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Visibility in smoke

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Contents
  1. 1Why visibility matters
  2. 2How smoke reduces visibility
  3. 3Measuring smoke density
  4. 4Estimating visibility
  5. 5Effects on people and escape
  6. 6Use in design and regulation
  7. 7Limits and common misconceptions
  8. 8See also
  9. 9Notes
  10. 10References

Visibility in smoke is the greatest distance at which a person can still see and recognise an object, such as an exit sign, a door or a wall, through the smoke of a fire.[1][2] It is also called the visibility distance. Smoke reduces visibility because its particles absorb and scatter light, and irritant gases in the smoke make the eyes water.[1] Poor visibility does not kill directly, but it can delay or prevent escape and so expose people to heat and toxic gases.[3]

In fire safety engineering, visibility is one of the main tenability criteria used to judge whether people can still escape. Australian and New Zealand verification methods note that it is generally the first criterion to be exceeded in design calculations.[4][5] Engineers estimate it from the amount of soot in the smoke with a simple relation found in experiments by Jin. For a light-emitting sign, the visibility in metres is about 8 divided by the extinction coefficient of the smoke; for a light-reflecting sign, it is about 3 divided by that coefficient.[6] Typical design limits lie between 5 and 30 m, depending on the country, the size of the space and how well the occupants know the building.[4][7]

Why visibility matters

Smoke is a mixture of solid particles, liquid droplets and gases. Black smoke usually contains much soot, while white smoke often consists of fine droplets.[8] Close to the fire the smoke is optically dense and hot; as it rises, it mixes with air and becomes thinner and cooler.[8] Among the harmful properties of smoke, Introduction to Fire Safety Engineering lists its toxicity and the fact that it blocks the view.[8]

Experience in Japan showed why the loss of view matters. From the 1960s, a growing number of people died from smoke in fire-resistant buildings. Toxic gases or low oxygen were the final cause of death, but many people were trapped early in the fire by relatively thin smoke. Loss of visibility was therefore an indirect but fatal cause.[9] NUREG-1805, the Fire Dynamics Tools report of the U.S. Nuclear Regulatory Commission, adds that a little smoke makes people walk faster, while more smoke slows them down. Smoke can also act as a psychological barrier, so that people avoid a smoke-filled route and may become trapped.[3]

Near the fire, visibility is usually the deciding factor for safe escape. Introduction to Fire Safety Engineering notes that visibility near a fire can fall below 1 m within tens of seconds. As long as visibility is more than 5 to 10 m, the dose of toxic gases in that smoke is often of secondary importance.[10] Further from the fire this can be different: carbon monoxide concentrations can be very high while visibility is still more than 20 m. Soot particles settle and spread less far through a building than the gases do.[10]

How smoke reduces visibility

Smoke reduces visibility in two ways. Particles between the object and the eye absorb and scatter the light coming from a sign. At the same time, light from the general lighting of a corridor or room is scattered by the smoke towards the eye, which reduces the contrast between the sign and its background.[9][11] An object can only be seen when its contrast with the background exceeds a threshold. Under daylight conditions, a contrast of 0.02 is often used for a black object seen against a white background.[6] For this reason, more light does not always help: light that falls on the smoke is scattered and lowers the contrast.[12]

Side view of a smoky corridor: light from an exit sign weakens on its way to an eye, while ceiling light scattered by smoke also reaches the eye.
Smoke weakens the light from an exit sign and scatters light from the room lighting towards the observer, which lowers the contrast of the sign. Based on Jin (2002), pp. 2-42–2-43.

Visibility therefore depends on many factors. These include the absorption and scattering by the smoke, the lighting of the room, whether a sign emits or reflects light, the wavelength of the light, and the visual acuity of the observer.[6] NUREG-1805 adds the size and colour of the particles and whether a sign is lit from behind or from the front. It also notes that the same observer, under the same conditions, can give results that differ by 25 to 30 per cent at different times.[3] Visibility is thus an indication of the conditions rather than a precise distance.[12]

Irritant smoke makes things worse. In tests in a 20 m long corridor, subjects walked through irritant white smoke from wood cribs and through less irritant black smoke from kerosene.[13] In thick irritant smoke they could keep their eyes open only for a short time, and their tears prevented them from reading the signs.[13] Visual acuity fell sharply once the extinction coefficient exceeded about 0.25 m−1.[14][6] NUREG-1805 also notes that visibility is generally much better at floor level, but that operating sprinklers tend to bring the smoke closer to the floor.[1]

Measuring smoke density

The amount of light that smoke removes is described by its extinction coefficient. Bouguer's law states that the intensity of light falls exponentially with the length of the light path through smoke, and the extinction coefficient sets how fast it falls.[15] It is measured as the natural logarithm of the ratio of the incident to the transmitted light intensity, divided by the path length, and has the unit m−1.[11] When base-10 logarithms are used instead, the result is the optical density per metre, which equals the extinction coefficient divided by 2.3.[15] Because both quantities have the unit m−1, it is important to know which of the two a source or a computer program uses.

