by Shan Raffel
Every fire sends out signals that can assist the firefighter in determining the stage of fire development and, most importantly, the changes that are likely to occur. This skill is essential to ensure the safest and most efficient firefighting strategy and tactics are employed. Being able to “read a fire” is the mark of a firefighter who can make decisions based on knowledge and skill, not guess work or luck.1
The foundations for “Reading the Fire” were laid in Sweden in the late 1970s. Swedish Fire Engineer Krister Giselsson was at the forefront of redefining the accepted knowledge of fire development and extinguishing techniques. He, with firefighter Mats Rosande, recognized that changes in building construction and increasing fuel loads from the incorporation of plastics into almost every part of the interior furnishings were leading to fires that developed to a scale never before experienced. Using a scientific foundation, they developed practical solutions to educate and equip firefighters with strategies, tactics, and tools that allowed them to take back control of the emerging new modern fireground.
By the late 1990s, firefighters were commonly being taught to recognize what was then referred to as the “signs and symptoms of flashover” and the “signs and symptoms of backdraft.” These were useful aids in recognizing the fire phenomena that could occur in the rooms of origin.
My research and personal experience led me to believe that there was a set of undescribed fire events that did not fit into the category of flashover or backdraft. Often, there were no indicators prior to a very sudden, extreme, and unexpected change in internal conditions. In fact, in 2005, the term “Fire Gas Ignition” was used to describe the sudden ignition of accumulated smoke (unburned fuel):
Fire Gas Ignition (FGI): ‘an ignition of accumulated fire gases and combustion products, existing in, or transported into, a flammable state.’ There are a wide range of events that can be conveniently grouped under the heading (FGI), and such phenomena can generally be defined as any ignition usually caused by the introduction of an ignition source into a pre-mixed state of flammable gases or the transport of such gases towards a source of ignition or the transport of a fuel-rich mixture of gases into an area containing oxygen and an ignition source. The ignition is not reliant on the action of airflow/oxygen in the direction of an ignition source, which is clearly recognized as a backdraft event.2
The Four Key Fire Behaviour Indicators
Although I found the indicators listed under the “signs and symptoms” useful for the classroom setting, I saw the need for a simple model that could assist in rapidly determining the location of the fire, the stage of fire development, and likely changes. In 1999, I was applying a simple mnemonic that focused on the four key fire behavior indicators: Smoke, Air., Heat, and Flame (SAHF “SAFE”). (1)
Accurate fire development size-up is an essential skill for developing the safest and most efficient method of attack. In particular, a “Tactical Ventilation” plan cannot be safely developed or implemented until a SAHF assessment is carried out.
Tasked teams should also use a SAHF risk assessment in their area of operations. This is particularly critical prior to making any openings. This information should be relayed to the rapid intervention team (RIT) and the incident commander (IC) so that a more accurate profile of the fire can be developed and maintained.
The indicators can be read individually, but a more complete fire profile can be developed by reading as many of the indicators as possible from the various vantage points inside and outside of the structure.
Context – Building and Environmental Factors
Around 2005, Battalion Chief Ed Hartin added a “B” in front of the four fire behavior indicators to reinforce the need to read them the context of the building.
“B” – Building Factors
A sound understanding of building construction is fundamental knowledge that all firefighters must possess. The built environment is constantly changing as the demand for energy efficiency and cost-effective construction increases. Detailed information on building construction is beyond the scope of this article, but I will briefly discuss some of the key factors that can significantly affect fire dynamics.
Construction Type and Occupancy
Understanding the various construction types in your area is essential in understanding how fire is likely to develop and the potential for structural collapse. The use of the occupancy will give information on the likely life risk and contents that could be encountered.
The thermal properties of the walls and ceilings are dependant on their density, insulative properties, and their thermal capacity. The thermal capacity is a measure of the amount of heat energy required to raise the temperature of 1 g of the substance by 1 C. If the substance has a low thermal capacity, it will heat up rapidly. If the linings have good insulation properties, the heat energy is less likely to be conducted into the walls and ceiling and out of the room. The higher the density of the linings, the more material there is to absorb the heat energy.
Construction materials with a high thermal mass will absorb and store the heat energy generated by the fire. This can slow down the growth rate of the fire as energy is absorbed into the mass of the lining by conduction. However, once the linings have absorbed the heat energy, they will tend to hold it and eventually radiate this energy back into the compartment. Water applied to the surface of this lining will rapidly evaporate, but the surface temperature will rise rapidly as the stored heat energy is conducted back to the surface.
