Class B fire extinguishers, designed for flammable liquid fires like gasoline, oil, and paint, contain one of several different agents depending on the type. The most common are dry chemical powders (sodium bicarbonate, potassium bicarbonate, or monoammonium phosphate), carbon dioxide gas, foam concentrates, or clean agents like Halotron. Each works differently to knock down a flammable liquid fire, and the choice matters for both effectiveness and cleanup.
Dry Chemical Powders
The majority of Class B extinguishers you’ll encounter in homes, offices, and workshops use dry chemical powder. There are three main formulations, and they don’t all work the same way.
Sodium bicarbonate is the classic “BC” dry chemical, essentially a fine-grained cousin of baking soda. When it hits a flame, it breaks down and releases carbon dioxide while also interrupting the chemical chain reaction that keeps the fire burning. It neutralizes the highly reactive particles (called free radicals) in the flame, starving the fire of the energy it needs to sustain itself.
Potassium bicarbonate, sometimes marketed as Purple-K, works through the same chain-reaction interruption but is roughly twice as effective per pound as sodium bicarbonate on flammable liquid fires. The potassium reacts with free radicals in the flame to form stable compounds like potassium hydroxide, pulling those reactive particles out of the combustion process. Purple-K is widely used in industrial settings and by airport fire crews for this reason.
Monoammonium phosphate is the powder inside the familiar red ABC extinguisher, the most common type sold for home use. A typical residential model carries a 2A:10B:C rating. This agent smothers the fire with a powdery residue while also interrupting the chain reaction. Its advantage is versatility: it handles Class A (ordinary combustibles), B (flammable liquids), and C (electrical) fires. The trade-off is that it’s slightly less effective on pure Class B fires than potassium bicarbonate.
Carbon Dioxide (CO2)
CO2 extinguishers contain pressurized liquid carbon dioxide stored in a heavy steel cylinder. When discharged, the liquid converts to gas and comes out as an extremely cold white cloud. Unlike dry chemicals, CO2 doesn’t need a separate propellant. It provides its own pressure for discharge.
CO2 suppresses fire primarily by cooling the burning gases below the temperature needed to sustain combustion. It also dilutes the oxygen and fuel vapors around the flame, slowing the rate of heat release. Reducing oxygen to about 10 percent by volume makes combustion impossible for nearly all fuels. Because carbon dioxide is heavier than air, it settles over the burning liquid and forms a temporary blanket.
The biggest limitation is re-ignition. Once the CO2 cloud dissipates, hot fuel can reignite because there’s no lasting barrier on the liquid’s surface. CO2 also poses a suffocation risk in enclosed or low-lying spaces like basements and pits, where the heavier-than-air gas can pool invisibly. For these reasons, CO2 extinguishers work best in well-ventilated areas or where you need a clean discharge that won’t damage electronics or equipment.
Foam Agents
Foam extinguishers contain a water-based solution mixed with surfactants that produce a thick, stable blanket of bubbles when discharged. The traditional high-performance version is aqueous film-forming foam (AFFF), which uses fluorocarbon surfactants to achieve an extremely low surface tension, in the range of 15 to 25 millinewtons per meter. That low surface tension allows the foam to spread rapidly across the surface of burning gasoline, oil, or other hydrocarbon liquids and form a thin aqueous film that seals in vapors.
This vapor-sealing film is what sets foam apart from other Class B agents. It both smothers the fire and cools the fuel surface, and the physical barrier stays in place after the flames go out, dramatically reducing the chance of re-ignition. However, the fluorocarbon surfactants in traditional AFFF contain persistent environmental pollutants, which has pushed the industry toward fluorine-free alternatives. Newer formulations use silicone-based or hydrocarbon surfactants that approach similar performance without the environmental toxicity.
Clean Agents
Clean agent extinguishers are designed for environments where residue would cause as much damage as the fire itself: server rooms, aircraft cockpits, museums, and medical facilities. Halotron I is the most widely available handheld clean agent rated for Class B fires. It discharges as a rapidly evaporating liquid that leaves no powder, foam, or residue behind.
In testing against standardized heptane pool fires, Halotron I outperformed several other clean agents. These extinguishers cost significantly more than dry chemical models, but the fact that they require zero cleanup is a major advantage in sensitive spaces. CO2 and Halotron extinguishers both fall into the “no cleanup required” category.
How Class B Ratings Work
The number before the “B” on an extinguisher’s label tells you the size of flammable liquid fire it can handle. Extinguishers earn that number by successfully putting out a heptane fire (heptane is one of the main components of gasoline) in a standardized steel pan. A 10-B rated extinguisher, for example, has proven it can extinguish 31 gallons of burning heptane in a 25-square-foot pan. Higher numbers mean larger fires: a 20-B handles roughly double that area.
When you see a rating like 2A:10B:C, that tells you the extinguisher covers Class A, B, and C fires, with a 10-B capacity for flammable liquids. For most homes and small offices, a multi-purpose ABC dry chemical extinguisher with at least a 10-B rating is the standard recommendation.
Cleanup and Residue Concerns
What’s inside the extinguisher matters long after the fire is out, because the residue from different agents behaves very differently. ABC dry chemical powder (monoammonium phosphate) is mildly corrosive when it gets wet, which means it can damage electronics, metal surfaces, and machinery if not cleaned up promptly. The best approach is to vacuum the bulk of the powder with a HEPA-filtered shop vacuum rather than sweeping, which just pushes fine particles into the air. After vacuuming, wipe remaining residue with a damp cloth. Avoid flooding the area with water first, as that turns the powder into a sticky paste that’s harder to remove and more corrosive.
CO2 and clean agent extinguishers leave nothing behind. This is their primary selling point for protecting valuable equipment, though they cost more and typically offer lower B ratings than comparably sized dry chemical units.
Health Effects of Inhaling Dry Chemical
All dry chemical powders are mild respiratory irritants. Inhaling the discharge can cause coughing, shortness of breath, and irritation of the lungs, eyes, and skin. These effects are typically short-lived in healthy people. If you or someone nearby is coughing after an extinguisher discharge, move to fresh air. People with asthma, emphysema, or bronchitis are more vulnerable, as the fine particles can aggravate these conditions. Chronic, repeated exposure to the dust over long periods can potentially cause a condition called pneumoconiosis, a type of lung disease caused by inhaling fine particulate matter, though this is an occupational concern rather than a risk from a single home use.