Fire is more than just a source of heat and light. Fire is a powerful chemical reaction that can sustain life and cause destruction. But what exactly is fire, and how does it work? Understanding what fire is, how it starts, and how it spreads is essential for safety at home, in the workplace, and at school. In this article, we’ll break down what fire is, explain the four stages of fire, introduce the fire tetrahedron, and go over the different fire classifications.
What is fire
Fire, simply put, is a chemical process of combustion involving the oxidation of a fuel source at a high temperature. It releases energy and produces heat and light.
Flames are produced following the chemical reaction between oxygen and another gas.
Flames are intensified by increasing the rate of combustion.
Fire treahedron
Four elements, also known as the fire tetrahedron, must be present in order for a fire to exist.
These fire tetrahedron elements include:
- Oxygen
- Heat
- Fuel
- Chemical reaction
When one of the four elements is removed, the fire can then be extinguished.
Understanding the elements of fire is an essential component of firefighting education and suppression efforts.
The stages of fire
There are four stages of fire:
- Ignition: At this stage, a fire extinguisher can control the fire.
- Growth: Additional fuel ignites, causing the size of the fire to increase rapidly.
- Fully developed: This is when temperatures reach their peak, causing damage.
- Burnout: The fire gets less intense.
Fires grow quickly and can become dangerous in a matter of minutes, which is why understanding the stages of fire is so important. In the ignition stage, the fire is still small and manageable — this is the best time to act. If not controlled early, the fire enters the growth stage, where it rapidly spreads as it consumes more fuel. This growth is often exponential, meaning the fire can double in size every few seconds. Once it reaches the fully developed stage, it becomes much harder to control, causing maximum damage and risk. Recognizing a fire early and using the right extinguisher, if available, and call 911 can make all the difference in stopping a small flame from becoming a major disaster.
And to select the right type of extinguisher to put out a fire, you must understand the different classes of fires.
Fire classifications
Not all fires are the same, and using the wrong method to put one out can be dangerous. For example, trying to use water on a grease fire or an electrical fire can actually make things worse. Each type of fire — whether it involves wood, liquids like gasoline, electrical equipment, or even cooking oils — needs a specific kind of extinguisher to be put out safely. Understanding these classifications helps with response, reduces risks, and helps prevent fires from spreading or causing more damage.
Fires are classified into five groups:
- Class A fires consist of ordinary combustibles, such as wood, paper, cloth, trash and plastics.
- Class B fires are fueled by flammable or combustible liquids, such as grease, oil and gasoline.
- Class C fires are also known as energized electrical fires. These fires involve electrical equipment, such as motors, transformers and appliances.
- Class D fires comprise of combustible metal fires, involving metals such as potassium, sodium, aluminum and magnesium.
- Class K fires involve cooking oils and greases, such as animal and vegetable fats.
Ultimately, having a basic understanding of what fire is and its types of classifications will lessen the chances of on-duty firefighter injuries and fatalities.
This article, originally published Jan. 26, 2017, has been updated with a video, additional resources and current information. FireRescue1 is using generative AI to create some content that is edited and fact-checked by our editors.
