Sudden Blast

Unanticipated smoke explosion and building collapse nearly kills three firefighters.

Portsmouth, VA Near-Miss Incident

Firefighter Eric Kirk gives a firsthand account of a near-miss incident involving a smoke explosion in the June 2009 issue of FireRescue magazine. On a December morning in 2007, firefighters in Portsmouth, Virginia responded to a fire in a church. On arrival, the building was well involved and defensive operations were initiated to protect exposures and confine the fire. Over the course of the fire, smoke extended into an attached, three-story, brick building and formed a flammable fuel/air mixture. Subsequent extension of flames from the church to the interior of the exposure resulted in ignition and explosive combustion of this fuel (smoke)/air mixture.

Incident Photos from

Smoke Explosion

This post expands on Smoke is Fuel (Hartin, 2009), a sidebar that I wrote for FireRescue that accompanies Eric’s article examining the Portsmouth, VA smoke explosion incident.

Smoke explosions have resulted in three firefighter fatalities in the United States since 2005, two in Wyoming (see NIOSH Report F2005-13) and one last year in Los Angeles California (NIOSH report pending). In addition, there have been a number of near miss incidents including this one in Virginia and another in Durango, Colorado (see NIOSH Report F2008-02)However, many firefighters have not heard of or misunderstand this fire behavior phenomenon.

The terms backdraft and smoke explosion have typically been used to describe explosions resulting results from confined and rapid combustion of pyrolysis and unburnt products of incomplete combustion. Describing a backdraft incident at a Chatham, England Mattress Store in 1975, Croft (1980) states “this is not an entirely new phenomenon, the first formal description of what have been called ‘smoke explosions’ having been given in 1914” (p. 3).

As an explanation of many contradictory statements in reference to explosions that are reported to have occurred in burning buildings, where it is also testified that explosives were non-existent, we may cite so-called “smoke explosions.”

Distinct from, yet closely allied with explosions of inflammable dust, are explosions caused by the ignition of mixtures of air with the minute particles of unconsumed carbon and invisible gaseous matter in smoke from the imperfect combustion of organic substances…

These “smoke explosions” frequently occur in burning buildings and are commonly termed “back draughts” or “hot air explosions” (Steward, 1914).

As discussed in my earlier post, Fires and Explosions, the term Smoke Explosion was a synonym for Backdraft. In fact, if you look up the definition of smoke explosion in the National Fire Protection Association (NFPA) 921 (2008) Guide for Fire and Explosion Investigation, it says “see backdraft” (p. 921-15). However, today it identifies a different, and in many respects more dangerous extreme fire behavior phenomenon. Smoke (or Fire Gas) Explosion is described in fire dynamics textbooks such as Enclosure Fire Dynamics (Karlsson and Quintiere) and An Introduction to Fire Dynamics (Drysdale) and Enclosure Fires (Bengtsson). Of these, the text Enclosure Fires by Swedish Fire Protection Engineer Lars-Göran Bengtsson provides the best explanation of how conditions for a smoke explosion develop. However, this phenomenon is less well known among firefighters and fire officers. In fact many well known fire service authors continue to use backdraft and smoke explosion interchangeably.

A smoke or fire gas explosion occurs when unburned pyrolysis products accumulate and mix with air, forming a flammable mixture and introduction of a source of ignition results in a violent explosion of the pre-mixed fuel gases and air. This phenomenon generally occurs remote from the fire (as in an attached exposure) or after fire control.

Conditions Required for a Smoke Explosion

The risk of a smoke explosion is greatest in compartments or void spaces adjacent to, but not yet involved in fire. Infiltration of smoke through void spaces or other conduits can result in a well mixed volume of smoke (fuel) and air within its flammable range, requiring only a source of ignition.

Smoke explosions create a significant overpressure as the fuel and air are premixed. Several factors influence the violence of this type of explosion:

  • The degree of confinement (more confinement results in increased overpressure)
  • Mass of premixed fuel and air in the compartment (more premixed fuel results in a larger energy release)
  • How close the mixture is to a stoichiometric concentration (the closer to an ideal mixture the faster the deflagration)

For additional information on transient, explosive, fire phenomena see earlier posts: Gas Explosions and Gas Explosions Part 2.

