Chapter 1: Overview of Combustion Toxicology
Published:16 Oct 2015
D. A. Purser and R. L. Maynard, in Toxicology, Survival and Health Hazards of Combustion Products, ed. D. A. Purser, R. L. Maynard, and J. C. Wakefield, The Royal Society of Chemistry, 2015, ch. 1, pp. 1-10.
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Chapter 1 summarizes the main aspects of the subject areas addressed in the book, which is divided into five main sections: the science of combustion, the toxicity of combustion products, clinical toxicology and management of combustion product exposures, examples of unusual conflagrations, and public health aspects of fire management. For each section a short guide to the topics covered in the different chapters is presented.
There can be no doubt that the discovery of how substances can be caused to burn is the greatest of all human discoveries. Until the discovery of nuclear fission, combustion was the only known means of causing matter to release large amounts of stored energy. Combustion remains the overwhelmingly most important means of releasing such energy; indeed even nuclear power would be impossible without combustion: metals have to smelted and fabricated before nuclear reactors can be constructed. Just how important combustion is can be illustrated by considering a lump of coal, a piece of wood or a litre of oil: how could one release the energy within these materials without combustion? From very early times to the present, from the need for warmth in caves to the apparent need for high speed motor cars, combustion has played an essential role in man's development. The essence of the discovery: that combustion releases heat and that heat, a form of energy, can do work is known to everybody.
When organic substances burn, they release heat; they also release chemical products of combustion. The atoms forming the molecules of organic matter cannot be destroyed by combustion, but they can be caused to separate from their original combinations and to form other combinations. Material comprising only molecules containing carbon, oxygen and hydrogen cannot, on burning, fail to produce molecules containing these elements. The principle products of combustion are carbon dioxide and water, but depending on the efficiency of the combustion process, other substances are produced: carbon monoxide, for example. Already we have identified a toxicologically active product of combustion. Carbon monoxide is a very poisonous gas and is responsible for a high percentage of deaths occurring in accidents involving fires. Combustion is often not very efficient and a range of gases and particulate materials are produced: smoke and ash. Some products remain at the source of the fire; others are carried into the atmosphere along with the heated air produced by the fire. Air pollution is produced. Even well regulated combustion of the type seen in modern internal combustion engines produces pollutants. Everybody has seen black smoke being emitted by old diesel powered vehicles; even the “cleanest” new motor car produces carbon dioxide.
The toxic effects of combustion products resulting from various sources are a major cause of morbidity and mortality. All people are exposed, every day, to air pollutants produced by combustion of organic material. Some are exposed to high concentrations: forest fires provide an example. Others are exposed to lower concentrations: levels of air pollution in the countryside of developed countries like the UK provide an example. Some people are exposed to potentially dangerous levels of pollution during their work: those working with diesel engines in confined spaces, those working as fire-fighters provide examples. And some are exposed as a result of accidents: those trapped in a burning building and those exposed to carbon monoxide being emitted by a faulty coke boiler provide obvious examples. Just how dangerous smoke from fires actually is may easily be forgotten. In fact it is very dangerous indeed: incapacitation by inhalation of smoke and consequent inability to escape is the major cause of deaths in fires. In the UK alone 200 deaths and 2500 injuries requiring hospital treatment are, each year, caused by accidental exposure to smoke from uncontrolled fires.1
Figures 1.1 and 1.2 show annual fire deaths and injuries per million population in the UK from smoke and burns, by far the majority of which occur in domestic dwellings. In the 1950s most deaths (7.2 per million) and injuries (35.7 per million) resulted from burns, with very few deaths (1.9 per million) and injuries (3.3 per million) attributable to toxic smoke exposure. Between the late 1950s and the early 1970s, although the incidence of injury and death from burns remained approximately constant, there was a more than fourfold increase in deaths and a fivefold increase in injuries from exposure to toxic smoke.
