What Is a Class B Fire? Causes, Extinguishers, and Prevention Explained (2026)

You’ve probably seen fire extinguishers labelled for different fire classes — but many people still don’t fully understand what a Class B fire actually means. Using the wrong extinguisher on a flammable liquid fire can make the situation dramatically worse, not better. In this guide, you’ll learn exactly what Class B fires are, what causes them, which extinguishers work, and how to prevent them at home and in the workplace.

Key Takeaways

Class B fires involve flammable liquids, gases, and petroleum-based substances such as gasoline, oil, paint, and propane — common in workplaces, garages, and industrial environments.
Class B fires spread rapidly because flammable vapours ignite easily when exposed to sparks, heat, or open flames.
CO2, foam, and dry chemical extinguishers are the correct agents for Class B fires.
Water should not be used on Class B fires — it spreads burning liquid and intensifies the fire.
Proper storage, ventilation, and employee training prevent the majority of Class B fire incidents.
Fire classification knowledge reduces emergency response mistakes and supports workplace compliance.

What Is a Class B Fire?

Class B fires are fires involving flammable liquids, gases, and petroleum-based materials such as gasoline, oil, paint, and propane. Unlike Class A fires — which involve solid combustibles like wood and paper — Class B fires burn from liquid or gaseous fuel sources that do not leave an ash residue after combustion.

What makes Class B fires particularly hazardous is the role of vapour. Flammable liquids do not actually burn in their liquid state — their vapours ignite when they mix with air at the right concentration. This means a petrol spill at room temperature can produce enough vapour to ignite from a spark several metres away. According to the NFPA, flammable and combustible liquid fires account for a significant share of industrial fire fatalities annually — Source: NFPA, 2023.

For a complete overview of all fire categories, read our guide on types of fire extinguishers explained.

Why Are Class B Fires Dangerous?

Class B fires are dangerous because flammable vapours ignite rapidly, spread quickly, and are difficult to control without the correct extinguishing agent. A small petrol spill that ignites can engulf a large area within seconds as vapour ahead of the liquid catches and the flame travels back to the source.

Moreover, many Class B materials are stored in large quantities in workplaces — fuel depots, paint storage rooms, chemical plants, and vehicle workshops all carry significant Class B fire loads. The UK Health and Safety Executive identifies flammable liquid fires as a leading cause of serious workplace injuries in industrial settings — Source: HSE, 2022. For industrial fire prevention strategies relevant to high-risk environments, a site-specific fire risk assessment is the recommended starting point.

What Causes Class B Fires?

Class B fires are caused by ignition sources coming into contact with flammable liquid vapours or gases. The most common causes include:

Fuel leaks and spills — petrol, diesel, or solvent spills near electrical equipment or open flames
Static electricity discharge — particularly during fuel transfer operations without proper earthing
Electrical sparks — from switches, motors, or faulty equipment in areas storing flammable liquids
Smoking materials — a lit cigarette near a solvent-heavy area is a high-risk ignition scenario
Improper storage — unsealed containers, overfilled storage tanks, or flammable liquids stored near heat sources

For example, a workshop mechanic decanting petrol into an unsealed container near a running engine creates both a vapour concentration and a heat source — a textbook Class B ignition scenario. For flammable liquid storage safety guidance, consult your local fire safety regulations and NFPA 30 standards.

What Are Common Examples of Class B Fires?

Class B fires occur across a wide range of industrial, commercial, and domestic settings. Recognising them helps you respond with the correct agent immediately.

A fuel station forecourt where a petrol spill near a running engine ignites from exhaust heat
A paint storage room where solvent vapours accumulate and catch from a light switch spark
A vehicle workshop where brake fluid or engine oil ignites during a repair operation
A chemical plant where a ruptured pipeline releases flammable gas that ignites at a nearby source
A domestic garage where stored petrol cans positioned near a boiler unit create a vapour ignition risk

All of these share the same characteristics: a liquid or gas fuel source, rapid vapour ignition, and the requirement for a smothering or oxygen-displacing extinguishing agent — not water.

[Insert image: Diagram showing three Class B fire scenarios — fuel station, paint storage room, and vehicle workshop — with vapour spread illustrated | Alt text: “Identify Class B fire examples in fuel station paint storage and vehicle workshop settings”]

How Is a Class B Fire Different From Other Fire Classes?

Fire classes categorize fires by fuel type, and each class requires a different extinguishing approach. Using an agent designed for one class on another can actively worsen the emergency.

Fire Class	Fuel Type	Common Examples	Correct Extinguisher
Class A	Ordinary combustibles	Wood, paper, cloth	Water, foam, ABC powder
Class B	Flammable liquids/gases	Petrol, oil, paint, propane	Foam, CO2, dry powder
Class C (UK)	Flammable gases	Butane, LPG, methane	Dry powder; isolate gas first
Class D	Combustible metals	Magnesium, lithium	Specialist dry powder only
Class F	Cooking oils and fats	Deep fryer oil, lard	Wet chemical, water mist
Electrical	Live electrical equipment	Wiring, switchgear	CO2, dry powder

For a detailed comparison of all fire classes, read our Class A fire safety guide or the full fire classification overview.

