Fire in the 4800 block of South Paulina starts out looking as a routine fire. But notice the increasingly dark smoke early in the video (at :45) that becomes lighter again (at about 1:20). At about 3:00 minutes and later the smoke over the front door (to the right of the ladder) is possibly pulsating.
Less than a minute after a firefighter ventilates a top story window (3:46 elapsed time), a large backdraft engulfs the house (4:12 elapsed time). Four firefighters suffered facial burns. One of the firefighters spent more than a week at the University of Chicago Medical Center. Three of the injured firefighters were standing near the front door. The rush of flames blew out the front door. The fourth injured firefighter was hit by flames while he was on a ladder. The biggest risk of harmful or fatal injury is when a person inhales the hot air. An injured airway can swell and block air to the lungs.
A backdraft is a situation which can occur when a fire is starved of oxygen; consequently combustion ceases but the fuel gases and smoke remain at high temperature. If oxygen is re-introduced to the fire, by opening a door to a closed room, combustion can restart often resulting in an explosive effect as the gases heat and expand like a blast or explosion. Ventilating a roof or an upper window helps the gases escape upward and helps the heat escape upward.
Characteristic signs of a backdraft situation include yellow or brown smoke, smoke which exits small holes in puffs (a sort of breathing effect) and is often found around the edges of doors and windows, and windows which appear brown or black when viewed from the exterior. These darker colors are caused by incomplete combustion. If the room contains a lot of soot, it indicates that the room lacks enough oxygen to permit combustion. Firefighters often look to see if there is soot on the inside of windows and in cracks around in the room. The window might have cracked because of the heat. The windows of the structure may also have a slight vibration due to the pressure differentials. The surrounding environment (e.g. the hallway outside the suspected backdraft room) will be extremely hot.
A flashover is similar to a backdraft, but does not involve the introduction of oxygen to an oxygen-starved fire area as a cause of the increased combustion. A flashover is the near simultaneous ignition of all combustible material in an enclosed area. When certain materials are heated they undergo thermal decomposition and release flammable gases. Flashover occurs when the majority of surfaces in a space are heated to the autoignition temperature of the flammable gases. Flashover normally occurs at 500 °C (930 °F) or 1,100°F for ordinary combustibles, and an incident heat flux at floor level of 1.8 Btu/ft²*s (20 kW/m²).
An example of flashover is what happens after a piece of furniture starts to burn in a domestic room. The fire involving the initial piece of furniture can produce a layer of hot smoke which spreads across the ceiling in the room. The hot buoyant smoke layer grows in depth (downward), as it is bounded by the walls of the room. The radiated heat from this layer heats the surfaces of the combustible materials (other non-burning furniture, toys clothing) in the room, causing them to give off flammable gases via pyrolysis or material decomposition caused by high temperature. When the surface temperatures become high enough, these gases ignite (at a flashpoint temperature) and the entire room becomes engulfed in flames. Autoignition is also known as kindling point, and is defined as the temperature at which a material will spontaneously ignite in a normal atmosphere without the initiation of a spark or flame.