Well-insulated linings will hold the heat energy in the fire compartment, and this may accelerate fire growth. Combustible linings will produce pyrolysis gases as they are heated, and this will contribute to the fuel accumulation in the over-pressure region near the ceiling. Non-Combustible linings will tend to absorb the heat energy in the early stages of the development.Linings such as rendered cement will absorb a lot of energy and act as heat sinks. Once that energy has been absorbed, it will be more difficult to cool and may continue to release the energy even after the initial fire has been knocked down.
Double-glazing and well-sealed construction can in some cases hinder growth by restricting air supply to the fire. On the other hand, lightweight construction can often ‘self-vent,” leading to rapid fire progression.
My current approach to simplifying the context of the building is to view it in terms of the fire development that is likely in that style of construction and occupancy.
Flashover will occur in most buildings if sufficient air is available. Buildings with large single-glazed windows are likely to fail and supply additional air in the early stages of development. Lightly insulated construction will allow early heat transfer, which could lead to the early failure of structural components and thus supply air and allow super-heated fire gases to spread through the structure. Heavy brick or cement rendered walls will absorb a lot of energy, which could delay flashover.
Backdraft is more likely to occur in energy-efficient buildings with good insulation and sealed windows or double/triple glazing. Developing fires may consume the available oxygen before the fire is able to flash over. As the oxygen level decreases, the combustion process may switch from flaming to smouldering. While the temperature of the room is high enough to support pyrolysis, combustibles in the compartment will continue to release flammable vapors.
Voids; ducts; shafts; balloon frame; a large, open plan; high ceilings; false or suspended ceilings are some of the factors that allow smoke to be transported and accumulate in areas adjacent to the compartment of origin, or some distance from it. Modifications can create unexpected openings or voids. Poor or damaged smoke/fire stopping can be found in original or modified buildings. The unburnt fuel in the smoke is often partially mixed with fresh air and can accumulate to flammable concentrations. If an ignition source progresses into this accumulated premixed smoke, the result can be a very sudden and extreme event.
Around 2008, Division Chief, Safety & Innovation, Peter McBride (Ottawa Fire Services) expanded the context to include critical environmental factors such as wind, velocity, and direction, low humidity, and extremes of temperature. Wind is the most critical of the environmental factors. Strong winds can accelerate fire growth and increase the likelihood of fire spread to exposures by ember attack. It can also magnify the effects of low humidity and extremes of temperature.3
Topography. The natural lay of the land or the built environment around the structure can cause variations in direction and velocity of the wind on the various sides of the involved structure. Another example of extreme impact is what is known as the “wind-driven fire.” Strong winds blowing into a closed room can increase the pressure inside the compartment. If the door is opened, the result can cause a rapid and sustained flow of super-heated gases into the passageway.
Extreme temperature variations can have a significant impact on how the smoke released from a structure behaves. Extreme low ambient temperatures can cause rapid cooling of discharged smoke that will decrease the buoyancy. When combined with “low atmospheric pressure,” an inversion layer can form that will prevent the smoke from rising. The construction commonly found in areas likely to experience extreme cold are less likely to show heat indicators that could be considered “common, normal or reliable” in the type of constructioncommonly found in moderate climates.4
The most significant impacts of extremes in temperature are on the physical welfare of firefighters. Fighting operations in very high ambient temperatures can become extremely arduous, and frequent rehabilitation periods combined with core cooling, rest, and rehydration are essential. Operations in extremely low temperatures can also significantly increase the risks faced by firefighters. Parts of the world that regularly encounter extreme temperatures are often well aware of the hazards and the control methods. The risk is usually higher in situations where extreme temperature environments are experienced as part of a rare event in geographical areas that are not familiar with how to manage these extremes.
Volume and Location
The volume and location of smoke are often the first indicators the human eye notices. In general, they can provide a guide to the fire size and location. In some cases, they can be unreliable and can give false indications as to the location, fire size, and stage of development. Smoke can travel through concealed voids and shafts and emerge in totally unexpected locations. Heated smoke is buoyant and will rise vertically. When it reaches horizontal obstructions, it will spread out and look for further openings to allow vertical travel. Most firefighters have experienced a structure releasing large volumes of smoke and later discovered that the actual fire area was smaller than anticipated or in a totally unexpected location. As with all fire behavior indicators, it is critical not to read this indicator in isolation.