The extinction coefficient is the product of the mass concentration of smoke particles and a specific extinction coefficient, which describes how much light one gram of smoke removes.[15] Seader and Einhorn measured about 7.6 m²/g for smoke from flaming combustion of wood and plastics, and about 4.4 m²/g for smoke from pyrolysis.[15] NUREG-1805 gives the same values in imperial units, 37,000 ft²/lb for flaming and 21,000 ft²/lb for smouldering combustion.[16] Introduction to Fire Safety Engineering uses a generic value of 3300 m²/kg for smoke from burning building and furnishing materials, together with base-10 optical density.[17] This value is close to 7.6 m²/g divided by 2.3, the base-10 form of Seader and Einhorn's value (calculated).

The mass of smoke particles depends on the fuel and on how well it burns. The soot yield is the fraction of the burnt fuel that enters the smoke as soot.[18] Plastics produce relatively much soot, wood much less, and ethanol so little that its smoke is hard to see.[18] Typical values for well-ventilated burning are 0.015 kg/kg for wood, 0.14 for PVC and 0.23 for polyurethane, and a mixed value of 0.1 kg/kg is often assumed for the contents of a room.[18] To find the concentration, the energy released by the fire is divided by the heat of combustion to give the mass of fuel burnt. This mass is multiplied by the soot yield and divided by the volume of the smoke.[19]

Five boxes joined by arrows: energy released, fuel burnt, soot mass, soot concentration, extinction coefficient, visibility.
Hand calculation of visibility: from the energy released by the fire to the soot concentration, the extinction coefficient and the visibility. Based on Van Rossum (2027), §6.3–§6.5, and Mulholland (2002), pp. 2-263 and 2-265.

Estimating visibility

Jin found that, for visibilities of about 5 to 15 m, the product of the visibility of a self-illuminated sign and the extinction coefficient of the smoke is almost constant.[11] The SFPE Handbook chapter on smoke production fits his data with two constants, one for each type of sign. This leads to a simple relation for the visibility of a sign:[6] S=CK

Symbol Description Unit
S visibility of a sign or object m
C visibility constant: 8 for a light-emitting sign, 3 for a light-reflecting sign –
K extinction coefficient of the smoke (natural logarithm) m−1

Light-emitting signs, such as illuminated exit signs, are two to four times more visible than light-reflecting ones.[6] NUREG-1805 uses the same constants, with 3 also for building components such as walls and doors seen in reflected light.[16] Jin's own chapter gives ranges, 5 to 10 for light-emitting and 2 to 4 for reflecting signs.[11] The relation is based on subjects who looked at signs through glass, so the irritant effect of the smoke on the eyes was excluded.[6] In irritant smoke, Jin proposed a reduced visibility above an extinction coefficient of about 0.25 m−1.[14]

Falling lines on log scales: visibility of light-emitting signs is 8 divided by K, of reflecting signs 3 divided by K, with 10 m and 5 m limits.
Visibility of light-emitting and light-reflecting signs against the extinction coefficient of the smoke, with the 10 m and 5 m design limits. Based on Mulholland (2002), p. 2-265, and Australian Building Codes Board (2022), p. 10.

The SFPE Handbook gives a worked example for a 200 g polyurethane foam pillow burning with flames in a room of 6 m by 6 m and 2.5 m high.[6] With a smoke yield of 0.03, the pillow produces 0.03 × 200 = 6 g of smoke. Spread evenly over the room volume of 90 m³, this gives a concentration of 0.067 g/m³.[6] With a specific extinction coefficient of 7.6 m²/g, the extinction coefficient is 7.6 × 0.067 = 0.51 m−1. A light-reflecting exit sign is then visible up to 3 / 0.51 = 5.9 m.[20]

The handbook also shows an alternative calculation. It uses a mass optical density of 0.22 m²/g, measured for polyurethane foam, which gives a base-10 optical density of 0.49 m−1.[20] Converted to an extinction coefficient, this gives an extinction coefficient of 1.12 m−1 and a visibility of only 2.7 m, which the handbook considers the more reliable result.[20] The example illustrates how strongly the answer depends on the smoke data chosen.

