Adapted from CBD-144. Toxic Gases and Vapours Produced at Fires.
Originally published December 1971. Sumi K, Tsuchiya Y.

Toxic gases and vapours produced at fires are responsible for a large number of fire deaths. If those resulting from clothing fires are excluded, more victims are claimed by the products of fire than by burns. Fire statistics reveal that of the total number of deaths at building fires, again omitting those due to ignition of clothing, approximately 50 per cent are due to combustion products. This figure should probably be even higher since it is difficult to be certain whether death has resulted from burns or toxic combustion products when victims are severely burned.

Smoke and toxic gases and vapours usually occur together at fires and it is difficult to distinguish clearly which product of combustion is responsible for the harmful effects. Before discussing them, smoke, gas and vapour should be defined as these terms are used in the present Digest. Smoke is particulate matter consisting of very fine solid particles and condensed vapour. It constitutes most of the visible part of the products of combustion observed at a fire. Gas is a product of combustion that remains a gas even when cooled to normal building temperatures. Vapour is a product of combustion that is gas when produced but reverts to solid or liquid at normal temperatures. Vapours will gradually condense on cool surfaces as they migrate from the fire.

The main danger from smoke is reduced visibility; that from toxic gases and vapours, their adverse effect on body functions. Smoke will often impede the escape of occupants from a burning building and result in prolonged exposure to the harmful effects of toxic products. Toxic gases and vapours can cause death if they are present in sufficient quantities and for a sufficient time. Certain ones can also trap occupants by acting as irritants. For example, small concentrations of products such as hydrogen chloride and ammonia cause direct irritation of the respiratory tract and the eyes. Although irritants may serve as warning agents and alert occupants to fire, under certain circumstances they can prevent victims from finding an exit even before reduced visibility from smoke traps them.

Fire authorities have long been concerned with the life hazard associated with toxic combustion products. Its seriousness for fire fighters was recognized by the fire service many years ago and almost all modern fire departments are now equipped with self-breathing apparatus. The danger to occupants of buildings is also recognized, but so far it has appeared impractical to limit the use of combustible materials, until recently essentially cellulose, either as building materials or as goods and furnishings. Of recent years various new materials, especially synthetic polymers, have found increasing use in buildings and their introduction has heightened the concern of fire authorities with reference to toxic combustion products. Part of this increased concern has arisen because of the lack of information on the toxic combustion products produced.

The toxic products responsible for fire deaths are usually not known because detailed pathological examination of fire victims is rarely conducted. Some information on the pathological response of man to various harmful gases and vapours produced at fires is available, however (2,3,4).

Carbon monoxide (CO) is produced as a result of incomplete combustion of materials containing carbon and is present in large quantities at most fires. Carbon monoxide that is inhaled causes asphyxiation by combining with haemoglobin in a reversible reaction to form carboxyhaemoglobin. Its formation at the expense of oxyhaemoglobin reduces the availability of oxygen for the cellular systems of the body. Anoxaemia induced by carbon monoxide does not, as with simple asphyxiants, cease as soon as fresh air is inhaled. After even moderate degrees of gassing, only about 50 per cent of the carbon monoxide is eliminated in the first hour under ordinary circumstances; complete elimination under the action of fresh air is not effected for many hours. The highest concentration of CO to which man may be exposed day after day without adverse effect is 50 ppm (Table I). Above this level, symptoms such as headache, fatigue and dizziness appear in healthy individuals.

Table 1. Physiological Response to Various Concentrations of CO.

Carbon dioxide (CO2) is produced in quantity at most building fires. Inhalation of carbon dioxide stimulates respiration and this in turn increases inhalation of both oxygen and possible toxic gases and vapours produced by the fire. Stimulation is pronounced at 5 per cent (50,000 ppm) concentration, and 30-minute exposure produces signs of intoxication; above 70,000 ppm unconsciousness results in a few minutes. The threshold limit for CO2, that is the concentration that can be tolerated by workers day after day without adverse effect, is 5,000 ppm.

Also see: Cameroon.pdf [2.4 MB]

Hydrogen cyanide (HCN) is produced when materials that contain nitrogen in their structure, e.g., orlon, nylon, wool, polyurethane, urea-formaldehyde and ABS (acrylonitrile-butadiene-styrene) are involved in fire. Hydrogen cyanide and other cyanogen compounds arrest the activity of all forms of living matter. They exert an inhibiting action on the use of oxygen by the living cells of the body tissues. The physiological response to various concentrations of hydrogen cyanide is presented in Table 2.

Table 2. Physiological Response to Various Concentrations of Hydrogen Cyanide

Hydrogen chloride (HCl) is produced when polyvinyl chloride (PVC) is decomposed at fires. If inhaled, HCl will damage the upper respiratory tract and lead to asphyxiation or death. The physiological response of man to various concentrations of HCl is given in Table 3.

