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Contents
  1. 1Components and formation
  2. 2Soot and smoke yield
  3. 3Visibility and optical properties
  4. 4Toxicity and inhalation
  5. 5Smoke movement and dilution
  6. 6Smoke control
  7. 7Measurement and modelling
  8. 8Limits and common misunderstandings
  9. 9See also
  10. 10Notes
  11. 11References

Smoke is a mixture of solid particles, liquid droplets and gases produced during combustion and thermal decomposition. In fire safety, the smoke volume also includes entrained ambient air, because its temperature and total volume determine smoke movement and exposure. This broader usage differs from definitions that reserve smoke for only the condensed-phase aerosol.[1][2]

Smoke can reduce visibility, deliver toxic and irritant products, carry unburnt fuel and transfer heat. Its appearance and behaviour vary with fuel, combustion mode, oxygen supply, mixing and time. White, black, hot or weakly visible smoke therefore cannot be interpreted by colour alone.[2][1]

The mixture is produced by flaming combustion, pyrolysis and smouldering, then transported by plume and building airflows. Entrainment dilutes concentration and temperature while increasing total mass flow. A small combustion source can consequently create a large smoke volume.[2][3]

Components and formation

Solid particles include carbonaceous soot and other condensed material, while droplets can form when low-vapour-pressure products condense. Gases include carbon monoxide, carbon dioxide, water vapour, unburnt fuel and many other decomposition products. Their proportions depend on material and combustion conditions.[2][4]

Flaming combustion produces smoke through a balance between growth processes in the fuel-rich flame and burnout with oxygen. Aromatic polymers such as polystyrene tend to produce more smoke than hydrocarbons with single carbon-carbon bonds in the cited comparison. Flaming smoke often contains substantial elemental graphitic carbon.[2]

Pyrolysis occurs when an external heat source decomposes a fuel surface. Vapour can then condense into light-coloured droplets as it rises and cools. Smouldering is self-sustaining surface oxidation rather than forced pyrolysis, and also produces droplet-rich smoke with different particle properties.[2][5]

White smoke commonly contains fine droplets and light aerosols, including condensed water or pyrolysis products, while water vapour itself is invisible. Black smoke usually contains more soot. These are tendencies in the cited descriptions, not a reliable classification of every fire.[1][2]

Smoke cloud containing soot, droplets, gases and entrained air with invisible water vapour noted.
Smoke is a mixture of particles, droplets, gases and entrained air. Based on Van Rossum (2027), §6.1 and Mulholland (2002), p. 2-258.

Soot and smoke yield

Soot is the carbonaceous particulate component associated with incomplete combustion. It contributes to dark colour, optical extinction and flame radiation. Soot is therefore one component of smoke rather than a synonym for the entire smoke mixture.[6][2]

Soot yield expresses the mass of soot produced per mass of fuel burned. It depends on fuel and combustion conditions, with more complete combustion generally producing less soot. Plastics tend to produce more than wood in Van Rossum's examples, while ethanol produces so little that the smoke can be difficult to see.[6]

Pre-flashover example values include 1.5% for wood, 23% for polyurethane, 14% for PVC, 16% for polystyrene and 19.5% for expanded polystyrene. These values apply to oxygen excess in the cited source and should not be used as universal material constants. Ventilation-limited and suppressed fires can have different yields.[6]

Suppressed or smouldering combustion can have a higher yield per kilogram while burning less fuel overall. Total soot production therefore depends on both yield and mass consumed. Comparing materials only by percentage can misstate the actual smoke source strength.[6]

Visibility and optical properties

Visibility in smoke is the distance over which a person can perceive an object or sign. It depends on optical density, object contrast, illumination, smoke scattering and the observer. Irritants can further reduce effective visibility by causing eye pain and impaired vision.[7][8]

Particles reduce the luminous flux reaching the eye from an object and scatter background light into that line of sight. Both effects reduce contrast. Additional lighting can therefore fail to improve visibility when scattering overwhelms the contrast needed for recognition.[8][7]

Optical density expresses light attenuation per metre and is related to particle concentration and a mass-extinction coefficient. Van Rossum gives a generic average of 3,300 m²/kg for smoke from burning building and contents materials. It is a modelling average rather than a universal property of every smoke aerosol.[9]

For approximately uniform smoke, Van Rossum gives practical visibility estimates of one divided by optical density for non-luminous objects and 2.5 divided by optical density for luminous objects. He cautions that these are rough rules. Different optical densities along the actual line of sight require integration and make visibility direction-dependent.[7]

Jin's experiments similarly find an approximately constant product of visibility and extinction coefficient for signs over selected ranges. The product depends on sign type, reflectance and illumination, and black smoke can give somewhat better visibility than white smoke at the same extinction coefficient. This reinforces that optical density alone does not define perception.[8]

Toxicity and inhalation

Fire toxicity includes asphyxiation, irritation and other physiological effects caused by combustion products. Carbon monoxide, hydrogen cyanide, low oxygen, carbon dioxide and irritant compounds can all contribute. Their yields depend on the same combustion conditions that shape the aerosol.[10][11]

