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Contents
  1. 1Toxic products of fires
  2. 2How toxic gases affect people
  3. 3Dose, concentration and time
  4. 4Influence of the type of fire
  5. 5Assessment in fire safety engineering
  6. 6Limits and uncertainty
  7. 7See also
  8. 8Notes
  9. 9References

Fire toxicity is the harmful effect on people of the gases, vapours and particles produced by a fire. Smoke is toxic to people, in addition to blocking the view and carrying heat.[1] In fires, toxic products cause three main effects: irritant gases that impair escape, asphyxiant gases that cause confusion and loss of consciousness, and lung irritants that can kill after the fire through lung inflammation.[2] The two major asphyxiant gases are carbon monoxide and hydrogen cyanide, while low oxygen and very high carbon dioxide concentrations also have asphyxiant effects.[3]

Toxic smoke is a leading cause of death in building fires. In the United Kingdom, about half of all fatal and a third of all nonfatal casualties of dwelling fires were reported as "overcome by smoke and toxic gases" rather than by heat and burns.[4] Over half of all fire fatalities have been attributed to inhaling carbon monoxide.[5] In fire safety engineering, toxicity is assessed by estimating the time at which occupants would receive an incapacitating exposure, and comparing it with the time they need to escape.[6] Toxicity is one of several tenability criteria, together with heat, radiation and visibility in smoke.[7]

Toxic products of fires

Studies of large and small fires and animal exposures have led to two basic findings. First, the smoke of even a single material contains a large number of potentially toxic products, and its composition varies strongly with temperature and oxygen supply.[8] Second, despite this complexity, the toxic effect of each smoke atmosphere was dominated either by an asphyxiant gas, carbon monoxide or hydrogen cyanide, or by irritants.[8] The effects of the individual gases were found to add up approximately, which allows toxicity to be predicted from a fairly simple chemical analysis.[8]

Carbon monoxide is always present to some extent in all fires, whatever the materials or the stage of the fire.[3] Hydrogen cyanide is always present to some extent when nitrogen-containing materials burn. These include acrylics, polyurethane foams, melamine, nylon and wool, which are found in most buildings.[3] Its concentrations are likely to be high in large post-flashover fires.[3] In fires, hydrogen cyanide is about 20 to 40 times more potent than carbon monoxide.[2]

More than twenty irritant compounds contribute to the irritancy of fire smoke. The most important are acid gases, such as hydrogen fluoride, hydrogen chloride, hydrogen bromide, nitrogen oxides and sulphur dioxide, and organic irritants such as acrolein and formaldehyde.[2] Irritant gases are often acidic compounds of the halogens chlorine, fluorine and bromine, and they hamper sight and breathing during escape.[7] The acute lethal potencies of individual fire products range over more than five orders of magnitude. Hydrogen cyanide and nitrogen dioxide are lethal at around 0.1 to 1 g/m³, carbon monoxide and hydrogen chloride at 1 to 10 g/m³.[9]

Burning lithium-ion batteries are a recent example of the influence of materials. A Dutch study of fires in indoor car parks found hardly any difference in the general toxic gases from burning electric and conventional cars. Electric vehicles did produce about 1.8 times as much hydrogen fluoride.[10] A battery pack of 100 kWh can emit 2 to 20 kg of hydrogen fluoride, depending on its state of charge.[10] The study concluded that the effect of this extra hydrogen fluoride on escape safety is nil, because smoke from any burning vehicle is already a harmful mixture.[11]

How toxic gases affect people

Exposure to toxic smoke and heat can impair a person in four ways. Smoke and irritants can impair vision, irritants can cause pain and breathing difficulties, toxic gases can cause asphyxia, and heat can cause pain and burns.[8] Up to a certain level these effects reduce the speed and efficiency of escape. At higher levels they can prevent escape altogether.[8] Exposure to dense, irritant smoke tends to be the first hazard that fire victims meet.[8]

Outline of a person with three labelled targets: eyes and upper airways, lungs, and brain via the blood.
The three main toxic effects of fire smoke: sensory irritation of eyes and airways, asphyxia through the blood and brain, and lung inflammation after the fire. Based on Purser (2002), pp. 2-87–2-88 and 2-91.

