Bioaccessibility of metals in urban playground soils.

Children ingest soil. The amount ingested varies with the child's behaviour, and daily ingestion rates have been calculated to be between 39 and 270 mg day(-1). During play, children ingest soil both involuntarily and deliberately, and it can be assumed that the latter may result in ingestion of a larger soil particle size fraction and a larger soil mass than the former. Measurements of soil metal contents commonly display the total metal content, where soil sieved to <2 mm has been digested with strong acids. This procedure does not consider differences in metal contents between size fractions or ingested soil masses. Moreover, it does not consider the difference between bioaccessible and total metal content, possibly resulting in an incorrect evaluation of the potential health risks from soil intake. Intervention and guideline values are commonly calculated via tolerable daily intake values, in turn derived from toxicological studies where the contaminant is administered to a test animal in feed or water. It is then assumed that the bioavailability of a contaminant in soil equals the bioavailability in the matrix used in the toxicology study. However, the complexity and heterogeneity of soil often results in a lower bioavailability than from food or water. The current study investigated the bioaccessibility of soil As, Cd, Cr, Ni and Pb from two different particle size fractions representing deliberate (<4 mm) and involuntary (<50 microm) ingestion, as well as from two different soil masses representing deliberate soil intake; 2 g for a child with pica behaviour and 0.6 g for a non-pica child. The bioaccessibility was investigated using an in vitro digestion model and urban playground soils collected away from any point pollution sources. The bioaccessibility (%) of the different metals increased in the order Ni=Cr=Pb<<As<Cd for the involuntary ingestion (<50 microm, 0.6 g) and pica behaviour (<4 mm, 2 g) scenarios, while the deliberate non-pica scenario (<4 mm, 0.6 g) displayed increasing bioaccessibility in the order Ni=Cr<Pb<As=Cd. The difference in bioaccessibility between elements was concluded to be due to differences in origin, sorption behaviour and pH dependence. The study also found that the bioaccessible amount of metal in ingested soil is not always related to particle size or to soil mass in soils with low contaminant levels. Factors such as pH dependence of the metal and the soil's clay content are also significant in determining bioaccessibility.