Fire News

What’s Inside a Fire Extinguisher?

Posted on by hannah
Beautiful photo of fire extinguisher training, a person using a fire extinguisher to put out the flames on an outdoor scene.,A red canister is being used by someone wearing black.
04
Dec

Fire extinguishers are a familiar sight in homes, workplaces, and public buildings, but have you ever wondered what’s actually inside them?

While they might look similar on the outside, the contents of a fire extinguisher vary depending on the type of fire it’s designed to tackle. Understanding what’s inside each extinguisher helps you choose the right one, use it safely, and comply with UK fire safety regulations.

In this guide, we’ll break down what’s inside each main type of fire extinguisher, how it works, and which fire classes it’s suitable for.

A Quick Refresher: Fire Classes in the UK

Before we dive into the contents, it helps to know the UK fire classes:

  • Class A – Fires involving solid materials like wood, paper, fabric

  • Class B – Flammable liquids like petrol, paint, alcohol

  • Class C – Flammable gases like propane, butane

  • Class D – Fires involving metals like lithium or magnesium (rare in general settings)

  • Class F – Cooking oils and fats (deep fat fryers)

  • Electrical fires – Not a formal class, but some extinguishers are safe for use on live electrical equipment

 

 

What’s Inside a Fire Extinguisher? By Type

1. Water Extinguisher (Class A)

Contents:

  • Pressurised water

  • Sometimes mixed with additives to improve performance

How it works:
Cools the burning material and removes heat from the fire triangle (heat, fuel, oxygen).

Use for:

  • Wood, paper, fabric, textiles

  • General office, retail, and school settings

Do NOT use on:
Electrical, flammable liquid, or oil fires

2. Foam Extinguisher (Class A & B)

Contents:

  • Aqueous Film Forming Foam (AFFF)

  • Water-based solution mixed with foaming agents

  • Pressurised with nitrogen or air

How it works:
Creates a cooling effect and forms a foam blanket that smothers the fire, cutting off the oxygen supply.

Use for:

  • Flammable liquids (e.g. petrol, diesel)

  • Paper, wood, and soft furnishings

Do NOT use on:
Live electrical fires unless clearly marked as safe

3. CO₂ (Carbon Dioxide) Extinguisher (Electrical & Class B)

Contents:

  • Pure, liquefied carbon dioxide under high pressure

  • Discharges as a cold gas

How it works:
Displaces oxygen around the fire and cools the fuel source. Leaves no residue.

Use for:

  • Live electrical fires (computers, servers, switchboards)

  • Flammable liquids (Class B)

Do NOT use on:
In confined spaces without ventilation, CO₂ can displace breathable air

4. Dry Powder Extinguisher (Class A, B, C, Electrical)

Contents:

  • ABC powder (commonly monoammonium phosphate or ammonium sulfate)

  • Propelled by nitrogen or CO₂

How it works:
Interrupts the chemical reaction of the fire and smothers it by coating the burning material.

Use for:

  • Solids, liquids, gases, and electrical fires

  • Multi-risk environments, car parks, fuel storage areas

Do NOT use in:
Enclosed indoor spaces. It creates a cloud of powder that can reduce visibility and affect breathing

5. Wet Chemical Extinguisher (Class F & A)

Contents:

  • Potassium acetate or potassium citrate-based solution

  • Slightly soapy liquid, pressurised with nitrogen

How it works:
Cools the fire and creates a soapy film (saponification) on hot oil or fat, sealing off the oxygen.

Use for:

  • Cooking oil and fat fires (deep fat fryers)

  • Also effective on paper and wood (Class A)

Do NOT use on:
Flammable liquids or electrical fires

6. Specialist Extinguishers (Class D – Metal Fires)

Contents:

  • Specific dry powders like graphite, copper, or sodium chloride

  • Designed for reactive metal fires

How it works:
Absorbs heat and forms a crust over burning metal to cut off oxygen.

Use for:

  • Metal fires (e.g. magnesium, titanium, lithium)

  • Found in laboratories and specialist industrial settings

Do NOT use on:
Other fire classes. Only use for Class D fires

Why Knowing the Contents Matters

Using the wrong type of extinguisher can make a fire worse. For example:

  • Using water on an electrical fire could cause electrocution

  • Using CO₂ in a small, unventilated room can reduce oxygen for people as well as the fire

  • Using foam or powder on cooking oil could cause hot oil to splatter dangerously

By knowing what’s inside, you can:

  • Select the right extinguisher for your environment

  • Use it correctly in an emergency

  • Stay compliant with UK fire safety laws

 

 

Final Thoughts

Fire extinguishers might look alike, but they’re filled with very different substances designed for specific fire types. Understanding what’s inside each extinguisher helps you make informed decisions about fire protection, avoid dangerous mistakes, and ensure your property is equipped with the correct extinguishers in the right places.

For expert advice on fire protection and prevention measures, contact Martyn Young Fireproofing Consultancy on 07585 896648