Stages of Fire | What are the stages of Fire

Stages of Fire | What are the stages of Fire

Fires evolve over several stages as the available fuel and oxygen are consumed. Each stage has its own characteristics and hazards that safety managers and fire-fighting personnel should understand.

Stages of Fire | What are the stages of Fire

Incipient Stage (Pre-Heat, Pre-Ignition)

The incipient stage is the first or the beginning stage of a fire also known as "ignition" and is usually represented by a very small fire, which often (and hopefully) goes on its own before the next stage is reached. In this stage, combustion has begun. This stage is identified by an ample supply of fuel and oxygen. Recognizing a fire in this stage provides your best chance at escape or suppression.

The products of combustion that are released during this stage normally include water vapor, carbon dioxide, and carbon monoxide. Temperatures at the seat of the fire may have reached 1000°F, but room temperatures are still close to normal.

Free-Burning Stage

The free-burning stage follows the incipient stage. The self-sustaining chemical reaction is starting up at this point. Greater quantities of heat are released and the fuel and oxygen supply rapidly consumed. Room temperatures can rise to over 1300°F. The free-burning stage can become dangerous in an enclosed compartment. The contents inside a compartment are heated because of the heat intensity. If the compartment isn't well ventilated at some point, the contents of the compartment will reach their ignition temperature. A flashover occurs when the contents inside a compartment reach their ignition temperature simultaneously and become involved in flames. It is not uncommon for room temperatures to exceed 2000°F following a flashover. Human survival, even for properly protected firefighters, is difficult if not impossible for a few seconds within a compartment following a flashover.

Decay (Smoldering, Glowing)

The smoldering stage follows the free-burning stage. As a free-burning fire continues to burn, the chemical reaction will eventually consume the available oxygen within the compartment and ultimately convert it into carbon monoxide and carbon dioxide. This causes the oxygen concentration within the compartment to decrease. The combustion by-products often occupy the container, so human survival is not feasible. When the concentration of oxygen drops by volume to 15 percent, the chemical reaction will not have enough oxygen to support free-burning combustion. The fires are clearly still alive and the gasoline is starting to shine. Smoldering fires can continue the combustion process for hours, particularly when insulated inside a compartment. Room temperatures can range from 1000–1500°F. The by-products of burning always fill the room, so human survival can not. An extreme hazard, called a backdraft, can develop during the smoldering stage. A backdraft occurs when oxygen is introduced into a smoldering compartment fire. The immediate availability of sufficient oxygen in the presence of sufficient fuel, heat, and chemical chain reactions causes flaming combustion again. In some cases, the backdraft is so violent that an explosion will occur. Human survival, even of properly protected firefighters, is usually not possible.


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