Reading Smoke: Part 1

By John Mittendorf

The mitigation of structural fires is a dynamic process that is normally based on numerous factors that can be summarized as follows:

Unfortunately, the preceding factors collectively combine to hamper the American fire service with 20,000 injuries and over 100 deaths per year. These figures are more than significant when statistics indicate fireground injuries and deaths are increasing — not decreasing! Consider the following: If the number of structure fires are slowly decreasing (and they are) but the number of fireground deaths and injuries remain about the same each year, then in reality, fireground deaths and injuries are increasing. How could this be when modern firefighters are subjected to superior training programs, and high tech equipment? Additionally, NFPA statistics indicate that 70 percent of structure fires occur in single family dwellings (SFD’s), and 70 percent of those fires are confined to one or two rooms. From a simplistic perspective, the answer is the routine SFD fire, as well as other types of structure fires, have dramatically changed.

As delineated in the "Reading a Building" chapter of my book, building construction now repetitively uses lightweight materials due to the increasing scarcity of wood. This has directly contributed to rapid collapse of lightweight structural members when they are exposed to fire. However, there is an additional factor that has also contributed to the increasing number of firefighter deaths and injuries — smoke! As smoke has also dramatically changed over the past several decades, let’s begin this article by considering the various characteristics of smoke and the importance of why it should be evaluated at every structure fire.

SMOKE CHARACTERISTICS

One of the more popular and relevant national training programs is titled “The Art of Reading Smoke,” and for good reason. In the past 40 years, smoke has dramatically changed from two basic viewpoints.

First, and from a simplistic perspective, smoke is nothing more than by-products from a fire. Therefore, as the whatthat is burning is changed, the composition of smoke is also changed, and as a result, we also change how smoke burns from two basic viewpoints — the degree of heat and combustibility of smoke. This is best illustrated by considering the basic elements of smoke. Fundamentally, smoke is a mixture of two basic elements — unburned solid particulates and fire gases.

Unburned particulates will primarily be comprised of the material or materials that are burning, and carbon. These particulates are capable of burning if their ignition temperature is sufficient.

Fire gases are a combination of a multitude of various gases that are released by the material or materials that are burning, and are also capable of burning if their ignition temperature is sufficient. However, fire gases can be much more complicated than particulates as burning materials are capable of yielding a wide range of fire gases. Common examples of some of the fire gases that are released during the combustion process are:

• Carbon monoxide

• Sulfur dioxide

• Hydrogen fluoride

• Hydrogen chloride

• Hydrogen cyanide

• Aldehydes

• Benzene

• Acrolein

• Sulfur dioxide

• and the list goes on....

All firefighters should be aware that all of the aforementioned fire gases can be detrimental or fatal to any human. However, if the average firefighter was asked to choose the most common and deadly fire gas, carbon monoxide would likely be chosen most often. Although carbon monoxide is well known for its ability to be present in smoke and cause death by asphyxiation if inhaled in sufficient quantities, it is becoming apparent that hydrogen cyanide may be the most dangerous fire gas encountered by fireground personnel. It is estimated that hydrogen cyanide is over 30 times more toxic than carbon monoxide!

This fact was recently underscored in Rhode Island in 2006 when a firefighter involved at a “routine” fire in a fast-food restaurant was sent to a hospital and diagnosed with toxic levels of cyanide. Interestingly, over the next 14 hours, two additional fires in the same area were responsible for four more firefighters being hospitalized and also diagnosed with toxic levels of cyanide, in addition to one of the firefighters suffering a heart attack. These incidents have been responsible for a new look at this common product of combustion which may be present in high quantities at structure fires. Let’s take a brief look at hydrogen cyanide and its inherent risk to fireground personnel.

Hydrogen cyanide can be found in either a liquid or gas form and is a powerful poison that reduces the capacity of blood to carry oxygen. Bodily organs that are vulnerable to cyanide poisoning are the brain, heart, and central nervous system. The toxicity of hydrogen cyanide is emphasized by its use in gas chambers and as a chemical warfare agent! Unfortunately, hydrogen cyanide is found in common materials such as wool, paper, wood, and cotton. However, it is also found in elevated levels in synthetic materials such as foam, pesticides, plastics, synthetic fibers, polyurethane, fiberglass insulation, and other similar modern materials.

These materials are common in the typical residential structure fire as well as other types of structure fires, resulting in hydrogen cyanide likely being present in readily detectable amounts in smoke from smoldering fires, free-burning fires, and overhaul operations. Stated from another perspective, hydrogen cyanide is an extremely toxic gas that is released during the combustion process from any material that contains nitrogen, which is commonly found in structural occupancies! Although there are numerous other toxic gases released during the combustion process, hydrogen cyanide is a prime example why ventilation should be an initial consideration at structure fires, and a primary reason why fireground personnel should not breathe smoke. That statement applies to structure fires, automobile fires, dumpster fires, and other similar incidents. Toxic gases that are inhaled by fireground personnel give credibility to the phrase "when firefighters breathe smoke, they can die."

Secondly, smoke that is routinely found on the modern fireground can be a willing candidate to suddenly flashover. Intriguingly, the term flashover was relatively unknown prior to the 1970’s as the term backdraft was well known and understood by most fireground personnel. However, in the 1980’s, flashover began to replace backdraft, and today, few firefighters are familiar with backdraft while most firefighters are familiar with flashover. Why? If we carefully look at the history of building construction from a simplistic viewpoint, it is easy to see why smoke has dramatically changed over the past years:

PRE-1970

Prior to the 1970’s, conventional building methods and materials were commonly employed. Additionally, the interior contents of residential and commercial buildings also primarily consisted of conventional materials such as wood, paper, wool, cotton, and so on. These materials normally burned with a gray-brown smoke (cellulose based materials) and the smoke would commonly burn around 1,500 to 1,600-degrees F. Because of the high ignition temperature of smoke from conventional materials, suppression personnel would routinely arrive at a structure fire to find pressurized levels of smoke and an environment that was oxygen deficient. These conditions often presented initial personnel with the potential of a backdraft from the sudden introduction of oxygen when entering a structure.

During this time frame, PPE was marginal, but was beginning to improve as a combined result of an aggressive focus to improve the protection of fireground personnel with the advantages of new technology. To summarize this general time frame, fires burned slower than the fires of today, and the term backdraft was familiar to most fire suppression personnel.

POST-1970

After the 1970’s and up to modern times, building construction and smoke from fires in residential and commercial buildings noticeably changed. Conventional materials have been steadily replaced by innovative products (defined as synthetic materials) that burn two to three times hotter and faster than conventional materials. Although building codes have significantly improved as compared to the pre 1970 building codes, they do not address the way modern materials burn and the resultant smoke that is released during a fire. Since the 1970’s, PPE has dramatically improved as a combined result of an aggressive focus to improve the protection of fireground personnel with the advantages of new technology. To summarize this general time frame, fires now characteristically burn faster and hotter than the fires of yesterday, and the term flashover is familiar to most fire suppression personnel.

In our next article, we will take a closer look at backdraft and flashover.

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Chief Mittendorf is the author of Truck Company Operations and Facing the Promotional Interview. To purchase, return to the Main Page and scroll down to Fire Engineering Books.