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Contents
  1. 1Sources in fires
  2. 2Effects on the body
  3. 3Time to incapacitation
  4. 4Combined effects with other gases
  5. 5Toxic potency of materials
  6. 6Treatment in design and regulation
  7. 7Limits of knowledge
  8. 8See also
  9. 9Notes
  10. 10References

Hydrogen cyanide (HCN) is a highly toxic gas that is released when materials containing nitrogen burn or decompose. Together with carbon monoxide, it is one of the two major asphyxiant gases in fires.[1] It is always present to some extent when nitrogen-containing materials such as acrylics, polyurethane foams, melamine, nylon and wool are involved in a fire, and these materials occur in most buildings.[1] Its concentrations are likely to be high in large post-flashover fires.[1] The gas is also called hydrocyanic acid.[2]

In fires, hydrogen cyanide is about 20 to 40 times more potent than carbon monoxide.[3] While carbon monoxide poisoning sets in slowly and insidiously, hydrogen cyanide poisoning tends to be rapid and dramatic.[4] Concentrations of 100 to 200 ppm can make fire victims lose consciousness quickly, after which they may die from carbon monoxide or other causes.[5] Its contribution to deaths in fires is hard to establish, because blood cyanide is often not measured and decomposes after death.[1][6] Hydrogen cyanide is therefore an important but often uncertain part of fire toxicity.[4]

Sources in fires

Hydrogen cyanide forms from materials whose molecules contain both nitrogen and hydrogen. A US Nuclear Regulatory Commission guide lists polyacrylonitrile, nylon, wool, polyurethane, urea-formaldehyde and acrylonitrile-butadiene-styrene (ABS) as examples.[7] Several of them, such as polyurethane, nylon and ABS, are plastics, which makes hydrogen cyanide an important product of plastics in fire.[7] In flames, hydrogen cyanide is formed in the oxygen-poor reduction zone, while nitrogen dioxide forms from the oxidation of hydrogen cyanide where oxygen is available.[8]

The temperature at which materials decompose affects the yield. Polyacrylonitrile starts to decompose between 525 and 625 K and then releases small amounts of ammonia and hydrogen cyanide.[9] Wool heated to 873 to 1198 K yields large amounts of hydrogen cyanide. For polyurethanes, the production of hydrogen cyanide and carbon monoxide increases with the decomposition temperature.[10] In tests at 900 °C, flexible polyurethane foam gave a clear atmosphere consisting mainly of hydrogen cyanide and carbon monoxide. At lower temperatures the same foam produced a dense yellow isocyanate smoke with low yields of both gases.[11]

The supply of oxygen matters as much as temperature. In early, well-ventilated flaming fires, most of the nitrogen in materials is oxidised to nitrogen oxides and nitrogen, and the yield of hydrogen cyanide is generally low. Acrylic materials, and to some extent rigid polyurethanes, are the exceptions.[12] Under hot, oxygen-starved conditions, the yield of hydrogen cyanide from all nitrogen-containing materials increases dramatically.[13] The generation efficiency of hydrogen cyanide from nylon increases with the equivalence ratio, that is, as the fire becomes shorter of air.[8]

Large-scale tests show the resulting pattern. In post-flashover fires, with the victim in a location remote from the fire, the atmosphere can contain 0 to 500 ppm of hydrogen cyanide together with up to 3 per cent carbon monoxide.[14] Above 800 °C with little oxygen, the main decomposition products break down into small molecules, including high concentrations of carbon monoxide and hydrogen cyanide.[14] The International Fire Engineering Guidelines (2005) note that smouldering polyurethane foam can also yield significant hydrogen cyanide.[15]

Effects on the body

Hydrogen cyanide acts by blocking the use of oxygen in the body. The cyanides combine in the tissues with enzymes involved in cellular oxidation. Oxygen is then unavailable to the tissues, and death follows through asphyxia.[2] The toxic agent is the cyanide ion. Unlike carbon monoxide, which stays mainly in the blood, the cyanide ion spreads through the body fluids and reaches the cells of vital organs.[7] Hydrogen cyanide is carried rapidly to the brain, which explains why its effects appear faster than those of carbon monoxide.[16]

