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Page 591 Appendix L Development of Data Used in Risk Assessment This appendix provides additional information on the data needed to estimate different elements in the risk-characterization steps of emission characterization, transport and fate, exposure assessment, and assessment of toxicity. Emission Characterization The best approach to characterizing emissions is to measure the flux from each manufacturing, storage, use, or disposal facility. However, such flux measurements are generally not available, because sources are not uniform across geography or time, because they are so large (e.g., a several-square-block manufacturing site) that no point for measuring flux is apparent, or because flux measurements are so difficult and expensive, and require such detailed knowledge of local meteorology, as to be impractical. Therefore, most emission data are calculated or estimated from industry-wide averages applied to such things as "emission factors," process rates, quantities of chemical present at given locations, or numbers of individual components. Some information that might be needed to estimate and characterize emissions from a facility is provided in Table L-1. (Not all information is needed for all calculation methods.) Transport And Fate Atmospheric-chemistry models are used to determine where emitted chemicals are transported and their characteristics when deposited. Several kinds of information are needed to estimate the transport and fate of pollutants:
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Page 592 TABLE L-1 Potential Data Needs for Calculation of Emissions Process Vents 1. Volumetric flow rate of vent gas 2. Vent-gas discharge temperature 3. Concentration of individual or aggregate HAP 4. Operating hours per year of unit operation 5. Molecular weight of gas 6. Efficiency of control device 7. Production rate during measurement Fugitive Emission 1. Numbers of pumps, valves, flanges, pressure-relief valves, open-ended lines, and compressors 2. Screening level 3. Weight % of HAPS in stream 4. Percent leaking equipment 5. Other HAPS characterization 6. Frequency of leak checking Loading Emission 1. Type of cargo carrier 2. Mode of operation 3. Annual volume of liquid loaded 4. Temperature of liquid loaded 5. Weight in percent of HAP in loaded material 6. True vapor pressure of HAP loaded 7. Molecular weight of HAP 8. Efficiency of control device Storage-Tank Emissions 1. Material stored 2. Diameter of tank 3. Rim seal type 4. Tank, roof, and shell color 5. Ambient temperature 6. Wind speed 7. Density and partial pressure of chemical 8. Molecular weight 9. Vapor pressure 10. Efficiency of control device 11. Type of storage tank 12. Annual throughput 13. Number and diameter of columns Emission Factors 1. Magnitude of input into the process 2. Production level Wastewater Sources 1. Volumetric flow rate of wastewater 2. Concentration 3. Production rate during flow determination 4. Production rate during concentration determination Source: EPA, 1991c.
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Page 593 • Data on emissions of pollutants that result from production, storage, use, and disposal (discussed in previous section). • Data on physical and chemical properties of pollutants (see Table L-2). For example, the vapor pressure of a chemical pollutant plays a major role in determining exchange of the chemical between the atmosphere and other environmental media. The vapor pressures of chemicals vary widely from those of gases (such as CO, CO2, and SO2), with vapor pressures of more than 1 atm, to those of aromatic compounds, organophosphates, dioxins, and other non-criteria pollutants, which are often in the range of 10-8-10-3 atm. VOCs generally have vapor pressures of greater than 10-3 atm and semivolatile compounds vapor pressures of 10-8-10-3 atm. Lead and other inorganic species are volatile as well. Water solubilityis important, because, with vapor pressure, it determines the distribution of a pollutant in the atmosphere. Water-soluble vapors, for example, might be efficiently scrubbed from air by rainfall or fog depositionprocesses that can minimize human exposure, at least by inhalation. Suspended dust or aerosol particles can adsorb vapors of the pollutant and may also play a major role in determining the rate of exchange of chemicals between the atmosphere and other environmental media. • Data on transformation, degradation, and sequestration of pollutants in the environment (Table L-2), including chemical, biologic, and physical data: Chemical data (e.g., for atmospheric oxidation and photochemical reactions). Chemical breakdown depends on molecular structure, and for some substances breakdown is rapid. If the chemical is susceptible to nucleophilic attack, oxidation, or hydroxylation, alterations can occur rapidly and change the potential exposure dramatically. Biologic data (e.g., on degradation by metabolic action of microorganisms). Alterations by biologically mediated reactions are enormously variable, and data are needed on products of alteration; for example, do emissions tend to become more toxic or less toxic? Physical data (e.g., on solubility and gravitational settlement). For particles, gravitational settlement or sedimentation increases with the aerodynamic diameter of the particle. Physical processes that occur in the atmosphere can affect particle-removal efficiency. Hydroscopic particles can increase in size because of the accumulation of water from the vapor phase in the atmosphere; this growth can help in their removal by sedimentation and washout. • Data on rate of removal of pollutants by various routes. For example, the rate of catalytic oxidation of SO2 decreases if the water concentration in the atmosphere falls below that necessary to maintain catalyst droplets. The critical point seems to be the percent relative humidity; above this, rates of catalytic oxidation increase dramatically. In clean air, SO2 emissions are only very slowly oxidized via homogeneous reactions of the gas phase to SO2 vapor. The development of the kind of information described here is important for the pre-