Smoke color can vary with the type and form of fuel package as well as the ventilation available. It may give a general indication of the efficiency of the combustion process in the room or structure.
Lighter colored smoke (sometimes almost white) is often produced when the fuel is heated to the pyrolysation temperature. At this temperature, the volatile components of the fuel are released and the solid carbon is left behind in the form of char. If the heat continues to increase and there is insufficient oxygen even for smouldering combustion, then the production of white smoke will continue. It is important to realize that as the fire develops, heat will be transferred to neighboring compartments, which can cause the pyrolysis of linings and adjacent combustibles. This white smoke will accumulate and drift into uninvolved sections of the building. Even through this smoke is at a lower temperature, it has a very high fuel content and the introduction of flames into these spaces can result in a very sudden and powerful ignition. Remember that when the application of water begins, the smoke discharged will be lighter in color because of the presence of water vapor.
Brown or yellowish smoke can be released in the early stages of the pyrolysis of timber products. This is caused when the lignin breaks down and the tar is released as a vapor.
Dark smoke often indicates rich conditions resulting from restricted air supply. Where flaming or smoldering combustion is occurring, the carbon in the product is released as soot in the smoke. The color can be very dark if the combustion process is hindered by a lack of air or excessively fuel rich conditions. If the air supply is good, some of the carbon will be burned in the reaction zone (flame), which will result in a slightly lighter smoke and a yellow flame.
When gray smoke is present, it indicates that at least some flaming combustion is present. Where there is a mixing of the darker smoke from flaming or smoldering combustion with the whiter pyrolysis smoke, the result can be a gray smoke.
Thickness (Optical Density)
The thickness can be a good indicator of the efficiency of the combustion process. In the early fuel-controlled stage, the rate of smoke production is less because of a relatively good air supply. As it progresses to ventilation controlled, the combustion become less efficient and smoke volume will increase. Severely ventilation-limited fire will produce large volumes of thick dark smoke. The smoke thickness is also a function of duration. So, it is possible for a relatively small fire to produce a large volume of thick smoke if it has been burning for a long time in ventilation-limited conditions.
Smoke seen expanding upwards and roiling indicates that the gases are at a high temperature. Even very dense fire products will be lighter than air when they are heated to high temperatures. In contrast, smoke released at a lower temperature has a tendency to slowly drift upwards or even downwards in cold conditions. This lower temperature and buoyancy could indicate relatively low compartment temperatures, or it could be caused by cooling that has occurred as the smoke has traveled some distance through uninvolved sections of the structure.
Height of Neutral Plane
As the fire develops, the neutral plane will lower and the thickness of the smoke gases will increase. Therefore, be prepared for the following potential conditions:
· A high neutral plane could indicate that the fire is in the early stages of development.
· A very low neutral plane could indicate very rich backdraft-like conditions.
· A sudden rise could indicate that ventilation has occurred.
· Gradual lowering could indicate a build-up in fire gases and an approaching flashover.
· Sudden lowering could indicate a sudden intensification of the fire.
(1-2) These photos show the lowering of the neutral plane as the fire progresses towards flashover. (Photo courtesy of Tim Watkins.) In summary, it is critical to consider all of the indicators together before jumping to conclusions. This is true of all indicators, but it is particularly important when considering smoke color.
Air is one of the four fire behavior indicators that may be observed at an opening or within a structure. Air, which is invisible, is typically characterized through the movement of the contrasting smoke, its velocity, turbulence, and height within the boundaries of a compartment or at an opening. When combined with the smoke indicators, it is known as the smoke/air track. It is vitally important that firefighters understand the likely pathways of air movement caused by planned or unplanned ventilation in all parts of the structure. (4)
Bi-Directional Flow Path
When an opening is created in the room on fire, the heated gases will flow out of the top of the opening and cool air will be drawn in through the bottom of the opening.
Fuel Controlled – Smooth Flow
If the smoke/air track is slow and smooth, it could indicate that the fire is in the early stages and most likely still fuel controlled.
