DOSE ESTIMATES

Doses to the public from FMPC releases were assessed through atmospheric, surface-water, and groundwater pathways. All important determinants of the dose received were included: the distance and direction of a person's residence and work location in relation to the center of the facility; the time of day and duration that the person spent in the area; the time spent indoors and associated breathing rates and air-turnover; the time spent outdoors and associated breathing rates; the use of contaminated water for drinking, irrigation, and swimming; and the ingestion of contaminated foodstuffs and soil. Those characteristics were allowed to vary for several periods and individual lifestyles (from preschool to employment). However, the RAC authors do not clearly present which parameters are most influential in determining dose, nor do they present the range or distribution of doses to the public from the FMPC releases. Rather, they develop 9 scenarios to represent people born from 1946 to 1970 who lived continuously in the 10-km area around the facility. These scenarios were presented as representative, but the doses presented were biased to the higher end by the focus on people born during the early operation of the facility (6 of 9 scenarios) and the assumption that people spent all this time in the area under consideration. It is unclear whether people who have lived near FMPC will be able to relate the scenarios to their own experiences. Moreover, given the paucity of information on the importance of various characteristics, it would be impossible for a given person to determine whether the difference between his or her experience and the scenarios might lead to a higher or lower dose.

The main purpose of estimating doses from the radon-decay products in the task 6 draft report is to be able to compare them with doses to the organs and tissues from other sources. The committee makes the following recommendations regarding the dose comparison. First, the methods used to calculate lung doses from radon decay products and from other sources must be consistent for the dose comparison to be meaningful. RAC has calculated the dose to the bronchial epithelium by using an NCRP method (NCRP 1984), but the average lung dose from other inhaled radionuclides is calculated by using the method from ICRP Publication 30 (ICRP 1988). Second, the uncertainties in such estimates should be acknowledged and quantified. In practice, the risk from radon-product exposure is estimated



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A REVIEW OF THE RADIOLOGICAL ASSESSMENTS CORPORATION'S FERNALD DOSE RECONSTRUCTION REPORT DOSE ESTIMATES Doses to the public from FMPC releases were assessed through atmospheric, surface-water, and groundwater pathways. All important determinants of the dose received were included: the distance and direction of a person's residence and work location in relation to the center of the facility; the time of day and duration that the person spent in the area; the time spent indoors and associated breathing rates and air-turnover; the time spent outdoors and associated breathing rates; the use of contaminated water for drinking, irrigation, and swimming; and the ingestion of contaminated foodstuffs and soil. Those characteristics were allowed to vary for several periods and individual lifestyles (from preschool to employment). However, the RAC authors do not clearly present which parameters are most influential in determining dose, nor do they present the range or distribution of doses to the public from the FMPC releases. Rather, they develop 9 scenarios to represent people born from 1946 to 1970 who lived continuously in the 10-km area around the facility. These scenarios were presented as representative, but the doses presented were biased to the higher end by the focus on people born during the early operation of the facility (6 of 9 scenarios) and the assumption that people spent all this time in the area under consideration. It is unclear whether people who have lived near FMPC will be able to relate the scenarios to their own experiences. Moreover, given the paucity of information on the importance of various characteristics, it would be impossible for a given person to determine whether the difference between his or her experience and the scenarios might lead to a higher or lower dose. The main purpose of estimating doses from the radon-decay products in the task 6 draft report is to be able to compare them with doses to the organs and tissues from other sources. The committee makes the following recommendations regarding the dose comparison. First, the methods used to calculate lung doses from radon decay products and from other sources must be consistent for the dose comparison to be meaningful. RAC has calculated the dose to the bronchial epithelium by using an NCRP method (NCRP 1984), but the average lung dose from other inhaled radionuclides is calculated by using the method from ICRP Publication 30 (ICRP 1988). Second, the uncertainties in such estimates should be acknowledged and quantified. In practice, the risk from radon-product exposure is estimated