Introduction to Fire Safety Engineering uses a similar rule of thumb with base-10 optical density. Visibility is 1 divided by the optical density for objects that do not emit light, and 2.5 divided by it for luminous objects.[12] In the book's example, a constant 1 MW fire in a shop of 20 m by 15 m and 4 m high burns 6 kg of fuel in 120 seconds. With a soot yield of 0.1, this releases 0.6 kg of soot into a smoke layer of 450 m³, giving an optical density of 4.4 m−1.[12]

Even an illuminated exit sign is then visible at only about 0.57 m, so escape through the layer is not realistic. With wood as the fuel, the optical density would be 0.66 m−1 and the visibility about 1.5 m.[12] Both formulas assume smoke that is spread evenly; where the density varies, the visibility depends on the direction of view and must be found by integrating along the line of sight.[12]

Effects on people and escape

Smoke affects behaviour before it blocks the view completely. In Jin's tests in a smoke-filled room, most members of the public began to show emotional effects at an extinction coefficient of about 0.1 m−1. Most researchers who knew the room well showed them only at 0.35 to 0.55 m−1.[21]

Jin took 0.5 m−1 as the limit at which escape becomes difficult even for people who know the escape route. For people unfamiliar with the building, he proposed 0.15 m−1. The corresponding visibilities are 4 m and 13 m.[21] Other researchers proposed values that vary widely, probably because of differences in the spaces and in the groups of people.[21]

Smoke also reduces walking speed. In Jin's corridor tests, walking speed fell gradually as non-irritant smoke became denser, but fell very rapidly in irritant smoke over the same range of densities. Subjects who could not keep their eyes open walked in a zigzag or step by step along the wall.[13] According to NUREG-1805, walking speeds in low-density irritant smoke were about the same as in non-irritant smoke.[16]

The design of escape signage follows from these results. The visibility of ordinary exit signs falls quickly from more than 10 m to 5 m as the extinction coefficient rises to about 0.4 m−1.[22] Changing the colour of a sign changes its visibility by a few tens of per cent at most. Doubling the visibility requires a much brighter sign.[23] In a smoke-filled passage, a row of flashing green lights on the floor, spaced 0.5 m apart, still guided people well at extinction coefficients up to 1.0 m−1.[24][22]

Use in design and regulation

Visibility is used as an acceptance criterion when the available escape time is compared with the required escape time.[4][5] The visibility is checked at a fixed height above the floor, which is 2 m in Australia and 2.0 m in New Zealand.[4][5] Limits differ between countries and documents:

Country and document Visibility limit Conditions
Australia, Fire Safety Verification Method Standard (2022) 10 m; 5 m in rooms under 100 m² or within 5 m of an exit measured 2 m above the floor
New Zealand, Building Code clause C4.3 (as reproduced in the C/VM2 commentary, 2013) 10 m; 5 m in rooms under 100 m² measured 2.0 m above the floor in C/VM2
Netherlands, explanation to the Bbl (non-enclosed spaces) 30 m for luminous objects smoke mixed over the height
United Kingdom, BS 9999 (as quoted in Van Rossum, 2027) not below 8 to 10 m –

In Australia, the Fire Safety Verification Method Standard (2022) treats visibility below 10 m as untenable at 2 m above the floor. In rooms under 100 m², or where the exit is within 5 m, visibility may fall to 5 m.[4] The accompanying handbook says that visibility should be calculated assuming back-lit exit signs, unless the design brief decides otherwise.[25] In New Zealand, Verification Method C/VM2 (second edition, 2025) uses visibility at 2.0 m above the floor as one of three tenability criteria, with the limits set in Building Code clause C4.3.[5] The commentary on the first edition reproduces these limits as 10 m, or 5 m in rooms under 100 m².[26] It adds that the British document PD 7974-6 also recommends a visibility of 10 m.[27]

In the Netherlands, Introduction to Fire Safety Engineering cites the explanation to the Dutch building decree (Bbl) for non-enclosed spaces. When the smoke is mixed over the height of the space, luminous objects must remain visible at 30 m or more.[7] The book calls this strict compared with the 8 to 10 m of the British standard BS 9999. It recommends setting the limit according to the space: in corridors 5 m is often accepted, while 30 m is common in large spaces but not needed in small ones.[7]

In practice, visibility often decides the available escape time. The C/VM2 commentary explains that, with its specified smoke yields, the visibility criterion is exceeded in most cases as soon as the smoke layer reaches 2.0 m.[28] In a worked hospital example in the commentary, visibility at 2.0 m fell below 10 m within 23 seconds in the patient room and after 86 seconds in the corridor.[29] With the visibility relation, a limit of 10 m corresponds to an extinction coefficient of 0.8 m−1 for a light-emitting sign. For a reflecting sign it corresponds to 0.3 m−1 (calculated with the constants 8 and 3).[6]

