Reading the Fire: Smoke and Air Track

In general terms, lighter colored smoke frequently contains a substantial concentration of unburned (and highly flammable) pyrolysis products.As discussed in the previous article, fire behavior indicators can be grouped into five general categories...


In general terms, lighter colored smoke frequently contains a substantial concentration of unburned (and highly flammable) pyrolysis products.

As discussed in the previous article, fire behavior indicators can be grouped into five general categories: building, smoke, air track, heat, and flame. A simple mnemonic for remembering the categories is B-SAHF ("be safe"). Click here for a poster illustrating B-SAHF.

This article focuses on two related sets of indicators; smoke and air track. Smoke, as a product of combustion is a fairly obvious indicator of fire behavior. Air track indicators are often included in the "smoke" category, but this article will make the case for thinking of them as different, but highly interrelated.

Smoke Indicators

Smoke indicators include: location, color, density, and volume. In many cases these indicators can be observed from the exterior of the structure, providing an early indication of what is going on inside. However, these indicators can also be observed (although some times not as easily) while working inside a burning building. The interrelationship of these factors is graphically illustrated in Figure 2 (if you would rather look at a list of smoke factors, click here for Figure 1a).

Volume and Location

In many cases visible smoke may be the only indicator that there is a fire in the building. The volume and location of smoke discharge provide some indication of fire location and extent, but alone may prove to be unreliable. Ventilation controlled fires tend to produce a greater volume of smoke than those that are fuel controlled, however it is important to consider volume and location in conjunction with the other fire behavior indicators to obtain a clearer picture of fire conditions.

Smoke volume is often referred to as either "heavy" or "light". However, this could be confused with density (does heavy smoke sink and light smoke rise?). If you use heavy and light consistently to refer to volume, it is likely that the firefighters and officers you work with will understand, but it is something to think about when looking to describe fire behavior clearly.

Smoke visible from the exterior of the building may provide a useful clue as to the location of the fire. However, it is important to place this observation in the overall context provided by the fire behavior indicators (looking at smoke alone may be misleading).

What do you make of the smoke indicators visible in Figure 2? A moderate amount of smoke is visible from several openings (less from the upper window and doorway). Smoke is optically dense and buoyant. It is difficult to tell the level of the neutral plane (at the doorway) due to the effect of the stream being directed through the door. Back to this photo in a bit with a look at the related air track indicators.

Location continues to be important when working inside. Consider the extent of smoke filling each compartment as you work your way through the building. The term "smoke logged" as used in Figure 1 refers to a compartment that is filled (or largely filled) with smoke.

Color

Smoke color can vary considerably depending on the nature of the fuel that is burning. Petroleum products, rubber, and many plastics will produce black smoke, while wood and other ordinary combustibles will commonly produce smoke ranging from light gray to yellowish, dark brown or even black when the fire is under ventilated. It is essential to remember that smoke color is only one of a number of indicators used to predict fire behavior and it must be considered in context.

In general terms, lighter colored smoke frequently contains a substantial concentration of unburned (and highly flammable) pyrolysis products. Under these conditions, smoke can ignite (given adequate oxygen and a source of ignition) and present a significant threat to firefighter safety. Dark smoke generally results from an under ventilated fire and/or combustion of petroleum products. In examining smoke color, it is also important to consider changes over time (e.g., smoke becoming darker or lighter). It may not be clear what is the major influence on smoke color (fuel or ventilation limitations), as with the other indicators it is important to consider color in context.

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