A modified ICRP 66 iodine gas uptake model and its parametric uncertainty

Intakes via inhalation may occur from radionuclides released in the form of a gas. The chemical characteristics pertaining to the release influence the intake and subsequent dose to an exposed individual. Gases are taken up or absorbed in the entire respiratory tract and the associated uptake mechanisms are quite different from deposition of particulates. Gaseous iodine can exist in various chemical forms, e.g., elemental iodine, inorganic, and organic iodine compounds. These different chemical species play an integral role in the gaseous uptake o f iodine in t he respiratory tract. Gas uptake in the various regions of the respiratory tract results in the intake of iodinated material into the body. The radioactive iodine taken up in the gas-exchange tissues is absorbed into the bloodstream of an individual and subsequently transferred to other organs. Iodine in the circulatory system can then be taken up by the thyroid gland, with resulting dose to the thyroid. The magnitude and uncertainty in regional gas uptake is important in the assessment of individuals exposed to airborne releases of radioiodine. The current ICRP 66 model is rudimentary and estimates regional gas uptake based on solubility and reactivity of the different radionuclides entering the respiratory tract. The modified model proposed here employs methodology and a mathematical structure to determine estimates of fractional gas uptake rather than defaulting to literature values, as in the current ICRP model. Model parameters have been assigned input distributions and estimates of uncertainty have been determined. A sensitivity analysis of these parameters has been performed to demonstrate the importance of each of these parameters. The sensitivity analysis ranks the model-input parameters by their importance to estimates of regional gas uptake. The model developed herein may be used for improved estimation of gas uptake in the respiratory tract and subsequent dose estimates from the different chemical forms of radioiodine.