A fire extinguisher holds an extinguishing agent (water, foam, dry powder, CO2, wet chemical or a clean agent gas) plus the pressure that pushes it out. What is inside depends on the type, and each agent suits different fires, so one extinguisher cannot be used on every fire. Each agent works by breaking the fire triangle of heat, fuel and oxygen: cooling (water), smothering (foam), displacing oxygen (CO2) or interrupting the chemical reaction (dry powder and clean agents).
This page covers what each type contains. For the gas inside them and the difference between a gas agent and a propellant, read which gas is used in a fire extinguisher.
| Type | What is inside | How it puts out fire | Used on |
|---|---|---|---|
| Water | Water, with a pressure source | Cools the fuel | Class A (wood, paper, textiles) |
| Foam (AFFF) | Foam concentrate mixed with water | Forms a film that smothers the liquid surface | Class B, some Class A |
| ABC dry powder | Monoammonium phosphate powder | Interrupts the chemical chain reaction | Class A, B, C |
| CO2 | Carbon dioxide, stored as a liquid under pressure | Displaces oxygen around the flame | Class B, C |
| Wet chemical | Potassium-based solution | Forms a soapy layer on hot oil and cools it | Class F / K (cooking oils and fats) |
| Clean agent | Gas such as FM-200 or HFC-236 | Interrupts combustion; evaporates without residue | Class A, B, C |
ABC extinguishers are versatile and can be used for Class A, B, and C fires. The powder agent (monoammonium phosphate) works by disrupting the chemical chain reaction of fire. These are ideal for general industrial use, workshops, and garages. However, they create a dust cloud that can cause visibility issues and require cleanup after use.
CO2 fire extinguishers are designed specifically for electrical fires (Class C) and flammable liquid fires (Class B). Carbon dioxide works by displacing oxygen, effectively suffocating the flames without leaving residue. This makes it an excellent choice for data centres, server rooms, and laboratories. Because of the rapid cooling effect, the nozzle can become extremely cold during discharge. For the properties of the gas itself, see which gas is used in a fire extinguisher.
Foam fire extinguishers are particularly effective for Class B fires involving flammable liquids such as gasoline and oil. They create a thick foam layer over the burning liquid, cutting off oxygen and preventing reignition. Foam extinguishers can also be used on some Class A fires, making them a valuable choice for fuel stations and industrial areas.
Kitchen fires, often involving cooking oils and fats, require a specialised approach. Wet chemical extinguishers contain a potassium-based solution that reacts with hot oils to form a soapy layer, effectively cooling and smothering the flames. These are ideal for Class F fires commonly found in commercial kitchens and restaurants.
Clean agent fire extinguishers use gases such as FM-200 or HFC-236, which evaporate quickly without leaving residue. They are used for enclosed spaces and sensitive equipment such as data centres, museums and aircraft. They replace older Halon extinguishers, which were phased out because of their environmental impact.
Water is the most widely used fire suppression agent that cools down fire and reduces heat. Water fire extinguishers are primarily effective for Class A fires (wood, paper, textiles). Never use water for electrical or oil-based fires, as it conducts electricity and spreads flammable liquids.
| Agent | Class A | Class B | Class C | Class F | Residue |
|---|---|---|---|---|---|
| CO2 | ✗ | ✓ | ✓ Safe | ✗ | None |
| ABC Dry Powder | ✓ | ✓ | ✓ | ✗ | Powder |
| Foam (AFFF) | ✓ | ✓ | ✗ | ✗ | Foam/water |
| Wet Chemical | ✗ | ✗ | ✗ | ✓ | Soap film |
| Clean Agent | ✓ | ✓ | ✓ Safe | ✗ | None |
Tags: Fire Extinguishers · Fire Safety · Fire Fighting Equipment
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Depending on type, a fire extinguisher contains water, foam concentrate, ABC dry chemical powder, CO2 gas, a wet chemical (potassium-based) solution, or a clean agent gas like FM-200, each suited to different fire classes.
The cylinder holds the extinguishing agent under pressure, along with a propellant in types that do not store their own pressure, a discharge valve, and a siphon tube that draws the agent out when triggered.
ABC fire extinguishers contain monoammonium phosphate dry chemical powder, which disrupts the fire's chemical reaction and works on Class A, B and C fires.
Wet chemical extinguishers contain a potassium-based solution (such as potassium acetate) that reacts with hot cooking oils and fats to form a cooling, smothering soap layer.
A foam extinguisher contains foam concentrate mixed with water. When discharged, it spreads a layer over burning liquid and cuts off the oxygen supply.
It depends on the type: CO2 extinguishers hold carbon dioxide, clean agent types hold gases like FM-200, and other types use nitrogen to push the agent out. See the full guide on which gas is used in fire extinguishers.
The gas inside each type, and agent vs propellant.
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