Asphyxiant gases act mainly on the brain by causing a lack of oxygen in the tissue. The body can compensate up to a certain dose, so a victim is often unaware of the danger.[3] Once this point is passed, the decline is rapid: signs similar to severe alcohol intoxication are followed by unconsciousness and death if exposure continues.[3] Carbon monoxide binds to the haemoglobin in the blood to form carboxyhaemoglobin, which reduces the oxygen supplied to the tissues.[3] Loss of consciousness is predicted at about 40 per cent carboxyhaemoglobin, but it can occur at about 30 per cent. Death is predicted at 50 to 70 per cent.[3][12]

The other main asphyxiants follow different patterns. While carbon monoxide poisoning sets in slowly and insidiously, hydrogen cyanide poisoning tends to be rapid and dramatic.[13] Oxygen concentrations below 15 per cent and carbon dioxide concentrations above 5 per cent are themselves asphyxiant.[3] Carbon dioxide matters mainly because it increases the rate at which carbon monoxide and hydrogen cyanide are taken up.[2] The effects of oxygen depletion and high carbon dioxide depend on the concentration once the body has adjusted, which takes a few minutes.[14]

Studies of fire victims confirm the central role of carbon monoxide. In a pathology study in Strathclyde, Scotland, lethal carboxyhaemoglobin levels above 50 per cent were found in 54 per cent of all fatalities. About 69 per cent had levels above 30 per cent, which can cause incapacitation.[3] Activity makes matters worse. Primates engaged in light activity were seriously affected at 25 to 35 per cent carboxyhaemoglobin, while resting animals were often unaffected at up to 40 per cent.[12]

Dose, concentration and time

The basic measures of an inhaled toxic exposure are the concentration of the toxic gas and the duration of the exposure. Their product, the Ct product, estimates the dose.[15] Toxicologists often use Haber's rule, which states that a given toxic effect occurs at a constant dose. The product of concentration and time is then a constant for that effect:[15]

W=C⋅t

Symbol Description Unit
W exposure dose that causes a given effect, for example incapacitation ppm·min
C concentration of the toxic gas in the inhaled air ppm
t duration of exposure min

The rule assumes that the toxic substance is taken up at a constant rate.[15] Carbon monoxide is taken up and also removed through the lungs, so its uptake slows down over long exposures. For short exposures at high concentrations, as in flaming fires, the uptake is nearly linear and the rule applies without serious error.[15][16] In experiments with active monkeys exposed to 1000 to 8000 ppm of carbon monoxide, unconsciousness occurred after a nearly constant dose of about 27,000 ppm·min.[15]

As a worked example, the time to incapacitation of these animals at a constant 4000 ppm of carbon monoxide follows from the rule. The dose is taken as the approximate constant of 27,000 ppm·min found in the experiments.[15] Dividing this dose by the concentration gives the time to incapacitation:

t=WC=270004000=6.75 min

The measured dose at this concentration was 26,868 ppm·min, which gives 6.7 minutes.[14] At 1000 ppm the same rule predicts 27 minutes, against 26.6 minutes from the measured dose of 26,600 ppm·min.[14] The predicted time is short because the concentration is high. Levels of several per cent carbon monoxide have been measured in full-scale room fires, and 4000 ppm can be fatal in less than an hour.[5]

Two charts of time to incapacitation against gas concentration: a smooth hyperbola for carbon monoxide and a steep drop for hydrogen cyanide.
Time to incapacitation of active monkeys against concentration: carbon monoxide follows a constant dose of about 27,000 ppm·min (left), hydrogen cyanide does not (right). Based on Purser (2002), pp. 2-87–2-88 and 2-104–2-105.

The rule does not hold for all effects. Irritation of the eyes and upper airways occurs as soon as exposure starts, and its severity depends on the concentration rather than the dose.[15] Hydrogen cyanide departs from the rule as well. At 180 ppm, monkeys were incapacitated after 2 minutes, a dose of 400 ppm·min, but at 100 ppm it took about 20 minutes and a dose of 2000 ppm·min.[14] Short exposures to high concentrations are therefore more dangerous than the dose alone suggests.[14]

To deal with concentrations that change during a fire, the dose received in each short period is divided by the dose that causes the effect. These fractions are added until the sum reaches 1, when the effect is predicted. This is the fractional effective dose.[14] For irritants, a fractional irritant concentration is used instead. It compares the current concentration with the concentration that impairs escape.[17] Escape impairment is predicted at a fractional irritant concentration of 1 and incapacitation at about 3 to 5.[2]

Influence of the type of fire

The yields of toxic products depend strongly on how a material burns. In general, materials give high yields under smouldering and under oxygen-starved flaming conditions, and lower yields under well-ventilated flaming.[2] At temperatures of 400 to 700 °C, typical of smouldering fires or the area near an early flaming fire, the widest range of toxic products is formed. Carbon monoxide yields are then high, with ratios of carbon dioxide to carbon monoxide approaching 1.[18]