Smoke inhalation exposes the respiratory system to this mixture. Effects can include breathing difficulty, respiratory tract pain, confusion and loss of consciousness. Heat and eye irritation can further impair escape, so toxicity influences both survival and the ability to reach safety.[10][12]

Carbon monoxide is often the principal asphyxiant, but atypical materials require other species to be assessed. Smouldering polyurethane foam, for example, can yield significant hydrogen cyanide in the 2005 guidance. A smoke assessment based only on one gas can therefore miss the dominant hazard.[13]

Dose depends on concentration, exposure time and breathing response. Visibility can be poor while toxic dose is small during a short escape, or good while carbon monoxide remains high after particles deposit. Visibility and inhalation hazard must therefore be evaluated separately.[1][14]

Smoke movement and dilution

Smoke movement results from buoyancy, expansion, wind, mechanical systems and pressure differences across openings. Hot smoke rises in a plume, entrains air and can accumulate as an upper layer. Later flow through vents and shafts can expose remote spaces.[15][16]

Entrainment increases the total mass and volume of smoke while reducing concentration and temperature. This dilution does not necessarily make the smoke harmless, because a greater volume can spread toxic products and energy to more locations. Source strength and receiver exposure both matter.[3][17]

Air entrained into a plume increases volume while smoke travels to another room.
Entrainment dilutes smoke while increasing its volume and spread. Based on Van Rossum (2027), §5.3 and Australian Building Codes Board et al. (2005), p. 2.5-2.

Particle and gas transport can differ. Soot can coagulate, evaporate or deposit on surfaces, and additional droplets can form through condensation. Gases may then remain hazardous farther from the fire even when visibility improves.[18][1]

Unburnt pyrolysis gases add a combustion hazard to the mixture. If they mix with oxygen under suitable thermal or ignition conditions, they can ignite as a smoke explosion or backdraft. Smoke is therefore not only a passive consequence of fire; it can become fuel for a later event.[1][19]

Smoke control

Smoke control uses natural ventilation, zone pressurisation, extraction, dilution and containment to limit where smoke travels and accumulates. The 2005 guidelines identify these as the principal smoke-management approaches. Each changes concentration, pressure or the available flow path.[20]

Extraction removes smoke and transported energy but requires a compatible make-up-air path. Excessive extraction at one outlet can cause plug holing, drawing clean air from below and wasting capacity. Outlet arrangement and layer depth therefore affect actual performance.[21]

Pressurisation aims to keep selected spaces clear by maintaining a pressure difference, while containment limits the region available to smoke. Dilution reduces concentration but increases the total air-smoke mixture. These approaches should be evaluated against the fire and building conditions rather than treated as interchangeable.[20]

Smoke control can also change combustion. Additional oxygen can support more complete burning and lower carbon monoxide yield, but it can also increase heat release and spread. The effect on the whole fire must be considered, not only the concentration at one receiver.[22][15]

Measurement and modelling

Smoke production can be described by mass produced per mass of fuel burned, but this conversion factor depends on combustion mode and test scale. Mulholland reports values from 0.001 to 0.17 for flaming combustion and from 0.01 to 0.17 for pyrolysis and smouldering in the reviewed experiments. Reduced ventilation can strongly change these results.[18]

Particle size distribution is another basic property. It influences light extinction, deposition and detectability, and often spans several orders of magnitude. A logarithmic size axis is therefore useful when comparing distributions.[18][23]

Concentration, particle size, optical density and toxic-gas content answer different questions. A measurement of one cannot be substituted for another without a model. Full exposure assessment needs the mixture composition and its time history at the receiver.[2][14]

Zone and field models estimate smoke production and movement from fire, plume and ventilation inputs. Their layer and mixing assumptions are simplifications, and local optical and toxic conditions can differ from model averages. Results should be checked against the intended receiver and time.[24][17]

Generic yield gives OD 4.4 and visibility 0.23/0.57 m; wood gives OD 0.66 and visibility 1.5 m.
Worked smoke example with two soot-yield assumptions. Based on Van Rossum (2027), §6.5.

Limits and common misunderstandings

Smoke is not only soot. Droplets and gases can dominate visibility, toxicity and movement, and water vapour itself is invisible. Treating every visible cloud as only carbon particles excludes much of the actual mixture.[1][2]

Colour does not establish composition or hazard. White smoke can contain irritants and toxic gases, while weakly visible smoke can still contain carbon monoxide. A clear line of sight does not prove that inhalation is safe.[1][11]

Dilution does not eliminate the source. It reduces concentration while increasing total smoke volume, so the same or greater mass of toxic products can reach more people. The receiver's concentration and exposure time must be assessed.[3][14]

Finally, smoke is not only a marker of fire. Unburnt components can ignite, and hot smoke can spread fire by radiation and convection. Its role includes fuel transport and heat transfer as well as toxicity and obscuration.[1][19]

The smoke source should be characterised before its transport is calculated. Fuel, combustion mode, mass-loss rate, soot yield and toxic-species yield provide different parts of that description. Missing one can lead to a smoke volume estimate that cannot support optical or toxic assessment.[18][13]