Indicators Smoke Explosion Potential

It is very difficult to predict a smoke explosion. However, the following indicators point to the potential for this phenomenon to occur.

  • Ventilation controlled fire (inefficient combustion producing substantial amounts of unburned pyrolysis products and flammable products of incomplete combustion)
  • Relatively cool (generally less than 600o C or 1112o F) smoke
  • Presence of void spaces, particularly if they are interconnected
  • Combustible structural elements
  • Infiltration of significant amounts of smoke into uninvolved exposures

Mitigating the Hazard

As with recognizing the potential for a smoke explosion, mitigation can also be difficult. The gases are relatively cool, so application of water into the gas layer may have limited effect. Tactical ventilation to remove the smoke is the only way to fully mitigate the hazard and establish a safe zone. However, use care not to create a source of ignition (such as the sparks created when using an abrasive blade on a rotary saw).

The best course of action is to prevent infiltration of smoke into uninvolved spaces using anti-ventilation (confinement) tactics. Anti-ventilation is the planned and systematic confinement of heat, smoke, and fire gases, and exclusion of fresh air (from the fire). In this case, anti-ventilation may involve pressurizing the uninvolved are to prevent the spread and accumulation of smoke.

Ed Hartin, MS, EFO, MIFIreE, CFO


Bengtsson, L. (2001). Enclosure fires. Karlstad, Sweden: Räddnings Verket.

Croft, W. (1980) Fires involving explosions-a literature review. Fire Safety Journal, 3(1), 3-24.

Drysdale, D. (1998). An introduction to fire dynamics (2nd ed.). New York: John Wiley & Sons.

Hartin, E. (2009, June). Smoke is fuel. FireRescue, 27(6), 54.

Karlsson, B. & Quintiere, J. (2000). Enclosure fire dynamics. Boca Raton, LA: CRC Press.

Kirk, E. (2009, June). Sudden blast: Unanticipated smoke explosion & building collapse nearly kills 3 firefighters. FireRescue, 27(6), 52-54.

National Fire Protection Association. (2008). NFPA 921 Guide for fire and explosion investigations. Quincy, MA: Author.

National Institute for Occupational Safety and Health (NIOSH). (2006) Death in the Line of Duty Report F2005-13. Retrieved June 22, 2009 from

National Institute for Occupational Safety and Health (NIOSH). (2009) Death in the Line of Duty Report F2008-02. Retrieved June 22, 2009 from

Steward, P. (1914). Dust and smoke explosions, NFPA Quarterly 7, 424-428.

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8 Responses to “Sudden Blast”

  1. Brian Says:

    Good article. How often do we as firefighters walk through the smoke without thinking about what we are walking in. Awareness is key. All of us need to think before we act, not the other way around.

  2. Stefan S Says:

    I believe one important difference between backdraft and smoke explosion, is that a smoke explosion requires an ignition source. A backdraft doesn’t (and we talk about a “gravity current” which is the air flowing into a compartment with a ventilation controlled fire, thus causing the backdraft). Charlie Fleischman at Canterbury University, New Zealand, made some research on backdrafts some years ago (theory and small scale experiments). Also, Daniel Gojkovic at Lund University picked up where Charlie ended and made some large scale experiments involving methane as well as carbon dioxide. I’m looking for a student to pick up where Daniel ended…
    Keep up the good work!

  3. hartin Says:

    Stefan, You are absolutely correct. I have updated the post to more clearly identify this distinction. Ed

  4. laurence delorme Says:

    hello Ed,

    very great thread which explains the differences between smoke explosion and,in France,it is very difficult to explain that backdraft and smoke explosion are 2 different “phenomena”.

    for me,it is different: cause when you have a backdraft,the air,causes the phenomenon.

    for the smoke explosion,an ignition source causes the explosion,totally different and Stefan explained it very well in his reply.

    i think we are confused in France cause first ,the people who translated the documents about backdraft ,put this sentence:smoke in the head of many people in the fire service,they read the documents and they think that backdraft=smoke explosion.just a mistake of translation which creates many other mistakes to identify these 2 different phenomena:smoke explosion and backdraft.

    thanks for reading me,i wrote too much….

    best regards.