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Page 594 TABLE L-2 Physicochemical Properties of Chemical and Its Atmospheric Environment Important in Transport-Fate Calculations Properties of Cbemical Properties of Environment Physical properties: Particulate load: Molecular weight For dust, other solid particulate matter Density For liquid aerosols Vapor pressure (or boiling point) Oxidant level Water solubility Temperature Henry's constant (air-water distribution coefficient) Relative humidity Lipid solubility (or octanol-water distribution coefficient) Amount and intensity of sunlight Amount and frequency of precipitation Soil sorption constant Meteorologic characteristics: Chemical properties: Ventilation Rate constants for Inversion Oxidation Surface cover: Hydrolysis Water Photolysis Vegetation Microbial decomposition Soil type Other modes of decomposition Particle properties: Size Surface area Chemical composition Solubility diction of risk associated with environmental pollutants. Such data could be used to identify the most probable routes through the environment and provide clues to the rate of degradation (alteration) from source to receptor. Knowing the probable routes and sinks, one can identify populations that should have special attention in an evaluation of potential health effects. More refined approaches might include selecting or developing models to estimate transport and fate of pollutants. • Data on types of models to predict the persistence, transport, and fate of pollutants, including their input requirements, degree of accuracy and precision, and method of validation. Several models of aerial dissipation have been reported. Exposure Assessment To evaluate human exposure for risk-assessment purposes, information is needed on the following: • Contaminants (e.g., types, in which media, at what concentrations, and for what durations).
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Page 595 • Exposed population (e.g., who is at risk, where, and under what circumstances; how long they are exposed and to what degree; and their intake of the contaminant from air, food, water, or through other relevant routes). These are described in more depth below. For the contaminant, the minimum data need include measured or estimated concentrations at the point of human contact for a specified duration. For air, concentration data are generated by sampling air and simultaneously or sequentially measuring the toxicant trapped at a given air flow rate and for a given period monitored. Beyond those generalities, analytical methods vary widely in specifics and in the key dimensions of accuracy (agreement with true value), precision (spread in data), and limit of detection. Errors can be large, particularly in trace analysis, so concerns are warranted about the quality of concentration data used in risk assessments. The following cautions are pertinent: • all data should be collected with validated methods under strict quality-assurance and quality-control standards. • A clear statement of uncertainty is fundamental to all analytic reports (Keith et al., 1983). Errors are likely to be greater with airborne trace-amount toxicants than with ''criteria pollutants," which tend to occur at much higher concentrations. This is because the relative accuracy of instruments often decreases at low concentrations. • A contaminant might be present but below the detection limit of the equipment. In this case, the concentration of the contaiminant should not be assumed to be zero. Rather, the detection limit (or some agreed-on fraction of it) should be used in the processing of data. • Vapors must be discriminated from particle-bound residues in air monitoring, especially for toxicants of low to intermediate vapor pressure. • Data on trace toxicants should be confirmed by mass spectrometry or other confirmatory method to increase confidence in the results. For the exposed population, the nature of the harm must be defined. It is important to assess the various degrees of exposure and the numbers within each identifiable set of the population, such as sets defined by age or health status. In the absence of personal monitoring data, geographic, behavioral (e.g., activity-pattern), and demographic considerations will often allow estimation of the exposure, although the estimated exposure might not be directly related to an individual's exposure. Because exposure to a specific chemical is rarely confined to a single route (although one route might dominate), the total exposure must be calculated by summing air (inhalation), dermal, and dietary (food and water) intakes. For example, pollutants that begin as "air pollutants" can generate substantial exposures through other media if they can move from air to water, soil, or vegetation.
