Formation of Hydrogen Cyanide in Fires

In fires, HCN is formed in the reaction zone from fuels containing nitrogen (N) and hydrogen (H) atoms.

However, HCN may be consumed if there is sufficient oxygen in the mixture and if the temperature is sufficiently high, making further oxidation of HCN possible. This oxidation can lead to a variety of compounds including equally undesirable compounds, such as oxides of nitrogen (so called NOx), although under ideal conditions should ultimately lead to CO2, H2O and N2. Most commonly in fires, however, CO would be a major product of the oxidation of HCN.

HCN more toxic than CO?

Although HCN is a narcotic gas like CO it has been suggested that HCN is about 35 times more toxic than CO. The influence of HCN on the victim is relatively quick because it is carried rapidly to the brain by blood. The most important determinant of incapacitation with HCN is the rate of uptake, which is directly related to the concentration of HCN in the air the victim is breathing. This is dealt with in more detail in the next chapter.

It has been speculated that HCN is generally not found in the blood of fire victims essentially for two reasons. The first reason is that traditionally one has concentrated on identifying and quantifying the amount of CO in the blood of fire victims. Thus, this may be a case of not having found HCN because it has not been looked for. In the literature there is ample evidence available that shows a wide variation in the concentration of carboxyhaemoglobin (COHb) necessary to cause death. Thus, the amount of COHb claimed to be responsible for the death of fire victims may range from 30% to 70%. This would indicate that in at least some cases where COHb has been assigned the cause of death there might well have been other important contributing factors.

The second reason is that the mode of action of HCN is different to that of CO. As HCN does not follow Habers rule very low concentrations are actually required to prompt a toxic reaction. This toxic reaction will generally not be death but unconsciousness. The person effected would essentially be unable to escape from the scene of the fire and would continue to breath, thereby inhaling increasing amount of CO and possibly eventually actually die from CO inhalation. Technically speaking the cause of death may well be CO poisoning but arguably had the person not been incapacitated by the HCN in the first place then they could have escaped from the fire and avoided the CO poisoning. Thus, the death has been reported to be poisoning due to CO even though the real reason to the death could be the incapacitation due to HCN.

The discoteque fire in Gothenburg

A potent modern day example of the importance of fire gases in causing the death of fire victims is the Gothenburg Disco fire in 1998. In this fire most of the 63 victims died due to toxic gases and not due to heat from fire. The fuel in this fire consisted of material that could potentially generate large amounts of HCN. Indeed, the report from this incident indicates that HCN was an important factor in the fatalities.

Small scale tests

A study has been conducted to investigate the relative importance of HCN to CO emissions from a series of materials commonly used either as building material or in the production of e. g. upholstered furniture used in domestic applications. A series of small scale tests have been conducted on five types of material chosen firstly for the presence of nitrogen in their chemical structure and secondly for their relevance as material present in domestic applications.

 
Photo of the DIN-furnace used for the small-scale experiments.


The specific materials included in this study were: wool, nylon, synthetic rubber, melamine and polyurethane-foam. None of the materials were combustion modified through the use of a flame retardant additive.

The reason to focus on HCN is clear. It is important as a potential killer or escape inhibitor in fires but as we presently lack a concrete comparison between the CO producing potential of common material relative to their HCN producing potential it has been difficult to establish the relative importance of HCN compared to that of CO. This is of particular importance as we become more acquainted with computer modelling using, for example, Computational Fluid Dynamics (CFD). In recent years it has become more common to use CFD methods to model fire behaviour in complicated situations as an alternative to time consuming and costly experiments. In recognition of the importance of CO, production models have been developed to calculate the production of CO as the fire being modelled develops. This study has, therefore, also been conceived to determine whether calculation of the production of CO using CFD is sufficient to establish the toxic hazard posed by any given enclosure fire, or whether it is necessary to model other species, such as HCN, as well.

As a starting point for this study a literature study has been conducted to establish what has been done previously in this field. Full results are available in SP Report 2000:27.

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