Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
EXECUTIVE SUMMARY Safety in mass transit vehicles is the concern of the Urban Mass Transportation Administration (UMTA), the sponsor of this project. Although UMTA is not primarily a regulatory administration, it does issue guidelines and recommended practices that provide technical assistance for the transit industry. An example of this function that is relevant to the present study concerns the use of polymeric materials in transportation vehicles and the potential fire hazard associated with them. In 1984 UMTA issued a set of recommended practices for the selection of materials to be used in rail transit vehicles. These recommend ed practices provide test methods and materials acceptance limits for flammability and smoke emission characteristics of construction materials. The recommended practices do not address the toxicity of combustion products likely to arise from the pyrolysis, smolder- ing, or burning of polymeric substances found in mass transit vehicles. To assist the transit industry in addressing the fire and toxicity characteristics of transit vehicle construction materials, UMTA requested that the National Research Council (NRC) undertake a study of this issue. The study was conducted by the NRC's National Materials Advisory Board (NMAB), with assistance from the Transportation Research Board. The objective of the study was to recommend to the sponsor guidance for the selection of construction materials for mass transit vehicles (e.g., buses, subway cars) that would minimize the risks of toxic effects on passengers in the event of a fire. This report, intended to provide the relevant technical information that formed the basis for arriving at the conclusions and recommendations of the committee, shows the noteworthy technological progress made in recent years toward understanding and quantify- ing the smoke toxicity factors involved in fire hazard assessment. That understanding has led to increased attention to the fire growth parametersignition, flame spread, heat release rate, mass burning rate, and transport of combustion products as they impact toxic hazard. While smoke toxicity data are not, in themselves, measures of toxic hand, they may be used, together with fire performance data, to estimate toxic hazard.
2 Consideration of the technical information presented in this report led to a rationale for the assessment of potential toxic hazards in the event of fire involving materials and/or products used in mass transit vehicles, which makes use of hazard assessment engineering calculations. These engineering calculations require the definition of a fire scenario' the use of a fire growth model, and toxicity data for the materials and/or products to be evaluated. · The defined fire scenario. The scenario should include the occupancy character- istics of the vehicle, the class of materials and/or products involved and the type or classification of the fire that is of concern. An example of a scenario is the case of a rapidly developing, well ventilated fire involving upholstered seating in a bus occupied primarily by healthy young adults. · The fire growth model. Fire growth models exist that, although not perfect in all respects, are perhaps satisfactory for the purpose of estimating the rate of temperature increase, the development of visual obscuration, and the concentration of combustion products as a function of time for a fire scenario. The latter quantity, expressed in units of g~m~~. min. is the amount of toxic smoke to which passengers would be exposed over time. It is to be noted that the fire growth models require as input, information on the heat release/mass burning rates of the combustible material involved in the fire scenario. These data are inferred from certain laboratory fire tests, some of which have been or are being adopted as standards both in the United States (American Society for Testing and Materials) and also internationally (International Organization for Standardization). Among such tests are those involving the cone calorimeter, the rate of heat release calorimeter, and the furniture calorimeter. · Toxicity data for the materials and/or products under evaluation. These data are obtained from laboratory tests that utilize a combination of analytical data along with exposure of rodents, and yield two useful types of information: I) the principal typefs) of intoxication, i.e.' asphyxiation, sensory or pulmonary irritation, etc. and 2) the lethal toxic potency (LC50) of the smoke in units of g~m~3over a given exposure period. Toxic potency data obtained from laboratory tests are, however, subject to the following limitations and/or considerations. 1. No single laboratory combustion device is appropriate for all materials and products under the conditions of all fire types and stages. Therefore, there can be no universal Smoke toxicity testy The laboratory combustion device used in a test should be chosen and operated to approximate as closely as possible the conditions of the type of fire being examined. (For example, laboratory scale combustion furnaces may, under certain conditions, produce less carbon monoxide (CO) per unit mass of sample burned than would occur in a real fire. Thus, laboratory LC50 values may need to be Adjusted for use in hazard calculations.) 2. All LCso values have an associated level of statistical confidence. Furthermore, interlaboratory comparisons suggest LC50 determinations can vary by a factor of about 2e5. 3. Although calculated from data based on exposure of rodents, LC50 values can be extrapolated to human exposure with reasonable confidence for asphyxiants and for pulmonary irritation. Sensory irritation is not addressed with current laboratory smoke toxicity tests. Also, its relevance to incapacitation of humans has not been demonstrated. (Hazard assessments currently tend to set threshold tenability levels for acid gases and other combustion products known to have irritant properties.)
3 <. 4. Currently employed laboratory smoke toxicity tests do not directly measure incapacitating effects of smoke inhalation. Incapacitation must be inferred from LC50 values. The current best strategy for evaluation of the potential toxic hazards in the event of fire involving materials and/or products involves the following elements: Identify each product/occupancy/fire scenario. 2. Using fire performance data for candidate materials obtained from relevant laboratory tests, engineering calculations appropriate to each fire scenario should be carried out to determine the sensitivity of the predicted toxic hazard to toxic potency. This is done by using a range of arbitrary toxic potency values. (Included in the calculations are extremely low I~C50 values for modeling hypothetical fire toxicants exhibiting either unusual toxic effects or unusual toxic potency.) a. If the predicted toxic hazard of a scenario is found to be relatively insensitive to toxic potency, and based on expert judgment considering all available information (chemical structure and composition, literature and experiential data, etc.), there may be no need to determine actual toxic potency values. Attention should be concentrated on other fire growth parameters. b. If the predicted toxic hazard of a scenario is deemed significantly sensitive to toxic potency, laboratory smoke toxicity testing should then be conducted to identify the principal toxic effects and to establish LC50 values for use in the engineering calculations. Smoke toxicity testing should be conducted as follows: · Select laboratory combustion device and operating conditions consistent with the type and stage of fire in the scenario. · Conduct analytical tests for CO, carbon dioxide, oxygen, hydrogen cyanide, hydrogen chloride,... · Estimate LC50 from analytical data using appropriate calculations. · Verify L`C50 and toxic effects with animal experimentts) to ensure that the monitored toxicants account for the estimated toxic effects and that there is no evidence for unusual or unexplained toxicity. · Use experimentally determined toxic potency data for candidate materials and/or products in the hazard engineering calculations to characterize their relative fire safety implications. Ultimately, fire risk assessment might be used in the evaluation of the life safety impact of materials used in transit vehicles. Although case studies of materials used in transit vehicles would appear to match well with the criteria for credible use of fire risk assessment, validation of the methodology is limited by the small number of fatal incidents in uncontrolled fires.
4 In addition to acknowledging the value of well-instrumented testing over a range of spatial scales, the committee's overall recommendation is that the selection of candidate materials for use in transit vehicles should be made following analysis of each material's fire properties and smoke toxic potency within the context of specific plausible fire scenarios.