Fires can be unpredictable. They start small but quickly grow out of control. Knowing the four stages of a fire—incipient, growth, fully developed, and decay—helps you and your loved ones stay safe.

In this guide, we’ll talk about what happens during each stage and when to call for help. Remember, the right knowledge could make all the difference in preventing a small fire from turning into a raging inferno.

What Are the Four Stages of Fire?

Fires go through four main stages: incipient, growth, fully developed, and decay. Each stage comes with its own set of challenges. Let’s take a closer look at them below: 

1. Incipient Stage

The incipient or ignition stage is the first phase of a fire. During this stage, heat, oxygen, and fuel (also known as the fire triangle) work together to start a fire. Since the fire is just starting, it’s usually easy to manage using tools like fire blankets and fire sprays. Think of something like a candle or a small ember from a campfire. That's a fire in the incipient stage. The fire is still contained and doesn't spread quickly during this phase. 

A fire in this early stage has small flames, low heat, and little smoke. The smoke usually doesn’t make it hard to see, so you can spot the fire quickly. This is also the best time to put it out. If you catch a fire early, you can stop it before it gets big enough to spread and cause serious damage. However, fires can spread quickly once they pass this stage, so it’s crucial to act fast. As the fire progresses, the smoke gets thicker, visibility drops, and the heat increases, making it harder to stop without professional help. 

In addition, the fire is mostly localized during this stage. This means the fire hasn’t spread to other parts of the room, and the heat hasn’t reached a level where it’s burning things around it. 

It’s also important to remember that fighting a fire at this stage takes time. Sometimes, people slowly react when they notice a fire. For these reasons, automated setups like kitchen fire suppression systems exist. These systems can sense a fire in the incipient stage and automatically suppress it, saving you precious time. 

2. Growth Stage

The fire will enter the growth stage if you don’t put it out during the incipient stage. During this phase, things start to get more serious. The initial heat causes nearby items to catch fire, and the flames spread. The fire’s heat is transferred through convection (rising heat) and radiation (spreading heat), making the fire bigger and hotter. You may also notice some smoke. The temperature increases as well. 

A huge danger during the growth stage is the possibility of a flashover. A flashover occurs when everything combustible in a room ignites all at once. The temperature in the room can range between 500°C (932°F) to 590°C (1,100 °F). The hot gases from the fire can also reach the ceiling and spread horizontally. The heat and smoke fill the room, causing all nearby combustibles to reach their ignition point at the same time. Once a flashover happens, the fire becomes uncontrollable and difficult to survive.

However, it's important to note that flashovers don’t always happen. They only occur if there’s enough oxygen and fuel. If the fire burns through the fuel too quickly or runs out of oxygen, it may not reach a flashover. For example, if the room is sealed tightly with no air coming in, the fire may burn out before a flashover occurs. Similarly, if the fire doesn’t have enough fuel to sustain it, it may not build up enough heat to cause a flashover. 

It is crucial to recognize the signs of an impending flashover. If the temperature in the room rises quickly, the smoke thickens, and the heat feels unbearable, a flashover could be imminent. In addition, the fire may have spread enough for everything in the room to catch fire at once. Find out how to spot a flashover here. 

3. Fully Developed Stage

A fire peaks in size and intensity during the fully developed stage. This is the most dangerous phase because the fire has burned through most, if not all, of the available fuel. Plus, the temperature has soared to its highest point, sometimes reaching 1,093°C (2,000°F). This is more than enough to cause severe burns. 

At this stage, the fire consumes oxygen quickly, making breathing difficult. Thick, dark smoke also fills the air. The fire burns intensely, producing so much heat that it can damage windows, melt structural materials, and cause severe damage to the building. It’s also during this phase that the fire’s energy release is at its greatest, as the fire burns everything in its path.

While the fire is fully developed in one part of a building, conditions might be different in other areas. For example, one room might have reached the fully developed stage while another is still in the growth stage. Another might be in the incipient stage with a small, manageable flame. However, the fire can quickly spread from one room to another. Even if a nearby room is in the early stage of a fire, the conditions in the building can be immediately dangerous to life and health (IDLH). 

In addition, the heat becomes so intense that unburned gases accumulate at the ceiling and can ignite as they leave the room. This can cause flames to appear at doors or windows, even if the fire is contained in a specific area. The thick and black smoke can also obscure the flames.

At this point, the fire is incredibly hard to control. It has caused a lot of damage, and firefighters need advanced techniques and equipment to put it out. The fire will continue to burn until it runs out of fuel.

