By David W. Dodson
Published Thursday, March 19, 2009
| From the September 2007 Issue of FireRescue
Incident commanders, safety officers and first-due decision makers have discovered that "reading smoke" is an essential skill for situational awareness and predicting fire behavior. When applied to a building fire, the process of reading smoke can help fire officers understand the fire's exact location, size and potential for rapid spread. Further, the information gained by reading smoke becomes a strong foundation for developing and adjusting incident action plans and improving firefighter safety.
Some fire officers are steadfast in their belief that smoke-reading cannot be taught; they believe that it's an intuitive skill born of experience. However, I believe that experience, coupled with a little science, help form the basis for reading smoke-and that can be taught.
The process of reading smoke is actually quite easy, although you may have to abandon the oversimplified "heavy smoke/light smoke" mentality that permeates radio reports. The reality is a bit more complex. In fact, smoke leaving a building has four key attributes: volume, velocity (pressure and flow characteristic), density and color. A comparative analysis of these attributes can help you determine the size and location of the fire, as well as the potential for a hostile fire event like flashover. But before we examine each attribute, let's first address the underlying science of smoke.
In a simpler time, smoke was viewed as the particulates (solids) suspended in a thermal column. In today's world, that oversimplification is dangerous. When you see smoke leaving a building, you should interpret it as an aggregate of solids, aerosols and fire gases that are toxic, flammable and volatile.1
The solids suspended in the thermal plume include carbon (soot), dust and airborne fibers. Aerosols (suspended liquids) typically include a whole host of hydrocarbons (oils/tar) and moisture. Lastly, there are perhaps hundreds of named fire gases in smoke. Carbon monoxide leads the list of gases produced during incomplete burning. Other notable gases (in both quantity and flammability) are hydrogen cyanide, benzene and acrolein. Smoke can also contain polyvinyl chloride, hydrogen chloride, hydrogen sulfide, phosgene, nitrogen dioxide, ammonia and phenol, to name a few. Figure 1 below shows the properties of common smoke gases. Clearly, it doesn't take a scientist to see how dangerous smoke can be. The bottom line: Hot smoke is extremely flammable and will ultimately dictate fire behavior.
Well-ventilated, open flaming is actually a good thing; the burning process is more complete, and the solution is quite simple (wet stuff on the red stuff). Unfortunately, structure fires are rarely well ventilated. The heat gets absorbed into other materials (contents and the walls/ceiling), which, in turn, break down and begin off-gassing without flaming. It is here that smoke flammability begins.
Within a box (room), the off-gassed smoke displaces air, leading to an "under-ventilated" (vent-controlled) fire. Under-ventilated 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 hot, accumulating smoke can be considered a fluid, dynamic fire load that can ignite when mixed with air. Thus, firefighters crawling through hot, thick smoke are actually crawling through fuel, and their protective clothing is becoming saturated with that fuel.
Smoke gases above their flashpoint (with air mix) need only a sudden spark or flame to ignite. Distal to the actual fire, a simple glowing ember or failing light bulb can spark the ignition. Smoke gases above their ignition temperature need only a proper air mix to ignite. Ignition of accumulated smoke also changes basic fire-spread dynamics; instead of flame spreading across surfaces of contents, the fire spreads with the smoke flow.
Once you understand this smoke science, you are prepared to read smoke. Let's now turn to the four attributes of smoke: volume, velocity, density and color.
Smoke volume by itself indicates very little about a fire, but it sets the stage for understanding the amount of fuels that are off-gassing within a given space. This can help you understand the relative size of the event. For example, a small fast-food restaurant can be completely filled with smoke from a small fire. Conversely, it would take a significant fire event to fill the local big-box store with smoke. In other words, the volume of smoke leaving a building can help you form an impression of the fire. "Light smoke showing" from more than one location of a very large building is a significant warning sign. Once again, reading smoke volume is just a starting place; other attributes will more specifically paint the fire's picture.
Smoke leaving a building will have certain speed and flow characteristics. The combination of the two is what we call velocity. Smoke speed is an indicator of pressure that has built up within the building. From a tactical standpoint, you must 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 or volume. When watching smoke leave the building, know that pressure caused by heat will typically rise and slow gradually after it leaves the building. Smoke pressure caused by volume (the box is overfilled) will immediately slow and balance with outside airflow.
In addition to speed, the smoke will have one of two flow characteristics: turbulent or laminar. The smoke coming off flame tips will always be turbulent (rapidly expanding and very agitated). As smoke leaves the flame tips, it will flow up to the ceiling and begin migrating away from the fire. The heat of the expanding smoke will be absorbed as smoke flows across the ceiling, walls, doors, etc. As the heat is absorbed, the smoke will become laminar-calm, orderly or free-flowing. Viewed from outside a building, laminar smoke flow means that the box is still absorbing heat. Conversely, turbulent smoke flow seen outside the building means that the box cannot absorb any more heat-the precursor to flashover. The most important smoke observation is turbulent versus laminar smoke flow (see photo labeled "Velocity"). Smoke that's turbulent is ready to ignite and indicates an imminent flashover environment delayed by improper air mix.
