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INTRODUCTION This report deals with current understanding of combustion-product toxicity and fire hazard assessment. In studying these issues, the Committee on Fire Toxicology examined the number of fire-related deaths in the United States and possible causes of these fatalities. A short primer on fire and fire hazard was developed, to provide a better understanding of the status of fire hazard models and test methods. The Committee has studied the hazards associated with fires and reviewed the test methods now used to evaluate the toxicity of combustion products. Finally, the Committee developed guidelines for hazard assessment and prepared specific case studies based on them. About 5,000 people die every year in the United States as a result of fire. Technical improvements have occurred--e.g., in building fire codes, firefighting techniques, flammability standards for mattresses, the use of fire detection devices in homes, the use of sprinkler systems in public buildings, and public fire- safety awareness--and the annual number of fire deaths has been decreasing for 20 years. But it is commonly believed that the fire death rate might have decreased even more if new materials of synthetic origin had not come into use. Most fire-related deaths are due to inhalation of toxic gases in smoke, not to fire or heat itself. Carbon monoxide (CO) is thought to be the most common cause of fire-related death. Because of its high affinity for hemoglobin, relatively small concentrations of CO can saturate the blood, form carboxyhemoglobin (COHb), and deprive tissues of oxygenation. In general, COHb 12
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13 concentrations above about 30-40% seriously impair the ability of humans or animals to perform a task, and concentrations above 50-60% can be fatal. Some people can function even at COHb concentrations up to 50%, because a hypoxia challenge, whether caused by oxygen (O2) depletion or CO accumulation, increases cerebral blood flow. In the face of decreasing O2 availability, however, this compensatory mechanism eventually fails to deliver enough O2 to the brain, and the victim loses consciousness. If the victim survives, severe necrologic disorders can sometimes be seen after a period of apparent recovery. It was recently suggested that hydrogen cyanide (HCN) can also contribute importantly to the overall toxic hazard of fire. The neuropathologic patterns after HCN and CO exposure appear to be similar; thus, despite the difference in mechanisms of inducing hypoxia, the two gases might damage neural tissue in an identical manner. In addition to toxic gases, fire generates many respiratory irritants--such as hydrogen chloride, acro- lein, and sulfur dioxide--that can cause necrosis and pulmonary edema. Delayed deaths have reportedly occurred after what appeared to be mild exposures to these gases. A review of the major combustion products and their indi- vidual contributions to the toxic hazard affords only partial insight into the total toxic hazard in fire. The overall hazard is associated with exposure to mixtures of the individual combustion products and with their effects in preventing escape from the fire environment. The irritant effects of fires on survivors can be classified as early and late. The early effects are usually associated with damage to the upper airways and the respiratory tract in general. The most common late sequela of a single exposure is some degree of pulmonary obstruction. Studies of firefighters have shown, with some variability, that a long-term consequence of repeated fire exposure can be the development of an obstructive, restrictive, or mixed ventilatory defect. With regard to the possible increase in cancer incidence in firefighters, the results of several studies have suggested higher than normal incidences of a variety of cancers, but there seems to be no conclusive relationship between the type of cancer and exposure history. The roles of the technical improvements in fire prevention and detection mentioned above in fire safety and fire loss are not well understood. It is therefore
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14 not possible to determine whether the use of "new materials" is associated with different or increased hazard. However, even in the absence of a demonstrable increase in the role of toxicity in fires, the concern over new materials warrants attention. It is appropriate to continue the deliberate process of developing methods for analyzing and predicting product response to fire under expected conditions of use. Such methods will be useful both for selecting materials for specific uses and for substantiating regulatory positions. The hazards presented by fire are best assessed through consideration of all the characteristics of fire. The hazard associated with smoke depends both on how rapidly a material produces smoke and on the toxic and irritant potency of the smoke once produced. In general, smoke hazard cannot be characterized unless both kinds of information--production rate and potency--are taken into account. Smoke production rate is a function of a material's fire properties and of the environment in which the fire takes place. Therefore, hazard depends on the situation: laboratory measurements of materials them- selves do not predict hazard until the measured properties are evaluated in the context of how the material is to be used and how it might burn--i.e., in a given fire sce- nario. The two means of providing information on the fire scenario are full-scale simulations and mathematical fire models. The expense and cumbersomeness of full-scale fire simulation make reliance on mathematical models desirable, and the Committee believes that models have been developed to the point where they can be used-- cautiously--as a basis for fire hazard assessment.