Reading the Fire:
Smoke Indicators Part 2

In many cases, smoke may provide the dominant indication that there is a fire in the building (but keep in mind that if you can see smoke on the exterior, there are also air track indicators). Have a look at Figure 1 and see what smoke indicators you observe.

Figure 1. Smoke Indicators


Note: Terry Moody Photo, Commercial Fire Ranlo, NC.

My prior post, Reading the Fire: Smoke Indicators began the process of developing or refining an existing concept map of smoke indicators. As a starting point, I have identified location, optical density (thickness), color, physical density (buoyancy), and volume as basic categories of smoke indicators (see Figure 2). However, you may choose to approach this somewhat differently.

Figure 2. Basic Categories of Smoke Indicators


These five categories provide a simple framework for examining smoke indicators, but considerably more detail may be developed within each category. Observation of smoke indicators can often provide an indication of the location and extent of the fire as well as its burning regime. However, it is essential that smoke indicators be integrated with other categories of indicators in the B-SAHF scheme to gain a clearer sense of fire conditions and likely fire behavior. Remember that looking at smoke alone may be misleading.


The concept of smoke location applies to both the exterior and interior of the structure. From the exterior, consider the following:

  • Is smoke visible (this one is simple)?
  • What side(s) and level(s) of the building is the smoke visible from?
  • Is the smoke discharge from substantial openings such as open windows or doors?
  • Is smoke discharge limited to a single opening or is it visible from multiple openings?
  • Is the smoke discharge from points of normal building leakage (e.g., gaps around windows and doors, normal ventilation openings such as attic vents)?
  • Does the location of smoke discharge change over time?

These questions identify potential linkage to building factors indicators (i.e., actual openings) and air track (discharge of smoke and intake of air). Location may also be interrelated with other smoke indicators such as volume. For example, discharge of smoke from the top of a doorway has considerably different implications if the ceiling height is 7.3 m (24′) rather than 2.4 m (8′).

  • Smoke location continues to be important on the interior of the building.
  • Is smoke confined to a single compartment or group of compartments?
  • Is smoke present in void spaces?
  • Is smoke on one level or multiple levels?
  • Are there changes to the location of smoke over time?

Some indicators can be classified into more than one category. For example, smoke at the ceiling level could be considered from the perspective of location or (potentially more importantly) as an indicator of volume.


The language used to describe fire and smoke conditions in radio communications (e.g, size-up report or report on conditions) is often subjective and ambiguous. For example, what exactly is heavy smoke and how is that different from light smoke. I gained a new appreciation of the ambiguity of these terms when teaching fire officers who had English as a second or third language. Heavy and light commonly refer to weight, but in this case are frequently applied to volume or possibly optical density (or in other cases both of these characteristics). In an effort to provide clarity, I use the terms large and small in relation to volume. Note that this is still subjective, but I think a bit clearer than heavy and light.

From the exterior, consider the volume of smoke discharged. However, it is important to keep in mind that this may not be an indication of how much smoke is in the building. Volume becomes a bit more complex, but more important inside the building. The level of the neutral pressure plane at openings is related to both volume and air track. After entering a compartment, the level of the hot gas layer is a key indicator of smoke volume, but it must be considered in relation to building factor such as ceiling height. Firefighters often consider the height of the hot gas layer above the floor, but may not consider the depth of the hot gas layer down from the ceiling (which is equally important as an indicator of smoke volume). Compartments that are completely filled with smoke are said to be smoke logged. As with other indicators, it is essential to consider changes over time. In particular:

  • Is the volume of smoke discharged from the exterior increasing or decreasing?
  • Is the level of the neutral plane or hot gas layer raising or lowering?

Movement of the hot gas layer can indicate changes in volume or air track, both of which are important indicators of changing fire conditions, burning regime, and potential for extreme fire behavior such as ventilation induced flashover or backdraft.

Optical Density (Thickness)

As with the earlier discussion of heavy and light smoke, density can be a bit confusing as it is used in two different contexts. Most commonly, dense smoke is so thick that you can’t easily see through it. Optical density refers to obscuration, how difficult it is to see through the smoke. Thick or optically dense smoke contains a high concentration of particulates and is difficult to see through. High particulate concentration can also give the smoke the appearance of having texture (like velvet). Thickness is influenced by burning regime and the type of fuel that is burning. Ventilation controlled conditions and/or combustion of many synthetic fuels can result in development of optically dense or thick smoke.

