Bioaccessibility and Bioavailability of 232Th and 238U in Human Health Risk Assessment

Abstract

Contamination of thorium and uranium is a global environmental problem that can contribute to human health concerns. Numerous epidemiological and laboratory studies have demonstrated that environmental and occupational thorium and uranium exposure can induce myriad health problems. Thorium-232 (232Th) and uranium-238 (238U) coexist naturally and are ubiquitous in the environment. Regarding natural or anthropogenic thorium and uranium contamination, the major sources of concern are mining, groundwater, phosphate fertilizers, nuclear and rare earth elements (REEs) facilities, and military activities. In natural or anthropogenic scenarios, thorium and uranium exposure may cause health risks because of their chemotoxicity and radiotoxicity. Therefore, possible radiobiological and toxicological risks must be addressed, even for small amounts of thorium and uranium exposure. On a global scale, multiple studies have focused on oral exposure to 232Th and 238U in soils. However, research is scarce on the oral exposure of 232Th and 238U in concrete. Therefore, the primary objective of this case study was to determine the potential health risks from acute incidental ingestion of 232Th- and 238U-contaminated soil and concrete. The objective was tested by conducting several studies below: 1. To investigate the feasibility of the Unified BARGE Method (UBM) in vitro assay to predict 232Th and 238U bioaccessibility in co-contaminated soil and concrete. 2. To investigate the absolute- (ABA) and relative-bioavailability (RBA) as well as histopathological and hematological effects of acute oral exposure to 232Th and 238U in co-contaminated soil and concrete. 3. To develop 232Th and 238U computational oral physiologically based pharmacokinetic (PBPK) models using the Simulation, Analysis, and Modelling (SAAM II) program to solve the complete system of the first-order linear differential equation and to evaluate the suitability of UBM in vitro method to predict ABA and RBA of 232Th and 238U by developing the in vitro-in vivo correlations (IVIVC). 4. To estimate the committed effective dose (Deff) and excess lifetime cancer risk (ELCR) from acute incidental ingestion of 232Th and 238U in co-contaminated soil and concrete. To simulate contaminated soil and concrete, the IAEA-312 and SC were used as surrogates. The concrete was spiked with Th- and U-nitrates at various contamination levels (~ 50, 100, and 200 mg kg1). The bioaccessible of 232Th and 238U in IAEA-312 and SC have been determined using UBM in vitro bioassay. The 232Th-fba values from IAEA-312 in the GP and GIP were 1.96% ± 0.24% and 1.22% ± 0.23%, respectively. In contrast, 232Th-fba values from SC in GP and GIP ranged from 0.24% ± 0.04% – 0.29% ± 0.02% and 0.008% ± 0.001% – 0.021% ± 0.003%, respectively. In the case of 238U-fba from IAEA-312, 4.91% ± 0.22% in GP and 6.42% ± 0.55% in GIP were found, whereas 238U-fba values from SC in GP and GIP ranged from 2.80% ± 0.17% – 3.08% ± 0.08% and 25.73% ± 0.49% – 32.64% ± 0.37%, respectively. These findings shows that 232Th is preferentially released by the GP fluids, whereas the solubilization of 238U occurs predominantly in GIP. Our findings suggest that the composition and concentrations of soil and concrete, precipitate formation in the GP and GIP assays, and pH of the medium might influence the fba. Similar to previous studies, the fba relies on the in vitro method. In addition, the ABA and RBA bioavailability of 232Th and 238U in IAEA-312 and SC were determined by employing in vivo SD rats. The ABA values of 232Th and 238U post-exposure rats' blood samples were 0.028% and 0.750%, respectively. Whereas the RBA values of 232Th and 238U were determined as 11.8%  0.14% – 13.7%  0.29% and 22.0%  0.86% – 30.8%  1.01%, respectively. Post-acute 232Th and 238U ingestion from IAEA-312 and SC demonstrate noticeable histopathological and hematological alterations, implying that intake of 232Th and 238U in IAEA-312 and SC can lead to erythrocytes and proximal tubules damage and also elevate hematological attributes. The PBPK model in humans simulating the blood concentration–time profiles post-acute oral IAEA-312 and SC was established based on the established PBPK model in SD rats. Based on IVIVC, these results provide strong evidence that the 232Th- and 238U-ABA values tend to underestimate the actual magnitude, and UBM provides a robust tool for use in the risk assessment of 238U in SC. The 232Th in SC did not meet the criteria as the bioaccessibility values were not correlated with the bioavailability values. The relationship between fba and RBA derived for SC is a useful advance for developing exposure narratives for contaminated concrete and other contaminated soils. Based on the assumptions and estimates, the findings revealed a relatively low radiation risk from 232Th and 238U in IAEA-312 and SC post-acute exposure regardless of workers and children aged 5. Given the lower-than-reference values for both Deff and ELCR of 232Th and 238U, incidental ingestion of those radionuclides from IAEA-312 and SC thus poses minimal radiation risks. Based on these findings, I discussed relatively new information/data on the effects of 232Th and 238U from co-contaminated soil and concrete incidental ingestion. This study would be beneficial for an adequate understanding of the health implications caused by the acute oral exposures of 232Th and 238U in co-contaminated soil and concrete, especially in the bioavailability and toxicity assessment. Moreover, by understanding the cause and effect of incidental ingestion of 232Th and 238U (particularly from the standpoint of bioavailability and influence on the blood), our findings provide insight into the choice of a more reliable in vivo bioassay to estimate the RBA and hematotoxicity of 232Th and 238U in contaminated soil and concrete. Nonetheless, further studies are still necessary, particularly at high 232Th and 238U doses in contaminated soil, concrete, or any relevant materials. The present study provides fresh perspectives for future research into the health implications of acute oral exposures to 232Th- and 238U-contaminated soil and concrete.