Which phenomenon may occur when fresh air re-enters a space with accumulated fuel-rich gases, potentially causing an explosive ignition?

Which phenomenon may occur when fresh air re-enters a space with accumulated fuel-rich gases, potentially causing an explosive ignition?

• A. Flashover
• B. Ventilation-limited burn
• C. Backdraft
• D. Smoke poisoning

Answer: (C)

Backdraft happens when a fire in a confined space burns out of oxygen and accumulates hot, fuel-rich gases near the ceiling. If fresh air suddenly enters, those gases mix with the oxygen and reach flammable limits, igniting explosively. The result is a rapid, violent ignition that can send a blast of flame and pressure outward from the opening. This is different from flashover, which is the entire room igniting from heat, and from a ventilation-limited burn, where the fire is kept in check by oxygen levels rather than an explosive air-fuel mix forming at an opening. Smoke poisoning is about inhalation hazards, not an ignition phenomenon.