Which Fire Extinguishers Are Best for Class B Fires?

CO2, foam, and dry chemical extinguishers are the most commonly recommended agents for Class B fire emergencies, each working through a different suppression mechanism suited to liquid and gas fuel fires.

Foam extinguishers (cream label) are often the first choice for Class B liquid fires. The foam forms a physical blanket over the burning liquid surface, cutting off its oxygen supply and preventing vapour release that would reignite the fire. They are rated for both Class A and Class B fires, making them versatile for mixed-risk environments.

CO2 extinguishers (black label) displace oxygen around the fire, suffocating combustion without leaving residue. They are ideal where liquid fires involve sensitive equipment — vehicle engines, electrical panels, server rooms — because they cause no secondary damage. However, CO2 provides no post-fire cooling, so reignition is possible if the fuel source remains.

ABC dry powder extinguishers (blue label) are the most versatile type, rated for Class A, B, and C fires. They interrupt the chemical chain reaction of combustion. The limitation is that powder leaves significant residue and does not cool the fuel — reignition risk remains if liquid temperature stays high. For a full breakdown, read our guide on PASS method for fire extinguishers.

Why Should You Never Use Water on a Class B Fire?

Water should not be used on Class B fires because it does not mix with burning liquids and can spread them rapidly across a wider area. When water contacts a burning fuel surface, it sinks below the lighter liquid, converts to steam violently, and propels burning droplets outward — dramatically enlarging the fire’s footprint. This is one of the most dangerous mistakes made during flammable liquid fire emergencies.

How Do You Respond Safely to a Class B Fire?

Safe response to a Class B fire requires activating the alarm, calling emergency services, and using the correct extinguisher only if the fire is small and contained. If the fire involves a large fuel source, pressurised gas container, or has spread beyond a single point of origin, evacuate immediately and do not attempt suppression.

If the fire is small enough to tackle, use the PASS method:

Pull the safety pin to unlock the extinguisher handle
Aim the nozzle at the base of the fire — the fuel surface, not the flames
Squeeze the handle to discharge the agent in controlled bursts
Sweep side to side across the base until the fire is extinguished

After suppression, monitor the area for reignition — particularly with CO2 or powder agents that provide no lasting cooling effect. For emergency evacuation planning that incorporates Class B fire scenarios, ensure your evacuation plan specifies assembly points clear of fuel storage areas.

How Can You Prevent Class B Fires?

Class B fire prevention depends on controlling both the fuel source and the potential ignition sources in the same environment. The most effective measures include:

Store flammable liquids in approved sealed containers in designated, ventilated fire-resistant storage areas
Maintain adequate ventilation in areas where vapours can accumulate — garages, spray booths, chemical stores
Enforce no-smoking rules within and around flammable liquid storage and handling areas
Earth fuel transfer equipment to prevent static discharge ignition during decanting operations
Inspect equipment regularly — leaking seals, cracked fuel lines, and faulty valves are primary ignition risk factors
Train all relevant staff on Class B fire hazards, correct extinguisher selection, and emergency procedures

For a workplace fire safety checklist that covers Class B-specific controls, your fire risk assessment documentation should specify storage quantities, ignition source controls, and emergency response procedures. Under NFPA 30 and equivalent UK standards, businesses storing flammable liquids above defined thresholds have specific compliance obligations — Source: NFPA 30, 2021.

What Should You Do Next?

The most important next step is to audit your environment for Class B fire risks — identifying where flammable liquids and gases are stored, used, or transferred, and whether the correct extinguishers are in place.

Take these four actions today:

Check your extinguishers — confirm you have foam or CO2 units in areas where flammable liquids are present, and verify their last service date
Review your storage — ensure flammable liquids are in sealed, labelled containers in ventilated, fire-resistant storage away from ignition sources
Train your team — every person working near Class B materials should know the correct extinguisher, the PASS method, and the evacuation route
Schedule a fire risk assessment — a qualified assessor will identify specific Class B risks and confirm your extinguisher provision meets legal requirements

For guidance on how to conduct a fire risk assessment for flammable liquid environments, consult a BAFE-accredited fire safety professional.

Conclusion

Class B fires involve flammable liquids and gases — including petrol, oil, paint, and propane — and are among the fastest-spreading and most dangerous fire types in workplace and industrial settings. The correct extinguishers are foam, CO2, and ABC dry powder. Water must never be used. Prevention comes down to safe storage, proper ventilation, and trained personnel who know exactly how to respond.

Check your flammable liquid storage today. Verify your extinguisher type. Confirm your team knows the PASS method. Those three steps, done before an emergency, are the difference between a contained incident and a serious one.

Written by: [SHANKAR PAREKAR], Fire Safety Specialist — [Brief expertise of 27 years of experience in commercial fire risk assessment and fire safety compliance across industrial and commercial sectors.]
Reviewed by: [SHIV DASS] ,[ BAFE-registered fire safety engineer and certified fire risk assessor with expertise in BS 5306 compliance and industrial fire suppression systems.]

Disclaimer: This article was initially drafted using AI assistance. It has undergone thorough revisions and factchecking by human editors and subject matter experts to ensure accuracy.