  • How thick is the smoke on the exterior of the structure?
  • How thick is the smoke inside the building?
  • Does thickness differ based on location (a potential connection with location indicators here)?
  • Is thickness changing over time?

A key indicator substantively related to thickness is texture. Increased particulate concentration, increases obscuration, but if sufficiently high, also results in the appearance of texture. Smoke that looks like velvet is the result of extremely inefficient combustion and contains a high concentration of unburned fuel.


Color has traditionally been an important but often misunderstood smoke indicator. Consider the following questions:

  • Which color smoke indicates the greatest hazard to firefighters: Creamy white or light tan, light gray, dark gray, brown, or black?
  • What color smoke indicates potential backdraft: White, yellow, brown, dark gray, or black?

The greatest challenge in making sense of smoke color is that it is the result of a number of interrelated factors including fuel type and burning regime. Light colored (e.g., white to light tan) smoke may contain a high concentration of unburned pyrolizate. While this smoke color is not typically associated with a high degree of hazard, this smoke is fuel and may present a significant threat. Yellowish smoke is typically associated with the backdraft phenomena. However, observation of a number of backdraft events points to considerably less certainty in the relationship between smoke color and backdraft.

Think about the range of smoke color that may be encountered in structural firefighting, but remember that all organic (carbon containing) fuel can produce black smoke under sufficiently ventilation controlled conditions.

Changes in color over time are a critical indicator of developing fire conditions and the effect of tactical operations.

In the early 1980s when my eldest daughter was five or six years old, she was observing live fire training in an acquired structure. During one evolution, she pointed out smoke conditions at a second floor window to a photographer who was taking pictures for a National Fire Protection Association (NFPA) training program. She stated “you see that light gray smoke coming from the window…it’s going to get black and then flames will come out”. Sure enough that was exactly what happened. The photographer was surprised. I was not. I pointed out that Heather had been to more fires than the photographer and that children pay attention to everything. There is a lesson here for us!

Physical Density (Buoyancy)

From a scientific perspective, density is mass per unit volume. Vapor density is the relative density of a gas or vapor in comparison to air (at standard temperature and pressure). The fire environment adds some complexity as increasing the temperature of a gas can cause it to expand, reducing its density (which is why hot smoke goes up). However, most of the constituents of smoke are heavier than air and will sink as they cool. Physical density refers to the buoyancy of the smoke.

Smoke that is buoyant will rise quickly and smoke that is not will hang low to the ground. Generally buoyancy is related to the temperature of the smoke, the higher the temperature, the less (physically) dense the smoke and the greater the buoyancy. Early in fire development smoke may not be that buoyant due to limited heat release. Development of a well defined hot gas layer indicates significant difference in temperature between the hot smoke and cooler air below. Later in the development of the fire, buoyancy may be affected by the operation of automatic sprinklers (or application of water from hoselines) or it may simply cool as it moves away from the fire. In evaluating the importance of physical density, consider the following questions:

  • What building factors might impact on buoyancy?
  • How does ambient temperature influence buoyancy?
  • What potential hazards does smoke with limited buoyancy (i.e., cool smoke) present?

Work in Progress

Hopefully we have been working on this project together and you have been developing or refining the smoke segment of your fire behavior indicators concept map. My current map is illustrated in Figure 3.

Figure 3. Smoke Indicators Concept Map v5.2.2.1


You can also download a printer friendly version of the Smoke Indicators Concept Map v5.2.2.1 (including notes made during development). As always, feedback is greatly appreciated.

Master Your Craft

Ed Hartin, MS, EFO, MIFireE, CFO

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6 Responses to “Reading the Fire:
Smoke Indicators Part 2”

  1. laurence delorme Says:

    hello Ed,

    great article about smoke and the color,i learnt many interesting things.

    if you agree,i could post your article on my blog and do some translations in french? regards.

  2. hartin Says:

    Thanks for your feedback. No problem with cross posting the article and translating into French. If you do, let me know the URL and I will add it to my site as well.

  3. laurence delorme Says:


    as soon as i finish the translation,i will post a link.

    thanks for your agreement.


  4. laurence delorme Says:


    here is the link to the translation of your article entitled:smoke indicators part 2:


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    […] discussed in Reading the Fire: Smoke Indicators Part 2, these indicators can be displayed in a concept map to show greater detail and their […]

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