Ventilation Controlled – Turbulent Flow
As the fire grows larger, the demand for air increases as does the volume of super-heated smoke. Eventually, this leads to a situation where the discharged smoke blocks off the opening and restricts the ingress of air towards the fire base. This results in turbulence at the neutral plane in the opening.
Uni-Directional Smoke/Air Tracks
Smoke or flames seen leaving an opening from the very bottom to top of the opening in an indication of a total exhaust outlet. For the smoke or flames to be flowing directly out of an opening, there must be inlets that are at least equal in cross-sectional area or wind being driven into the inlet/inlets.
Look for open doors and windows that could be feeding the fire. As previously mentioned, each uni-directional exhaust opening will require inlets of at least equal cross-sectional area (unless the inlets are wind driven). It may be possible to reduce the rate of fire development if these inlets can be closed or restricted until hoselines can be put in place.
Strong winds being blown into a closed fire compartment can lead to a high-pressure zone in the compartment. Under normal wind conditions, a room with only one opening will display a bi-directional air track. This will be either fuel controlled (smooth flow) or ventilation controlled (turbulent). In the wind-driven scenario, the opening will aggressively alternate from a total inlet to a total exhaust outlet.
Smoke seen pulsing out of small openings can indicate a ventilation-controlled (or -limited) fire. This indicates that there are variations in pressure caused by a limited oxygen supply. As the oxygen level decreases, so does the combustion process. This, in turn, decreases the temperature and, consequently, the volume of the gases decreases. This causes air to be drawn in, the fire starts to increase, the temperature of the gases increase, and the expanding smoke is pushed out through the gaps under pressure until the air is consumed and the cycle starts again. In some cases, this could develop into a situation where the sudden opening of the compartment could lead to backdraft.
Whistling noises may indicate that air is being pushed in and out of the compartment through small gaps or openings because of pressure variations. This indicates a ventilation-controlled fire. It might be difficult to notice this with background noise.
The initial assessment should include looking for indicators of high temperatures. Building construction is a major factor in which heat indicators are likely to be evident. Heavy, well- insulated construction is less likely to show heat indicators in the early stages.
Many of the heat indicators may not be easy to identify in double- or triple-glazed windows.
Soot-stained windows with little or no flame showing
The blackening indicates rich conditions (backdraft potential) and is often accompanied by oily deposits on the inside of the window.
Cracking or Crazing of Glass
Rapid heat build-up can result in cracking of glass. Crazing (finer cracks) can be seen when the heat build-up has been slower and is often accompanied with blackening and oily deposits, indicating high-temperature, fuel-rich conditions.
Blistering or Discoloration of Paintwork or Cladding
This indicator is often present in lightweight internal doors. Heavy, well-insulated exterior doors are much less likely to show this indicator. Initially, feeling the door surface and door handle may help to detect developing heat conditions.
Sweeping a water spray across a lightweight door or surface can also be used to test for surface heat. If the door is hot (over 212 F/100C) the film of water on the upper section will rapidly evaporate. In some cases, it is possible to get an indication of the height of the neutral plane by observing the line at which the evaporation ceases.
Sudden Heat Build-up
This is frequently quoted as an indicator that flashover or backdraft is impending. If you are waiting for this indicator, it is likely that you have missed the other fire behavior indicators. This is a late indicator; it often occurs after some form of fire gas combustion (rollover) has commenced in the ceiling area. This may be difficult for the firefighter to see and will not give adequate warning time. By the time the firefighter wearing modern personal protective equipment senses the temperature increase, the situation will be very dangerous. Never rely on sensing sudden heat build-up.
One of the best ways to detect heat is with a thermal imaging camera. Where available, it should be a standard part of every size-up and internal operation. Another simple way of checking temperature is to place a small burst of water, on a very narrow pattern, into the overhead layer. If the water returns to the ground without any hissing, it is likely that the ceiling temperature is below 212°F (100°C) in that area. If on the other hand, the water does not come down, and a hissing sound is heard, it would indicate that the temperature is over 212°F (100°C). Firefighters can also carefully feel with the gloved hand to sense for heat build-up. If no excessive heat can be felt through the glove, slip the bottom of the glove back over the palm to expose the skin and cautiously life the hand overhead to feel the heat layer.
There is a tendency for firefighters to focus on any visible flame on arrival. There is nothing inherently wrong with this as long as it does not lead to a loss of situational awareness. Unless the structure is totally involved, it is still critical to read all of the fire behavior indicators to get a complete picture on the current stage of fire development.
