What specific conditions, often including dense, turbulent smoke and inward-moving air when an opening is made, are precursors to a backdraft?

A backdraft is a sudden and explosive ignition of superheated, unburned fuel gases and smoke when fresh oxygen is introduced into an oxygen-depleted, confined fire compartment. The specific conditions that precede a backdraft begin with a compartment fire, which is a fire burning within an enclosed space with limited ventilation. As the fire consumes the available oxygen, combustion becomes incomplete, meaning that not all the fuel can burn due to insufficient oxygen. The intense heat within the compartment causes solid fuels to undergo pyrolysis, which is the chemical decomposition of materials by heat in the absence of oxygen, releasing vast quantities of flammable gases and vapors, referred to as fuel gases. These superheated fuel gases, along with particulate matter, accumulate within the compartment, creating a highly fuel-rich and oxygen-deficient atmosphere. Dense, turbulent smoke is a critical visual precursor, indicating a high concentration of unburned fuel gases, high heat, and significant pressure within the compartment; its density shows a rich fuel-air mixture, and its turbulence suggests the gases are under pressure and rapidly expanding. The compartment environment is essentially oxygen-starved, suppressing visible flames but allowing the build-up of unburned, superheated fuel gases. The final and critical precursor is the sudden introduction of fresh air, which contains oxygen, when an opening is made to the confined space, such as opening a door or breaking a window. The 'inward-moving air' refers to this rapid influx of oxygen-rich air being drawn into the oxygen-depleted compartment due to pressure differences and the extreme heat. This sudden mixing of fresh oxygen with the superheated, unburned fuel gases creates an immediately ignitable mixture that rapidly and violently combusts, resulting in a backdraft.