Backdraft Indicators and Mitigation
In the late 1970s I was blown out of a building by a backdraft during live fire training in an acquired structure (pre NFPA 1403 Standard on Live Fire Training Evolutions). Fortunately, I only had minor injuries, but this ignited my passion for understanding fire behavior and developing the ability to read the fire and anticipate potential fire development and rapid fire progression events. One of the puzzling things about this event was that despite some textbook signs of potential backdraft, no one recognized this, not the students, not the instructors, and not the incident commander who was an experienced chief officer from an urban fire department. All the participants had declarative knowledge and were able to recognize the signs of backdraft when presented in a multiple choice question but could not recognize (or at least consistently recognize) them when presented during firefighting operations.
Backdraft Indicators
Bolliger (1995) performed a literature search to identify potential warning signs of backdraft. He identified that several possible warning signs had been collated from interviews with people who had been involved with a backdraft. “However, there is no factual basis for most warning signs. In fact, many of these [warning signs] contradict each other or are of absolutely no help” (Bolliger, 1995, p. 12). He illustrates this with alleged backdraft indicators related to the color and [optical] density of the smoke:
- Thick, dense, black smoke.
- A sudden change from thick, dense, black smoke to yellow or grayish yellow.
- Thick billowy clouds of yellow smoke.
- White cold smoke.
- Heavy dark smoke.
Smoke within this range of conditions is encountered at many fires that to not present the risk of potential backdraft. While some of these conditions may have a correlation to specific backdraft events, they are not (at least alone) a reliable indicator of backdraft potential.
A list of possible warning signs of potential backdraft can be synthesized from the work of Chitty (1994), Bolliger (1995), Gorbett and Hopkins (2007), Lambert (2014) Brown, Falkenstein-Smith, & Cleary (2021):
- Fires in concealed spaces such as ceiling voids and poorly ventilated rooms or compartments.
- Hot doors and windows, indicating that a fire has been burning inside the compartment, possibly with limited ventilation.
- Oily deposits on windows occurring from condensation of pyrolysis products onto cooler surfaces.
- Puffing or pulsating smoke discharge from the fire compartment (smoke discharge followed by air intake in an alternating flow).
- Rattling of windows (related to increases and decreases of pressure within the compartment or enclosure).
- Whistling or roaring sounds (related to increased pressure within the compartment or enclosure)
- Strong air intake when making a ventilation opening.
- A large amount of smoke with no visible fire or simply an orange glow.
- Blue ghosting flames in the smoke layer.
- No visible, audible, or tactile warning signs!
“These warning signs must be considered in the context of the specific scenario encountered and excessive weighting would not be given to any single sign. Encountering several of the signs together however would give a strong indication of potential for a backdraft” (Chitty, 1995).
One way to frame backdraft indicators is using the building, environment, smoke, air flow, heat, and flame (BE-SAHF) categories of fire behavior indicators. Note: This model was originally B-SAHF and was subsequently modified to include the environment (e.g., wind, ambient temperature, humidity) (Raffel & Walker, 2022).
Building
Key building indicators include limited ventilation openings and the presence of void spaces and a significant fuel load. Fuels with a high heat of combustion and/or substantial surface area may also increase risk of backdraft due to increased mass of pyrolysis products produced during a ventilation limited decay stage. Building construction characteristics that will allow the fire to burn for a long time without structural failure may also increase risk of backdraft due to increased pyrolysis production over extended burn times.
Environment
Ambient temperature and wind are both significant factors to consider in evaluating backdraft potential. The greater the difference between the ambient temperature and temperatures within the enclosure, the greater the strength and speed of the gravity current created when a horizontal ventilation opening is created. The influence of wind will depend on wind speed and direction, if the wind is blowing inline and towards a horizontal ventilation opening such as a door or window, the speed and turbulence of a gravity current will be increased.
A high difference temperature between ambient, external temperature and the interior of the enclosure and wind speed and direction can increase mixing of fresh air and fuel rich smoke, increasing backdraft risk.
