Which elements comprise the fire tetrahedron?

• A. Heat, fuel, oxygen, and chemical chain reaction
• B. Heat, fuel, oxygen, and humidity
• C. Heat, water, oxygen, and chemical chain reaction
• D. Fuel, oxygen, heat, and pressure

Answer: (A)

The fundamental idea here is that a fire needs four elements to keep burning: heat, fuel, oxygen, and the chemical chain reactions that propagate flame chemistry. Heat supplies the energy to reach and maintain ignition and to drive the reactions forward. Fuel provides the material that can burn. Oxygen acts as the oxidizer that accepts energy from the reaction. The chemical chain reaction, made up of free radicals that are generated during combustion, keeps those flame reactions going once ignition has occurred. If you remove any one of these four, the flame can no longer sustain itself.

Choices that include humidity, water, or pressure aren’t part of what sustains a flame in the same way. Humidity isn’t a required component of combustion, water cools and can extinguish, and pressure isn’t one of the fundamental sustaining elements. So the best answer is the one that lists heat, fuel, oxygen, and chemical chain reaction.