Exposure of the American People to Iodine-131 from Nevada Nuclear-Bomb Tests Review of the National Cancer Institute Report and Public Health Implications (1999) / Chapter Skim
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2 Review of the NCI Radiation Dose Reconstruction
2 Review of the NCI Radiation Dose Reconstruction
Pages 17-44
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From page 17... ...
· Whether there are identifiable weaknesses in the techniques or assumptions used to perform the dose reconstruction. · Whether the amount of detail presented in the report about thyroid dose estimates of representative individuals by county, by age group, and by milkdrinking habits is meaningful and useful to the public and the public-health establishment.
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From page 18... ...
.i BASIC ASSESSMENT Given the challenges posed by the paucity of the data relevant to dose reconstruction, the disparity between the original purpose of the data collection and the requirements for dose reconstruction, the need to derive amounts of I-131 ingested on the basis of measured deposition of gross beta activity, and the problems associated with drawing conclusions about counties and states based on incomplete information, this committee was generally impressed by the clarity and thoroughness of the NCI report and the obvious attention to detail. This committee does note, however, that some of the procedures used in dose reconstruction were not sufficiently detailed (for example, the discussion of kriging)
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From page 20... ...
in the dose estimates in these two counties is caused by uncertainties in the total amount of I-131 coming from the cumulative tests. In this case, the uncertainties in the county dose estimates reflect systematic variation only, and even though the 95 percent confidence intervals for the county estimates overlap, it is likely that if the dose estimate in Custer County is too high, the dose estimate in gingham County is too high as well.
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From page 21... ...
DOSES AND RISKS FROM OTHER RADIONUCLIDES Although the NCI report focuses on exposure to I-131, as its mandate required, it warrants noting that about 200 radionuclides are produced and released from atomic weapons tests, and exposure to these radionuclides could pose health risks to organs other than the thyroid. For the very large tests that were conducted by Russia and the United States, but not at the NTS, there was sufficient energy released to carry most of the radioactive debris into the stratosphere where it resided with a half-time of a few years.
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From page 22... ...
Tests conducted aloft, either tethered to a balloon or dropped from the air, contained substantially less debris, and the nearby fallout pattern for balloon or air shots was much less than it was for tower shots. Although the fallout debris was distributed through many layers of the atmosphere, the major part for tower shots would exist in two layers, one at lower elevations that distributed into the lower level wind flow at the time of the test and the other at much higher elevations.
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From page 23... ...
It is clear, however, that individual dose estimates must be more uncertain than are county averages. As seen in Chapter 6 of the NCI report, age at exposure is important in determining thyroid dose estimates, as is individual milk consumption.
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From page 24... ...
If this assertion can be interpreted as a 95 percent confidence interval for true dose, then it corresponds to an individual dose GSD of 2.27, which seems inconsistent with the range of uncertainties given for the county estimates; the average GSD over all counties is 2.24. The near equivalence of these county and individual numbers might indicate that the county GSD actually incorporates uncertainties in individual doses.
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From page 25... ...
The NCI report has prepared an excellent and thorough compilation of the events at the Nevada Test Site, including relevant configurations (air bursts, tower shots, underground tests) and explosive yields in kilotons (kt)
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From page 26... ...
In the mid-1980s, these data were used to form the Town Data Base by the Offsite Radiation Exposure Review Project (ORERP) , a study that reconstructed radiation doses from all radionuclides in the area within several hundred kilometers of the Nevada Test Site (Church and others 1990~.
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From page 27... ...
The real distinction between interpolating using kriging versus using the AIPC method is that kriging involves fitting parameters in a statistical model for the gummed-film measurements prior to performing the interpolation. Because kriging is the most important method used to obtain deposition estimates, much of this section is devoted to reviewing the NCI report' s application of this method.
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From page 28... ...
, the HASL gummed-film stations provide a set of measurements that have both characteristics, and thus, kriging is a reasonable technique for estimating deposition of I-13 1 from the Nevada Test Site. Kriging, in any specific setting, involves considerable modeling before interpolations are computed.
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From page 29... ...
estimate that the errors in deposition estimates for a given site and shot are probably no worse than a factor of 2 and that uncertainties in estimates of cumulative deposition are much lower. Nevertheless, that discussion along with the data in Table 7 of that reportindicate that, even for total deposition, the fraction of variability attributable to measurement error could still be a significant portion of total deposition variability, perhaps 50 percent, depending on how the rather vague comments about uncertainty are interpreted.
