85764 Neuherberg, Germany

Transcription

1 Measurement of Daily Urinary Excretion of Thorium in Unexposed German Adult Subjects and its Application in Testing some Aspects of the Current ICRP Biokinetic Model P. Roth 1, V. Höllriegl 1, U. Oeh 1, P. Schramel 2 GSF-National Research Center for Environment and Health, 1 Institute of Radiation Protection, 2 Institute of Ecological Chemistry, Ingolstaedter Landstrasse 1, D Neuherberg, Germany roth@gsf.de Abstract. The daily urinary excretion of Th was measured in a set of adult German subjects who were never in their lifetime occupationally exposed to Th and its related compounds. The Th concentrations in urine were measured using quadrupole type inductively coupled plasma mass spectrometer (ICP-MS), and the limit of detection achieved using this method was 1 ng/l Th in urine. The mean excretion for 23 adult subjects (12 males and 11 females) was obtained to be 6.2 ± 3.4 ng/d and in the range of ng/d. Using the reported daily dietary Th intake of 1.75 µg/d for the adult German population and also the new biokinetic model of Th, the expected daily excretion was calculated to be close to 0.3 ng/d. The comparison of two values suggests that perhaps the gastrointestinal absorption factor for the diet incorporated Th (f 1 value) may need upward revision. This aspect is discussed in depth in the paper. 1. Introduction Thorium is a naturally occurring radioactive element. Along with its chain of daughter products, Th ( 232 Th) emits six alpha particles and is therefore considered as one of the highly radiotoxic elements. It is used in industry for many purposes, e.g. the manufacture of gas mantles, welding rods, thermistors, and catalysts. Th is also recognised as potential fuel for the fast breeder reactors where it can be converted to the fissile material 233 U. The occupational workers handling Th and its compounds can get exposed to them leading to radiation hazard. There is thus an urgent need to study the incorporation of Th and the resulting internal radiation dose in thepopulations occupationally exposed to Th. The old ICRP biokinetic model of Th [1], which was based on animal experimental studies carried out by Stovar et al. [2] was replaced by the new ICRP model [3], which has developed to provide the best fit to the human data obtained in the last decade. The earlier biokinetic model of Th and the respiratory tract model [4] were found to have limitations due to which they were replaced. This leads to the revision of ALI values for 232 Th from 90 Bq to 1680 Bq [5]. It therefore becomes important that the new biokinetic model of a radionuclide is tested for its various aspects before its application in internal dosimetry. A reliable model is expected to endorse its characteristics at all levels of exposure. Therefore, the new ICRP biokinetic model was tested by using it to calculate the urinary excretions of Th for human subjects living in normal background and having daily dietary intake of 232 Th typically for a normal non-exposed adult German population. The predicted values were compared with the actually measured 232 Th excretion. 2. Materials and methods The calculations of the daily urinary excretion of Th based on the application of the new ICRP biokinetic model of Th for a daily dietary intake of 1.75 µg/d reported for adult German subjects [6, 7] showed that the daily excretion should be less than 0.5 ng/d. This required the development of analytical method capable of measuring Th at ng/l or lower levels of concentrations. An extremely sensitive analytical method was developed based on the use of quadrupole type of ICP-MS and applied to measure daily urinary excretion of Th in adult German subjects. Using this method it was possible to achieve the limit of detection of 1 ng/l. The details of the instrument conditions are given in Table I. 1

2 Table I: Instrument operating conditions ICP-MS Plasma 15 L/min Nebulizer flow 0.95 L/min Auxiliary flow 0.8 L/min RF power 1200 W CEM voltage 3.1 kv Nebulizer Gem tip cross flow (Perkin Elmer) Sample uptake 0.9 ml/min Sample cone Platinum tipped nickel Skimmer cone Platinum tipped nickel Reagents Th standard solution 1 g/l(spex Ind., U.S.A.) Ir standard solution 1 g/l(spex Ind., U.S.A.) HNO 3 Subboiling distilled H 2 O Ultrapure (Milli-Q, Millipore, Germany) 2.1. Sampling. Twenty four hours urine samples were collected from a set of 23 adult German subjects (12 males and 11 females) aged between 17 and 84 years who were never exposed (occupationally) to higher levels of Th and its compounds other than the exposure at normal background level Sample storage. Ten ml aliquots of these samples were acidified with 0.5 ml of HNO 3 and 0.5 ml of HCl and were stored at 4 C until their analysis. No other processing of samples was required. Due to the use of the small sample processing prior to analysis, the reagent blank was lower than 0.05 ng/l Analysis. The measurements in the sample were carried out using ICP Mass Spectrometer ELAN 5000 (Perkin Elmer Sciex). The operating conditions of the instrument are summarised in Table I. The standard addition method was applied for calibration and iridium (100 µg/l) was used as internal standard. The detection limit of the instrument was determined to be 1 ng/l (3s of the background signal), which could be also considered as the limit of detection for 232 Th. 3. Results and discussion Figure1 shows the distribution of the samples in various ranges of the daily urinary Th excretion. It is quite clear that most of the samples (more than half) lie in the excretion range of ng/l. The mean excretion was obtained to be 6.2 ng/l. But, Th is an element, which is non-essential to the human system, and therefore its excretion may be better represented by the median value of 5.1 ng/l (Table 2). This value of daily urinary excretion ranges between 3.5 ng/d reported by Dang et al. [8] for the urban Indian population and 9.2 ng/g Th reported for the Australian population by Hewson and Fardy [9]. It is however higher than the urinary Th excretion of 1.5 ng/d reported by Riedel et al. for a small number of adult German subjects [10]. The expected excretion of urinary Th was calculated for its unit daily dietary intake using the new ICRP biokinetic model of Th [3]. For the purpose of calculations, the gastro-intestinal absorption factor of 5.0E-4 was assumed. This factor is same as proposed by ICRP 1997 [11] for the non-specific types of sources (diet incorporated Th). In figure 2, the plot shows the daily urinary excretion with increasing period of ingestion of dietary Th for the unit daily intake. It may become clear that even for the highest age group of subjects representing the German population, with the daily intake of 1.75 µg Th [5, 6], the urinary excretion using the new ICRP biokinetic model may not exceed 0.4 ng/d Th. The contribution of Th to the daily urinary excretion may not contribute more than 0.1 ng/d from inhalation at the known rate of intake rate as reported by UNSCEAR [12] for Germany and other 2