Experiments with monkeys show the typical course of poisoning. At 147 ppm the first effects were minor, but at some time during a 30-minute exposure breathing increased sharply, up to four times the normal volume. Within one to five minutes of the start of this hyperventilation the animals lost consciousness.[4] The increased breathing drew in more gas and stimulated breathing further, a positive feedback.[16] Once exposure ended, recovery was rapid and almost complete within five to ten minutes.[16]

The response depends strongly on the concentration. Below about 80 ppm, effects in the animal experiments were minor for up to one hour. Between 80 and about 180 ppm, unconsciousness occurred at some time within 30 minutes. Above 180 ppm it followed within a few minutes.[16]

Limited human data show a similar pattern. Incapacitation occurred after 20 to 30 minutes at 100 ppm and after 2 minutes at 200 ppm, with rapid death above about 300 ppm.[16] Body size also plays a part. Children take up the gas faster because of their smaller size, and physical activity speeds up uptake in adults.[16]

A US Nuclear Regulatory Commission guide quotes the following effects from a 1971 source. The threshold limit value for workers is 10 ppm, and 20 to 40 ppm can be inhaled for several hours without appreciable effect. Concentrations of 120 to 150 ppm are dangerous within 30 to 60 minutes, and 3000 ppm or more is rapidly fatal.[7] The same guide notes that more than 180 ppm leads to unconsciousness within minutes, and that the fatal effect would normally come from carbon monoxide after hydrogen cyanide has made the victim unconscious.[2]

Time to incapacitation

Unlike carbon monoxide, hydrogen cyanide does not follow Haber's rule of a constant dose. For carbon monoxide, incapacitation occurs at about 27,000 ppm·min whatever the concentration. For hydrogen cyanide, the dose was about 2000 ppm·min at 100 ppm but only 400 ppm·min at 200 ppm.[16] Haber himself recognised this deviation in 1924.[16] The most important factor appears to be the rate of uptake, which depends on the concentration in the smoke and on breathing.[16] Concentrations above about 150 ppm are therefore more toxic than longer exposures to lower concentrations would suggest.[16]

From primate and human data, the SFPE Handbook derives an exponential expression for the time to incapacitation at a constant concentration. A simpler linear expression fits the data between 80 and 180 ppm, but the exponential form also covers higher and lower concentrations.[5] It fits the data with a regression coefficient of 0.984:[5]

tI,CN=exp⁡(5.396−0.023CHCN)

Symbol Description Unit
tI,CN time to incapacitation (loss of consciousness) at a constant concentration min
CHCN concentration of hydrogen cyanide in the inhaled air ppm

The expression is based mainly on data between about 80 and 180 ppm, where the time to incapacitation lies between about 2 and 30 minutes.[5] At 100 ppm it gives about 22 minutes and at 200 ppm about 2.2 minutes, which reflects the tenfold drop when the concentration doubles.[5] Real fires involve changing concentrations, and averaging the concentration over the exposure tends to overestimate the time to incapacitation.[5] The handbook therefore divides each minute of exposure by the time to incapacitation at that minute's concentration, and adds these fractions. Incapacitation is predicted when the sum reaches 1.[5]

As a worked example, the handbook considers a person exposed to 90 ppm of hydrogen cyanide for 15 minutes and then to 180 ppm for 2 minutes.[5] Both concentrations lie in the range where the expression was fitted.[5] The fraction for each period is its duration divided by the time to incapacitation at its concentration:

tI,CN(90)=exp⁡(5.396−0.023×90)=27.83 mintI,CN(180)=exp⁡(5.396−0.023×180)=3.51 minF=1527.83+23.51=0.54+0.57=1.11

The fraction exceeds 1, so the person is predicted to be incapacitated. After 15 minutes at 90 ppm the fraction is only 0.54, but the two minutes at 180 ppm add more than the whole first period. The handbook places incapacitation between 17 and 18 minutes.[5] Interpolating within the second minute at 180 ppm gives about 16.6 minutes, so the short peak is decisive in either case. The same approach is used in the fractional effective dose for the asphyxiant gases, in which a simplified form of the cyanide fraction also corrects for other nitriles and for nitrogen dioxide.[5]