Smoke
Smoke indicators present the greatest puzzle. As previously noted, color, volume, and optical density are frequently identified as potential backdraft indicators. They may be correlated to the occurrence of backdrafts, or they may not. This is a problem if you only think about smoke.
As fire becomes ventilation limited, the volume and density of smoke produced will increase. In the growth stage smoke will push through small vents and construction gaps and may become darker. However, as the oxygen concentration within the enclosure decreases and heat release rate diminishes, temperature will drop and so too will the pressure in the compartment. This results in reduced smoke discharge to the exterior and smoke discharge may cease entirely (Kerber, 2010). Nothing showing means nothing!
The color of smoke is influenced to a substantial extent by what is burning, and the extent of limited ventilation. Color may vary, light color smoke or black smoke may become dense grayish yellow. Yellow smoke is often associated with decay due to limited ventilation (and backdraft conditions). However, color alone is not a reliable indicator.
Smoke that is optically dense and has the appearance of texture may be a more significant indicator. But this too, when considered alone, can be misleading. For example, smoke from burning hydrocarbons or hydrocarbon-based products is generally black and extremely optically dense but does not necessarily indicate backdraft potential.
When a fire is burning in an elevated position within an enclosure and it is not fully smoke logged with smoke to the floor, changes in heat release rate and temperature due to limited ventilation may cause raising and lowering of the hot, less dense, upper layer due to changes in smoke volume.
Air Flow
Air flow (or air track) refers to the exchange of smoke and air in and out of an enclosure. It is likely that pulsing air flow is one of the most reliable indicators of backdraft potential. However, this too comes with a caveat. Pulsing air flow is one indicator of a ventilation-limited fire and does not always indicate backdraft conditions. An important consideration in evaluating pulsing air flow is to recognize that the interval between smoke discharge and intake of air can vary considerably (seconds to minutes) and it may be easy to miss this indicator when quickly performing 360-degree reconnaissance.
It is also important to note that air flow is also significantly influenced by opening size (e.g., cracks vs an open window or door) and proximity to the fire.
Heat
Temperature during the decay stage can initially be quite high (and continue to rise for some time). There may be visual indicators such as blackened windows, high velocity smoke discharge, and surfaces such as windows and doors are likely to be quite hot. However, if decay is due to ventilation-controlled conditions, the temperature will eventually drop (if the compartment remains sealed).
Research has indicated that for a given fuel, there is likely a critical temperature below which backdraft may not occur when ventilation is increased (Carvel & Wu, 2017 & Wu, Santamaria, and Carvel, 2020). However, there may be exceptions or specific fuel mixtures that present backdraft risk at lower temperatures than identified by prior research if a source of piloted ignition is present. For example, in the 62 Watts Street (NY) incident, firefighters describe the smoke as “warm” and post-incident fire modeling identified the temperature within the apartment as less than 100o C (212o F) (Bukowski, 1996).
Flame
In ventilation-controlled decay, flaming combustion is reduced and if the oxygen concentration falls below 13%-15%, flaming combustion will cease (Quintiere, 2016) but pyrolysis and heterogenous combustion will continue. However, do not be fooled! Flames may be present. Ignition of fire gases escaping from the compartment (as they mix with air) can provide a strong indication of fuel rich, oxygen deficient decay conditions. Also remember that conditions can vary considerably in different parts of the structure. Backdraft conditions can exist in a void space while you can see a fully developed fire with flames showing from several windows.
Recognizing Potential Backdraft Conditions
As noted by Chitty (1995), identification of potential backdraft conditions cannot be made based on a single indicator. Recognizing backdraft potential requires recognition of patterns of fire behavior indicators.
Developing expertise in recognizing these conditions presents a challenge as backdrafts are infrequent and a firefighter or fire officer may have the opportunity to see a backdraft once in their career (or maybe will never see one). Observing a backdraft provides a single view and may or may not provide the opportunity to observe relevant cues. In over 50 years in the fire service, I have seen three backdrafts during emergency incident operations, two provided the opportunity to observe key indicators. One (in which I was blown from the building), I did not see anything but smoke, and the backdraft occurred in an adjacent compartment to my working position.