Although a number of factors may be involved, the main cause of this increase was considered to be changes in living styles in the average British home, and in particular the replacement of traditional materials used for the construction of upholstered furniture and bedding by man-made materials, especially polyurethane foam filling and synthetic covering materials. Not only did the incidence of flaming fires increase, but when fires occurred, fire growth was rapid and involved the production of large volumes of irritant smoke, containing high concentrations of particulates, carbon monoxide and hydrogen cyanide. The dense smoke was therefore much more likely to impede the escape of occupants, who were then rapidly overcome by asphyxiant gases and heat.
Concerns regarding these issues led to the introduction of the upholstered furniture flammability regulations in 1988, which required improved resistance to ignition from small smouldering and flaming sources. This had no immediate effect, since significant replacement of old furniture in the housing stock took around a decade. However, another safety innovation introduced from this time was smoke alarms, gradual uptake of which coincided with a gradual decrease in smoke deaths, although the total number of fires and the number of smoke injuries continued to increase. From around 2000, by which time much old upholstered furniture had been replaced, the incidence of fires and of serious injuries also started to decrease so that these and the death rate gradually decreased towards the levels of the 1950s. Toxic smoke exposure remains the man cause of injuries and deaths in fires. These aspects are discussed further in Chapters 2 and 15.
Another influence may be the decline in the prevalence of cigarette smoking in the UK over this period (Figure 1.3). This has important implications for both the incidence of acute fire deaths and injuries (since “smokers' materials” are reported as a major cause of fatal fires in the annual fire statistics1 ) and the long term morbidity and mortality from smoking-related diseases.2,3 However, the decrease in incidence of smoking over the period up to 2000 in fact coincides with the increase in the numbers of fires in dwellings. It is possible that the further decline in smoking since 2000, coupled with the more recent unacceptability of smoking indoors, may be partly responsible for the decrease in the number of dwellings fires since this time.
This variety of causes of morbidity and mortality related to combustion products illustrates one aspect of their complexity: the wide range of toxic product concentrations of interest in evaluating effects, and the very wide range of exposure times that need to be considered. Figure 1.4 captures this, by showing the concentration ranges (expressed in μg m−3) and exposure periods (expressed in hours) of interest for several toxic substances occurring in combustion products mixtures. The figure illustrates the enormous range of concentrations and times of interest involving over 16 orders of magnitude from pg m−3 concentrations of dioxins over a 50 year exposure period to over 100 g m−3 of carbon monoxide over periods as short as a few seconds.
The top left of Figure 1.4 shows concentrations of asphyxiant gases, such as carbon monoxide and hydrogen cyanide, acid gases and organic irritants, such as hydrogen chloride and formaldehyde, and smoke particulates, which can be present at concentrations up to percent levels by volume or g m−3 by mass during fires. In order to understand the effects on the survival of fire victims it is necessary to study the effects of smoke and irritants on escape capability and the incapacitating physiological effects of asphyxiant gases during critical periods of from a few seconds to a few minutes, in order to calculate time to loss of consciousness and death. For subjects rescued alive from fires it is also important to consider the more permanent effects of these gases on cerebral and cardiovascular function, and the effects of irritants and smoke particulates on lung function and pathology (complicated by burn injuries).
Moving on to exposure periods of approximately 1–100 hours, for situations such as acute exposures to diluted smoke plumes from wildfires, the concern relates mainly to effects from smoke particulates and irritants, including nuisance odour, mild eye and respiratory tract irritation and possibly more serious acute effects on vulnerable individuals.
Long term exposure to ambient air pollutants has been shown to be associated with a significant effect on health. In this context the concentrations of interest are very low, for example in the μg m−3 range for pollutants, such as fine particulates (PM 2.5) or formaldehyde, and in the pg m−3 range for dibenzodioxins. Studies reviewed by the Committee on the Medical effects of Air Pollutants (COMEAP) in 2009 and 20104,5 led to the conclusion that current levels of fine particles (PM 2.5) in the UK are responsible for 29 000 attributable deaths each year. A large proportion of material monitored as PM 2.5 comes, directly or indirectly, from combustion processes. No threshold of effect for such findings has been discovered. The effects leading to an increased risk of death involve those on the cardiovascular system and on the risk of lung cancer. It is interesting that effects on the respiratory system appear to be less important. Short term increases in ambient concentrations of air pollutants are also associated with increases in deaths and hospital admissions: in this case the respiratory system is affected, in addition to the cardiovascular system. The European Commission has funded a recent review of these effects.6 In addition to these environmental exposures, tobacco smoking is the primary cause of preventable illness and premature death, accounting for approximately 100 000 death per year in the UK, representing 36% of all respiratory deaths, 28% of all cancer deaths and 14% or all circulatory deaths.7
With regard to some of these toxic substances, especially dioxins, the systemic health effects may result directly from inhalation exposure, but also from secondary ingestion following deposition of combustion products into water and taken up into foodstuffs, or even as a result of dermal exposure.8 These issues are discussed in Chapters 3 and 12.
