A backdraft is an air-driven event, unlike a flashover, which is thermally driven. Backdraft is usually defined as a deflagration resulting from the sudden introduction of oxygen into a ventilation-limited space containing unburned fuel and gases. When the air combines with the unburned fuel, rapid ignition can occur with devastating force. The normal oxygen level in air is approximately 21 percent. Below 14 percent, visible flame is reduced. When these fuels mix with air they ignite and burn quickly resulting in overpressure.