  5. Stefan S Says:

    I have to make a correction to my comment:
    Generally, a backdraft needs a ignition source too. But, the temperature of the fire gases are much higher, so the ignition source can be smaller (less energy is required to ignite the gases). And we are still talking about the gravity current (air flowing in etc.).
    Also, it should be noted that a smoke explosion is a premixed flame and a backdraft is a diffusion flame.
    The book “Enclosure fires” by Lasse Bengtsson explains this (and a lot more) very well. And I can recommend some of the papers by Fleischman and Gojkovic.
    Definitions of various fire phenomena are important for scientist and during education and training of fire fighters and fire officers. In a real fire, it is very (VERY!) hard to identify such phenomena because they exists in very complex environments. I believe we should focus on things important to the development and spread (and suppression, of course) of fires (fuel, geometry, flow of gases, etc.). Not necessarily the definitions.

  6. Tim McLaren Says:

    Since your class in Redmond Oregon, I have taken a great interest in the phenomena of smoke explosions and appreciate your research and information on the subject.

  7. g Cecyre Says:

    Bonjour Ed. etl Delorme, ne pas traduire
    mon texte français en francais car il va s’ensuivre beaucoup d’erreurs et d’incompréhension.

    IL n’y a pas de différence entre ici ,aux USA, ou en Europe un Backdrat (combustion spontanée) ce n,est pas réellement une explosion, Il y a un lieu plein de fumée chaude qui contiennent des gaz de combustion et des gaz combustibles trop riche pour brûler, il y a des braises sur le déclin qui sont y très en chaleur, comme 250 à 400 degrés de chaleur, mais beaucoup trop pauvre pour allumer ces gaz, On a besoin de près de 600 degrés Celssus pour qu’il y ait ignition,pensez gaz naturel ,Méthane, Une entrée d’air au sol va alimenter les braise et leurs faire reprendre vie, la flamme nue, elle qui a près de 1000 degrés Celsius , c’est la chaleur dont les gaz ont besoin pour s’embraser.Et nous remarquons que l’air qui entre peut ne pas tout être toute consumé par le feu ,et cette air chaude monte par convection pour aller diluer les gaz trop riche et produire une zone idéal dans la fourchette de la plage d’inflammabilité. Ce phénomène accomplit il y a une combustion rapide extrême, ou une combustion spontanée,immédiate. PLus le mélange du % de gaz est bas plus le backdraft sera mou, de faible intensité. il se produit normalement à la base du local partie inférieur, pour un plus violent et fort ,c’est un mélange de gaz en % plus riche. `remarque les signes précurseurs quand un backdraft se prépare la fumée se densifie prend de la vitesse, et refoule violemment vers l’extérieur ou il se forme une boule de 3 à 4 mètres de diamètre à la sortie eten même temps on voit la flamme qui allume de l,intérieur du local pour suivre jusqu’à l’extérieur sur un 10 à 12 pieds et les flammes remontent presque collé au bâtiment, observer.

    Pour un smoke-explosion, IL y a dans une maison , un local, une pièce, beaucoup de gaz de PYROLYSE, Gaz ou vapeur blanche légère ou grisâtre, froide et peu aussi être moyennement chaude, on peut trouver des contenants, tas de linges, matelas,coussin,(feu incandescent)et si on les retourne sans les avoir noyer dans l’eau,la flamme revie et elle allume ces gaz de pyrolyse. Il faut noyer ces matériaux généreusement et éventrer les matériaux pour vérification, on récolte de la petite fumée blanche et dès que retourné, les braises s’activent en consumant l’air qui lui est arrivé et de gaz combustibles légèrs qui sont au plafond et qui peuvent être Chauds normalement, ou froids à ce moment et quand apparaiît une flamme nue,1000 degrés, elle entre en contact direct avec ces gaz à l’interface air/gaz et il y a embrasement immédiat et subbit, ce smoke-explosion a un grande force de délacement car il a allumé des gaz LSI (UFL)en partie haute du local et la projection se fait sur une grande distance jusqu’à l’extérieur, le smoke-explosion est plus violent, qu’un backdraft. jespère que j’ai été assez clair. N.B. si vous voulez des textes je vous en enverrai à une adresse email.merci Georges

  8. g Cecyre Says:

    Je peux vous envoyer des textes déjà écrit que je n’aurai pas à écrire à chaque fois, juste les transférer sur éamil.merci bye bye

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