What is fire?  

The Fire Tetrahedron 

The fire tetrahedron, aka the “fire pyramid,” represents the four elements required to start and sustain a fire. It used to be called the “fire triangle” when it only had the first three elements before researchers identified the fourth: 

1. Fuel includes all nearby flammable objects, liquids, or gases. 

2. Heat causes different fuels to ignite at their specific flashpoints. 

3. Oxygen is steadily required for a fire to ignite and burn. 

4. Chemical chain reaction: A feedback loop where the fire creates heat and releases gases, and the heat ignites the gases, feeding the fire and creating more heat and combustible gases. This reaction sustains the fire. 

All four elements must be present for fire to exist. If you remove any of them, the blaze will be extinguished. 

Fire Classifications 

Fire is classified based on what it uses as fuel. There are five classes: 

• Class A: Ordinary fires involving wood, paper, fabric, and other “ordinary combustibles.” 

• Class B: Liquid and gas fires, such as those involving gasoline or alcohol, but excluding liquids or greases used during cooking. 

• Class C: Electrical fires caused, worsened, or sustained by energized electrical equipment. 

• Class D: Metallic fires that burn combustible metals, such as magnesium and sodium. 

• Class K: Grease and oil fires, specifically from cooking. 

Stages of Fire  

Stage 1: Incipient/Ignition 

• “Fuel, oxygen and heat join together in a sustained chemical reaction.” 

• Flames are small and have not spread. 

• The smoke levels are manageable and not a significant threat. 

• Low heat emission. 

Stage 2: Growth 

• “With the initial flame as a heat source, additional fuel ignites.” 

• The flame continues to grow with an ample supply of oxygen and fuel. 

• A layer of smoke forms above the flame, and hot gases collect at the ceiling. 

• The room temperature rises, bringing the fire closer to igniting every potential fuel source. 

Sub-Stage Event: Flashover 

• Before a fire progresses to the next stage, there is a moment called “flashover.” It is the near-simultaneous ignition of all fuel sources in an enclosed area. 

• No one survives flashover in a space where it happens. 

Stage 3: Fully Developed 

• The fire is big enough to encompass most or all available sources of fuel, and temperatures peak. 

• The flames are obscured by dark, dense, black smoke. 

Stage 4: Decay (The Longest Stage) 

• The fire slows with a decrease in fuel or oxygen. 

• New fuel or oxygen will allow the fire to regrow. 

Why is this important? 

These concepts provide a foundation for how different fire protection systems work to control or extinguish a fire. 

For example, some systems attack a specific portion of the fire tetrahedron or have “design objectives” to control fires—slowing them and their stages down—providing time for people to escape structures and firefighters to arrive. Other systems work to suppress or completely extinguish fires.