Limits and common misconceptions

The visibility relation is an approximation. The SFPE Handbook warns that its example assumes smoke that is well mixed through the room, while in reality the concentration is higher near the ceiling. The smoke yield is uncertain, the specific extinction coefficient comes from a few small-scale tests, and the validity range of the relation has not been widely studied.[20] Introduction to Fire Safety Engineering also calls its rule of thumb fairly inaccurate.[12]

A common misconception is that a visibility limit only concerns comfort. Smoke that stops people from seeing the exit also slows them down and lengthens their stay in the smoke, which increases the dose of toxic gases they inhale.[7] A second misconception is that good visibility means safe conditions. Far from the fire, the soot may have settled while the toxic gases are still present.[10]

A third pitfall is mixing the two definitions of smoke density. A base-10 optical density is 2.3 times smaller than the natural-log extinction coefficient of the same smoke.[15] Using an optical density in a relation written for the extinction coefficient therefore overestimates visibility by the same factor. The book's rules of thumb, applied to base-10 optical density, give smaller visibilities than Jin's constants for the same smoke (calculated).[12][6]

See also

Notes

  1. ↑ 1.0 1.1 1.2 Iqbal and Salley (2004), p. 18-1.
  2. ↑ Ministry of Business, Innovation and Employment (2025b), p. 70.
  3. ↑ 3.0 3.1 3.2 Iqbal and Salley (2004), p. 18-2.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 Australian Building Codes Board (2022), p. 10.
  5. ↑ 5.0 5.1 5.2 5.3 Ministry of Business, Innovation and Employment (2025b), p. 17.
  6. ↑ 6.00 6.01 6.02 6.03 6.04 6.05 6.06 6.07 6.08 6.09 6.10 Mulholland (2002), p. 2-265.
  7. ↑ 7.0 7.1 7.2 7.3 Van Rossum (2027), §7.7.
  8. ↑ 8.0 8.1 8.2 Van Rossum (2027), §6.1.
  9. ↑ 9.0 9.1 Jin (2002), p. 2-42.
  10. ↑ 10.0 10.1 10.2 Brandweeracademie (2020), as cited in Van Rossum (2027), §6.1.
  11. ↑ 11.0 11.1 11.2 11.3 Jin (2002), p. 2-43.
  12. ↑ 12.0 12.1 12.2 12.3 12.4 12.5 12.6 12.7 Van Rossum (2027), §6.5.
  13. ↑ 13.0 13.1 13.2 Jin (2002), p. 2-44.
  14. ↑ 14.0 14.1 Jin (2002), p. 2-45.
  15. ↑ 15.0 15.1 15.2 15.3 15.4 Mulholland (2002), p. 2-263.
  16. ↑ 16.0 16.1 16.2 Iqbal and Salley (2004), p. 18-3.
  17. ↑ Van Rossum (2027), §6.4.
  18. ↑ 18.0 18.1 18.2 Van Rossum (2027), §6.2.
  19. ↑ Van Rossum (2027), §6.3.
  20. ↑ 20.0 20.1 20.2 20.3 Mulholland (2002), p. 2-266.
  21. ↑ 21.0 21.1 21.2 Jin (2002), p. 2-47.
  22. ↑ 22.0 22.1 Jin (2002), p. 2-52.
  23. ↑ Jin (2002), p. 2-46.
  24. ↑ Jin (2002), p. 2-51.
  25. ↑ Australian Building Codes Board (2020b), p. 110.
  26. ↑ Ministry of Business, Innovation and Employment (2013), p. 28.
  27. ↑ Ministry of Business, Innovation and Employment (2013), p. 31.
  28. ↑ Ministry of Business, Innovation and Employment (2013), p. 43.
  29. ↑ Ministry of Business, Innovation and Employment (2013), p. 98.

References

  • Australian Building Codes Board. (2020b). Handbook: Fire Safety Verification Method (Print version 1.1).
  • Australian Building Codes Board. (2022). Fire Safety Verification Method Standard 2022.
  • Iqbal, N., & Salley, M. H. (2004). Fire Dynamics Tools (FDTs): Quantitative fire hazard analysis methods for the U.S. Nuclear Regulatory Commission Fire Protection Inspection Program (NUREG-1805). U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation.
  • Jin, T. (2002). Visibility and human behavior in fire smoke. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 2-42–2-53). National Fire Protection Association; Society of Fire Protection Engineers.
  • Ministry of Business, Innovation and Employment. (2013). Commentary for Building Code clauses C1–C6 and Verification Method C/VM2.
  • Ministry of Business, Innovation and Employment. (2025b). Verification Method C/VM2: Framework for fire safety design (2nd ed.).
  • Mulholland, G. W. (2002). Smoke production and properties. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 2-258–2-268). National Fire Protection Association; Society of Fire Protection Engineers.
  • Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.
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