Bioaccessibility of contaminants from ingested soil in humans. Method development and research on the bioaccessibility of lead and benzo[a]pyrene

For risk assessment of orally ingested contaminants it is often unknown how much of the contaminant is taken up by the human body. In most human risk assessments of soil contamination, oral bioavailability of a contaminant from soil is considered equal to the bioavailability of the contaminant from the matrix as used in toxicity studies upon which risk assessment is based. In toxicity studies typically food and liquid matrices are used. In literature it is suggested that oral bioavailability of contaminants from soil is significantly lower. As a consequence, risks may be overestimated substantially. A simple in vitro digestion model, representative for human physiology, was developed in order to investigate the effects of a soil matrix on oral bioavailability. This model allows measuring bioaccessibility of a contaminant, i.e. the fraction of the dose ingested that becomes available for absorption into the human body. The present report describes the optimisation of the in vitro digestion model. Furthermore, with the in vitro digestion model the effects of several parameters on bioaccessibility were investigated. Lead and benzo[a]pyrene were chosen as test contaminants. Effects of 1) type of contaminant, 2) contamination level, 3) type of soil, 4) pH of soil, 5) ageing of the soil, and 6) metal speciation on bioaccessibility were investigated. Also a start was made with investigating the differences between artificially and historically contaminated soil. It was tried to describe the data by means of a mechanism-based mathematical model. The results suggest in many cases a non-linear relationship between the level of lead contamination and the amount of contaminant mobilised from soil into digestion juice, i.e. chyme, or a relationship with a maximum level of benzo[a]pyrene in chyme, i.e. a precipitation level. In addition, bioaccessibility seems to depend on the type of contaminant and the type of soil. The other tested parameters seemed to have little or no influence on the bioaccessibility. Further experiments are needed in order to validate the mathematical model and to make the model applicable for forecasting bioaccessibility of a contaminant from a certain soil sample. Ultimately, the mathematical model may be employed to estimate bioaccessibility values based on contaminant and soil characteristics. The in vitro digestion model may be used as a tool to assess site specific bioaccessibility. The data should be interpreted with caution as long as validation to the in vivo situation has not taken place