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Page 596 A case in point is that of chlorinated hydrocarbons (polychlorinated biphenyls, toxaphene, DDT, etc.) in the Arctic; the mechanism was long-range transport in the air, but the exposure of indigenous peoples in the region is through the diet and results from the uptake of chemicals deposited in the food chain. Assessment Of Toxicity A risk analysis must include an assessment of the toxicity of a chemical, i.e., of the potential hazard the public health. Such analysis can be based on a combination of experimental toxicity and human data. Clearly, information on the incidence of disease associated with known exposures to toxicants is the most useful for human risk assessment. It is also the least available, however, because it depends on the occurrence of some unplanned or unforeseen event (e.g., an accident or malfunction in a manufacturing facility) or it is collected for a narrowly defined population (e.g., a workforce) exposed at magnitudes and for durations well beyond what the general population experiences. For ethical (and also sometimes legal) reasons, controlled dose-response studies in humans are rare. The human data that might be available for risk assessment are in three broad categories: • Clinical. Outcome and disease data are reported for members of the general population, including, if known: A description of the outcome(s). The diagnostic criteria used. A description of individual characteristics that might affect outcomes (age, pre-existing illness, etc.). Exposure history, including dose and time frames. The opinions of medical experts on the findings and the applicability of the results to the general population are also important in determining the usefulness of clinical evidence for risk assessment. • Toxicologic. Outcome and disease data are reported for persons (usually volunteers, not members of the general population) after exposure under controlled experimental conditions, including: Description of the hypotheses tested. The criteria used to select the study groups. The relevance of the outcomes to the general population or specified subpopulations (e.g., potential high-risk groups). The diagnostic and detection methods. The experimental conditions.
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Page 597 Personal characteristics that might affect exposure and outcome (e.g., age, sex, and pre-existing conditions). In addition, the method of exposure (nature and composition of toxic agent, routes of exposure, media and means of exposure, time of exposure, and doses) and statistical evaluation (e.g., point and range estimates, measures of association and significance, and dose-response and time-response relations) should be described. • Epidemiologic. Outcome and disease data are collected on groups of people in real-world settings. These data should be accompanied by: A description of the hypotheses tested. Criteria applied to select groups observed. Study methods and target-group participation rates. Diagnostic criteria for clearly defined outcomes. Exposure history and characteristics, including period and doses relevant to outcome studied. Evaluation of characteristics that might affect exposure and outcome (e.g., age, employment, activity patterns, and pre-existing health conditions). Appropriate statistical analyses of comprehensive outcome measures (e.g., point and range estimates, dose-response data, time-response analysis, and measures of association and significance) Interpretation of the findings, including analysis of generalizability, bias, and other confounding issues. References EPA (U.S. Environmental Protection Agency). 1991. Procedures for Establishing Emissions for Early Reduction Compliance Extensions. Vol. 1. EPA-450/3-91-012a. U.S. Environmental Protection Agency, Washington, D.C. Keith, L.H., G. Choudhary, and C. Rappe. 1983. Chlorinated Dioxins and Dibenzofurans in the Total Environment. Woburn, Mass.: Ann Arbor Science.
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Representative terms from entire chapter: