IAAI Certified Fire Investigator (CFI) Practice Test

Backdraft happens when a confined space is filled with hot, fuel-rich, oxygen-poor combustion products from a smoldering fire. When fresh air is suddenly introduced, the available oxygen mixes with flammable gases like hydrocarbons and carbon monoxide, allowing rapid oxidation. This causes a violent deflagration—a fast, subsonic flame front driven by heat transfer—often with a noticeable blast or surge of flames. The key here is the air influx triggering ignition of the trapped gases as a deflagration, not a detonation. Flashover involves a rapid heat-driven spread to all combustibles, a smoke explosion is about ignition of accumulated gases in a sealed space with an ignition source, and detonation is a supersonic, shock-wave–driven explosion, which isn’t the typical mechanism in a backdraft.