboratories (UL) played a pivotal role by establishing initial testing protocols focused on ordinary combustibles, such as wood and paper, to evaluate extinguisher effectiveness. By 1921, the National Fire Protection Association (NFPA) adopted its first official standard on portable fire extinguishers, introducing basic ratings that compared small-capacity units, like one-quart carbon tetrachloride extinguishers, to larger 2½-gallon models for combating these common fire types.[6] These early efforts emphasized empirical testing to ensure reliability, laying the groundwork for broader classification frameworks without yet formalizing distinct fire classes.[7] The modern classification system for fires was formalized in NFPA standards during the mid-20th century, with NFPA 10 evolving from its origins in 1919 to include detailed class specifications by the 1960s to categorize fires by fuel type—ordinary combustibles, flammable liquids, energized electrical equipment, and combustible metals, respectively—and specify appropriate suppression methods.[8] In Europe, harmonization efforts began in the late 1970s under the European Commission, with initial surveys by experts assessing national fire testing practices to facilitate unified standards through the European Committee for Standardization (CEN); however, substantial progress on a common classification system, including reaction-to-fire classes, accelerated in the 1980s and 1990s following the 1988 Construction Products Directive.[9] Australia followed suit in the 1980s with the development of standards like AS 1841 for portable fire extinguishers, aligning classifications with international influences while addressing local needs for bushfire and industrial hazards. Influential industrial accidents have shaped broader process safety standards, indirectly influencing fire safety frameworks. Post-2000 updates addressed emerging risks, including the addition of Class K in the 1998 revision of NFPA 10 (effective in subsequent editions) for cooking oil and fat fires, which required specialized wet chemical agents to prevent re-ignition, and its European equivalent Class F under EN 3 standards.[10] In the 2010s, rising incidents of electric vehicle (EV) battery fires, involving lithium-ion thermal runaway, spurred discussions on classification adaptations, with global reports documenting around 250 such events by 2022 and influencing research into hybrid suppression strategies, though no new universal class emerged.[11] Ongoing work by ISO/TC 21 on fire safety aims to harmonize extinguisher ratings and address emerging risks like lithium-ion batteries across regions. As of 2025, fire classification systems have seen no major overhauls since the early 2020s, with ongoing efforts focused on international alignment through ISO standards, such as ISO/TC 21 on fire safety, to harmonize extinguisher ratings and testing across regions for better global interoperability.Standards and Systems
United States (NFPA)
In the United States, fire classification for portable fire extinguishers is primarily governed by NFPA 10, the Standard for Portable Fire Extinguishers, developed and maintained by the National Fire Protection Association (NFPA). The 2026 edition of NFPA 10, which is the current standard as of 2025, establishes requirements for the selection, installation, inspection, maintenance, and testing of extinguishers to ensure they serve as an effective first line of defense against small fires.[5] This standard defines five distinct fire classes—A, B, C, D, and K—based on the fuel type involved, with labeling that includes pictorial symbols and alphanumeric ratings to indicate suitability and effectiveness.[12] Under NFPA 10, Class A fires involve ordinary combustible solids like wood, paper, cloth, rubber, and certain plastics, where extinguishers are rated numerically (e.g., 2-A) based on water equivalency for fire control.[12] Class B addresses flammable or combustible liquids and gases, such as gasoline, oil, grease, and propane, with ratings (e.g., 10-B) reflecting the square footage of flammable liquid fire that can be extinguished.[12] Class C pertains to fires involving energized electrical equipment, emphasizing non-conductive extinguishing agents to avoid shock hazards, a designation unique to the U.S. system that treats electrical risks as a separate category rather than reclassifying them under other fuels after de-energization.[12] Class D covers combustible metals like magnesium, titanium, zirconium, sodium, and potassium, requiring specialized dry powder agents.[12] Class K targets cooking appliances with vegetable or animal oils and fats, using wet chemical agents to saponify and cool the fire.[13] Enforcement of NFPA 10 occurs through federal and state regulations, with the Occupational Safety and Health Administration (OSHA) incorporating it via 29 CFR 1910.157, which mandates compliance for portable fire extinguishers in workplaces to protect employees from fire hazards.[14] States may adopt variations; for instance, the California Fire Code (2022 edition), based on the International Fire Code with local amendments, explicitly requires portable fire extinguishers to be installed and maintained in accordance with NFPA 10, alongside state-specific seismic and wildfire considerations. Testing and rating of extinguishers follow UL 711, Rating and Fire Testing of Fire Extinguishers, which outlines full-scale fire tests for each class to verify performance under controlled conditions.[15] Unlike the European EN 2 system, which lacks a dedicated electrical class and approximates U.S. Class C by using Class A or B extinguishers after isolating power, NFPA 10's structure prioritizes ongoing electrical conductivity risks.[3]European Union (EN)
In the European Union, fire classification is standardized under EN 2:1992, which categorizes fires based on the nature of the combustible material to ensure consistent selection and performance of firefighting equipment across member states. This standard defines five primary classes: Class A for fires involving solid materials, such as wood, paper, or textiles, that typically sustain burning with glowing embers; Class B for flammable or combustible liquids, like petrol or oils; Class C for gases, including propane or natural gas; Class D for combustible metals, such as magnesium or aluminum; and Class F for cooking oils and fats.[16][17][18] EN 2 was originally published in 1992 and amended in 2004 to incorporate Class F, reflecting the need to address specific kitchen-related hazards. Unlike systems in other regions, such as the United States, the EU framework does not designate a separate class for energized electrical equipment, instead treating electrical fires according to the primary fuel class involved, with de-energization recommended where possible.[17][19] Complementing EN 2, the EN 3 series establishes requirements for portable fire extinguishers, first issued in 1996, to verify their effectiveness against these fire classes through standardized performance tests. Key parts include EN 3-7 for characteristics and test methods, updated in 2004 with a 2007 amendment, and EN 3-8 for construction, pressure resistance, and mechanical tests, revised in 2021 to enhance safety and durability. These standards ensure extinguishers are rated for specific classes, such as 13A/55B for water-based units effective on solids and liquids.[20] Compliance with EN 3 is mandatory EU-wide under the Pressure Equipment Directive 2014/68/EU, which classifies most portable extinguishers as Category II or III equipment requiring conformity assessment and CE marking to confirm safety and interoperability. While the directive promotes harmonization, member states may adopt supplementary national guidelines, such as the UK's BS 5306-3:2017, which details installation, commissioning, and maintenance schedules to align with EN 3 while addressing local building regulations.[21][22][23] As of November 2025, the core provisions of EN 2 and EN 3 remain unchanged since the 2021 revision to EN 3-8, with ongoing efforts emphasizing sustainability in extinguisher testing and production, including the EU restriction on per- and polyfluoroalkyl substances (PFAS) in firefighting foams, adopted on October 2, 2025, and entering into force on October 23, 2025, to reduce environmental impact.[24][20]Australia (AS/NZS)
In Australia and New Zealand, fire classification for portable extinguishers is governed by the AS/NZS 1841 series of standards, which specify requirements for design, performance, construction, and testing to ensure reliability in diverse fire scenarios. The core document, AS/NZS 1841.1, outlines general requirements for non-aerosol portable fire extinguishers, originally published in 1997 and revised in subsequent editions, with the latest version in 2022 incorporating updates to materials and performance criteria. These standards define six fire classes: Class A for ordinary combustibles like wood and paper; Class B for flammable liquids; Class C for flammable gases; Class D for combustible metals; Class E for fires involving energized electrical equipment; and Class F for cooking oils and fats. Extinguishers are rated and labeled according to their effectiveness against these classes, as determined through standardized testing protocols. A distinctive feature of the AS/NZS system is the dedicated Class E designation for electrical fires, which requires extinguishers to demonstrate non-conductivity and safety when applied to live equipment, partially aligning with the U.S. NFPA approach but differing from the European EN standard that treats electrical hazards as unclassified extensions of other classes. Testing for Class A fires under AS/NZS 1841 incorporates considerations for regional risks, such as the rapid spread of bushfires in Australia and New Zealand, by evaluating extinguisher performance on cellulosic materials under high-heat conditions that simulate dry vegetation ignition. This adaptation ensures suitability for environments prone to wildfires, where extinguishers must maintain efficacy amid embers and radiant heat. The Class F category, similar to the EU's Class F, addresses high-temperature cooking media through saponification agents in wet chemical extinguishers.