Once flaming starts, hot and well-oxygenated flames burn most of these products to carbon dioxide and water. The ratio of carbon dioxide to carbon monoxide can initially reach 500 to 1000.[18] As carbon dioxide approaches 5 per cent and oxygen falls towards 15 per cent, combustion becomes less efficient and the ratio drops to 50 to 100.[18] In hot post-flashover fires above 800 °C with little oxygen, the products break down into small molecules. The smoke can then contain high concentrations of carbon monoxide and hydrogen cyanide.[18]

The SFPE Handbook summarises the toxic hazards of four types of fire, based on large-scale fire simulation tests.[18] The concentrations for post-flashover fires depend on the position relative to the fire compartment.[18] The escape time available ranges from ample, if occupants are alerted to a smouldering fire, to very restricted or none near a post-flashover fire.[18]

Range bars of carbon monoxide concentration for four types of fire, from 0.15 % in smouldering fires up to 4 % in oxygen-starved flaming fires.
Ranges of carbon monoxide concentration and typical time to incapacitation for four types of fire in large-scale tests. Based on Purser (2002), p. 2-123.
Type of fire Main toxic hazards Time to incapacitation
Smouldering, victim in room of origin or remote carbon monoxide 0 to 1500 ppm, oxygen 15 to 21 %, irritants, smoke hours
Flaming, victim in room of origin carbon monoxide 0 to 1 %, carbon dioxide 0 to 10 %, oxygen 10 to 21 %, irritants, heat, smoke a few minutes
Small oxygen-starved flaming fire, victim in room of origin or remote carbon monoxide 0.2 to 4 %, carbon dioxide 1 to 10 %, oxygen below 12 %, irritants, heat, smoke a few minutes
Post-flashover, victim remote carbon monoxide 0 to 3 %, hydrogen cyanide 0 to 500 ppm, some irritants, smoke and possibly heat less than 1 minute near the fire, elsewhere depending on dilution

The location of victims also varies. Nearly 75 per cent of deaths due to smoke inhalation occur in places remote from the room of fire origin.[5] In the United States, more than half of all fire deaths are reported to occur remote from the room of origin, mostly in post-flashover fires that have spread beyond it.[19] In the United Kingdom, by contrast, 65 per cent of deaths occur in the room of origin, in fires that have not spread beyond it.[19] These differences reflect differences in culture and building styles.[19]

Assessment in fire safety engineering

A toxic hazard assessment first describes the growth of the fire, the production of toxic products and their spread through the building. It then derives curves of concentration against time for the main toxic gases, smoke and temperature.[20] From these curves, the time at which occupants would be impaired, incapacitated or killed is calculated with fractional dose and concentration models.[20]

Flowchart from fire growth and toxic yields to gas concentrations over time, then FED and FIC, then time to incapacitation compared with escape time.
Steps of a toxic hazard assessment: from fire growth and product yields to the time to incapacitation, compared with the time needed to escape. Based on Purser (2002), pp. 2-85 and 2-95.

In practice, many engineering methods focus on carbon monoxide. The International Fire Engineering Guidelines (2005) name carbon monoxide as the principal toxic species in most fires. They state that the analysis can generally be restricted to it unless atypical materials are involved, such as smouldering polyurethane foam, which can yield significant hydrogen cyanide.[21] In New Zealand, the Verification Method C/VM2 (second edition, 2025) measures tenability at 2.0 m above the floor. It uses the fractional effective dose of carbon monoxide, the fractional effective dose of thermal effects, and visibility.[22] The dose for carbon monoxide follows ISO 13571 and includes the effects of carbon monoxide, carbon dioxide and oxygen.[22]

The commentary on the first edition of C/VM2 (2013) explains these choices. Other gases were left out because of a lack of confidence in the available data, which depend strongly on the test method and ventilation.[23] Research has also shown that carbon monoxide reaches toxic levels before other species and is present in all fires.[23] The commentary requires an escaping occupant to receive a fractional effective dose of carbon monoxide of no more than 0.3. It uses a reference dose of 35,000 ppm·min, increased for the faster breathing caused by carbon dioxide above 2 per cent.[24]

In Australia, the Fire Safety Verification Method Standard (2022) uses a fractional effective dose of thermal effects of 0.3 and a visibility limit of 10 m. It sets no separate limit for toxic gases.[25] Both the Australian and New Zealand documents note that visibility is generally the first tenability criterion to be exceeded.[25][22] The Australian handbook adds that carbon monoxide and hydrogen cyanide production is sensitive to the materials and the combustion regime.[26]

