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An important step in the assessment of dose to man from environmental releases of radionuclides is the estimation of dose per unit intake. Large uncertainties may be associated with estimates of dose due to variability in human physiological and metabolic characteristics. An evaluation of variability in three principal biological parameters (thyroid mass, uptake and retention of ingested iodine in the thyroid gland) used in calculation of dose to the human thyroid gland from ingested I3'I has been performed for several age categories. The resulting imprecision in predicted thyroid dose commitment per unit intake for each age group is estimated using Monte-Carlo sampling procedures. The ratio of the 99th percentile estimate of dose to the mean estimate is approx. 3 for each of the age groups. The importance of this uncertainty in dose estimates is discussed, including potential implications for radiation protection.

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An attempt has been made to quantify the variability in human biological parameters determining dose to man from ingestion of a unit activity of soluble 137Cs and the resulting imprecision in the predicted total-body dose commitment. The analysis is based on an extensive review of the literature along with the application of statistical methods to determine parameter variability, correlations between parameters, and predictive imprecision. The variability in the principal biological parameters (biological half-time and total-body mass) involved can be described by a geometric standard deviation of 1.2-1.5 for adults and 1.6-1.9 for children/adolescents of age 0.1-18 yr. The estimated predictive imprecision (using a Monte Carlo technique) in the total-body dose commitment from ingested 137Cs can be described by a geometric standard deviation on the order of 1.3-1.4, meaning that the 99th percentile of the predicted distribution of dose is within approximately 2.1 times the mean value. The mean dose estimate is 0.009 Sv/MBq (34 mrem/mu Ci) for children/adolescents and 0.01 Sv/MBq (38 mrem/muCi) for adults. Little evidence of age dependence in the total-body dose from ingested 137Cs is observed.