Uncertainty of the Iodine-131 Ingestion Dose Conversion Factor

Historical environmental releases of 131I at a number of Department of Energy nuclear weapons facilities and major nuclear accidents (e.g. Chernobyl) have contributed to a significant portion of environmental dose. Several conversion factors are utilised in the estimation of effects on humans from these releases, e.g. dispersion coefficients, consumption rates, uptake factors, transport factors, dose conversion factors, and risk coefficients. Assessments of effect on exposed persons are no more accurate than the individual components of the analysis. Only the component dealing with the behaviour of radioiodine in the human body is considered here. A probabilistic determination of the 131I adult ingestion dose conversion factor (DCF) was generated to assess the uncertainty of the internal dosimetry required to estimate radioiodine dose. The 131I DCF was found to vary from the maximum to the minimum value by a factor of about 4 with a median value of 4.3 x 10-7 Sv.Bq-1. The current deterministic estimates of the 131I DCF published by the DOE and EPA are 4.9 x 10-7 Sv.Bq-1 and 4.76 x 10-7 Sv.Bq-1, respectively, about 12% higher than the median probabilistic value. However, the National Cancer Institute's recently published value of 3.5 x 10-7 Sv.Bq-1 is about 19% lower than the median probabilistic value. The DCF model was found to be most sensitive to the parameters of thyroid mass and thyroid uptake fraction.