Backdraft demonstrations may provide an opportunity to observe some of the potential indicators of a backdraft, but these demonstration frequently do not provide a fully realistic context and may be misleading.
Similarly, videos of explosive, rapid fire progression events may provide an opportunity to observe key indicators but remember that video often only provides a single view and it may be difficult if not impossible to determine exactly what phenomena resulted in the explosion.
Does this mean it is impossible to recognize potential backdraft conditions? I don’t think so. Difficult, but not impossible. Remember that as the number of potential indicators increases, the likelihood of potential backdraft conditions increases as well.
Backdraft Mitigation
When confronted with potential backdraft conditions, what action should be taken to mitigate the risk. There are several possible options.
- Increase ventilation.
- Apply water.
- Do not take offensive action.
Ventilation
Many early fire service texts on strategy and tactics identified ventilation as a method of preventing backdrafts and smoke explosions (which in the perspective of the day were one and the same). However, some did not specify what type of ventilation and where it should be performed (Casey, 1968; Clark, 1974). In Fireground Tactics, Fried (1972) specified that vertical ventilation was the most appropriate ventilation tactic for mitigation of potential backdraft or smoke explosion conditions.
The proper procedure is to open up directly over the fire. This allows the hot gases to move upward though the opening and away from the fire. The counter current of air flowing downward toward the fire serves only to brighten the flames but will not cause a back draft. The opening acts as an inlet for air and an outlet for hot smoke and gases (Fried, 1972, p. 65).
Zhao, Li, and Wu (2021) and Zhao, Li, and Wu (2021) recommend a similar strategy (but using a window high in the compartment).
A strategy for avoiding backdraft is suggested. When fighting a fire that breaks out in a ventilated confined compartment, an upper window in the compartment should be opened. Then, hot gas flows out, while the temperature and the combustible concentration decrease. Meanwhile, fresh air cannot flow in because of the hot gas expansion. This ventilation strategy could prevent oxygen reaching the flammability limit and avoid backdraft occurrences (Zhao, Li, & Wu, 2021, p. 12).
With the compartment opening area increasing, backdraft time decreases. It indicates that when fighting a backdraft fire, the upper part of the compartment should be opened to discharge the high-temperature gas in the compartment. In this way, the backdraft can be avoided or delayed (Zhao, Li, & Wu, 2021, p. 176).
The experiments conducted and data presented by Zhao, Li, and Wu (2021) and Zhao, Li, and Wu (2021) do not support the idea of upper level ventilation mitigating or reducing the risk of backdraft. While vertical ventilation represents conventional wisdom on reducing backdraft potential, there may be some issues with the logic of this approach.
Zhao, Li, and Wu (2021) state that with horizontal ventilation high in the compartment, hot gas flows out, while the temperature and the combustible concentration decrease. If pressure within the enclosure is higher than outside, and the hot smoke is less dense and more buoyant than the fresh air outside, and an opening is created high in the enclosure, smoke will flow out and up (high pressure to low and rising due to lower density). Initially, the flow will be unidirectional out, but eventually air will flow into the enclosure establishing a bi-directional flow (smoke out and air in).
If smoke flows from the enclosure and there is no intake of air from the outside, pressure may be equalized, but there is no significant change in the concentration of fuel, oxygen, and non-combustible products within the compartment.
Once a bi-directional flow is established at the ventilation opening, the oxygen concentration within the enclosure will increase. If prior to ventilation, the concentration of fuel and oxygen within the enclosure was above the flammable range (given the temperature and concentration of gases), the introduction of oxygen will potentially bring the mixture within the flammable range. If the temperature is above the auto ignition temperature (AIT) or the increase in oxygen results in a piloted ignition source, a backdraft may occur.
If the opening is high in the compartment, the occurrence of backdraft may be delayed, but consequently, a greater amount of mixing may occur prior to ignition, worsening the backdraft (Stefen Svensson personal communication May 26, 2024).
Application of Water
The application of water for cooling makes intuitive sense. However, research has shown that the application of water in the form of mist decreases the fuel mass fraction (by increasing the mass fraction of non-flammable products) and as a result, decrease backdraft risk by dilution rather than the thermal mechanism of cooling (Gottuk, Williams, and Farley, 1997).
