Uncertainty Analysis of the ICRP Human Respiratory Tract Model applied to Interpretation of Bioassay Data for Depleted Uranium

Measurements of uranium excreted in urine have been widely used to monitor possible exposure to depleted uranium (DU) by personnel who have served in areas where DU munitions were used in conflicts. A study has been carried out to provide guidance on the most appropriate parameter values to be used in assessing intakes and doses from urine samples taken at long times (100 to 10,000 days) after possible intakes, and to assess the overall uncertainty in the results. As part of this study, information relating to the application of the ICRP Human Respiratory Tract Model has been reviewed, and from it, central values, ranges and distributions were assigned to parameter values defining respiratory tract deposition of appropriate DU aerosols, and clearance of DU from the respiratory tract. An analysis was carried out of the sensitivity to variation in each parameter value, of committed lung dose and effective dose calculated from a measurement of DU in urine. Generally, the most important parameters were those defining dissolution of the DU in the respiratory tract: the fraction that dissolves rapidly and the slow (long-term) dissolution rate. The overall uncertainty in dose (ratio of maximum to minimum) was estimated to be about 50 at 10 days after intake and about a factor of 7–10 at 1000–10,000 days. Nevertheless, the maximum estimated dose from a measurement of 1 nanogram DU in urine was less than 1 millisievert up to 5000 days after intake. This report complements reports that consider uncertainties in the ICRP model that describes the behaviour of uranium after entry to the blood, and the possible effects on urinary excretion of alteration in kidney function resulting from uranium toxicity. This study was partly funded by the Defence Science and Technology Laboratory under Contract No RD034-9781