4. Decay Stage

The decay stage is the final phase of a fire, where things start to wind down. During this phase, the fire’s intensity drops. The fire has burned through most of the fuel, and there isn’t enough oxygen or fuel to keep it at full strength. The temperature falls, and the fire becomes less intense.

While this might sound like the danger is over, it’s not the time to relax. Even in the decay stage, the fire can still be dangerous. The fire can reignite if there’s any fuel and oxygen left. A fire in decay can still flare up because of hidden embers or small pockets of fuel that haven’t fully burned. That’s why firefighters check for any remaining hot spots and cool down the building.

Firefighters also watch out for backdrafts or smoldering during the decay stage. A backdraft occurs when oxygen suddenly rushes in. This causes the remaining gases to combust, which can lead to a flare-up. On the other hand, smoldering occurs when the fire burns slowly and without flames.

Even if the fire seems under control, some areas might have enough heat or fuel to reignite the flames. Firefighters have to be extra careful at this point. They’ll use ventilation techniques to remove lingering gases and completely cool down the area so the fire doesn’t start again. 

Wildfires enter the decay stage when they have burned most of the trees, plants, and other fuels in their path. Firefighters try to control this stage by creating firebreaks or using water. But just like structure fires, the risk of flare-ups remains, especially if the wind changes or more fuel becomes available.

What to Do When There’s a Fire

The fire has just begun and is still small and manageable. This is the best time to take action to prevent the fire from growing. Here’s what to do during the incipient stage: 

1. Alert Others

Let everyone know as soon as you spot the fire. Hit the fire alarm pull station if you’re in a building. Doing this gets people moving and helps clear the building quickly. Fire alarms also trigger automatic setups like fire sprinklers.

2. Use Fire Prevention Tools

Grab a fire blanket, fire spray, or fire extinguisher if the fire is small. Check out how to use a fire blanket here. Meanwhile, here’s a guide on how to use a fire spray. 

If you’re using an extinguisher, make sure you’re using the right one, depending on the fire type. For instance, Class C fire extinguishers are used for electrical fires, not water-based Class A fire extinguishers. 

Pull the pin, aim at the base of the fire, and squeeze. Then, sweep from side to side. If it doesn’t work, get out fast. Find out more about the PASS extinguisher technique here. 

3. Contain the Fire

If you can’t put out the fire, try to contain it as you evacuate. Follow the RACE (Rescue, Alert, Contain, Evacuate) acronym as much as possible. Close doors and windows to prevent the fire from spreading, and help those with difficulty evacuating. 

4. Evacuate

Get out if the fire’s growing or getting out of control. Don’t wait to see what happens. Know your escape routes, stay low to avoid smoke, and don’t use elevators. Once outside, stay far from the building. 

When to Call for Help

You should call for help at the first sign of a fire before it has a chance to grow. The earlier you call for help, the better the chances of containing the fire. Here’s a breakdown of when to call for help: 

Incipient Stage 

• Call for help if you see smoke, flames, or any signs of a fire.
• The fire is small and likely manageable at this stage. But you shouldn’t wait. Contact the fire department so they can prepare to respond if the fire grows.
• Early intervention helps prevent the fire from growing, and firefighters will likely extinguish it quickly, especially if it's in the incipient or ignition stage. 

Growth Stage

• If you didn’t call when the fire started and it’s now at the growth stage, call for help immediately. 
• The fire is mostly difficult to contain at this point, so you need professional help. 
• Firefighters are trained to handle fires that are quickly growing. They can also use advanced equipment and techniques to prevent it from getting worse.

Fully Developed Stage 

• The fire is fully developed at this point, so you can’t control it on your own.
• If you haven't already called for help, do so immediately. Inform the authorities the fire is quickly spreading. 
• This is the most dangerous stage, so you must leave it to the professionals. 

Decay Stage 

• The fire has consumed most of the available fuel. It’s starting to burn out. The intensity decreases, and the flames reduce, though hot spots or embers may remain.
• Call for help if you haven’t already done so. Hot spots or embers might reignite even if the fire seems to die down.
• Firefighters will make sure the fire is completely extinguished and prevent flare-ups. 

Conclusion

Fires can be unpredictable, but knowing the stages helps you stay ahead. Whether it’s a tiny spark or a full-blown blaze, understanding what to do helps. Even if things seem to calm down, don’t take any chances—fires can flare up when you least expect it. The key to staying safe is knowing what’s happening and acting fast. Always trust your instincts, and don’t wait for things to get out of control.