Table 3. Physiological Response to various Concentration of HC1

There are three common oxides of nitrogen: nitrous oxide (N2O), nitric oxide (NO), and the two forms of the dioxide (NO2 and N2O4). Nitrogen dioxide, which is very toxic, can be produced from the combustion of cellulose nitrate. Nitric oxide does not exist in atmospheric air because it is converted into dioxide in the presence of oxygen. These compounds are strong irritants, particularly to mucous membranes and thus when inhaled will damage tissues in the respiratory tract by reacting with moisture to produce nitrous and nitric acids. The physiological response of man to various concentrations of nitrogen dioxide is given in Table 4.

Table 4. Physiological Response to Various Concentrations of Nitrogen Dioxide

The quantities of toxic gases and vapours produced by combustion depend on the material involved and the environmental condition. Some are already known; others can often be predicted from a knowledge of the chemical composition and molecular structure of organic compounds. A basis for prediction is important, both to research work in identifying combustion products and to designers. Fire and building officials could also benefit from a knowledge of the chemical composition of materials because chemical composition dictates which toxic products will be produced from the combustion of a given material. The following examples will illustrate how formation of the main toxic combustion products may be predicted from a knowledge of chemical composition of materials.

Polyethylene is a polymer consisting of carbon and hydrogen atoms. When this material is burned under ideal conditions with a hot fire and ample supply of oxygen, the main products are carbon dioxide and water. Under adverse conditions, for example when the oxygen supply is limited, carbon particles and carbon monoxide, which is toxic, will also be formed. Carbon monoxide is the main toxic gas produced from the combustion of polyethylene and other organic materials that are made up of carbon and hydrogen atoms.

Polystyrene also is made of carbon and hydrogen atoms. When this polymer is decomposed by heat, the major product is styrene, the compound from which it was produced. In a fire, styrene is broken down further to smaller molecules that react with oxygen to form the usual combustion products. The main toxic product from the burning of polystyrene is also CO. The product, styrene, is almost as toxic as CO, but it is produced in much smaller quantities (5).

PVC is made of carbon, hydrogen and chlorine atoms. When this polymer is decomposed by heat the chlorine atoms are broken off and each combines with a hydrogen atom to form hydrogen chloride (HCl), which is not only toxic but also very corrosive. Phosgene (COCl2), which is very toxic, is produced in negligible amounts from the combustion of PVC. Hydrogen chloride is more toxic than CO and may be produced in greater quantity than CO when PVC is involved in a fire.

When plexiglas is heated, the major decomposition product is methylmethacrylate, the compound from which it was synthesized. In a fire, methylmethacrylate is broken down to smaller molecules that react with oxygen to form the usual combustion products. The main toxic combustion product of plexiglas is CO. The toxicity of methylmethacrylate is of the same order as that of CO, but it is produced in much smaller quantities.

Cellulose, the main constituent of wood, is made of carbon, hydrogen and oxygen atoms. The burning of cellulose produces hydrocarbons and compounds made of these three elements, in addition to the usual combustion products. Some of the oxygen-containing vapours, especially the aldehydes, are very toxic. It is generally accepted, however, that CO is responsible for most deaths at fires involving cellulose materials because it is produced in much greater quantities than other toxic gases.

Several important synthetic fibres have a high acrylonitrile content of 80 to 85 per cent. These synthetic materials are made of carbon, hydrogen and nitrogen atoms, and when they are decomposed in a fire they produce hydrogen cyanide (HCN), a very toxic gas, plus CO and other combustion products.

Hydrogen cyanide also is produced from the combustion of ABS (acrylonitrile-butadiene-styrene) pipe and acrylic carpeting materials, which are synthesized using acrylonitrile. It is also produced by the combustion of other compounds that contain nitrogen in their structure such as urea-formaldehyde, nylon, wool and polyurethane.

Many investigators have undertaken studies on toxic combustion products of organic materials with the object of making a realistic assessment of hazard. One method of evaluating the toxic gas-producing potential of materials is by burning a material in an enclosure and subjecting animals to the resulting atmosphere. Work has also been conducted in which toxic combustion products are identified and quantitatively analysed. The relatively small progress that has been made over the years indicates the complexity of the problem. Some progress has recently been made, however, largely owing to advances in analytical techniques such as gas chromatography and mass spectrometry.

Toxic gases and vapours produced by combustion are responsible for the majority of deaths at building fires. For this reason fire authorities would like to consider regulations to restrict use of materials that produce large amounts of smoke and toxic gases. At present, there are very few regulations limiting the use of materials based on this property, but it is conceivable that more regulations will be adopted in the future.

