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David Dodson: The Art of Reading Smoke

BY DAVID W. DODSON

One of the “basics” that is gaining headway is the ability to “read smoke” to help predict fire behavior within a structure. The ability to read smoke has been around for many decades--the fire officers handling America’s fire epidemic in the 1970s became quite proficient at the skill.  Unfortunately, these sound tacticians felt that the ability to read smoke was based on experience and intuitiveness and couldn’t necessarily be taught except for repeated practice at actual fires.  Further, the skills these fire officers developed in reading smoke don’t readily apply to today’s fires. Low-mass synthetics and the consumer trend toward “big box” purchasing have led to a more volatile fire environment. To make matters worse, we are responding to fewer fires; the experience teacher is arguably diminished. To get back to the basics, we need to teach fire officers how to rapidly interpret smoke issuing from a building so that appropriate tactical choices can be made. For example, the first-due officer who can rapidly read smoke can make better decisions about aggressive fire attack or search and rescue priorities. While far from complete, this article will capture the essence of “reading smoke” and offer some tried and tested interpretations to help fire officers make better rapid decisions on the fireground.   

Reading smoke is not difficult--although for most fire officers, it will take an effort to break the “heavy smoke or light smoke” mentality that has come out of rapid “size-up” radio reports.   “Smoke” leaving a structure has four key attributes: volume, velocity (pressure), density, and color. A comparative analysis of these attributes can help the fire officer determine the size and location of the fire as well as the potential for a hostile fire event like flashover. Before we can look at the meaning of each attribute, we must understand the underlying science behind what is seen in smoke.

“SMOKE” DEFINED

In a simpler time, smoke was viewed as the particulates (solids) that are suspended in a thermal column. Fire gases and aerosols were listed as separate products of the combustion process. In today’s world, that oversimplification is dangerous. When a fire officer sees smoke leaving a building, the smoke needs to be interpreted as an aggregate of solids, aerosols, and fire gases that are toxic, flammable, and volatile.[1] The solids that are suspended in the thermal plume include carbon (soot), ash, dust, and airborne fibers. Concerning aerosols typically include a whole host of hydrocarbons (oils/tar). Fire gases are numerous with carbon monoxide, hydrogen cyanide, acrolein, hydrogen sulfide, and benzene leading the list. The bottom line is, Hot smoke is extremely flammable and will ultimately dictate fire behavior.    

Fire officers who focus on the fire (flaming) to determine tactics are being set up for a “sucker punch.” Open flaming is actually a good thing--the products of combustion are minimized to basically carbon, carbon dioxide, and water vapor. Within a building, the heat from flaming is absorbed through materials (contents and the walls/ceiling). These materials break down and begin off-gassing without flaming (pyrolysis). It is here that smoke flammability begins. Within a box (room), the off-gassed smoke displaces air, leading to what is termed an “underventilated” fire. Underventilated fires don’t allow the open flaming to complete a reaction with pure air--leading to increasing volumes of CO as well as the aforementioned smoke products.[2] The smoke is looking to complete what was started. Two triggers, the right temperature and the right mixture, may cause the smoke to ignite. Smoke gases above their flashpoint (with air mix) just need a sudden spark or flame to complete the ignition. Distal to the actual fire, a simple glowing ember or s failing light bulb can spark the ignition. Smoke gases above their ignition temperature just need a proper air mix.  Ignition of smoke that has pressurized a room or “box” will likely result in an explosive surge. Ignition of accumulated smoke also changes basic fire spread dynamics--instead of flame spread across surfaces of contents, the fire spreads with the smoke flow. The fire officer who watches what the smoke is doing will make better decisions than the one focused on flaming, simply because the smoke will tell you how nasty the fire is about to become as opposed to how bad it currently is. A compartmentalized fire that is ventilation-limited is looking for air. Arriving firefighters who open the front door for an aggressive attack provide that air. As smoke leaves the door, a volume switch begins to take place. Air is now becoming available to the fire, and a sudden growth in fire spread becomes imminent. With this understanding, we can look at the four attributes of the smoke.

ATTRIBUTES OF SMOKE

The four attributes of smoke are volume, velocity, density, and color.

Volume

Smoke volume by itself tells very little about a fire, but it sets the stage for understanding the amount of fuels that are “off-gassing” within a given space. A hot, clean-burning fire will emit very little visible smoke, yet a hot, fast-moving fire in an “underventilated” building will show a tremendous volume of smoke. The changes in today’s contents (low mass) can develop large volumes of smoke even though little flame is present. The volume of smoke can help set an impression about the fire. For example, a small fast-food restaurant can be totally filled with smoke from a small fire. Conversely, to fill the local “big box” store full of smoke would take a significant fire event. Once a container is full of smoke, pressure begins building if adequate ventilation is not available. This can help us understand smoke velocity.

