Archive for June, 2013

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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by: Jason A. Sutula

firefighters

The movie Backdraft opened in 1991 while I was still in high school. I will admit that there was something about that movie that peaked my interest in fire and fire investigation. I cannot give the movie full credit for why I eventually chose the fields of Fire Protection Engineering and Fire Investigation, but it definitely deserves some credit as an influence in my life. I have vivid memories of watching the fire move throughout the movie as if it was alive and with a very devious personality. Only a few short years later, during my education in the Department of Fire Protection Engineering, did I discover that many of those wonderful scenes were far from the reality of fire behavior and were just the product of some amazingly creative camera work.

Backdraft does do a great job of reiterating just how dangerous a fire can be, both to occupants of a structure involved in a fire as well as the fire department personnel who risk life and limb to perform rescue and extinguishment operations. Unfortunately, the movie does not do a great job of portraying or explaining just what a “backdraft” is.

The term backdraft is defined in NFPA 921: Guide for Fire & Explosion Investigations, 2011 Edition as, “A deflagration resulting from the sudden introduction of air into a confined space containing oxygen-deficient products of incomplete combustion.” We can examine this definition by starting with the basic components of a fire. We need something to burn (a fuel), a supply of oxygen (air), and enough heat energy to allow for the combustion process to begin and then continue. If you are missing one of these basic components, you will have no fire.

A backdraft, then, is nothing more than a specialized type of fire. To produce conditions that will allow for a backdraft to develop, you need a confined space (in a residential fire, typically this will be a room) to hold all of the components of the fire. If a fire starts in our hypothetical room and cannot get enough oxygen (imagine a fire burning in a room with the windows and doors completely closed), unburned fuel will escape the fire and fill the room. Once a door or window is then opened to this room, colder fresh air containing oxygen will flow into the room through the bottom of the opening while the hotter combustion products from the fire will flow out the top of the opening. Where these two flows meet, a flammable mixture of fuel and oxygen will develop in the room. The mixed zone will grow and spread into the room and eventually reach the area where the small fire was originally located. At that place in the room, all three conditions for fire are present, and the result is an extremely rapid ignition and spread of flame from the point of origin out toward the opening of the room.

The speed at which the burning flame travels is much more rapid than the speed at which an individual can react to or move away from the fully engulfing flames. Fire service personnel tasked with rescue operations are particularly vulnerable to backdrafts as they open doors within structures to look for possible victims. A tragic example of this phenomenon was reported in a case study report put out by the Building and Fire Research Laboratory (BFRL) at the National Institute of Standards and Technology (NIST) where the lives of two District of Columbia firefighters were claimed by fire in a townhouse at 3146 Cherry Road NE, Washington D.C. The report clearly shows the dangers associated with under-ventilated fires and also demonstrates that these types of fires can be modeled with computational fluid dynamics to better understand the hazard. For more information, the NIST report can be found by following this link: http://fire.nist.gov/CDPUBS/NISTIR_6510/6510.htm

by: Jason A. Sutula

house fire

I was recently at a conference speaking with several individuals who were not fire investigators and did not work within the field of Fire Science or Fire Protection Engineering. The conversation eventually turned to the type of work that I do for a living, which led to several stories that I enjoy telling related to past fire investigations. Judging by each individual’s reaction at the end of the conversation, I could not help but wonder if I had instilled a new-found fear related to the speed at which a residential fire will spread throughout a modern family or living room.

In particular, the group I was with was surprised when I casually broke out my favorite metaphor that the polyurethane foam stuffed couches, love-seats, and easy chairs found in their respective living rooms were nothing more than large blocks of solidified gasoline. That statement certainly got their attention and opened their eyes to some understanding of why fire injuries and deaths continue to occur, and why the amount of time available to safely escape from a residential fire has decreased substantially over the last 30 years.

As I continued my conversation, I realized that I have developed a bad habit of using many “terms of the art” that other fire investigators would immediately understand, but people not associated with the field might have a hard time picking up on the meaning. I decided at that point that my next series of blog articles would cover some of these fire investigation terms in hopes of bringing further understanding. This article will kick off the fire investigation definition series and will begin with the phenomenon of “Flashover”.

One of the main fire phenomenon responsible for shortening the amount of time available to safely escape from a residential fire is the phenomenon of Flashover. In short, Flashover is a transition from a local fire in a compartment (e.g., your living room) into a fully developed fire that encompasses the entire room and all of its contents. NFPA 921: Guide for Fire & Explosion Investigations, 2011 Edition defines Flashover as, “A transition phase in the development of a compartment fire in which surfaces exposed to thermal radiation reach ignition temperature more or less simultaneously and fire spreads rapidly throughout the space, resulting in full room involvement or total involvement of the compartment or enclosed space.” This definition can be broken into three basic parts: a compartment, a fire, and time.

For a fire to transition to Flashover, a fire must first be initiated in a compartment. Compartments are readily found in all structures and can include full rooms, cabinets, bureaus, closets, wall cavities, etc. A compartment provides a means by which to trap the energy released by the fire. The trapped energy will allow for the overall temperature of the gases in the compartment to increase and for the heat transfer through radiation to increase as well.

The fire itself is an important component. As a fire grows and spreads over the surface of a fuel package in the room (e.g., your living room couch), the rate of energy released by the fire increases exponentially. With the growth of the fire accelerating, the trapped hot gases in the compartment rise to a critical temperature. Additionally, the hot gases radiate energy toward the unburned fuel packages (e.g., other furniture, carpet, drapes, etc.), which raises the surface temperatures of those fuel packages closer and closer to a temperature where they will begin to burn. The critical gas temperature from the fire research literature that most scientists refer to for the onset of Flashover is approximately 1,112 °F (600 °C). Once the critical temperature and radiant energy levels are reached, all of the remaining fuel packages ignite throughout the entire compartment over a very short amount of time. The entire compartment is filled with fire and hot gases, which will expand rapidly through any opening from the compartment to adjacent rooms or to the outside of the building.

There are other variables that come into play with whether or not a particular fire in a compartment will achieve Flashover, such as the amount of oxygen available in the room (i.e., ventilation, window and door openings) and whether or not the fire growth is interrupted by fire fighting efforts or a sprinkler system activation. With enough of the final component, time, the transition will occur, and the after effects can be extremely dangerous to occupants trapped in the residence as well as fire service personnel. A future blog post will explore the dangers associated with Post-Flashover conditions.