Reliability of the ICRP's dose coefficients for members of the public, II. Uncertainties in the absorption of ingested radionuclides and the effect on dose estimates

Data on the gastrointestinal absorption of 12 elements have been reviewed. In each case, absorption is expressed as the fraction of the ingested element absorbed to blood, referred to as the f₁ value, applying to intakes of unspecified chemical form by average population groups. The level of confidence in individual absorption values has been estimated in terms of lower and upper bounds, A and B, such that there is judged to be roughly a 90% probability that the true central value is no less than A and no greater than B. Ranges are proposed for intakes by adults, 10-year-old children and 3-month-old infants. Uncertainty in f₁ values (B/A) ranged from 10% to factors of 100-400. The lowest uncertainties were for the well absorbed elements, H, I and Cs, for which there are good data, and the greatest uncertainties were for less well absorbed elements for which few data are available, particularly Zr and Sb. Ranges were generally wider for children and infants than for adults because of the need to allow for the likelihood of increased absorption with only limited data in support of the proposed values. The largest ranges were for 3-month-old infants, reflecting lack of knowledge on the time-course and magnitude of possible increased absorption in the first few months of life. For each age group, ICRP values of absorption tend towards the upper bound of the ranges, indicating a degree of conservatism in the calculation of ingestion dose coefficients. Examination of the effect of the proposed confidence intervals for f₁ values on uncertainties in dose coefficients for ingested radionuclides showed that there was no direct relationship. For some radionuclides, uncertainties in effective dose were small despite large uncertainties in f₁ values while for others the uncertainties in effective doses approached the corresponding values for uncertainty in f₁ values. These differences reflect the relative contributions to effective dose from cumulative activity in the contents of the alimentary tract, which in many cases is insensitive to uncertainties in f₁ and cumulative activity of the absorbed radionuclide in systemic tissues, which is proportional to f₁. In general, uncertainties in effective dose for children and infants exceeded those in adults as a result of greater uncertainties in f₁ values for the younger age groups. However, this effect was reduced in some cases by shorter retention times of absorbed nuclides in body tissues and organs.