Factors controlling the oral bioaccessibility of anthropogenic Pb in polluted soils

In human risk assessment, ingestion of soil is considered a major route of toxic Pb exposure. A large body of research has focussed on the measurement of the 'total' Pb contents in sediment, soil and dust as a measure for the exposure to lead. We report that Pb bioaccessibility (i.e. the maximum bioavailability), determined with an in vitro test, does not necessarily depend on the total Pb content. In contrast, the Pb bioaccessibility is initially controlled by the chemical form and particle size of the Pb source, which in turn determine its solubility. Furthermore, when anthropogenic Pb resides within the soil, it may form new, more stable, minerals and/or binds to organic matter, clay, reactive iron or other reactive phases, changing its bioaccessibility. The bioaccessible Pb fraction of 28 soils, polluted with various Pb sources (including residues of Pb bullets and pellets, car battery Pb, city waste and diffuse Pb), was determined with an in vitro-test and varied from 0.5% to 79.0% of total Pb. The highest Pb bioaccessibility (60.7% to 79.0%) was measured in soils polluted with residues of Pb bullets and pellets (shooting range), while the lowest Pb bioaccessibility (0.5%-8.3%) was measured in soils polluted with city waste (including remnants of Pb glazed potsherds and rooftiles, Pb based paint flakes, and Pb sheets). Bioaccessibility of Pb was correlated with pH, organic matter and reactive Fe. These results indicate that soil characteristics play an important role in the oral bioaccessibility of lead in polluted soils. Instead of basing human risk assessment solely on total Pb contents we propose to incorporate in vitro bioaccessibility tests, taking factors such as soil pH, organic matter content and reactive iron content into account. This approach will result in a better insight into the actual risks of Pb polluted soils to children.