Information on toxic gases and vapours produced by combustion of materials commonly found in buildings is still very limited. Some quantitative data on the toxic compounds mentioned in this Digest are now available, but data on many other toxic combustion products produced in small quantities are very scarce. Knowledge is also lacking of differences in burning rates (and resulting rates of production of toxic gases) of materials under comparable conditions of burning and differences in rates of condensation of products as they migrate away from the fire.
Carbon monoxide is produced in quantity at most building fires because most organic materials contain carbon in their chemical structure. Materials that contain nitrogen, such as acrylic fibre, nylon, wool and urea-formaldehyde foam, could produce dangerous quantities of HCN in addition to CO. When these materials are involved in fire, the resulting atmosphere could be more toxic than that from the combustion of an equal amount of material whose toxic product is mainly CO. Materials that contain a large proportion of chlorine, such as PVC, could also be very harmful at fires. They would produce HCl in addition to CO.

The present state of knowledge of toxic combustion products is probably not sufficient to guide the designer in his choice of combustible materials to be used in buildings. This Digest was prepared to keep him abreast of the problem and the efforts that are being made in this field.

References

1. The BOCA Basic Building Code/1970. Building Officials Conference of America, Inc., Fifth Edition. 413 p.
2. Henderson, Y. and H. W. Haggard. Noxious Gases and the Principles of Respiration Influencing their Action. Chem. Catalog Co. Inc., 1927, 212 p.
3. Autian, J. Toxicologic Aspects of Flammability and Combustion of Polymeric Materials. J. Fire and Flammability, Vol. 1, No.239, July 1970.
4. Documentation of the Threshold Limit Values. American Conference of Governmental Industrial Hygienists. Third Edition, 1971, 286 p.
5. Sumi, K. and Y. Tsuchiya. Toxic Combustion Products of Wood and Polystyrene. National Research Council of Canada, Division of Building Research, BRN 76, Sept. 1971.

Originally published July 1978. Sumi K, Tsuchiya Y.

Fire statistics reveal that inhalation of thermal decomposition products (smoke, gases and vapours) is responsible for the majority of fire deaths. Many new materials release harmful decomposition products very rapidly, and some of them are much more toxic than those generated by traditional materials. With their increased use as both building materials and furnishings, the problem of toxic products of combustion has become a subject of very real concern. This Digest briefly discusses two major types of laboratory assessment of fire toxicity and some of the problems encountered in developing standard methods of evaluation.

General information regarding toxic products of combustion presented in an earlier Digest is condensed in Table I. Smoke and toxic gases and vapours usually occur together at fires, so that it is difficult to distinguish the contribution of the two types of combustion product to the hazard. It is useful, however, to define them. Smoke is particulate matter consisting of very fine solid particles and condensed vapour. It constitutes most of the visible part of the products of combustion observed at a fire. Gas is a product of combustion that remains in a gaseous state even when cooled to normal building temperatures. Vapour is a product of combustion that is gas when produced but reverts to solid or liquid at normal temperatures. Vapours gradually condense on cool surfaces as they migrate from the fire.

Table 5. Main Harmful Products of Materials and General Harmful Effect* (*More details in CBD 144)

The main danger from smoke is reduced visibility; that from toxic gases and vapours, their adverse effect on body functions. Smoke can impede the escape of occupants from a burning building, prolonging exposure to the harmful effects of toxic products. Toxic gases and vapours can cause death if they are present in sufficient quantities and for a sufficient time. Some can also trap occupants by acting as irritants. For example, small concentrations of hydrogen chloride and ammonia cause direct irritation of the respiratory tract and the eyes. Although irritants may serve as warning agents and alert occupants to fire, they can under certain circumstances prevent victims from finding an exit even before reduced visibility from smoke traps them.

The life hazard associated with toxic combustion products was recognized by the fire service many years ago and almost all modern fire departments are now equipped with self-breathing apparatus. The danger to occupants of buildings is also recognized, but so far it has appeared impractical to reduce this risk by limiting the use of materials that have a high propensity for releasing harmful products. In recent years various new materials, especially synthetic polymers, have found increasing use in buildings and their introduction has heightened the concern of fire authorities with reference to toxic combustion products. Part of this increased concern has risen because of the lack of information on toxic combustion products and the problem of assessing their potential hazard.

At present, two main types of laboratory study of fire toxicity are undertaken: chemical analysis of decomposition products and biological tests involving animals. The two approaches complement each other in the development of information relevant to toxic hazard at fires.

The types and quantities of toxic gases and vapours produced by combustion depend on the materials involved and on environmental conditions. Some toxic products are already known; others can often be predicted from a knowledge of the chemical composition and molecular structure of the organic compounds.

Detailed analysis of decomposition products from synthetic materials is very difficult because the materials break down into many compounds. Analysis of the pyrolysis products of cellulose, for example, has revealed some 175 different organic compounds. Because of this the researcher must arbitrarily decide how detailed the analysis should be, and despite recent advances in analytical techniques the work continues to be very time-consuming. At present, detailed analysis cannot keep pace with the rapid development of new organic materials.