Combined effects with other gases

In a fire, hydrogen cyanide is never inhaled alone. Carbon monoxide reduces the transport of oxygen in the blood, while hydrogen cyanide reduces the ability of the tissues to use the oxygen delivered.[17] On theoretical grounds little interaction would therefore be expected. The consensus view is nevertheless that the two gases have at least some additive effect. In primate experiments, near-toxic levels of carbon monoxide slightly shortened the time to incapacitation by hydrogen cyanide.[17]

For hazard calculations, the handbook proposes to treat carbon monoxide and hydrogen cyanide as directly additive on a fractional dose basis.[18] The uptake of both gases is assumed to increase in proportion to the faster breathing caused by carbon dioxide.[18] For most practical fires, carbon monoxide remains the most important asphyxiant. Hydrogen cyanide and low oxygen add to its effects and may shorten the time to incapacitation considerably in some situations.[18]

The combination also shapes how fire victims die. In the Strathclyde study of fire deaths in Scotland, high blood cyanide levels were usually found together with lethal carboxyhaemoglobin levels, so the separate role of cyanide could not be established.[4] Because of its rapid "knockdown" effect, hydrogen cyanide at 100 to 200 ppm can cause a quick loss of consciousness, after which the victim dies from accumulating carbon monoxide.[5] Hydrogen cyanide is thus one of the narcotic gases in smoke inhalation, together with carbon monoxide, carbon dioxide and low oxygen.[2]

Toxic potency of materials

The release of hydrogen cyanide makes nitrogen-containing materials more toxic than other materials in some fire conditions. Under nonflaming conditions, materials that release hydrogen cyanide at low temperatures, such as polyacrylonitrile, modacrylic and rigid polyurethane foam, have toxic potency factors of 3 to 8 relative to a normal material.[19] Under post-flashover conditions, the potency of all materials rises because of higher yields of hydrogen cyanide and carbon monoxide. For nitrogen-containing materials it then ranges from about 2.5 for flexible polyurethane foam to about 11 for modacrylic and polyacrylonitrile.[19]

Even below post-flashover temperatures, hydrogen cyanide can dominate. For wool decomposed at 700 °C, the toxicity is likely to be dominated by the high hydrogen cyanide yield, and the same is considered true for most other nitrogen-containing materials.[20] The handbook stresses that data for this type of fire are scarce.[20] Hydrogen cyanide is also flammable, with flammability limits of 5.6 to 40 per cent in air, far above toxic concentrations.[21]

Treatment in design and regulation

Fire engineering guidance often lists hydrogen cyanide among the toxic species to consider. The International Fire Engineering Guidelines (2005) name carbon monoxide, carbon dioxide, hydrogen cyanide and low oxygen as the toxic species often considered for life safety.[22] In practice, however, the same guidelines state that the analysis can generally be restricted to carbon monoxide unless the materials are atypical.[15]

In New Zealand, the first edition of Verification Method C/VM2 specified design yields for carbon monoxide, carbon dioxide and soot, and stated that hydrogen cyanide production need not be considered.[23] The commentary explains that only carbon monoxide, carbon dioxide and oxygen were included because of a lack of confidence in the data for other species. Carbon monoxide was also found to reach toxic levels before other species.[24] The second edition (2025) keeps a fractional effective dose of carbon monoxide that includes only carbon monoxide, carbon dioxide and oxygen.[25] In Australia, the Fire Safety Verification Method handbook (2020) notes that the production of carbon monoxide and hydrogen cyanide is sensitive to the materials and the combustion regime.[26]

Fire models can track hydrogen cyanide where the user supplies the data. In the zone model CFAST, the fuel molecule may contain nitrogen, and the yield of hydrogen cyanide is limited by the nitrogen in the fuel.[27] Any nitrogen not converted to hydrogen cyanide is released as nitrogen gas.[27] The result therefore depends entirely on the yield chosen by the user.[28]

Limits of knowledge

The importance of hydrogen cyanide in fire deaths can be underestimated. The dynamics of its uptake, distribution and breakdown in the body are poorly understood, and few blood measurements exist for injured and dead fire victims.[5] Carboxyhaemoglobin is routinely measured in fire victims, but blood cyanide is measured less often and usually a day or more after exposure.[6] Cyanide decomposes by about 50 per cent within one to two days in a body. Blood levels of 2.0 to 2.5 µg/ml in fresh samples are considered capable of causing incapacitation and 3.0 µg/ml lethal, with lower thresholds for stored samples.[6]

