Human Metabolism and Ecological Transfer of Radioactive Caesium

Sammanfattning: The whole-body content of radiocaesium was measured in a South-Swedish urban group of people residing in the city of Lund (55.7^°N, 13.2^°E) between 1960 and 1994. The results from the survey have been analysed in order to estimate the ecological half-time, T_eco,eff, of fallout radiocaesium and the aggregate transfer from ground deposition to man in the region. After 1987, the biological half-times, T_e, of ¹³⁷Cs and ⁴⁰K in man were also determined in the reference group through whole-body content measurements in combination with 24-hour urine sampling. Relationships between 24-hour urinary excretion and body burden of ¹³⁷Cs in the group together with data from the literature were then applied to urine samples collected in 1994 and 1995 from adult subjects living in the highly contaminated region of Bryansk, Russia, in order to estimate their average body burden of ¹³⁷Cs. The equivalent biological half-time for ¹³⁷Cs in females of the Lund reference group was, on average (±1 WSE), 66±3 d, which agrees with other findings, whereas the value for the males, 81±4 d, was, on average, significantly lower than what is found in the literature. This is partly explained by the elevated mean age and relatively low mean body muscle mass of the males investigated in the group during the post-Chernobyl study period. The effective ecological half-time for ¹³⁷Cs from Chernobyl was found to be 1.8± 0.2 y. The aggregate transfer of ¹³⁷Cs from deposition to mean activity concentration in man was estimated to be 1.7 Bq kg^-1/kBq m^-2. These values may be compared with an effective ecological half-time of 1.3 years found in the reference group in the 1960s, and an aggregate transfer factor of 9.8 Bq kg^-1/kBq m^-2. The average committed effective dose from ingested ¹³⁷Cs Chernobyl fallout in the study group was estimated to be 0.02 mSv and from the nuclear weapons fallout to 0.20 mSv. The estimates of whole-body content of ¹³⁷Cs in the Russian subjects obtained through creatinine normalisation of the ¹³⁷Cs content in urine were, on average, a factor of 2 higher than estimates obtained through a potassium normalisation procedure; 55±8 (1 SE) kBq for creatinine and 29±4 (1 SE) kBq for potassium normalisation in the year 1994. This was due to a significant difference in the relationship between the urinary potassium and creatinine concentration between the Swedish and Russian group. The elevated potassium concentration found in the rural Russian subjects indicated a higher dietary intake of potassium, which greatly influences the accuracy and applicability of the potassium normalisation method when estimating body burden of ¹³⁷Cs through urine assay. It is therefore recommended not to use the potassium normalisation procedure in cases where the daily intake of potassium is unknown. The estimated body burden of ¹³⁷Cs in the Bryansk group in 1994 corresponds to a contribution to the estimated annual effective dose of 1.8 mSv y^-1, which is approximately 200 times greater than the average annual dose received from ingested ¹³⁷Cs by the subjects of the Lund reference group during the period 1987-1994.