These few statistics illustrate the enormous ongoing adverse morbidity and mortality costs of exposure to combustion products.
1.2 Summary of the Main Aspects of the Subject Areas Addressed in the Book
In order to cover the different aspects of fires, combustion products and toxic health effects we have divided this book into five sections:
The Science of Combustion.
The Toxicity of Combustion Products.
The Clinical Toxicology and Management of Combustion Product Exposures.
Examples of Unusual Conflagrations.
Public Health Aspects of Fire Management.
“The Science of Combustion” consists of five chapters addressing the chemical and physical aspects of combustion processes, fire development and fire types, the composition of combustion product “fire effluent” or “smoke” atmospheres, and the measurement and calculation of the yields and concentrations of toxic substances from a range of fuels in a variety of fire scenarios. An important aspect of fires is that the rates of production and composition of the combustion products vary considerably depending upon the combustion conditions. In Chapter 2, titled “Fire Types and Combustion Products”, David Purser describes different fire types and fire stages in terms of fire scenario development, combustion conditions, the range of combustion products formed and the behaviour of different fuels depending on elemental and structural composition. In Chapter 3, titled “Estimating Yields and Quantities of Mass Releases of Toxic Products from Fires”, he further develops this theme. The chapter describes methods for generating and measuring toxic products and yields under different fire conditions and presents yield data for a range of fuel types and fire scenarios. The chapter then explains how these can be used as input into engineering calculations for mass releases of combustion products both within and beyond enclosures such as buildings. Some of the technical issues with respect to valid test methods for burning materials under defined combustion conditions, and some challenges in sampling and measuring the composition of the combustion product atmospheres, are flagged up in this chapter. They are then described in more detail by T. Richard Hull and Anna Stec in Chapter 5, titled “Generation, Sampling and Quantification of Toxic Combustion Products”. In Chapter 4, titled “Products of Combustion and Toxicity from Specific Types of Fires”, James Wakefield describes a wider range of fire types and combustion products, including those involving industrial fires, from which specific industrial chemicals are released into the combustion product mixtures from the general building or vehicle materials. When large conflagrations such as these occur, large fire effluent plumes are released to the outdoor environment. In order to assess the hazards from such plumes, the mass releases from the source fire (calculated using methods such as those described in Chapter 3) are used as input to smoke plume dispersal calculation models, which are described by David Hall and Angela Spanton in Chapter 6.
Having generated our combustion products in the first section of the book, we next examine their toxicology in the following eight chapters. In Chapter 7 David Purser considers the application of animal models and human studies to prediction of combustion toxicity in humans. Combustion product atmospheres contain a complex mixture of gases and particulates interacting to produce a series of toxic effects on exposed subjects. Our understanding of these effects, which substances are the major actors and how they interact, is derived from a variety of sources. These include human fire incident investigations, animal exposures to combustion product atmospheres generated by burning a range of materials under different fire conditions, and exposures of rodents, primates and human volunteers to individual toxic substances (or specific mixtures of substances) known to occur in fire effluents. Prof. Purser explains how the main effects of acute exposure have been found to occur in a sequence consisting of immediate eye and upper respiratory tract irritancy followed by incapacitation due to the effects of asphyxiant gases during exposure, then followed by respiratory tract and lung inflammation some hours after exposure. These studies have shown that the major effects of exposure to fire effluents can be explained in terms or a small number of key irritant and asphyxiant substances. The quantification of the effects of these substances and their interactions is described and their incorporation into toxic potency calculation models for combustion product mixtures.