The toxicity data also have limits. Human exposure data are limited, and some reports suggest that people may survive higher concentrations for short periods. One person survived an accidental exposure to 444 ppm, and a 10-minute lethal concentration of 539 ppm has been suggested for humans.[16] The handbook considers the primate data a reasonable model for humans that probably errs slightly on the side of safety.[16] The fractional dose method is expected to give reasonable predictions only while the concentration is stable or increasing.[5]

See also

Notes

  1. ↑ 1.0 1.1 1.2 1.3 Purser (2002), p. 2-100.
  2. ↑ 2.0 2.1 2.2 2.3 Iqbal and Salley (2004), p. F-54.
  3. ↑ Purser (2002), p. 2-91.
  4. ↑ 4.0 4.1 4.2 4.3 Purser (2002), p. 2-103.
  5. ↑ 5.00 5.01 5.02 5.03 5.04 5.05 5.06 5.07 5.08 5.09 5.10 5.11 5.12 5.13 Purser (2002), p. 2-105.
  6. ↑ 6.0 6.1 6.2 Purser (2002), p. 2-106.
  7. ↑ 7.0 7.1 7.2 7.3 Iqbal and Salley (2004), p. B-68.
  8. ↑ 8.0 8.1 Tewarson (2002), p. 3-128.
  9. ↑ Beyler and Hirschler (2002), p. 1-125.
  10. ↑ Beyler and Hirschler (2002), p. 1-128.
  11. ↑ Purser (2002), p. 2-124.
  12. ↑ Purser (2002), p. 2-153.
  13. ↑ Purser (2002), p. 2-156.
  14. ↑ 14.0 14.1 Purser (2002), p. 2-123.
  15. ↑ 15.0 15.1 National Research Council of Canada et al. (2005), p. 2.4-12.
  16. ↑ 16.00 16.01 16.02 16.03 16.04 16.05 16.06 16.07 16.08 16.09 16.10 16.11 Purser (2002), p. 2-104.
  17. ↑ 17.0 17.1 Purser (2002), p. 2-109.
  18. ↑ 18.0 18.1 18.2 Purser (2002), p. 2-110.
  19. ↑ 19.0 19.1 Purser (2002), p. 2-98.
  20. ↑ 20.0 20.1 Purser (2002), p. 2-157.
  21. ↑ Beyler (2002b), p. 2-176.
  22. ↑ National Research Council of Canada et al. (2005), p. 1.5-4.
  23. ↑ Ministry of Business, Innovation and Employment (2013), p. 154.
  24. ↑ Ministry of Business, Innovation and Employment (2013), p. 43.
  25. ↑ Ministry of Business, Innovation and Employment (2025b), p. 17.
  26. ↑ Australian Building Codes Board (2020b), p. 111.
  27. ↑ 27.0 27.1 Peacock et al. (2026), p. 12.
  28. ↑ Peacock et al. (2026), p. 11.

References

  • Australian Building Codes Board. (2020b). Handbook: Fire Safety Verification Method (Print version 1.1).
  • Beyler, C. L. (2002b). Flammability limits of premixed and diffusion flames. 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. 2-172–2-187). National Fire Protection Association; Society of Fire Protection Engineers.
  • Beyler, C. L., & Hirschler, M. M. (2002). Thermal decomposition of polymers. 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. 1-110–1-131). National Fire Protection Association; Society of Fire Protection Engineers.
  • 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.
  • 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.
  • Peacock, R. D., McGrattan, K. B., Forney, G. P., & Reneke, P. A. (2026). CFAST – Consolidated Fire and Smoke Transport: Vol. 1. Technical reference guide (Version 7, NIST Technical Note 1889v1). National Institute of Standards and Technology. https://doi.org/10.6028/NIST.TN.1889v1
  • 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.
  • Tewarson, A. (2002). Generation of heat and chemical compounds in fires. 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. 3-82–3-161). National Fire Protection Association; Society of Fire Protection Engineers.
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