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From page 30... ...
Incorporation of measurement error into the kriging generally requires outside information concerning the magnitude or variance of measurement error at each data location and can complicate assessments of prediction error. Turning to the number and location of the HASL stations, the adequacy of the monitoring system for the production, at the county level, of reasonable estimates either of deposition from a specific shot or of total deposition, depends greatly on the basic structure of the spatial correlation among measurements.
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From page 31... ...
Although it seems unlikely that important differences, such as those in wide-area average deposition estimates, would result, estimates at the small scale could be affected considerably. For example, the explicit incorporation of random error in the gummed-film estimates at the HASL locations might lead to much less variability in the interpolation estimates at the county level.
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From page 32... ...
The relationship between distance from the test site and particle size is based on a limited amount of data, which are summarized by Simon (1990~. The mass interception fraction for wet deposition was found to depend on the amount of rainfall or standing crop biomass.
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From page 33... ...
In the 1950s there were just as many, or possibly many more, people living on small dairy farms as there were people who owned one or two cows. It is important to distinguish more clearly the consumption of fresh milk from cows on small dairy farms from milk that came from true backyard cows.
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From page 34... ...
The uncertainty analysis is designed to estimate the variability of the county average concentrations of I-131 in fresh cows' milk, not to give more-individualized estimates of variability. Because it is the variability of the county level estimates that is at issue, the method used seems to exaggerate the importance of uncertainties in the milk transfer coefficients and probably in the mass interception factors as well.
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From page 35... ...
Going from Figure 4.25, which shows integrated concentrations of I-131 in fresh cows' milk, to Figure 6.2, involves multiplying the estimates of each county's integrated concentrations in fresh cows' milk by the estimates described in Chapter 5 of the NCI report for the fluid-use milk production of each county. The estimates in Figure TS.3 for integrated concentrations in volume-weighted mixed milk are obtained from volume-weighted fresh milk by application of milk transfer functions, VOL(i,j)
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From page 36... ...
· State statistics on the amount of milk used for non human consumption on State statistics on the amount of milk used in manufacture of food prod Per capita milk consumption in each state. · Population in each county.
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From page 37... ...
The uncertainty analysis provided on pages 6.4-6.6 of the NCI report starts with the uncertainties in the county estimates of time-integrated concentrations of milk. These initial uncertainties, as discussed above, might be too large because they appear to neglect the effect of pooling in averaging out variations in such factors as milk transfer coefficients, fm, except for Category 1 milk, where it is more reasonable that intake depends on the production of only a few cows.
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From page 38... ...
The data used in estimating infants' and children's consumption of milk are necessarily somewhat less certain than are data for overall per capita consumption, but again the likelihood of important systematic errors in these data seems small. FROM INTAKE TO THYROID DOSE National Cancer Institute county estimates of thyroid dose resulting from consumption of cows' milk are developed according to age and sex, which influence both the amount of milk consumed and the dose conversion factor (mrad/ nCi)
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From page 39... ...
To give perspective to the estimated average thyroid dose, this committee observes that standards for radiation exposure of the public evolved during the 1950s perhaps because of the presence of radioactive fallout from weapons testing in the United States and elsewhere. As developed in Appendix E of this report, the standards available during the Nevada Test Site operations were for protection of radiation workers, with limits for thyroid exposure that ranged from 15 to 30 rem per year.
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From page 40... ...
Charles Land of NCI. The material states that the GSD for the collective dose estimate is 1.4, so the per person thyroid dose could vary by a factor of 2 in each direction (that is, the average individual doses lie between 0.01 and 0.04 Gy)
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From page 41... ...
This committee, by independently validating the "order of magnitude" of the NCI's estimate of collective dose, is confident that there has been no gross overor underestimation, but it is less confident that the uncertainty in the estimate has been realistically determined by NCI. It should, however, be noted that within these collective and average doses are concealed large differences in individual doses that depend on factors such as lifestyle and age at the time of testing.
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From page 42... ...
42 Cal o o _.
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From page 44... ...
estimates are important, no clear statements in the report distinguish between systematic and between-county uncertainties. It is likely that within-county uncertainties produce a significant portion of overall uncertainty in thyroid dose, which means that the level of detail presented (county estimates, rather than estimates over much broader regions)
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