Class B fires involve flammable liquids, combustible liquids, and flammable gases. The NFPA specifically lists petroleum greases, tars, oils, oil-based paints, solvents, lacquers, alcohols, and flammable gases under this category. What unites all these materials is that they burn as vapors rather than as solids, which changes how the fire behaves and how it needs to be fought.
Flammable and Combustible Liquids
The bulk of Class B fire hazards come from liquids. These range from everyday substances like gasoline and alcohol to industrial chemicals like acetone, benzene, toluene, and methanol. Fuel oils, kerosene, turpentine, and lacquers also fall into this category. Even cooking greases and certain hydraulic fluids qualify.
The key distinction between “flammable” and “combustible” liquids is their flash point, which is the lowest temperature at which the liquid produces enough vapor to ignite. Flammable liquids have flash points below 100°F, meaning they can catch fire at room temperature or below. Gasoline, acetone, and ethyl alcohol all fall into this group. Combustible liquids have higher flash points (100°F and above) and generally need to be heated before they become dangerous. Fuel oils and kerosene are common examples.
Both types are Class B fuels regardless of flash point. The practical difference is that flammable liquids are dangerous in more everyday conditions, while combustible liquids typically require an additional heat source before they pose an ignition risk.
Petroleum-Based Products
Many Class B materials are petroleum derivatives. This includes motor oil, lubricating oil, transformer oil, petroleum grease, and tar. Oil-based paints and solvents used in construction, automotive work, and manufacturing also belong here. These substances are common in garages, workshops, and industrial settings, which is why Class B fire preparedness matters in those environments.
Flammable Gases
Class B fires also include flammable gases like propane, butane, and natural gas. These materials are already in vapor form, which makes them especially quick to ignite when they mix with air and encounter a spark or flame. Gas leaks in kitchens, workshops, or near outdoor grills are typical scenarios where a Class B gas fire could start.
Why Class B Fires Burn Differently
The critical thing to understand about Class B materials is that the liquid itself doesn’t burn. A layer of vapor sits above the surface of any flammable liquid, and it’s this vapor mixed with air that ignites. That’s why a puddle of gasoline at a warm temperature is far more dangerous than the same puddle on a cold day: heat increases the amount of vapor the liquid releases.
This vapor-based combustion is also why you should never use a direct stream of water on a Class B fire. Water can splash or spatter the burning liquid, spreading the fire across a wider area. If the burning liquid is in a container, water can raise its level and cause it to overflow. The correct approach is smothering: cutting off the fire’s oxygen supply rather than trying to cool the fuel.
How to Identify the Right Extinguisher
Fire extinguishers rated for Class B fires are marked with a red square containing the letter “B” and a pictogram of a fuel can. Three main types work on Class B fires:
- CO2 extinguishers (black label) displace oxygen around the fire and are effective on Class B and electrical fires. They leave no residue, making them popular in offices and around electronics.
- Dry powder extinguishers (blue label) work on Class A, B, and C fires by interrupting the chemical reaction that sustains combustion. They’re versatile but leave a cleanup mess.
- Foam extinguishers (cream-colored label) spread a blanket of foam over the burning liquid’s surface, sealing off the vapor layer from oxygen. They handle Class A and B fires.
If you work around any of the materials listed above, checking that your nearest extinguisher carries a “B” rating is a basic but important step. Many multipurpose extinguishers are rated ABC, covering flammable liquids along with ordinary combustibles and electrical fires.
Where Class B Fires Are Most Common
Class B fire risks concentrate wherever flammable liquids and gases are stored or used. Gas stations, auto repair shops, painting operations, industrial plants, restaurant kitchens (cooking oils and greases), and home garages with stored gasoline or solvents are all high-risk settings. Laboratories handling chemicals like ethanol, methanol, acetone, or ether also face significant Class B hazards.
Proper storage matters as much as having the right extinguisher. Keeping flammable liquids in approved containers, away from heat sources, and in well-ventilated areas reduces the chance that vapors accumulate to dangerous concentrations in the first place.