Volume and Location
Incidents in which flames are externally visible on arrival obviously make it easier to determine the seat of fire and likely the direction of spread. It is important to look for signs of multiple seats of fire and to realize that visible flame may have spread some distance from the original source.
Smoke autoigniting outside an opening indicates that the internal conditions are above the autoignition temperature (AIT) and too rich to support full flaming combustion inside the compartment. When the super-heated rich smoke leaves the compartment, it is able to dilute down from a mixture within the flammable range. If it does not cool down to below the AIT during this process, the fuel content can ignite spontaneously. When these conditions are encountered, it is critical to realize that increasing the air available to the room will result in a sudden and possibly violent increase in fire intensity (backdraft). Under these conditions, doors or windows should be closed until hoselines are in place. Careful application of water into the space before ventilation can reduce the temperature of the smoke to below the AIT and reduce the likelihood of sudden intensification.
Pockets of Flame Forming in the Smoke Layer (Ghosting)
If internal crews begin to see pockets of flame forming above the neutral plane, this is an indicator that the unburned fuel in the smoke layer is approaching the AIT. Cool the smoke, and consider withdrawal.
Once the accumulated unburned fuel begins to ignite, it is common for the flames to roll across the ceiling, resulting in a rapid increase in radiative heat. This could lead to flashover or FGI. The application of water in the ceiling space may stop or delay the progression of the rollover. If it is not possible to place water on the burning surfaces to slow the heat release rate, the crew should retreat to a point of safety.
Traditional teaching tells us that the color of the flame can give an indication of the particular product that is burning. While this is true in situations where a single product is burning, it is important to realize that the same product can burn with different colored flames depending on the combustion process. For example, liquid petroleum gas that is premixed with air will produce a blue-colored flame (because of the presence of carbon dioxide. If the fuel and air are mixed by the process of diffusion, then the flame will be yellow because of the presence of carbon particles from a less efficient combustion process. LPG burning in an oxygen-deficient or a fuel-rich environment can produce a red flame.
Another example is the combustion of particleboard in a compartment. When the air supply is good, it will burn with a yellow flame. If the oxygen concentration is reduced, the flame will become a reddish-orange color.
In a compartment fire, yellow flames generally indicate a reasonable air supply. As the combustion process becomes less efficient (less oxygen), the flames will start to turn orange and then red.
The shape or form of the flame can also give an indication of the type of combustion occurring. The reddish-orange flames that result from the rich combustion are often turbulent and have a short wave form. The ignition of accumulated pyrolysis products produces a very light-yellow flame, sometimes almost clear. Amazingly in this case, the wave form is larger, and the flames will move downward seeking the higher oxygen concentration. As with all of the indicators in the SAHF assessment, it is important to observe the initial flame color and then note any changes.
“Reading the Fire” is like learning a language. Every fire is “speaking” to us through the fire behavior indicators. Sometimes the indicators are so clear that the fire is shouting at us! But if we don’t understand the language, it does not matter how loud the message is. Sometimes the fire is talking softly; and even if we understand the language, we have to be paying close attention, or we will miss the message.
Sometimes the fire tells lies. It deceives us by telling only part of the story, concealing critical information. So, not only must we know the language of fire, we must also be aware that it will not always tell us everything in the early stages. Firefighters regularly work in a dangerous and rapidly changing environment. We have to make decisions in seconds with very limited information and take actions that may save or endanger lives. The best we can do is to base those decisions on our knowledge of building construction and visible fire behavior indicators at each stage of the incident. There are no easy answers; but as scientific research enlightens our knowledge of fire behavior, we must be prepared to challenge our traditional thinking and open our minds to new possibilities.
1.” Reading the Fire,” Raffel, Shan, Fire Australia Journal,2001; 11-13.
2. Grimwood, Paul; Hartin, Ed; McDonough, John; Raffel.,Shan. 2005. 3D Fire Fighting, Training, Techniques and Tactics. Stillwater: Fire Protection Publications.
3. McBride, Peter. 2008. Environmental factors – fire dynamics size-up. [interv.] Shan Raffel. 17 June 2008.
4.—. 2015. Smoke/Air track and relationship to flow path terminology. [interv.] Shan Raffel. 22 October 2015.
Original Article here.