What phase of a fire does the introduction of oxygen cause a rapid resurgence to a fully involved state?

The phase of a fire where the introduction of oxygen leads to a rapid resurgence to a fully involved state is the decay stage. During the decay stage, the fire has already consumed most of the fuel available and is diminishing due to a reduction in heat and available fuel. However, if additional oxygen is introduced at this point—such as through a sudden ventilation opening—it can reignite combustion very quickly, resulting in a resurgence of fire activity.

The decay stage is critical because it illustrates the importance of fire dynamics, emphasizing how changes in airflow can alter the behavior of a fire. In contrast, during the growth stage, the fire is still expanding but has not reached the point of diminishing returns regarding fuel. The fully involved stage represents a fire that has reached maximum intensity, and while it may sustain itself, it does not necessarily respond to new oxygen in the same explosive manner as the decay stage. The smoldering stage, on the other hand, typically involves low-temperature combustion that is not reliant on a vigorous flame and is often characterized by slow combustion that can be re-ignited with an adequate oxygen supply but doesn't exhibit the same dramatic resurgence as seen when oxygen is added during the decay stage.