Current Views on the Oral Bioavailability of Inorganic Mercury in Soil: Implications for Health Risk Assessments

Due to the presence of mercury at a number of major contaminated sites in the United States, the bioavailability of inorganic mercury in soil following ingestion has emerged as an important public health issue. Studies of the leachability/solubility of inorganic mercury in soil have shown that it is largely immobile, thereby suggesting that it will not be readily available for absorption in the gastrointestinal tract. Ignoring the effect of the soil matrix on decreasing bioavailability may result in a substantial overprediction of risks due to ingestion of contaminated soil. This paper discusses current knowledge about the oral bioavailability of inorganic mercury in soil and offers suggestions about how these data may be applied in human health risk assessment. Though precise estimates are not available, in vivo and in vitro estimates of the bioavailability of different inorganic mercury species in different matrices suggest that the bioavailability of mercury in soil is likely to be significantly less, on the order of at least three- to tenfold, than the bioavailability of mercuric chloride, the species used to derive the toxicity criteria for inorganic mercury. Because bioavailability can vary significantly with soil type, soil aging, the presence of co-contaminants and other factors, it is suggested that whenever the fiscal aspects justify a more precise estimate of bioavailability, site-specific estimates be developed. To develop a database for identifying a less expensive and more efficient method for estimating bioavailability, it is suggested that in vivo studies be conducted concurrently with in vitro studies. However, due to the lack of precision associated with the derivation of the most widely-used health guidance value for inorganic mercury (the USEPA RfD), additional work to address the uncertainties in the RfD is recommended.