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Uncertainty and Variability
Pages 246-260

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From page 246...
... Comprehensive assessments also involve treatment of the variability and uncertainty associated with those data.1 This chapter addresses how, in the context of waste incineration, uncertainty and variability are defined, characterized, and treated in the risk-assessment and riskcommunication process. This process includes consideration of hazard identification, dose-response characterization, emission-source characterization, exposure assessment, risk characterization, and risk communication.
From page 247...
... As uncertainty and variability become larger, it becomes more difficult for interested or affected parties to decide how to interpret results and assign relevance to the magnitude of estimated risk. If the range is too large, different people might base their interpretation of the results on their prior opinion of waste incineration.
From page 248...
... conceptual model (the influence diagram) , formulation of the computational model, estimation of input values, and calculation and interpretation of results, including uncertainties.
From page 249...
... Uncertainty analysis involves the determination of the variation or imprecision in the output based on the collective variation of the model inputs, whereas sensitivity analysis involves the determination of the size of the changes in model output as a result of changes of known size in individual model inputs. Data and Modeling Adequacy Uncertainty in model predictions arises from a number of sources, including specification of the problem, formulation of the conceptual model, formulation of the computational model, estimation of input values, and interpretation of the results.
From page 250...
... Influence of Uncertainty on Perception of Risk The decision to spend money to identify, estimate, and manage risk carries with it an implicit valuation of the risk being controlled. Because of the uncertainty inherent in risk characterization and risk management, it is important to consider how individuals and societies value uncertainty in knowledge of adverse consequences.
From page 251...
... Thus, assessment of health risk for waste-incineration facilities should include consideration of such variations, including emissions resulting from off-normal activities, in addition to routine stack and fugitive emissions. Because they involve unusual events on which there is little advance information, assessing the frequency of occurrence and progression of off-normal emissions is likely to be a highly uncertain process.
From page 252...
... . Environmental Transport and Human Exposure Some form of exposure assessment is required in a number of health-related assessments, including risk assessments, status and trends analyses, and epidemiologic studies.
From page 253...
... Despite their integrative nature, there can be considerable disagreement on the validity of animal data for use in assessing human hazards. In all epidemiologic studies, the focus of attention is on health effects in human populations.
From page 254...
... For lipophilic contaminants such as dioxins, furans, and polychlorinated biphenyls and for such metals as lead and mercury, exposures through food have been demonstrated to be major contributors to total dose in non-occupationally exposed populations (Travis and Hester 1991~. Overall uncertainties in estimating potential doses through food chains are much larger than uncertainties associated with direct exposure pathways (McKone and Ryan 1989; McKone and Daniels 1991~.
From page 255...
... Travis and Arms meat concentration versus (1988) intake for beef cattle, Bt Biotransfer factor for 7.9 x 10-9KoW 6 day/kg(milk)
From page 256...
... Biologic models Biologic models provide qualitative or quantitative descriptions of the site and mechanism of action. They attempt to define how and where an agent acts to produce disease, how sites and mechanisms differ across species, and what effect the differences have in the prediction of human disease.
From page 257...
... A clear assignment of the variation in input values to variability or uncertainty requires a careful and clear summary and justification of the assumptions used for each aspect of the modeling in which these input variables are used. Second, a sensitivity analysis may be used to assess how model predictions are affected by variation in input values.
From page 258...
... · Propagate the uncertainties through the model to generate a probabilitydensity function of predicted outcome values. From the probability-density function of predicted values of the outcome variable, derive confidence limits and intervals to provide a quantitative statement about the effect of input uncertainty on the model predictions.
From page 259...
... CONCLUSIONS There are many reasons for uncertainty and variability in the information used to assess possible health effects of waste incineration. There are large variations from facility to facility with regard to types of waste combusted, operating practices, allowable emission levels, emission-control technologies, types of chemicals emitted, environmental characteristics, proximity to other sources of contaminant exposure, frequency of off-normal emissions, and the biologic and behavioral characteristics of the people who might be exposed to the contaminants in the environment.
From page 260...
... Decisionmakers should consider individual and societal values regarding uncertain adverse consequences by eliciting individual or societal preferences, using decision analysis, and applying theories of science policy, social-welfare economics, and ethics. · Assessments of public-health risk posed by waste incineration should consider, through the use of sensitivity analyses or otherwise, the importance of emissions resulting from off-normal activities in addition to routine stack emissions or fugitive emissions.


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