CREDIBILITY OF UNCERTAINTY ANALYSES FOR 131I PATHWAY ASSESSMENTS

We would like to make your readers aware of numerous concerns we have with respect to the paper by A. A. Simpkins and D. M. Hamby on Uncertainty in transport factors used to calculate historic dose from 131I releases at the Savannah River Site. The paper by Simpkins and Hamby concludes by saying their uncertainty analysis would add credibility to current dose reconstruction efforts of public exposures to historic releases of 131I from the operations at the Savannah River Site, yet we have found their paper to be afflicted with numerous errors in assumptions and methodology, which in turn lead to grossly misleading conclusions. Perhaps the most egregious errors are their conclusions, which state that: a. the vegetable pathway, not the ingestion of fresh milk, was the main contributor to thyroid dose for exposure to 131I (even though dietary intake of vegetables was less in the past than at present), and b. the probability distribution assigned to the fraction of iodine released in the elemental form (Uniform 0, 0.6) is responsible for 64.6% of the total uncertainty in thyroid dose, given a unit release of 131I to the atmosphere. The assumptions used in the paper by Simpkins and Hamby leadmore » to a large overestimate of the contamination of vegetables by airborne 131I. The interception by leafy and non-leafy vegetables of freshly deposited 131I is known to be highly dependent on the growth form of the crop and the standing crop biomass of leafy material. Unrealistic assumptions are made for losses of 131I from food processing, preparation, and storage prior to human consumption. These assumptions tend to bias their conclusions toward an overestimate of the amount of 131I retained by vegetation prior to consumption. For example, the generic assumption of a 6-d hold-up time is used for the loss from radioactive decay for the time period from harvest to human consumption of fruits, vegetables, and grains. We anticipate hold-up times of many weeks, if not months, between harvest and consumption for most grains and non-leafy forms of vegetation. The combined assumptions made by Simpkins and Hamby about the fraction of fresh deposition intercepted by vegetation, and the rather short hold-up time for most vegetables consumed, probably caused the authors to conclude that the consumption of 131I-contaminated vegetables was more important to dose than was the consumption of fresh sources of milk. This conclusion is surprising, given that the consumption rate assumed for whole milk was rather large and that the value of the milk transfer coefficient was also higher and more uncertain than most distributions reported in the literature. In our experience, the parameters contributing most to the uncertainty in dose for the 131I air-deposition-vegetation-milk-human-thyroid pathway are the deposition velocity for elemental iodine, the mass interception factor for pasture vegetation, the milk transfer coefficient, and the thyroid dose conversion factor. In none of our previous investigations has the consumption of fruits, vegetables, and grains been the dominant contributor to the thyroid dose (or the uncertainty in dose) when the individual also was engaged in the consumption of even moderate quantities of fresh milk. The results of the relative contribution of uncertain input parameters to the overall uncertainty in exposure are counterintuitive. We suspect that calculational errors may have occurred in their application of the software that was used to estimate the relative sensitivity for each uncertain input variable. Their claim that the milk transfer coefficient contributed only 4% to the total uncertainty in the aggregated transfer from release to dose, and that the uncertainty in the vegetation interception fraction contributed only 3.3%, despite relatively large uncertainties assigned to both of these variables, violates our sense of face validity.« less