The book Introduction to Fire Safety Engineering gives the reasoning behind this emphasis on visibility. Predicting the toxicity of gases is far more complex than predicting visibility, and visibility is more often the governing criterion, although not always.[1] When escape routes pass only briefly through smoke, for less than 30 to 60 seconds, toxicity is usually not decisive.[7] It becomes important when much smoke is inhaled, for example on a long route through smoke or when poor visibility slows people down.[7] Far from the fire, carbon monoxide concentrations can be very high while visibility exceeds 20 m, because soot settles out sooner than the gases.[1]

Limits and uncertainty

Toxicity limits for design cannot be checked directly. Experiments exposing people to complex and dangerous smoke mixtures cannot be performed for ethical reasons.[27] The methods rest partly on human data and partly on animal studies, and they describe healthy young adults.[27] About 15 per cent of children and 5 per cent of adults have asthma, and the elderly are particularly sensitive to asphyxiant gases.[27]

Some people have died at carboxyhaemoglobin levels below 30 per cent. In people with angina, as little as 2 per cent reduces the time to the onset of chest pain during exercise.[27] Safe exposure levels for the whole population must therefore be well below those derived from healthy animals.[27]

The input data are also uncertain. Yields of toxic products vary with temperature, oxygen supply and the efficiency of combustion, so data from one test may not represent a real fire.[18] The results of small-scale toxicity tests on single materials cannot be assumed to apply to the same material in a large fire.[17] Physical activity, body size and breathing rate change the uptake of carbon monoxide considerably. Children take up carbon monoxide much faster than adults.[16]

See also

Notes

  1. ↑ 1.0 1.1 1.2 Van Rossum (2027), §6.1.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 Purser (2002), p. 2-91.
  3. ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 Purser (2002), p. 2-100.
  4. ↑ Purser (2002), p. 2-83.
  5. ↑ 5.0 5.1 5.2 Gottuk and Lattimer (2002), p. 2-54.
  6. ↑ Purser (2002), p. 2-85.
  7. ↑ 7.0 7.1 7.2 7.3 Van Rossum (2027), §7.7.
  8. ↑ 8.0 8.1 8.2 8.3 8.4 8.5 Purser (2002), p. 2-86.
  9. ↑ Purser (2002), p. 2-93.
  10. ↑ 10.0 10.1 Van der Graaf et al. (2021/2023), p. 26.
  11. ↑ Van der Graaf et al. (2021/2023), p. 30.
  12. ↑ 12.0 12.1 Purser (2002), p. 2-101.
  13. ↑ Purser (2002), p. 2-103.
  14. ↑ 14.0 14.1 14.2 14.3 14.4 14.5 Purser (2002), p. 2-88.
  15. ↑ 15.0 15.1 15.2 15.3 15.4 15.5 15.6 Purser (2002), p. 2-87.
  16. ↑ 16.0 16.1 Purser (2002), p. 2-102.
  17. ↑ 17.0 17.1 Purser (2002), p. 2-89.
  18. ↑ 18.0 18.1 18.2 18.3 18.4 18.5 18.6 18.7 Purser (2002), p. 2-123.
  19. ↑ 19.0 19.1 19.2 Purser (2002), p. 2-84.
  20. ↑ 20.0 20.1 Purser (2002), p. 2-95.
  21. ↑ National Research Council of Canada et al. (2005), p. 2.4-12.
  22. ↑ 22.0 22.1 22.2 Ministry of Business, Innovation and Employment (2025b), p. 17.
  23. ↑ 23.0 23.1 Ministry of Business, Innovation and Employment (2013), p. 43.
  24. ↑ Ministry of Business, Innovation and Employment (2013), p. 45.
  25. ↑ 25.0 25.1 Australian Building Codes Board (2022), p. 10.
  26. ↑ Australian Building Codes Board (2020b), p. 111.
  27. ↑ 27.0 27.1 27.2 27.3 27.4 Purser (2002), p. 2-90.

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.
  • Gottuk, D. T., & Lattimer, B. Y. (2002). Effect of combustion conditions on species production. 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. 2-54–2-82). 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.).
  • National Research Council of Canada, International Code Council, Department of Building and Housing, & Australian Building Codes Board. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
  • Purser, D. A. (2002). Toxicity assessment of combustion products. 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. 2-83–2-171). National Fire Protection Association; Society of Fire Protection Engineers.
  • Van der Graaf, J., Hessels, T., & Rosmuller, N. (2023). Fire safety of indoor car parks accommodating electrically powered vehicles. Nederlands Instituut Publieke Veiligheid. (Original work published 2021)
  • Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.
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