[25] The standards are developed jointly by Standards Australia and Standards New Zealand, with oversight from technical committees including fire protection experts and industry stakeholders. Compliance is enforced through harmonized national frameworks, such as Australia's model Work Health and Safety (WHS) Act 2011, which mandates employers to provide appropriate fire-fighting equipment as part of risk management duties, with state regulators like Safe Work Australia monitoring adherence via inspections and penalties for non-compliance. In New Zealand, equivalent provisions fall under the Health and Safety at Work Act 2015, administered by WorkSafe New Zealand. As of 2025, the system remains stable, with the 2022 revision of AS/NZS 1841.1 serving as the current benchmark, though ongoing reviews address emerging risks like electric vehicle fires without major amendments since then.[26]International and Other (ISO)
The International Organization for Standardization (ISO) establishes global benchmarks for fire safety equipment through standards like ISO 7165, which addresses the classification and performance of portable fire extinguishers. Published in 2017, ISO 7165 specifies requirements for extinguisher construction, testing, and rating to ensure reliability across diverse applications, defining fire classes A through D that correspond to ordinary combustible solids (A), flammable liquids and gases (B), energized electrical equipment (C, though not distinctly separated), and combustible metals (D).[27] This framework aligns closely with both the European EN 3 series and the U.S. NFPA 10 systems in its core categorization, emphasizing performance metrics such as discharge duration and extinguishing capability without dedicated provisions for cooking oils and fats or advanced electrical hazards.[28] The standard's non-binding nature allows for national adaptations, promoting interoperability in international trade and emergency response while prioritizing safety and efficacy over regional enforcement.[27] Beyond ISO's foundational work, several non-Western countries have developed fire classification systems that incorporate or parallel international norms while addressing local contexts. In China, the national standard GB 4351-2023 regulates portable fire extinguishers, classifying fires into five categories (A-E) that mirror the European EN approach: A for solids, B for liquids and gases, C for gases, D for metals, and E for electrical equipment, with detailed performance tests for agent efficacy and environmental durability.[29] Japan's system, governed by the Fire and Disaster Management Agency under the Fire Service Act, employs a four-class structure (A-D) for extinguishers, treating electrical fires as a subset of Class C alongside flammable liquids, and emphasizes compact, aerosol-based units suited to urban density and seismic risks.[30] Similarly, India's Bureau of Indian Standards outlines classifications in IS 15683:2018, adopting an NFPA-like model with Classes A (ordinary combustibles), B (flammable liquids), C (electrical), D (metals), and an additional E for specific electrical configurations, focusing on construction quality and tropical climate resilience.[31] These standards facilitate alignment with global practices but incorporate region-specific testing for humidity, agent stability, and manufacturing scalability.[32] Efforts toward global harmonization are led by the ISO Technical Committee 21 (ISO/TC 21), which develops and revises standards for fire protection equipment to bridge discrepancies between regional systems and enhance cross-border compatibility.[33] This committee's work includes integrating performance-based criteria from diverse sources, such as aligning extinguisher ratings with evolving threats, though progress on specialized classifications remains incremental. In 2024, ISO/TC 21 discussed proposals to incorporate guidance for lithium-ion battery fires into existing frameworks, recognizing their unique thermal runaway risks, but as of 2025, no formal adoption has occurred, leaving such hazards addressed primarily through supplementary national guidelines.[33] These initiatives underscore ISO's role in fostering unified terminology and testing protocols without overriding local sovereignty. Despite these advancements, gaps persist in the adoption of ISO and aligned standards in developing regions, where economic constraints, limited regulatory infrastructure, and varying enforcement capacities hinder widespread implementation.[34] In many low- and middle-income countries, fire classification systems prioritize basic A and B categories due to prevalent informal settlements and resource scarcity, often resulting in improvised equipment over standardized extinguishers. Cultural adaptations further shape applications; for instance, in African savanna regions, traditional fire management emphasizes controlled burns for agricultural renewal and pastoralism, integrating indigenous practices with ISO-inspired classes to address grassland and crop residue fires rather than urban or industrial scenarios.[35] This blend highlights the need for flexible, context-aware standards to improve global fire safety equity.Fire Classes by Type