Velocity

The speed and flow characteristic of smoke that leaves a building is referred to as velocity. In actuality, smoke velocity is an indicator of pressure that has built up within the building. From a tactical standpoint, the fire officer needs to know WHAT has caused the smoke pressure. From a fire behavior point of view, only two things can cause smoke to pressurize within a building: heat and volume. When you watch smoke leave the building, know that pressure caused by heat will typically rise and slow gradually after it leaves the building. Pressure caused by volume saturation will immediately slow and balance with outside air flow.

In addition to speed, smoke will have a flow characteristic:  turbulent or laminar.  If the velocity of the smoke leaving an opening is turbulent (other descriptions may include agitated smoke, boiling smoke, or “angry” smoke), a flashover is likely to occur. Turbulent flow is caused by rapid molecular expansion of the gases within the smoke and restriction of this expansion by the “box” (container). This expansion is being caused by radiant heat feedback from the box itself--simply, the box can’t absorb any more heat. This is the precursor to flashover. If the “box” is still absorbing heat, the heat of the smoke is subsequently absorbed, leaving a more stable or “laminar” smoke flow. Other words for laminar can include smooth or straight-line flow.  The most important smoke observation is turbulent vs. laminar smoke flow. Smoke that is turbulent is ready to ignite and indicates a flashover environment delayed by improper air mix.

Comparing the velocity of smoke at different openings of the building can help the fire officer determine the location of the fire: Faster smoke will be closer to the fire seat. Remember, however, that the smoke velocity you see outside the building is ultimately determined by the size of the exhaust opening. Smoke will follow the path of least resistance and lose velocity as the distance from the fire increases. To find the location of fire by comparing velocities, you must only compare like-size openings (doors to doors, cracks to cracks, and so on).  A veteran commander of hundreds of fires once told me to find the fastest smoke from the smallest opening--that’s where the fire is. In my own experience, I’ve found this to be a pretty accurate shortcut. 

Density

Whereas velocity can help you understand much about a fire (how hot and where), density tells you how bad things are going to be. Density of smoke refers to its thickness. Since smoke is fuel--airborne solids, aerosols, and gases that are capable of further burning--thickness tells you how much fuel is laden in the smoke. In essence, the thicker the smoke, the more spectacular the flashover or fire spread. Smoke thickness also indicates “fuel continuity.” Practically applied, thick smoke will spread a fire event (like flashover) farther than less dense smoke. We already know that turbulent smoke is a flashover warning sign, yet thick, laminar-flowing smoke can ignite because of the continuity of the fuel bed to a flaming source. One other point regarding smoke density: Thick, black smoke within a compartment reduces the chance of life sustainability because of smoke toxicology. A few breaths of thick, black smoke will render a victim unconscious and cause death within minutes. Further, the firefighter crawling through zero-visibility smoke is actually crawling through ignitable fuel. Modern fire tests are showing that smoke-cloud ignition can happen at lower temperatures than fires of even 10 years ago.[3] We can thank plastics and low-mass materials for making our job more dangerous. 

Color

Most fire service curricula teach us that smoke color indicates the “type” of material that is burning. In reality, this is only true for single-fuel or single-commodity fires. In typical residential and commercial fires, it is rare that a single fuel source is emitting smoke--the smoke seen leaving a building is a mix of colors. For a first-arriving fire officer, smoke color tells the stage of heating and helps us find the location of the fire within a building. Virtually all solid materials will emit a white “smoke” when first heated. This white smoke is moisture (natural products) and various vapors like ammonia and phenols (synthetics). As a material dries out and breaks down, the color of the smoke will change. Wood materials will change to tan or brown; plastics and painted/stained surfaces will emit a grey smoke. As materials are further heated, the smoke leaving the material will eventually be all black (carbonization). When flames touch a surface, the surface will off-gas black smoke almost immediately. Therefore, the more black the smoke, the hotter the smoke. Black smoke that is high velocity and very thin (low density) is indicative of flame-pushed smoke; the fire is nearby. 

Smoke color can also help you find the location of a fire. As smoke leaves a fuel that is ignited, it heats up other materials, and the moisture from those objects can cause black smoke to turn grey, or even white, over distance. As smoke travels, carbon and hydrocarbon content from the smoke will deposit along surfaces and objects, which also lightens the smoke color. That leads to the question, Is the dirty-white smoke you see a result of early-stage heating or late-stage heating smoke that has traveled some distance? To answer, just look at the velocity. White smoke that has its own pressure (push) is indicating distance. White smoke that is slow or lazy is most likely indicative of early-stage heating. One more important note about smoke color--namely, brown smoke. Unfinished wood gives off a distinctive brown smoke as it approaches late-stage heating (just prior to flaming). In many cases, the only unfinished wood in a structure is the wall studs, floor joists, and roof rafters/trusses. This can tell you that the fire is transitioning from a contents fire to a structural fire. Using our knowledge of building construction--especially lightweight structural components and gusset plates--the issuance of brown smoke from gable-end vents, eaves, and floor seams become a warning sign of impending collapse. Remember also that engineered wood products like oriented strand board (OSB) and laminated veneer lumber (LVL or “Micro-lam”) lose strength when heated. The glues of these products break down with heat and don’t necessarily need flames to come apart.[4]  Brown smoke from structural spaces containing OSB or LVL can indicate that critical strength has been already lost.          

