The Fire Tretrahedron

Firefighter fighting fire

The Fire tetrahedron explains the basic elements that are required to have a combustion. A tetrahedron – sorry for the geometry lesson – is a triangle with four triangular sides. Think of a pyramid with a triangle as the base instead of a square. There are four basic elements required to have a fire. The tetrahedron is composed of four sides and is the further development of the fire triangle, which only shows the elements required for an ignition.

Fire Teatrahedron showing the four elements required for fire

The Elements of Fire

For fire to exist, the right conditions need to exist. These conditions are referred to as elements. These elements are oxygen, heat, fuel and the chemical chain reaction. In the tetrahedron the chemical chain reaction generally forms the bottom of the pyramid, with the other elements depicted on the sides. Most fire extinguishing systems, such as sprinklers, extinguishers and suppression systems work on the principle of removing one or more of these elements from the fire.


Without oxygen a fire cannot start, likewise, if the oxygen is removed fire cannot be supported. Technically this element should be referred to as an oxidizer, rather than just oxygen. Oxygen is the most common oxidizer, but combustion without oxygen is also possible. Fluorine is an example of another oxidizer that could support a combustion without oxygen present. Oxygen then really is an oversimplification, but for the purpose for the tetrahedron, more than sufficient. Oxygen is a supporter of combustion and seeing that 21% of the atmosphere are oxygen, it can be easy to see why it is the most common.

Upper Explosive Limits

An example of how necessary Oxygen is to a fire would be a room that is oversaturated with a flammable gas. If there is a gas leak, the room can get filled with the flammable gas and displace all oxygen from the room. That would mean that you could theoretically create a spark and nothing would happen, as there is no oxygen present to support the combustion. That level of saturation would be referred to being above the UEL (Upper Explosive Limit) of the flammable gas in question.


For an ignition, a fuel needs to be present. Fuel can take many shapes and does not refer to gasoline specifically. Fuel could be a solid, such as wood, a liquid, such as grease, a gas, such as propane or anything in between. A sufficient amount of fuel needs to be available to sustain the combustion process.

Lower Explosive Limit

To refer back to the flammable gas example, the LEL (Lower Explosive Limit) is the minimum amount of the gas required to become a fuel for the fire. Below that limit, the gas is not available in sufficient quantity, so once again a spark could NOT light a fire. The dangers for gases specifically are within the LEL and UEL. Fuels for fire are organized by their class and assigned a letter. That is how the correct extinguishing agent can be picked for the hazard.


The term heat refers to the fuel’s flashpoint, the lowest temperature at which the fuel will ignite. There is a surprisingly small amount of heat required to start a fire. The action of lighting a match creates enough heat through friction to support the combustion process and light the match. Heat can come from many sources, such as the sun or artificially created. Many extinguishing methods work by lowering the temperature of the fuel, one major reason why water is so effective.

Chemical Chain Reaction of the Fire

The fourth element, which moves the fire from the fire triangle, the conditions needed for ignition, to the tetrahedron, the sustained combustion, is the chemical chain reaction of the fire. The chemical chain reaction provides the necessary heat to sustain the combustion. So long as this heat is provided, the fire will continue to burn and grow. It will only stop when it is deprived of one or more sides of the fire triangle.

Extinguishing a Fire

As noted above, extinguishing a fire requires starving the fuel of one or more elements of the tetrahedron. Most standard extinguishing methods work on this principle. It is important to use the correct agent for the applicable fire. We have an overview here of the different classes of fires and their corresponding extinguishing agents. Water-based sprinkler systems and other suppression systems also work along the same principles.


Safety training can be very beneficial to most organizations. A team with knowledge about how fires start and how they can be prevented can protect your facility from a disaster. Nexus Fire & Safety offers comprehensive training programs for organizations. This includes fire warden training, extinguisher training, and custom programs for specialized facilities. We are ready to assist with any questions that you may have and offer 24-hour support to keep your safety systems in top shape!