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Fire Investigation Mythunderstandings
Examining Long-Held Truths About Fire Dynamics, Physical Indicators of Incendiary Fires, and Fire Investigation Techniques

By Cathleen E. Corbitt-Dipierro

In this series, we will examine physical characteristics of the post-fire environment that have traditionally been interpreted as definitive indicators of incendiary fire, misunderstandings about fire dynamics, and misuse of fire investigation techniques.

Mythunderstanding #2:  The area of greatest damage and the lowest point of burning is always the area of origin.
“This isn’t necessarily true,” begins Mark A. Teufert, Special Agent Certified Fire Investigator and Training Program Manager for ATF’s State and Local Training Branch. “There are too many variables to make this blanket statement.  But, it’s what we all hear in training as a methodology: trace the fire back from least to most damage and that’s your area of origin.  So, in the fire investigation courses at the ATF National Academy, we set up a compartment burn that challenges this statement, making it clear that it’s a method, not an absolute truth.”

The test burn takes place in a two room compartment connected by a hall.  The details of the burn setup are confidential, but the result is that the fire produces the greatest amount of damage in a location that is not proximate to the area of origin.  “Most of the students we ask to determine the origin of this fire select the area of origin as the area with most damage and attribute other damage to flame extension.  But, that’s not how it happened.  This test burn illustrates the critical role of ventilation and fuel load in determining damage patterns.  The fire is drawn toward the ventilation source and heavily burns fuel items right near that ventilation source.”

interfireVR illustrates this same principal in an environment with a staircase.  In interfireVR, the fire starts in the corner of the living room where there is a space heater next to a couch.  A large opening to the dining room was only about six feet away from the fire origin.  The living room was open to a stairwell that led to the upstairs hall and at the top of the stairs was a bathroom with an open door to the hall and an open window to the outside.  The damage in this faraway bathroom was equal to or greater than the damage in the dining room right next to the area of origin.  Why?  The open window entrained air right outside, drawing the fire with it.

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The living room from the interfireVR burn.  The area of origin is on the couch in front of the space heater.  Note the dining room at the left.  Photo is taken from the stairs leading up to the second floor.

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The dining room adjacent to the living room in the interFIRE VR burn.  Note extent of damage compared to the upstairs bathroom (see below).

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The upstairs bathroom from the interfireVR burn.  This bathroom was at the top of the stairs from the living room.  The door to the bathroom was open and the bathroom window (at left, on the wall with the toilet paper holder) was open.  Note the smoke staining extending all the way to the floor.

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The yellow- and blue-colored melted plastic is the remains of two toothbrushes in a holder above the sink in the upstairs bathroom.

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The remains of the plastic hooks that once held the shower curtain.

“Configuration is also of critical importance when determining area of origin,” adds David T. Sheppard, PhD., a Senior Fire Protection Engineer at the ATF Fire Research Laboratory.  “For example, fall down of something like curtains could cause an area of low burning.  That’s not an area of origin, but it can easily be mistaken for one or be taken as ‘evidence’ of multiple fires.  Another fire effect that can cause low burning is radiant line of sight, which is responsible for 1/3 of the heat of a fire.”

Typically, the difficulty with burn pattern interpretation happens when there is a strong factor that tips the fire dynamics physics in a different direction that might “naturally” occur.  An open door at the end of the hall can affect the ventilation of the fire, possibly pulling it away from the true area of origin to more heavily damage the area near the ventilation source.  Once ignited by radiant heat or flame impingement, an unusually heavy fuel load in one part of the room can cause greater damage than less loaded areas.  The good news is that these conditions can be observed at the scene and their effect included in the fire flow analysis.  But, the investigator must look for them and actively analyze their potential effects, not blindly go by the adage of “least to most, lowest part, there’s your area of origin.”

And then, there’s the postflashover scene.  When a fire proceeds through flashover and into the full room involvement phase, the chaotic air flow and “firestorm”-like conditions can alter, obscure, or obliterate the original fire patterns, as well as cause unusual physical damage.  According to John D. DeHaan, the full room involvement stage typically produces heat flux readings at floor level of 170 kW/m2, which is sufficient to ignite most floor coverings and construction materials, creating extensive areas of low burning that may or may not be proximate to the area of origin.  Full room involvement can burn exposed wood on floors, window sills, and baseboards, even taking advantage of small gaps in wall construction to draw in air that causes ventilation effects.

Some of these potential postflashover effects mimic effects that can be caused by ignitable liquids, such as curling of vinyl floor tile and spalling of concrete. In fact, according to Dr. Sheppard, “Anytime a room reaches flashover, concrete will spall.” Unusual effects, like charring of the undersides of furniture, burning of floor coverings under furniture, burning under doors, and burning holes in floors have been observed with full room involvement fires. The myth that effects like burning under furniture can only be caused when an accelerant is poured under the furniture does not hold true when everything in the room is on fire. In Kirk’s Fire Investigation, John D. DeHaan notes that, in a postflashover fire, the turbulent mixing between combustion gases and fresh air being drawn into the room causes high and variable heat fluxes that can cause irregular damage to floors and floor coverings. They can also cause more intense burning around ventilation openings and melt synthetic fabrics and carpets. In fact, in a post-flashover fire, the fire becomes ventilation-controlled and the most intense fire, and therefore the most intense thermal damage, may be near the air supply, such as open windows or doors. Ventilation-controlled fires can, near ventilation sources, produce the characteristic V-pattern that is typically seen as an indication of fire origin. In a ventilation-controlled fire, if a V-pattern is found near a door, for example, the investigator must consider whether this might have been an effect of the postflashover burning, rather than an area of origin. As these effects show, flashover makes the determination of the first material ignited much more difficult, as heavy burning may now occur in many places.

The key in tracing fire flow and finding the area of origin is, in reality, far more complex than the myth of most damage and lowest burning.  The investigator must observe and take into account all the physical properties of the compartment, including ventilation, fuel load, configuration, the stage of the fire (and whether or not flashover was achieved)—and the interaction between all of these factors plus the unique characteristics of that environment.

For more information on areas of most damage and interpreting fire damage patterns, consult:

NFPA 921: Guide To Fire and Explosion Investigations. 2004 Edition.  Sections,, 6.2.5,

DeHaan, John D.  Kirk’s Fire Investigation, 6th Edition.  2006. pp 292, 293, 54.

Lentini, John J.  Scientific Protocols for Fire Investigation.  2006.  Pp. 72-89, 445-451, 461-462.

Quintiere, James G.  Principles of Fire Behavior.  Chapter 9 (Compartment Fires).

Last month's Mythunderstanding: #1: Spalling of Concrete

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