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A REVIEW OF THE RADIOLOGICAL ASSESSMENTS CORPORATION'S FERNALD DOSE RECONSTRUCTION REPORT from the radon decay-product exposure, not from a dose estimate. The committee notes that the risk per unit dose for radon decay products in this report has been specifically calculated to be consistent with the risk per unit exposure from the BEIR IV report (NRC 1988) and is different from the risk per unit dose for other radionuclides. The lung doses from radon decay products and uranium in figure 46 of the RAC report are not directly comparable. Dose reconstruction is necessarily both a retrospective activity and a site-specific one that often requires the reconstruction of events that occurred decades earlier. Reconstruction, depending as it commonly does on incomplete and often imperfect data, has its uncertainties. The sources of uncertainty include the need to estimate critical parameters in the modeling process when data are scarce or not available; the need to extrapolate observations made at one time to other times for which necessary data are absent; the need to assess the reliability of individual-specific information on lifestyle and residence, for example, in translating estimates of health risk from one population to another to which the risks might not be fully applicable; and the need to allow for the approximate nature of mathematical models of complex physical and biologic processes. Identification of those sources of uncertainty and estimation of their possible effects on the results of a dose reconstruction make up what is referred to as uncertainty analysis. The intent of such analysis is to determine the level of confidence that can be placed in the results of the reconstruction itself. Uncertainty analysis provides a measure of the credibility to be assigned to the reconstruction and is an integral part of the reconstruction process. The RAC report has gone to exceptional lengths to assess and allow for uncertainties in the components that define exposure (uncertainties in the size of the environmental radiation releases and environmental transport). However, uncertainties pertaining to interindividual variation in the intake of radioactive materials were taken into account inadequately. In particular, as previously stated, 9 hypothetical “representative” scenarios pertaining to variations in when and where persons lived and to some lifestyle and dietary habits were defined. The variations were then assumed to define each individual's intake and metabolism of the radionuclides in question without error (that is, no uncertainty was attached to these factors). Even if one assumes that a hypothetical individual's lifestyle and dietary factors have no uncertainty because they are “givens” for a particular scenario, uncertainties are still

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A REVIEW OF THE RADIOLOGICAL ASSESSMENTS CORPORATION'S FERNALD DOSE RECONSTRUCTION REPORT attached to almost all these factors. To name a few, for the factors pertaining to time spent indoors and outdoors, there would be an uncertainty range attached to the “indoor particle factor” (ratio of indoor to outdoor particle concentrations) that would be set at 0.7:1. There would likewise be a distribution of air turnover rates (0.4 h -1 assumed) in houses, schools, and so on. In the estimated radionuclide intakes from various classes of foods and water, the assumption is made that absolute intakes were constants for a given age and sex (for example, a 7-year-old girl would eat x amount of vegetables per year without variation) and that the radionuclide contamination would be an equivalent constant (for example, for all possible mixes of vegetable types, meat types, or fish types). There would clearly be distributions of radionuclides that would impart uncertainty to the estimates that should have been considered in arriving at overall estimates of uncertainty in doses. Physiologic factors would also contribute uncertainty: variations in breathing rates, lung size, and particle-expulsion efficiency; variations in absorption, metabolism, and retention times of radionuclides; and so on. Although the RAC report considers only the 9 hypothetical “representative” persons with fixed characteristics, if the doses to the Fernald population are to be modeled realistically, uncertainty estimates have to factor in uncertainties in the assessed or imputed lifestyles and intakes and uncertainties in the dose-conversion factors. For instance, individual reports of the percentage of time spent indoors or the degree of indoor vs. outdoor physical activity might be imprecise, inaccurate, or both. Intensive studies to validate reported dietary intakes of nutrients have typically reported validity coefficients in the range of only 0.35-0.7 (for example, Willett and others 1985; Rimm and others 1992); that indicates appreciable uncertainty in the assessment of dietary intake. According to the report (table S-14, page S-25), uncertainties in the estimation of biologic factors involved in estimating risks associated with uranium and thorium include the following:   5th percentile 95th percentile Uncertainty in RBE 0.5 1.5 Uncertainty in statistics 0.5 1.5 Other uncertainties 0.5 2 Combined uncertainty 0.33 2.6

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A REVIEW OF THE RADIOLOGICAL ASSESSMENTS CORPORATION'S FERNALD DOSE RECONSTRUCTION REPORT The uncertainty due to the above factors alone results in an 8-fold difference between the 5th percentile and the 95th percentile. In the case of radon (table S-17, page S-27), the range of uncertainties is as follows:   5th percentile 95th percentile Statistics 0.59 1.7 Dosimetry 0.71 1.4 Modeling and smoking 0.5 2 Other 0.5 2 Combined 0.33 3.3 As mentioned previously, the committee believes that the imputed range of uncertainty for radon dosimetry, given above, is unrealistically narrow. A final comment should be made about why the newer respiratory tract model of the International Commission on Radiological Protection (ICRP 1994a) was not used in the RAC report. The principal reason is that at the time of the task 4 report (1993), in which mathematical models were developed to describe the movement of radionuclides released from the site into the environment and to calculate the radiation doses to residents, the new ICRP lung model was not yet available. In the present work, low- and high-linear energy transfer (LET) dose coefficients were developed, as well as age-related differences in the dose coefficients. Radiation-risk programs from the US Environmental Protection Agency (EPA) and ICRP report 30 (ICRP 1988) models were used in adjusting dosimetry for low and high LET. Oak Ridge National Laboratory reports were used to develop age-dependent effects on dose coefficients. Under the circumstances, that is acceptable, and a change in methods would not substantially affect the outcome of this project, given the inherent range of uncertainties. Although an assessment of the magnitude of the intraindividual and interindividual uncertainties compared with the magnitude of uncertainties in the environmental-release and transport components has to our knowledge not been performed, the committee suspects that the 2 magnitudes are similar. Accordingly, the committee concludes that the magnitude of dose uncertainties is probably substantially underestimated in the RAC report.