A basic understanding of the characteristics and behavior of fire is important to anyone aspiring to be a firefighter. This section reviews key concepts relating to fire. Understanding fire behavior requires a good working knowledge of both chemistry and physics. This information is designed to give the potential firefighter candidate a good idea of what the firefighter profession is all about—the confinement, control, and extinguishment of fire.


Fire is a rapid, self-sustaining oxidation process generating heat and light. The components of a fire and the types of fire are discussed below.

Oxidation: a chemical reaction between an oxidizer and fuel.

Oxidizer: generally, a substance containing oxygen that will chemically react with fuel to start and/or feed a fire. Examples include oxygen in the air, fluorine gas, chlorine gas, bromine, and iodine.

Fuel: materials that burn. Most common fuels contain carbon, hydrogen, and oxygen. Examples include wood, paper, propane gas, methane gas, and plastics.

Combustion: a rapid oxidation reaction that can produce fire. The oxygen in the air (21 percent) is generally the oxidizer, chemically reacting with the fuel. All combustion reactions give off heat and are therefore exothermic reactions. Combustion is commonly called fire. If combustion is confined and a rapid pressure rise occurs, it is called an explosion.

Explosion: a rapid expansion of gases (fuel and oxygen) that have mixed prior to ignition. Common explosions encountered by firefighters are chemical and mechanical explosions. Chemical explosion: a rapid combustion reaction classified as a detonation or deflagration, depending on the rate of propagation.

Detonation: a reaction that propagates at the speed of sound (1,088 feet per second in air) producing a shock wave. Examples include high explosives (dynamite, blasting agents).

Deflagration: a reaction that propagates at less than the speed of sound. Examples include low explosives (gunpowder) and combustible gases and dusts.

Backdraft: an explosion caused by the sudden influx of air into an oxygen-starved area filled with a mixture of combustible gases (primarily carbon monoxide) that are heated above their ignition temperature.

Mechanical explosion: a physical explosion. Examples include a boiler explosion and a boiling liquid expanding vapor explosion (BLEVE).

BLEVE: a container failure in the form of an explosion caused by the weakening of the container shell from the heat from a fire, corrosion, or mechanical damage. If the contents inside the container are flammable, a dramatic fireball results. Pyrolysis: a decomposition reaction in a solid material, not fast enough to be self-sustaining, usually brought on by the introduction of heat. It is the precursor to combustion. Characteristics of pyrolysis include the discoloration or browning of the surface of the material and the emission of smoke vapors.

Exothermic reaction: a chemical reaction that generates heat. New substances formed have less heat energy than was in the reacting materials. An example is combustion.

Endothermic reaction: a chemical reaction causing the absorption of heat. New substances formed by the chemical reactions contain more heat energy than prior to the reaction. An example is spontaneous combustion.

Spontaneous combustion: an endothermic chemical reaction causing self-ignition. Examples include a pile of rags dipped in linseed oil, alkyd enamel resins, or drying oils not properly stored or discarded and wet hay inside a barn loft.

Fire triangle: a model used to help in the understanding of the three major elements necessary for ignition: heat (thermal energy), fuel, and oxidizer (oxygen). It visually depicts the ignition sequence.

Fire tetrahedron: a model that expands on the one-dimensional fire triangle. The fire tetrahedron visually shows the interrelationship among the three components of the fire triangle and further clarifies the definition of combustion by adding a fourth component (chemical chain reaction), depicting the concept of the rapid, self-sustaining oxidation reaction. The fire tetrahedron depicts the growth of ignition into a fire.

Heat (Thermal Energy)

Heat is defined as thermal energy. There are several types of heat, or thermal, energy.

Chemical energy: heat energy from oxidation reactions. Fire is an example.

Electrical energy: heat energy (resistance) developed by electrical current moving through a conductor (copper wire). Examples of electrical energy include arching, sparks, static electricity, and lightning.

Mechanical energy: heat energy developed from solid objects rubbing together causing friction. Mechanical heat energy is also created in the diesel engine (adiabatic process) to initiate combustion via the rapid compression of the fuel-air mixture in the cylinders.

Nuclear energy: heat energy released from the atom through fission (break-up of the nucleus) and fusion (combining of two or more nuclei) processes.

Solar energy: the heat energy of the sun in the form of rays that travel towards the Earth at the speed of light. These infrared and ultraviolet rays can be harnessed to heat homes, reflected by mirrors, and concentrated through a magnifying glass to ignite finely divided solid particles.