Differences in the biological half lives of inhaled Co-58 and Co-60

In-vivo measurements were initiated 4 days following an airborne exposure to highly contaminated radioactive debris. The material was produced in an explosive plugging operation in a nuclear steam service system. The nuclides of primary concern were Co-60 and Co-58. Total body scans obtained on days 4 and 39 indicated that the primary site of deposition was the chest. Early clearance was complete by day 6. From day 6 through day 200 only single component exponential clearance functions were observed. The effective half lives for clearance are estimated to be 57 days and 3.9 years for Co-58 and Co-60, respectively. The corresponding biological half times are 280 days and 14 years. The shorter biological clearance rate for Co-58 was confirmed by limited excreta analyses. Reasons for the separation of the two cobalt isotopes are discussed. The effects of long biological clearance times and separation of isotopes on exposure evaluation and dose commitment are considered