If temperatures within the enclosure are greater than 300o C, it is reasonable to assume that all water introduced into the enclosure will be turned to steam. If sufficient steam is produced, the mass fraction of fuel will be reduced below the critical fuel mass fraction and will not result in an explosion when air is introduced. Gottuk, Williams, and Farley (1997) found:
“A comparison of the fire compartment temperatures for similar tests with and without water injection shows that the thermal effect of water injection was insignificant with respect to backdraft development. In general, the temperature decreased marginally by about 35o C due to water addition… The fact that all tests had temperatures greater than the AIT and only tests with fuel mass fractions of 0.15 or less resulted in no explosion, indicate that the use of water injection suppressed the backdraft explosions primarily by means of diluting the atmosphere, thus reducing the fuel mass fraction…” (p. 945).
It is important to keep in mind that the experiments conducted by Gottuk, Williams, and Farley (1997) were in the context of shipboard firefighting and were performed in a steel compartment using diesel fuel. While not supported by experimental evidence, they speculated that in a compartment lined with gypsum board (more typical of the compartment linings of a residential or commercial building) that water spray would reduce gas temperature more significantly as the gypsum board serves as more of an insulator than steel. As gas temperature is reduced to or below 100o C cooling may become a more significant mechanism.
Weng and Fan (2002) drew similar conclusions based on experiments conducted in a metal compartment using methane as the fuel.
Water mist is an effective mitigating tactic that is able to suppress backdraft in compartment fires primarily by means of diluting the gas in the compartment and reducing total hydrocarbons mass fraction, rather than by a thermal mechanism of cooling. The more water mist injection mass, the less total hydrocarbons mass fraction, and the less intensity of backdraft in the compartment with the same opening geometry (p. 277).
It is important to note that both research projects used water mist in metal compartments (one steel and the other stainless steel) and neither used solid fuels typical of those found in buildings. In both experiments examining backdraft mitigation with water mist, water was applied prior to any increase in ventilation.
Water mist as used for fire suppression typically has droplet sizes ranging from 10 to 100 microns (O’Connor, 2022). Gottuk, Williams, and Farley (1997) did not specify the droplet size used in their experiments. In the experiments conducted by Weng and Fan (2002) the mean droplet size was 38 microns. By comparison, water droplets produced by a combination nozzle at a nozzle pressure of t bar (100 psi) are approximately 300 microns. Droplets produced by lower pressure combination nozzles are much larger and droplets produced by solid streams impacting compartment linings are larger still.
It is a stretch to simply generalize based on evidence provided by this limited number of experiments conducted in contexts that were considerably different than typically encountered when backdraft conditions exist in a building. However, it is reasonable to project if water can be applied into the area where backdraft conditions exist without significantly increasing ventilation, the risk of backdraft is likely to be reduced.
No Offensive Action
While not exactly mitigating the potential for occurrence of a backdraft, taking no offensive action may be a reasonable choice to mitigate the potential risks in some circumstances. Consider the following example.
You have been called to a fire in a furniture store at 02:00. The store is constructed of masonry with a built up roof (tar and gravel) over a metal deck supported by metal trusses. On Side Alpha (front) the store has the main entrance doors and large display windows. There are no openings on Side Bravo (left) and Delta (right). On Side Charlie (rear) there is an egress door and several overhead doors at the loading dock. There are no exposures.
You observe smoke stained windows on Side Alpha with smoke pushing from around the doors and other openings. The building appears to be fully smoke logged. On Side Charlie, you observe pulsing air flow with smoke pushing from around the doors and then stopping.
When presenting this scenario to groups of firefighters, someone invariably stays “go to the roof and ventilate”. But it is important to answer the strategic question before making tactical choices. Are these defensive fire conditions? It is unlikely that the store is occupied, and the contents are not salvageable. Based on the type of construction, even if the fire was extinguished, the building is likely to be torn down. In this case, a defensive strategy is the appropriate option.