Plume entrainment then links the source with layer formation. It determines how much air joins the combustion products and therefore affects both concentration and volume. Treating smoke production as fuel mass alone would omit the process that creates most of the moving mixture.[17][3]

A receiver-focused analysis follows the actual sightline and breathing location. Optical density can vary along the route, and particle deposition can change composition before the gas arrives. These local conditions are more useful for exposure assessment than an unqualified building-average smoke density.[7][18]

Test conditions should remain attached to yield and visibility data. Specimen orientation, oxygen concentration, external heat flux and smoke-collection method can all affect results. Comparing values without those conditions can create false agreement or disagreement between materials.[18][8]

For control-system review, nominal airflow should be converted into an actual smoke balance. Make-up air, leakage, plug holing and pressure differences can alter the effective path. The result should be checked at occupied locations and during the period when the system is expected to operate.[21][20]

The possibility of delayed combustion should also remain in the scenario. Unburnt gases can accumulate under oxygen restriction and ignite after mixing later, producing a smoke explosion or backdraft. A model that treats smoke only as an inert transport tracer would miss this hazard.[19][25]

A complete report therefore separates production, entrainment, transport, deposition and reception. It identifies which components are evaluated for visibility, toxicity and heat transfer, and which can burn later. This allows the same smoke mixture to be assessed for several different outcomes without losing its physical meaning.[1][10]

Different smoke metrics should not be collapsed into one index. Mass production describes source strength, optical density describes attenuation, toxic yield describes species production and visibility describes perception. A defensible assessment selects the metrics required for the decision rather than treating smoke as universally heavy or light.[2][9][14]

The smoke's thermal role should also remain in the energy balance. Hot gases and particles emit radiation, while convection transports energy to boundaries and adjacent rooms. These pathways can expose people and fuel even where the visible plume has become diluted.[26][3]

Where source or transport data are weak, alternative scenarios should make the uncertainty visible. Changing yield, ventilation, layer volume and deposition can reverse the predicted visibility or dose. A range of outcomes is more informative than a precise but unsupported smoke condition.[18][27]

The definition used in a report should also be stated. Aerosol-only and broader smoke definitions answer different questions about particles and the complete smoke volume. Declaring that scope prevents soot, gas composition and entrained air from being omitted by accident.[2][1]

See also

Notes

  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 Van Rossum (2027), §6.1.
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 Mulholland (2002), p. 2-258.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 Van Rossum (2027), §5.3.
  4. ↑ Drysdale (2002), p. 1-95.
  5. ↑ Ohlemiller (2002), p. 2-200.
  6. ↑ 6.0 6.1 6.2 6.3 Van Rossum (2027), §6.2.
  7. ↑ 7.0 7.1 7.2 7.3 Van Rossum (2027), §6.5.
  8. ↑ 8.0 8.1 8.2 8.3 Jin (2002), p. 2-43.
  9. ↑ 9.0 9.1 Van Rossum (2027), §6.4.
  10. ↑ 10.0 10.1 10.2 Purser (2002), pp. 2-85–2-86.
  11. ↑ 11.0 11.1 Gottuk and Lattimer (2002), p. 2-54.
  12. ↑ Purser (2002), p. 2-86.
  13. ↑ 13.0 13.1 Australian Building Codes Board et al. (2005), p. 2.4-11.
  14. ↑ 14.0 14.1 14.2 14.3 Purser (2002), p. 2-88.
  15. ↑ 15.0 15.1 Van Rossum (2027), §6.6.
  16. ↑ Australian Building Codes Board et al. (2005), p. 2.5-6.
  17. ↑ 17.0 17.1 17.2 Australian Building Codes Board et al. (2005), p. 2.5-2.
  18. ↑ 18.0 18.1 18.2 18.3 18.4 18.5 18.6 Mulholland (2002), p. 2-259.
  19. ↑ 19.0 19.1 19.2 Van Rossum (2027), §4.10.
  20. ↑ 20.0 20.1 20.2 Australian Building Codes Board et al. (2005), p. 2.5-7.
  21. ↑ 21.0 21.1 Van Rossum (2027), §6.8.
  22. ↑ Gottuk and Lattimer (2002), p. 2-55.
  23. ↑ Mulholland (2002), p. 2-260.
  24. ↑ Van Rossum (2027), §9.2.
  25. ↑ Van Rossum (2027), §4.9.
  26. ↑ Tien, Lee, and Stretton (2002), p. 1-73.
  27. ↑ Van Rossum (2027), §1.7.

References

  • Australian Building Codes Board, National Research Council of Canada, International Code Council, & Department of Building and Housing New Zealand. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
  • Drysdale, D. D. (2002). Thermochemistry. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
  • Gottuk, D. T., & Lattimer, B. Y. (2002). Effect of combustion conditions on species production. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
  • Jin, T. (2002). Visibility and human behavior in fire smoke. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
  • Mulholland, G. W. (2002). Smoke production and properties. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
  • Ohlemiller, T. J. (2002). Smoldering combustion. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
  • Purser, D. A. (2002). Toxicity assessment of combustion products. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
  • Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.
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