An important aspect of acute exposure to specific substances and evaluating their toxic effects is the concentration, exposure dose, toxicity relationships for different substances, and the extent to which they follow Haber's Law. Aspects of this are addressed in Chapter 7, and a detailed consideration of relationships to Haber's Law is provided by Robert Maynard and David Purser in Chapter 8, titled “Haber's Law and its Application to Combustion Products”.
The remaining six chapters in this section consider the toxicology of specific substances, or groups of substances, of particular significance. Chapter 9, titled “Carbon Monoxide”, by Robert Maynard, Isabella Myers and John Ross, and Chapter 10, titled “Hydrogen Cyanide – Physiological Effects of Acute Exposure during Fires”, by David Purser contain detailed descriptions of the uptake dynamics, calculation of blood and tissue concentrations, and physiological and other toxic effects for these mayor asphyxiant gases. In Chapter 11 Ken Donaldson, Amanda Hunter, Craig Poland and Steve Smith describe the mechanism of action of combustion-derived nanoparticles. David Purser covers the health hazards from dioxins and carcinogens in Chapter 12, and James Wakefield those from irritant gases in Chapter 13.
Analysis of the life-threatening exposures of building occupants during fires is important for building design hazard assessments, understanding the condition of survivors at the fire scene and in the emergency room, and for forensic investigation of fatal fire incidents. For these applications it is necessary to be able to model and predict the sequence of effects on occupants during fire incidents and their severity. In Chapter 14, titled “Acute Effects of Combinations of Toxicologically Active Substances and Heat on Fire Victims in Buildings and During Exposures to Outdoor Smoke Plumes”, David Purser describes the “fractional effective dose” modelling method with worked examples for actual fire scenarios.
The three chapters in the third section of the book cover clinical aspects of combustion toxicology and management of combustion product exposures. Timothy Marrs has contributed a chapter on the clinical management of cyanide poisoning, which complements Chapter 10 on hydrogen cyanide uptake and toxicology. David Baker has addressed the subject of irritant gases and acute lung injury, complementing descriptions of the toxicology of irritant combustion products in Chapter 7 and individual irritant gases in Chapter 13.
Although the severe acute health effects during and after exposure at fire scenes are well established, the significance of acute exposures to dilute outside smoke plumes from large fires is more controversial. Typically reported are nuisance odour, and minor eye and throat irritation, or breathing problems, but estimation of the predicted effects can be challenging. Wildland fires provide situations where populations, including the public and volunteer fire-fighters, may be exposed for periods from hours to several days to relatively dense smoke plumes from large areas of burning vegetation. Urban fire-fighters wear breathing apparatus during rescue and fire-fighting activities in buildings, but often have no respiratory protection in the immediate fire surroundings, when “damping down” during the last stages of a fire, or when investigating a contaminated post-fire scene. If there are significant acute or chronic health hazards from exposure to diluted smoke plumes they should show up in these groups. Jamie McAllister has reviewed this important subject in Chapter 17, titled “Health Effects in Groups Exposed to Wildland and Urban Fires”.
The two chapters in the fourth book section consist of descriptions of some unusual major conflagrations. Thomas Waite, Catherine Keshishian and Virginia Murray in Chapter 18, titled “The Buncefield Fire”, have contributed a chapter on this major incident, the largest fire in Europe since World War II, which had minimal health effects despite producing a large smoke plume over southern England for several weeks. Michaela Kendall, Mitchell Cohen and Lung-Chi Chen have described the consequences of the World Trade Center fires. The content of the effluent plume from this incident has led to considerable chronic health effects on exposed subjects. However, the main effects appear to be related not so much to the combustion products, which largely dispersed as a high altitude plume rather as at Buncefield, but owe more to the mineral dust and fibre plume released from the collapsing buildings. This illustrates an important point not covered elsewhere in the book, that mineral and related particulates can be carried up into energetic fire plumes during building fires, and then deposited in the surrounding areas depending on parameters such as the particle size, density and settlement velocity.
The final section consists of a chapter by Virginia Murray on the public health aspects of fire management in Chapter 20, titled “Providing Advice to Those Exposed to Combustion Products”.