3 European countries. This means that the daily urinary excretion of Th for German population contributed by dietary intake along with the contribution due to inhalation may not exceed 0.5 ng/d. This calculated value of expected excretion is an order of magnitude lower than the observed median excretion of 5.1 ng/d No. of samples in each range Ranges of daily urinary excretion FIG.1. Distribution of daily urinary excretion values of 232 Th for adult German subjects. Table II. Results of analysis of urine samples obtained from adult German subjects No. of subjects: 23 (12 males and 11 females) Age Group: years Range of daily urinary 232 Th excretion: ng/d Mean (SD): 6.2 ± 3.4 ng/d Median (95% confidence interval): 5.1 (± 1.5) ng/d GM (SGD): 5.3 (1.82) ng/d The possible implication of the observed higher urinary excretion, in comparison to the excretion values calculated using the new ICRP model [3], could be that the internal radiation dose may be very significantly overestimated when using the new model. It is therefore important to study the possible reason of the discrepancy between the measured and the calculated values. One possible reason could be the assumption by ICRP (1997) of rather low gastro-intestinal absorption factor of 5.0E-4. The second possible reason could be that at the LOD (limit of detection) of 1 ng/l for 232 Th, the instrument is still not adequately sensitive to accurately measure the daily urinary excretion which lies in the vicinity of the LOD. So more sophisticated versions of the ICP-MS such as sector field ICP-MS with almost two orders of magnitude better sensitivity may be employed to confirm 3

4 these results in order to complete the validation of the new ICRP model. It would be also interesting to carry out human absorption studies for the diet incorporated Th. 1e-3 Th accumulation under chronic ingestion Excretion rates of Th (fraction/day) 1e-4 1e-5 Urine 1e Days after start of intake FIG. 2. Daily urinary excretion for constant chronic unit intake. 4. Conclusion The daily urinary excretion of Th was measured using ICP-MS for adult German subjects who were never occupationally exposed to this radionuclide. The mean and median excretion for 23 subjects was obtained to be 6.2 and 5.1 ng/d respectively. The urinary excretion values were also calculated by applying the new ICRP model to the daily intake of Th reported for the German population. The calculated values were less than 0.5 ng/d in comparison to the ten times higher measured excretion values. There is an urgent need to investigate this disagreement between the measured and calculated urinary excretion values, since the present observation of an order of magnitude higher measured value in comparison to that calculated using the new ICRP biokinetic model would imply the significant overestimation of dose when using the new model. More sensitive analytical method with very low reagent blank value needs to be utilised to provide truly reliable data on renal Th excretion. The gastrointestinal absorption factor for humans for the diet incorporated Th also needs to be studied to achieve a definite conclusion. REFERENCES 1. International Commission on Radiological Protection. Limits for Intakes of Radionuclides by Workers. ICRP Publication 30, No. 1. Pergamon Press, Oxford (1979). 2. Stover, B.J., Atherton, D.R., Keller, N., Buster, D.S., Metabolism of the Th-228 Decay Series in Adult Beagle Dogs. Radiat. Res., 12: , (1960). 4

5 3. International Commission on Radiological Protection. Age-dependent Doses to Members of the Public from Intake of Radionuclides: Part 3 Ingestion Coefficients. ICRP Publication 69. Annals of the ICRP, 23, No.1, Pergamon Press, Oxford (1995). 4. International Commission on Radiological Protection. Human Respiratory Tract Model for Radiological Protection. ICRP Publication 66. Pergamon Press Oxford, (1994) 5. Phipps, A.W., Silk, T.J., Fell, T.P., The Impact of Recent ICRP Recommendations of Dose Coefficients, Annual Limits on Intake, and Monitoring programmes for Thorium. Radiat. Prot. Dosim., 79(1-4): , (1998). 6. Frindik, O., Essentielle und toxische Inhaltsstoffe in der täglichen Gesamtnahrung. Bundesforschungsahnsalt für Ernährung, BFE-Bericht R-83-02: 313, (1983). 7. Beyer, D., Biehl, R., The Significance of Natural Faecal Thorium Excretions for t he Surveillance of Inhaled Thorium Class Y Compound.; Fachverband für Strahlenschutz e. V.; FS T:82, (1989). 8. Dang, H.S., Jaiswal, D.D., Sunta, C.M., Daily Intake of Thorium by an Indian Population. Sci. Tot. Environ.: 57:73-77, (1986). 9. Hewson, G.S., Fardy, J.J., Thorium Metabolism and Bioassay of Mineral Sand Workers. Health Phys., 64: , (1993). 10. Riedel, W., Gawalik, D., Tschammer, A., The Excretion of Thorium in Urine of Unexposed Persons. Fachverband für Strahlenschutz e. V.; FS T: , (1998). 11. International Commission on Radiological Protection. Individual Monitoring for Internal Exposure of Workers. ICRP Publication 78. Pergamon Press, Oxford (1997). 12. United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). Sources and Effects of Ionising Radiation. New York, United Nations (2000). 5