The main objective of this thesis was the development of a method to estimate the bioavailability of ingested soil-derived uranium and the resulting internal dose to humans. The ingestion of small amounts of soil by humans occurs involuntarily and deliberately. For the involuntary ingestion of soil, for example via food, the amount of soil ingestion differs between 10 mg/day and 100 mg/day for adults and children, respectively. For the deliberate ingestion of soil, e.g. healing soil, amounts of up to 40 g/day over several weeks are reported. Uranium is ubiquitous in soil, and involuntary or deliberate soil ingestion is therefore accompanied by the ingestion of uranium. This leads to an increase of the internal dose due to the ionizing radiation from the radioactive decay of ingested uranium and its progeny. To estimate the internal dose after ingestion of soil-derived uranium, its bioavailability must be known. However, when work on this PhD started, no method was available to reliably determine the bioavailability of uranium for such a scenario. Therefore, a new method was developed. First, an already established in vitro solubility assay was used to determine the bioaccessibility (DF) of uranium of an edible soil low in uranium. Second, the actual bioavailability (fA) of uranium of this soil was determined by means of a human study. In this study, ten human volunteers ingested a specific amount of this soil and the bioavailability (fA) of uranium was estimated for all ten volunteers from their urinary excretion. By determining bioaccessibility (DF) and bioavailability (fA), it was possible to calculate the fAsol factor, which is the ratio of fA and DF, and which describes the transfer of soluble uranium to the circulatory system. The fAsol factor was calculated to be 0.53% (GM) ranging from 0.06% (2.5th percentile) to 4.43% (97.5th percentile). Knowing fAsol it is possible to obtain realistic data on the bioavailability (fA) of uranium from virtually any soil, without the need of further human studies. Only the same in vitro solubility assay, which was applied in the current study, has to be performed for any soil of interest to determine its specific bioaccessibility (DF). The corresponding bioavailability (fA) is thereafter simply calculated by adopting the fAsol factor determined earlier. The fAsol factor obtained was compared to values from the literature and found to be in good agreement. It can therefore be assumed that the fAsol factor does not depend on the duration of exposure or the amount of ingested uranium. In other words, it can be used for acute as well as chronic ingestion scenarios with high and low amounts of soil-derived uranium. Thus, in the course of this 8 thesis a robust method was developed to estimate the bioavailability and the resulting internal dose for different ingestion scenarios with different amounts of soil-derived uranium.

Finally, this newly developed method was applied on highly uranium-contaminated soils, i.e., on original soils from former uranium mining sites located in Eastern Germany. The bioavailability of uranium from these soils was determined and used to calculate internal doses after an assumed realistic scenario of soil ingestion. Based on the assumed exposure scenario a committed effective dose of 0.6 μSv (GM) ranging from 0.3 μSv (2.5th percentile) to 3.0 μSv (97.5th percentile) was estimated for the most uranium-contaminated soil. It is concluded that this ingestion of soil-derived uranium does not imply any major health risk to humans due to the additional internal dose.