Comparing the velocity of smoke at different openings of the building can help you 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 compare like-size openings (doors to doors, cracks to cracks, etc.). A veteran commander of hundreds of fires once told me to "Find the fastest smoke from the most restrictive (smallest) opening-that's where the fire is." In my own experience, I've found this to be a pretty accurate shortcut.
While velocity can help you understand much about a fire, density tells you how bad things are going to be. Smoke's density refers to its thickness. Since smoke is fuel 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 will be.
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, but thick, laminar-flowing smoke can ignite because of the continuity of the fuel bed to a flaming source. Another point regarding smoke density: Thick, black smoke within a compartment reduces the chance of life sustainability because of smoke toxicity. 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. Recent fire tests show that smoke can ignite 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.
Most fire service curricula teach us that smoke color indicates the type of material that's burning. In reality, this is true only for single-fuel or single-commodity fires. In typical residential and commercial fires, it's rare that a single fuel source is emitting smoke; after all, the smoke seen leaving a building is usually a mix of colors.
For a first-arriving fire officer, smoke color tells the stage of heating and helps us find the fire's location. Virtually all solid materials will emit a white "smoke" when first heated. This white smoke is mostly moisture. As a material dries out and breaks down, the color of the smoke changes. Wood materials change to tan or brown, whereas plastics and painted/stained surfaces emit a gray smoke, as a result of the mixing of moisture (white) and carbons/hydrocarbons (black). As the materials heat up, the smoke leaving the material eventually becomes all black, regardless of the material type. Therefore, the blacker the smoke, the hotter the smoke. Black smoke that's high velocity and very thin (low density) indicates a well-ventilated fire.
Smoke color can also help you find the location of a fire. As smoke leaves an ignited fuel, it heats up other materials, and the moisture from those objects can cause black smoke to turn gray, or even white, over distance. As smoke travels, carbon content from the smoke will deposit along surfaces and objects, which also lightens the smoke color. That leads to the question: Is the white smoke you see a result of early-stage heating, or is it late-stage heating smoke that has traveled some distance? To answer, just look at the velocity. White smoke that has self-sustaining speed (push) indicates a hot fire; however, the smoke you see has traveled some distance. The same thing happens when smoke is pushed through cracks and seams-the carbon and hydrocarbons get filtered off. White smoke that is slow or lazy most likely indicates early-stage heating.
Pure brown smoke can be significant. 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 pieces in a structure are the wall studs, floor joists and roof rafters/trusses (see photo labeled "Color"). Therefore, brown smoke can signal the fire's transition from a contents fire to a structural fire. Using our knowledge of building construction-especially lightweight structural components and gusset plates-brown smoke issuing from gable-end vents, eaves and floor seams becomes a warning sign of impending collapse. Remember: Engineered wood products, such as oriented strand board (OSB) and laminated veneer lumber (LVL or Micro-lam), lose strength when heated. According to my research, the glues of these products break down with heat and don't necessarily need flames to come apart. As a result, brown smoke from structural spaces containing OSB or LVL may indicate that critical strength has been already lost.
By combining these smoke attributes, we can make some basic observations about the fire. Compare smoke velocity and color from various openings to help find the fire's location. Faster/darker smoke is closer to the fire seat, whereas slower/lighter smoke is farther 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), neutralizing color and velocity prior to exiting the building.
Finally, smoke that is high volume, turbulent velocity, super thick and black should be called "black fire." Black fire is a sure sign of impending autoignition and flashover. 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 F! Treat black fire as actual flames: Vent and cool.
There are several other factors that can influence your interpretation of smoke's volume, velocity, density and color. First, analyze all smoke observations in proportion to the building. For example, smoke that is low-volume, slow-velocity, very thin and light-colored may indicate a small fire, but 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. Wind, thermal balance, fire streams, ventilation openings and sprinkler systems can also change the appearance of smoke, and should be taken into consideration when analyzing the attributes.
It's also important to note how fast the smoke attributes are improving or worsening. Smoke that's getting darker and increasing in velocity and density within seconds indicates rapid fire growth, and firefighters could be over-run by the fire. Smoke that rapidly thins can indicate that burning has become more complete.
Practice Makes Perfect
Obviously, the first step in learning to read smoke is to grasp the basics of smoke science and the interpretation of volume, velocity, density and color. From there, you must practice its application. With practice, the ability to read smoke will improve exponentially. But how do you practice reading smoke in an environment with few fires? The answer is grounded in motivation and a bit of inventiveness. I use raw fireground video footage. A quick search on the Internet (try "fire action video") will result in hundreds of sources for raw fireground footage that you can purchase or download for free. Your training officer may have a library of old fire footage or interesting case studies, and you can check with your local TV stations, which occasionally "purge" unused fire footage.
A great cold-weather drill: Sit with your crew, slip in a video and practice vocalizing observations related to volume, velocity, density and color. 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 watch smoke coming from a restaurant grease hood, a fireplace chimney or a 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 good "read."
Finally, 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 arbitrarily aggressive. 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 savable victims.
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.
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