Knowing the meaning of each attribute helps us paint a picture of the fire.  By combining these smoke attributes, some basic observations about the fire can be made before firefighters enter a structure. Compare smoke velocity and color from various openings to help find the location of the fire. Faster/darker smoke is closer to the fire seat, whereas slower/lighter smoke is further away. Typically, you’ll see distinct differences in velocity and colors from various openings. In cases where the smoke appears uniform--that is, same color/velocity from multiple openings--you should start thinking that the fire is in a concealed space (or deep-seated). In these cases, the smoke has traveled some distance or has been pressure-forced through closed doors or seams (walls/concealed spaces), which “neutralizes” color and velocity prior to exiting the building.

BLACK FIRE

“Black fire” is a good phrase to describe smoke that is high-volume, turbulent velocity, ultra-dense, and black.  Black fire is a sure sign of impending autoignition and flashover. In actuality, the phrase “black fire” is accurate--the smoke itself is doing all the destruction that flames would cause, charring, heat damage to steel, content destruction, and victim death. Black fire can reach temperatures of more than 1,000 degrees! Firefighters should treat black fire just as actual flames--vent and cool!

Wind, thermal balance, fire streams, ventilation openings, and sprinkler systems change the appearance of smoke. All smoke observations must be analyzed in proportion to the building.  For example, smoke that is low volume, slow velocity, very thin, and light colored may indicate a small fire--only if the building or “box” is small. This same observation from several openings of a “big box” store or large warehouse can indicate a large, dangerous fire.

PRACTICING THE “READING SMOKE” SKILL

Some firefighters may view the reading-smoke process as complicated or time-consuming. Trust me, once you capture the basics and start practicing, your ability to read smoke will improve exponentially--and you will be able to read smoke in mere seconds! As stated, you must practice!  How do we practice reading smoke in an environment with fewer fires? The answer is grounded in desire and a bit of inventiveness. I use raw fireground video footage. These videos are available from several sources, and many can be found on the Web. The next time your crew meets for a meal, slip in a video and vocalize volume, velocity, density, and color observations.  Be sure to compare the attribute differences around the building. One other technique I use to practice reading smoke may seem silly, but it works. I simply watch smoke coming from a restaurant grease hood, fireplace chimney, or smoke stack. Although it’s not difficult to understand the source of the smoke, the process of vocalizing what you see can improve your recognition speed. If you simply vocalize “how much, how fast, how thick, and what color,” you’ll build your speed and improve your smoke awareness. The faster you can recognize the attributes, the faster you can get a “read.”

Remember, reading smoke is not a tactic but a tool to help you make better tactical choices. In essence, the “reading smoke” approach allows us to be more “intellectually aggressive” as opposed to arbitrary aggressiveness. In the end, we still need to make the “box” behave (vent), control the fire (cool the flames and hot smoke), and aggressively search for victims. With all the challenges and changes in our emergency service world, it’s easy to see why we’ve lost the ability to read smoke. Take this information, and move it up the on the training priority list. You’ll be amazed at how powerful it can be in predicting fire behavior, deciding tactics, and preventing firefighter injuries or deaths. Oh, and don’t forget to pass it on!   

Endnotes

[1] Fire Protection Handbook, 19th Edition, Volume II, Section 8, National Fire Protection Association, Quincy, MA, 2003.

[2] Quintiere, James G., Principles of Fire Behavior, Delmar Publishers, a Division of Thomson Learning, Clifton Park, NY, 1998.

[3] Fire Protection Handbook, 19th Edition, Volume I ,Section 3, National Fire Protection Association, Quincy, MA, 2003.  This author compared the Handbook 2003 fire behavior models to data presented in the 1980s.  Recent National Institute of Standards and Technology and Underwriters Laboratories fire studies confirm this point.

[4] This is based on the author’s research in talking with numerous wood product manufactures as well as “backyard” testing with components during live-fire training.

BIO

DAVID DODSON is a fire service author and lecturer. He has 25 years of “street duty,” serving as a battalion chief, training/safety officer, and emergency manager for several Colorado fire departments. He is a past recipient of the George D. Post Fire Instructor of the Year award.

He has served on national boards including the NFPA Firefighter Occupational Safety Technical Committee, International Society of Fire Service Instructors (ISFSI), and the Fire Department Safety Officers ‘Association (serving as president). He owns and instructs for Response Solutions, LLC, a company dedicated to firefighter safety through training, procedural development, and consulting.