Ordinary Combustibles (Class A)
Class A fires involve ordinary combustible solid materials, including wood, paper, textiles, cloth, rubber, and many plastics, which typically burn by smoldering or flaming and leave behind ash or char residue. These fires often exhibit deep-seated burning characteristics, where combustion penetrates deeply into porous fuels like wood or upholstery, allowing heat to persist and potentially reignite if not fully extinguished. This behavior distinguishes them from surface fires and necessitates thorough penetration of suppression agents to reach the fire's core.[4][36] The primary hazards of Class A fires stem from their potential for high heat release rates, which can exceed several megawatts in enclosed spaces and drive rapid fire spread through structures, leading to extensive property damage. Additionally, these fires generate significant smoke volumes containing toxic particulates, carbon monoxide, and other irritants, which impair visibility, complicate evacuation, and pose acute respiratory risks to occupants. Class A incidents are prevalent in structural settings, such as residential and commercial buildings, where they account for the majority of fire-related property losses.[37][38] Effective suppression of Class A fires relies on water-based agents, such as streams from hoses or multipurpose dry chemicals that mimic water's cooling effect, to reduce fuel temperatures below ignition thresholds and prevent rekindling from residual heat. All major regional standards, including NFPA in the United States and EN in the European Union, consistently classify these fires as Class A to guide uniform response protocols. Structure fires, primarily involving Class A materials, accounted for approximately 34% of reported fires in 2023, according to NFPA data.[4][1][38][39]Flammable Liquids and Gases (Class B/C)
Class B fires involve the combustion of flammable or combustible liquids, such as gasoline, solvents, oils, and paints, as well as gases like propane and hydrogen, which readily vaporize and ignite. In the United States under NFPA standards, both liquids and gases fall under Class B, while in the European Union (EN) and Australia (AS/NZS), flammable liquids are classified as Class B and flammable gases as Class C. These materials are characterized by their low flash points, enabling ignition at relatively low temperatures; for instance, gasoline has a flash point of approximately -43°C, allowing vapors to form and burn even in cool conditions.[12][40][41] The behavior of these fires is marked by rapid flame spread due to the volatile nature of the fuels, where vapors mix easily with air to propagate quickly across surfaces or in open spaces. A significant risk is the potential for a boiling liquid expanding vapor explosion (BLEVE), particularly with pressurized containers of liquefied gases like propane, where heat from an external fire causes the vessel to rupture, releasing superheated vapors that ignite into a massive fireball. Autoignition temperatures further contribute to unpredictability; gasoline, for example, can self-ignite at around 280°C without an external spark. If electrical equipment is involved and energized, these incidents may overlap with Class C/E classifications, requiring de-energization before suppression.[42][43][44] Hazards extend beyond the fire itself, including inhalation of toxic fumes and vapors from burning solvents or gases, which can cause respiratory distress or poisoning in exposed individuals. In confined spaces, such as storage tanks or industrial enclosures, vapors accumulate, heightening explosion risks from even minor ignition sources. These fires are prevalent in industrial settings like refineries, chemical plants, and fuel handling facilities, where leaks or spills amplify dangers. Common examples include gasoline spills during vehicle refueling at service stations or ruptures in natural gas pipelines, leading to widespread fires and potential evacuations.[45]Energized Electrical Equipment (Class C/E)
Fires involving energized electrical equipment are classified as Class C in the United States under NFPA standards, where electricity serves as the ignition source or sustains combustion without the equipment being de-energized.[1] In Australia, these are designated as Class E per AS/NZS standards, emphasizing the electrical hazard from live sources like wiring or devices.[46] In the European Union, under EN 2, electrical fires are not assigned a separate class but are instead addressed based on the underlying fuel type, such as solids or liquids.[47] These fires present significant hazards, including the risk of electrocution from contact with live conductors and arc flash explosions that can release intense heat exceeding 35,000°F, causing severe burns or ignition of nearby materials.