Now shift gears and consider a multi-unit residential occupancy at 02:00 with backdraft conditions presenting in a first floor unit. This presents a significantly different situation. Taking offensive action to mitigate the potential for a backdraft (or other ventilation caused rapid fire progression) is essential.
Practical Approaches to Mitigating Backdraft Potential
Application of water into the area of the building with potential backdraft conditions is likely to be the most effective offensive method for mitigating potential backdrafts. However, as previously noted, it is essential to minimize air intake and prevent development of gravity current that may result in a backdraft prior to reduction of the mass fraction of fuel though vaporization of water inside the enclosure. Methods to accomplish this task may include:
- Use of one or more piercing nozzles.
- Cobra cutting extinguisher (or similar device)
- Brief and limited opening of a door and application of water with an attack line.
Tactical ventilation can be performed following mitigation through the application of adequate water into the enclosure.
References
Bolliger, I. (1995). Full residential -scale backdraft. Retrieved May 28, 2025, from https://bit.ly/3FoDT48.
Brown, C., Falkenstein-Smith, R., & Cleary, T. (2021). Reduced-scale compartment gaseous fuels backdraft experiments, NIST Technical Note 2183. Retrieved April 10, 2025, from https://bit.ly/44iTT1q.
Bukowski, R. (1996). Modeling a backdraft: The 62 Watts Street incident. Retrieved June 2, 2025 from https://bit.ly/43EaScK.
Carvel, R & Wu, C-L 2017, An experimental study on backdraught: the dependence on temperature, Fire Safety Journal, 91, 320-326. doi: 10.1016/j.firesaf.2017.04.003
Casey, J. (1968). The fire chief’s handbook (3rd ed). New York: The Ruben H. Donnelley Corporation.
Chitty, R., (1994). A survey of backdraft. Retrieved May 28, 2025, from https://bit.ly/3T2wKtf.
Clark, W. (1974). Firefighting principles and practices. New York: Dunn Donnelly Publishing Corporation.
Gorbett, G. & Hopkins, R. (2007). The current knowledge & training regarding backdraft, flashover, and other rapid fire progression phenomena. Retrieved June 2, 2025, from https://bit.ly/3FAkWeO.
Gottuk, D.; Williams, F.; & Farley, J. (1997). The development and mitigation of backdrafts: a full-scale experimental study. Fire Safety Science, 5, 935-936. doi: 10.3801/IAFSS.FSS.5-935
Kerber, S. (2010). Impact of ventilation on fire behavior in legacy and contemporary residential construction. Retrieved January 21, 2023, from https://bit.ly/3D4ujPj.
Lambert, K. (2014). Backdraft: fire science and firefighting, a literature review. Retrieved April 11, 2025, from https://bit.ly/3Et9Do5.
O’Connor, B. (2022). Water mist systems overview. Retrieved June 2, 2025, from https://bit.ly/3ZazyIt.
Quintiere, J. G. (2016). Compartment Fire Modeling. In M. J. Hurley (Ed.), SFPE Handbook of Fire Protection Engineering, 5th ed., 1062–1104). Springer. https://doi.org/10.1007/978-1-4939-2565-0.
Raffel, S. & Walker, B. (2022). Reading the fire. Queensland, Australia: Aus-Rescue Pty Ltd.
Weng, W. & Fan, W. (2002). Experimental study on the mitigation of backdraft in compartment fires with water mist, Journal of Fire Sciences, (20)4, 259-278. doi: 0.1106/073490402029761.
Wu, C.L., Santamaria, S. & Carvel, R. (2020). Critical factors determining the onset of backdraft using solid fuels. Fire Technology 56, 937–957. https://doi.org/10.1007/s10694-019-00914-9.
Zhao, J., Li, Y., & Wu, J. (2021). Effect of the opening area of compartment on the backdraft time, Journal of Civil Engineering, (9) 5, 173-176. doi: 10.11648/j.ajce.20210905.14.
Zhao, J., Li, Y., Li, J., Huang, Y., & Wu, J. (2021) Experimental study on the backdraft phenomenon of solid fuel. PLoS ONE, 16(8): doi: 10.1371/journal.pone.0255572.