Science and Judgment in Risk Assessment (1994) / Chapter Skim
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Appendix F: Uncertainty Analysis of Health Risk Estimates
Appendix F: Uncertainty Analysis of Health Risk Estimates
Pages 453-478
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From page 453... ...
We focus here on the uncertainties due to the input parameters for a given health risk assessment model. Prepared by Christian Seigneur and Elpida Constantinou, ENSR Consulting and Engineering, 1320 Harbor Bay Parkway, Alameda, California 94501, and Thomas Permutt, Department of Epidemiology and Preventive Medicine, University of Maryland, 655 West Baltimore Street, Baltimore, Maryland 21202 for Leonard Levin, Electric Power Research Institute, 3412 Hillview Avenue, Palo Alto, California 94303.
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From page 454... ...
This methodology is applied here to the uncertainty analysis of the carcinogenic health risks estimated as due to the emissions of a coal-fired plant. UNCERTAINTY ANALYSIS METHODOLOGY Overview A health risk assessment model combines a number of models to simulate the transport and fate of chemicals in air, surface water, surface soil, groundwater and the foodchain.
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From page 455... ...
When dealing with a complex model, such as a multimedia health risk assessment model, sensitivity analysis should be performed for each individual model component as well as for the overall model.
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From page 456... ...
The value of the sensitivity index may be a function of the value of the other model input parameters except for cases where the relationship between the model output and the input parameters is linear. Even though the sensitivity index, as defined above, sufficiently describes the effect on the model result for a given change in the input parameter, it does not provide a measure of the range of variation in the model output, given the expected range of variation of the input parameter.
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From page 457... ...
Parameterization of the Mode Response Surface Construction A multimedia health risk assessment model typically involves a large number of input parameters and comprises several individual models for simulating fate and transport, exposure, dose, and health effects. Such a model can be computationally very demanding and performing an uncertainty analysis for a large number of parameters may, therefore, not be feasible.
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From page 458... ...
Then use the pairs of parameter sets X'...Xk and corresponding model results Y,....Yk to construct the response surface. A simple example of a response surface can be that of the atmospheric transport model, in which the air concentration, Ca can be expressed in terms of four independent influential parameters and six constant parameters as follows: where: Ca Qe vS Ca = QeFlF2 F
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From page 459... ...
are implicitly included in the constant parameters of the response surfaces. Selection of the Probability Distributions for the Input Parameters Once the influential parameters have been identified, and the response surfaces for each model component constructed, probability distributions must be selected to represent each one of the parameters.
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. If the uncertainty analysis procedure was performed correctly, this probability distribution should represent a more complete and realistic characterization of the anticipated health risks, as it provides a range of possible values accompanied by their corresponding likelihoods instead of a single, deterministic point estimate.
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Carcinogenic and noncarcinogenic health effects were calculated in each of the subregions considered in the study area. The results subject to the uncertainty analysis presented in this report correspond to the carcinogenic health effects in the subregion of maximum risk.
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UNCERTAINTY ANALYSIS Sensitivity Analysis Sensitivity analysis of the individual model components as well as the overall multimedia health risk assessment model was performed to help identify the influential parameters. A total of 49 parameters were examined, and 22 were selected to be included in the final uncertainty analysis, based on their calculated sensitivity/uncertainty indexes.
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From page 463... ...
Figure 4 provides a graphical presentation of the derived ISC-LT response surface. The complete set of equations of the simplified multimedia health risk assessment model is presented below: · Atmospheric Transport Model (Component 1~: Ca = (xQeF1F2Al]
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R = D~CPF~ + D2CPF2+A7~ where: R = Total Carcinogenic risk; CPF~ = Inhalation cancer potency factor; CPF2 = Ingestion cancer potency factor; A7j= Constant j for model component 7 It should be noted that the full set of equations presented above applies only to the arsenic case. Cadmium and chromium are not considered carcinogenic through noninhalation pathways.
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In a health risk assessment, the uncertainty associated with the health effect parameters (i.e., cancer potency factors in the case of the present application) is of major importance.
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From page 466... ...
CPF by a triangular distribution defined by the best estimate, upper, and lower bounds. Monte Carlo Analysis- Health Risk Probability Distribution The derived response surfaces were combined in a simplified spreadsheet model which was coupled to the software package BRISK (Palisade Corporation, 1991)
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From page 467... ...
The results indicated that the deterministic risk value calculated in the original risk assessment study was a conservative estimate, corresponding to a higher risk percentile on the estimated risk probability distribution. ACKNOWLEDGMENTS This work was performed under contact No 3081-1 with the Electric Power Research Institute, Palo Alto, California.
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From page 468... ...
~ . 1~ ~ Input Data 1i Uncertainty Analysis Uncertainty in Model Inputs .
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469 Leo< _ ~d l o MU =01 o=~ · ~ ~ ~ ~ / E ._ \ ~ \ · 1 .
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470 8 ~` I' SCIENCE AND JUDGMENT IN RISK ASSESSMENT To -o. FIGURE 4 ISC-LT Response Surface __
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From page 472... ...
, Lake Water Exchange Rate 18 19 20 22 Arsenic Chemical Decay Coefficient in Lake i Surface Soil Depth I ~ Exposure Duration | Exposure Starting Time j Cation Exchange Capacity ~23 Arsenic Chemical Decay Coefficient in Soil 1 1 !
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From page 473... ...
I 4s ~ Inhalation Cancer Potency Factor-Arsenic I CPF1l (kg~/mg) l 46 Ingestion Cancer Potency Factor - Arsenic I, CPF21 (kg~/mg)
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From page 478... ...
478 TABLE 3 Probablity Distribution Selection SCIENCE AND JUDGMENT IN RISK ASSESSMENT .
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