[48] Common occurrences involve faulty wiring, overloaded circuits, or malfunctioning appliances like motors and transformers, where the electrical energy propagates the fire even after initial ignition.[49] Such incidents often combine with other fire classes—for instance, if solid combustibles are involved, the fire may exhibit Class A characteristics alongside the electrical risk—necessitating de-energization of the power source as the first priority when safe to do so.[50] Suppression guidelines for Class C/E fires prioritize non-conductive agents to prevent electrical shock; carbon dioxide (CO₂) extinguishers displace oxygen and cool the area effectively without residue, while dry chemical extinguishers create a barrier that interrupts the chemical reaction.[1] Water-based methods are strictly prohibited due to conductivity risks.[49] Post-2020 updates, including NFPA research and USFA guidelines, have increasingly emphasized protections for electric vehicle (EV) charging stations, recommending automatic shutoff systems and specialized suppression to mitigate arc flash and thermal runaway in high-voltage setups.[51]Combustible Metals (Class D)
Class D fires involve the combustion of combustible metals and metal alloys, which require specialized suppression techniques due to their unique reactivity. These fires are uniformly classified as Class D across major international standards, including those from the National Fire Protection Association (NFPA) in the United States and the International Organization for Standardization (ISO).[12][50] The materials typically include alkali metals such as sodium, potassium, and lithium; alkaline earth metals like magnesium; and other reactive metals including titanium, zirconium, and aluminum.[52][53] These fires burn at extraordinarily high temperatures, often exceeding 2000°C, with magnesium combustions reaching beyond 2500°F (1371°C) and potentially up to 7000°F (3871°C) in extreme cases.[53][54] The combustion process generates intense heat and can be self-sustaining once initiated, making these fires particularly challenging to control.[54] Such incidents commonly arise in industrial settings like aerospace manufacturing and metal processing, where fine powders, shavings, or molten forms of these metals are handled.[55] Key hazards of Class D fires stem from the metals' reactivity; alkali metals like sodium and potassium undergo explosive reactions with water or traditional foam agents, producing hydrogen gas and intensifying the blaze.[54][56] Additionally, the combustion releases toxic fumes, creating respiratory and health risks for responders and nearby personnel.[57] Effective suppression relies on dry powder agents, such as sodium chloride-based formulations blended with flow enhancers, which are applied to smother the fire by forming a molten crust that excludes oxygen from the burning metal surface.[58][53] Unlike other fire classes, cooling is not the primary goal; instead, the technique emphasizes gentle, even application to avoid scattering embers or disrupting the agent layer, often using scoop-and-sweep motions from a safe distance.[14] NFPA 484 outlines specific industrial guidelines for preventing and managing these fires in combustible metal operations.Cooking Oils and Fats (Class K/F)
Class K (or Class F in European and Australian systems) fires involve combustible cooking media, such as vegetable and animal oils and fats used in commercial and residential kitchens. These fuels include common examples like canola oil, which has an auto-ignition temperature of approximately 424°C, soybean oil, olive oil, and animal fats such as lard.[59] In the United States, Class K was introduced in the 1998 edition of NFPA 10, Standard for Portable Fire Extinguishers, to address the specific challenges of high-temperature cooking oils that standard extinguishers could not effectively suppress.[1] The European standard EN 2, revised in 1992, added Class F for similar fires involving cooking oils and fats, while Australia's AS 2444, updated in 2001, also designates Class F for these hazards.[18][60] These fires exhibit unique behavior due to the high temperatures required for ignition and their potential for self-sustaining combustion. Vegetable oils and fats auto-ignite above 350–400°C, often when overheated in deep fryers or on stovetops, leading to rapid flame spread and a high risk of re-ignition even after initial suppression because the oil remains hot enough to vaporize.[61] Such incidents are particularly common in commercial kitchens, where deep-fat frying equipment operates continuously, increasing exposure to overheating conditions. The oils' low viscosity when hot allows flames to propagate quickly across surfaces, distinguishing these fires from ordinary liquid fuel fires.[12] Hazards from Class K/F fires include severe splatter burns from oil expulsion, especially if water is mistakenly applied, causing a violent steam explosion, as well as dense smoke that reduces visibility and poses respiratory risks. In the United States, an average of 7,610 commercial kitchen structure fires occurred annually from 2014 to 2018, according to NFPA data, many involving cooking oils.[62] Suppression requires specialized wet chemical agents, typically potassium-based, which react with the hot oil through saponification to form a thick soap-like foam blanket that cools the fuel, smothers the fire, and prevents re-ignition by sealing out oxygen.[63] These agents are delivered via portable Class K/F extinguishers or integrated automatic hood suppression systems in commercial kitchens, which detect heat or flames and discharge over cooking appliances, ducts, and hoods to contain the fire.[64]Emerging Hazards (Lithium-Ion Batteries)
