When assessing the risks to children and adults due to contaminated soil and/or house dust, it has become clear over the past 25 years that ingestion does not equal the absorbed dose. That is, not all of the chemical in these media will be absorbed due to a myriad of chemical interactions with the different types of soil and dust present in the environment. To characterize the likely absorbed dose, many factors need to be considered. Theoretically, physiochemical characteristics should govern interactions between the contaminant and soil matrix and may affect solubilization, gastrointestinal absorption, and subsequently absolute oral bioavailability. Once solubilized and bioaccessible, a chemical must undergo absorption from the gut into systemic circulation. Absorption is most affected by chemical ionization, lipophilicity, and solubility. However, nearly 3 decades of research have not given sufficient insight to develop a universal model for predicting bioaccessibility. In this paper, we examine 2 markedly different chemicals and the factors that influence the bioavailability: dioxin (TCDD) and hexavalent chromium (Cr[VI]), which are discussed, based on previous work from our group. Scenarios involving childhood and adult exposures are addressed.
TCDD is uncharged and highly lipophilic, chemical properties expected to give high oral bioavailability, yet only 25% to 50% oral bioavailability has generally been reported for TCDD present in contaminated soil in animal studies. Strength of TCDD binding to soils may increase over time, termed “aging,” which reduces bioavailability of environmental TCDD and possibly risk estimates. Oral bioavailability of TCDD was determined by measuring the amount deposited in the liver of rats after ingestion of contaminated soil and reported to be approximately 43%. It is unclear if rodent studies properly predict the human gastrointestinal tract.
Conversely, hexavalent chromium (Cr[VI]) represents a charged, water-soluble metal known to be absorbed by anion transport. After acute ingestion by human volunteers, Cr(VI) was 6.9% bioavailable, whereas Cr(III) was less than 1% bioavailable regardless of pH. Cr(VI) is rapidly reduced in the liver and serum to Cr(III), which is incapable of cellular transport, absorption, and toxicity. Environmental and physiological reduction of Cr(VI) represents primary protective aspects of chromium toxicity, reducing the potential risk from oral exposure. Cr(VI)-sequestering capacity of erythrocytes and reducing capacity of whole blood may limit systemic toxicity. Due to its low oral toxicity, a number of human studies have been conducted. Overall, no significant increases in urinary Cr excretion were found above background in treatment groups that have been exposed to reasonable doses, suggesting that Cr is not sufficiently bioavailable for biomonitoring using urine at doses of 200 μg/d in soil.