Firefighter evaluates wildland fire in the autumn forest

Indiana Dunes National Lakeshore

line drawing of heat oxygen and fuel to make fire
wood burning
Old Cabin

Wood, dead leaves, and even structures can be considered fuel for a fire. From top: Fire Triangle, Crater Lake National Park, and Dinosaur National Monument

The Fire Triangle

Fire is a chemical reaction, called combustion. This involves the rapid oxidation of combustible materials—any substance which will ignite and burn—accompanied by a release of energy in the form of heat and light. An external source of heat generally is needed to start a fire.

The three-sided fire triangle shows that oxygen, heat and fuel in the proper proportions are necessary to create a fire. If any one of these three elements is removed, a fire cannot exist. Air supporting a fire must be at least 16 percent oxygen. The air that surrounds us contains about 21 percent oxygen. Heat and temperature are closely related. Heat is a type of energy in disorder, whereas temperature is a measure of the degree of that disorder.

The final component of the fire triangle is fuel. Fuel is considered any material capable of burning. This would include living vegetation, branches, needles, standing dead snags, leaves, human-built wooden structures, etc.