Lithium-ion batteries have emerged as a significant fire hazard in modern applications, including electric vehicles (EVs), portable electronics like vapes and e-bikes, and stationary energy storage systems, due to their propensity for thermal runaway—a self-sustaining exothermic reaction that rapidly escalates temperatures within battery cells.[65] This process releases flammable electrolytes and generates intense heat capable of melting metal enclosures, distinguishing these fires from conventional types by their prolonged and unpredictable nature.[65] The characteristics of lithium-ion battery fires stem from the battery's chemistry, where damage, overcharging, or manufacturing defects trigger thermal runaway, leading to the venting of combustible gases and potential explosions.[66] During this event, cells can eject flaming debris, and the reaction propagates to adjacent cells, exacerbating the fire's spread.[66] Notably, these batteries emit toxic compounds, including hydrogen fluoride gas, which poses severe respiratory and environmental risks, with toxicity levels varying by battery chemistry such as nickel-manganese-cobalt (NMC) or lithium-iron-phosphate (LFP).[67] Classification of lithium-ion battery fires remains challenging, as they do not align perfectly with established fire classes and are typically addressed under Class B for the involvement of flammable liquids like electrolytes or as a combination of Class A, B, and C hazards, particularly when energized electrical components are present.[68] Proposals for a dedicated Class L to specifically denote lithium-based fires have been discussed in industry forums, but as of 2025, no universal adoption has occurred across major standards like NFPA or ISO, leaving responders to apply multi-class strategies.[68] Key hazards include the high likelihood of re-ignition, which can occur up to days or even weeks after initial suppression due to residual energy in damaged cells, complicating post-fire safety.[65] Toxic emissions further endanger responders and bystanders, while incident rates continue to climb; for instance, the New York City Fire Department reported 277 lithium-ion battery fires in 2024, up from 268 in 2023, reflecting broader trends in EV and consumer device usage.[66][69] In the UK, fire services noted a 46% increase in such incidents over recent years, underscoring the global scale of this emerging risk.[70] Response protocols prioritize containment and intensive cooling over full extinguishment, as complete suppression is often impractical given the batteries' self-heating properties.[66] Firefighters are advised to use copious amounts of water—sometimes tens of thousands of gallons for large packs—or specialized dry chemical extinguishers like ABC types, while avoiding Class D agents unsuitable for electrolyte fires; emerging tools include thermal blankets and agents like Lith-Ex for targeted cooling below runaway thresholds.[71] Post-incident monitoring for re-ignition is essential, with proper PPE to mitigate toxic exposure.[66] For stationary installations, the NFPA 855 standard (2026 edition) outlines preventive measures such as hazard mitigation analyses and fire suppression systems tailored to lithium-ion energy storage, aiming to reduce initiation risks before fires occur.[72]Comparisons and Applications
Regional Equivalencies
Fire classification systems vary by region, but mapping equivalencies facilitates global understanding and application of fire safety measures. The primary systems include the United States' NFPA 10 standard, the European Union's EN 3 series, Australia's AS/NZS 1850 and AS 2444 standards, and the international ISO 7165 standard. These mappings align fire types across systems, though differences in lettering for certain hazards persist. The following table summarizes the equivalencies for major fire types across these systems, based on current standards as of 2025, which have remained consistent since revisions around 2020 with no fundamental changes to class definitions.[73][74]| Fire Type | US (NFPA) | EU (EN) | AU (AS/NZS) | ISO |
|---|---|---|---|---|
| Ordinary combustibles (e.g., wood, paper) | A | A | A | A |
| Flammable liquids (e.g., gasoline, oil) | B | B | B | B |
| Flammable gases (e.g., propane, hydrogen) | B | C | C | C |
| Energized electrical equipment | C | Unclassified (use non-conductive agent) | E | Unclassified (use appropriate class) |
| Combustible metals (e.g., magnesium, sodium) | D | D | D | D |
| Cooking oils and fats | K | F | F | F |