Fire Investigation Definition Series – Post-Flashover

Posted: June 20, 2013 in Fire Investigation
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by: Jason A. Sutula

carbon monoxide warning

The majority of residential fires I have investigated have had at least one room or compartment that has reached flashover. These rooms are heavily damaged by the fire, leaving behind a scene that can be hard to recognize from what it was before the fire. The hazards associated with the post-flashover conditions that led to the damage are frightening. Once a room has achieved flashover, every fuel surface within the room that can burn, will burn. This results in a room that is filled with much more fuel than can be burned with the amount of air (i.e., oxygen) that is available in the room. In addition to the dramatic increase in the temperature of the atmosphere (as mentioned in a previous article, these temperatures are in excess of 1112 °F), incomplete combustion takes place in this environment, which will produce many different toxic gases in very large quantities.

Surprising, the majority of all fire deaths are not due to the excessive temperatures created by flashover conditions or burns to the body. Instead, the leading cause of death from fire is smoke inhalation. More specifically, carbon monoxide, the silent killer, is the main culprit. A post-flashover room transitions quickly into a carbon monoxide pump, pushing the colorless, odorless, and tasteless gas throughout an apartment, single-family home, hotel, or other structure. Carbon monoxide is lighter than air and small enough (as molecules go) to easily slip around the cracks between walls, windows, and doors. Most fire victims did not have adequate warning or enough time to escape from the home before breathing in enough of the carbon monoxide gas to lose consciousness.

When inhaled, carbon monoxide will combine with hemoglobin in the blood to form carboxyhemoglobin (abbreviated as COHb). The measure of this value is usually reported in medical records as a percent COHb. At 15-20 percent COHb, the first symptom of exposure, a headache, is typically reported. At 30-40 percent COHb, loss of consciousness can occur. Death is associated with concentrations of 50-70 percent COHb.

There are many variables that can affect how quickly carbon monoxide will form carboxyhemoglobin in blood. The most important variable is the exposure concentration of the carbon monoxide. Prior to flashover, a fire will produce a very small amount of carbon monoxide. After flashover, the amount of carbon monoxide can increase to several percent by volume. A 0.5 percent concentration of carbon monoxide (5000 parts per million or ppm) can result in a 30 percent COHb in only seven minutes for a person who is walking.

More information on post-flashover conditions and carbon monoxide toxicology can be found in the “Effect of Combustion Conditions on Species Production” and “Assessment of Hazards to Occupants from Smoke, Toxic Gases, and Heat” chapters in The SFPE Handbook of Fire Protection Engineering, Fourth Edition 2008.

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Comments
  1. Reblogged this on 1st Responders Auxiliary and commented:
    Carbon Monoxide, Carbon Monoxide Poisoning, Fire, Fire Investigation

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