Comprehensive chemical analysis is necessary for the identification of unusual toxicants, and is usually undertaken following evidence that a given material is capable of releasing extremely harmful decomposition products. For most practical applications, however, it is not necessary. Testing for a few of the most important known toxicants often gives sufficient information. Some compounds such as carbon monoxide, hydrogen chloride, hydrogen cyanide, sulphur dioxide and oxides of nitrogen are recognized as harmful products; others such as water vapour and the hydrocarbons contribute little or no toxic hazard. It is usually sufficient to decompose materials under specified conditions and determine the resulting concentrations of a few of the most important toxicants. From this information a reasonable indication of the toxicity of the mixture of products can be obtained. This is the essence of the toxicity index concept.

The evaluation of fire toxicity of materials by exposing small animals, notably mice and rats, to decomposition products is being undertaken in several countries, but not in Canada at the present time. Such studies include both sophisticated research dealing with the effects of harmful decomposition products on the biological system and simple screening tests using animals for evaluation of toxicity.

Fire toxicology is much less advanced than many other areas of toxicology. Although standard approaches have been developed for evaluating toxicity of food additives, drugs, cosmetics and pesticides, no standard method is yet available for evaluating the combustion/pyrolysis products released by materials. Experimental data show that ranking of relative toxicity based on biological assessment is significantly different for different decomposition procedures, a fact that should not be surprising since the influence of experimental conditions on ranking of materials has been observed in studies of other fire characteristics such as rate of heat release, spread of flame, and smoke density. The development of a test that will rank materials according to toxic hazard in actual fires is needed, but it does not seem possible at present. A test that will determine toxic hazard under one or more specified conditions, with little relevance to actual fires, may be the best that can be hoped for in the near future.

Another complication of designing screening tests is that of controlling the temperature and oxygen concentration to which animals are exposed. It is important to ensure that the test determines the effect of toxic products alone, and that complications introduced by temperature stress or reduced oxygen concentration are avoided. Separating the apparatus that produces the thermal decomposition products from the animals is probably the best means of coping with the problem. Its disadvantage is the possible loss of important toxic components during transfer of the combustion products to the animals.

In exposing small rodents to decomposition products and observing their response, the most common endpoints of the experiments are death or incapacitation. Incapacitation seems a more meaningful endpoint than death since it is related to escape capability. The experimental results are often reported on the basis of LD50 or LC50, the dose or concentration of products required to kill or incapacitate, respectively, 50 per cent of the animals. Various methods of assessing incapacitation have been reported, but at the present time there is no agreement as to the best way to determine it.

The principal advantage of the chemical method of assessing toxicity is its convenience. Any laboratory engaged in fire research will be equipped to duplicate the very wide range of environmental conditions under which materials are decomposed in actual fires. As conditions can be reproduced exactly, results can be verified. The method holds potential for limiting specific elements in various products. For example, it could be used to control the amounts of chlorine or nitrogen in materials for specific applications.

There are limitations, however. As toxicological data are not available for many of the compounds produced as thermal decomposition products, it is not always possible to assess toxicity adequately. Neither is it possible to analyse all the combustion products found in a fire atmosphere, and small quantities of extremely harmful products may be overlooked. Another unknown factor is the effect of any interaction of decomposition products on the over-all toxicity of a mixture of products.

It is for these reasons that there is need for animal exposure tests, for which special facilities and expertise outside the usual range are necessary. This method permits comparison of all materials and study of all toxic components generated under specified conditions. Such an approach could identify materials that generate extremely harmful decomposition products when they burn and thus provide a basis for ensuring that they will not be marketed. It is difficult, however, to develop a method of ranking materials in order of toxic hazard by means of animal exposure tests.

Although considerable effort is being directed towards developing recommended procedures or standard methods of evaluating the fire toxicity of materials, there is as yet no accepted standard method. Until it becomes available it is difficult to have definite recommendations and regulations concerning the use of materials known to generate significant amounts of toxic decomposition products.

1. Sumi, K. and Y. Tsuchiya. Evaluating toxicity of decomposition products from analytical data. Proceedings, International Symposium on Flammability and Fire Retardants 1976, p. 241.
2. Fire Toxicology: Methods for Evaluation of Toxicity of Pyrolysis and Combustion Products. National Academy of Sciences, Report No. 2, August 1977, 29p.

Also read the papers “Polyurethane products in fires: Acute toxicity of smoke and fire gases” by Landry et al and “Toxicology of fire and smoke” by Levin and Kuligowski.

A pesticide warehouse containing mainly organophosphate insecticides and organochlorine pesticides is involved in a fire. What type of combustion products would you expect from this fire? What advice would you give to the Fire Service personnel who were involved in fighting this fire?