EVALUATION OF METHODS FOR ASSESSING THE ORAL BIOAVAILABILITY OF INORGANIC MERCURY IN SOIL

The risks associated with environmental exposures to inorganic mercury are typically assessed based on toxicity studies conducted with the s oluble salt, mercuric chloride (HgCl2). Evidence indicates, however, t hat inorganic mercury is present in soil as a variety of compounds and that oral absorption of inorganic mercury decreases with a decrease i n the solubility of the mercury compound being studied. Thus, while Hg Cl2 is approximately 15-20% bioavailable, the bioavailability of cinna bar (HgS) may be 30- to 60-fold less. The solubility and, hence, bioav ailability of inorganic mercury in soil is expected to be substantiall y less than that of HgCl2, due to the presence of less soluble compoun ds and their interactions with soil constituents. Quantification of th is difference in bioavailability is important in assessing potential r isks associated with exposure to mercury-containing soil. A review of available studies supports the expectation that mercury bioavailabilit y in soils will be reduced. This paper reviews methods for assessing s oil metal absorption with consideration of the characteristics of the oral absorption of elemental and inorganic mercury that should be eval uated in designing additional studies. Because of the very slow elimin ation of mercury in some species, it is recommended that a repeated-do se study be conducted. Such a study would yield an estimate of relativ e bioavailability based on a comparison of tissue mercury concentratio ns in animals ingesting soil with those of animals receiving HgCl2. Th e dose, age, gender, and species of animal selected are not expected t o affect relative bioavailability estimates; however, it is recommende d that studies be conducted in two animal species. Rats should be used because they have been used in many studies of mercury absorption and toxicity. A species of large animals such as monkeys, swine, or dogs should also be used to provide confirmation in a species with greater similarities to humans in gastrointestinal physiology and anatomy. Oth er critical factors in designing these studies, such as selection and characterization of soil samples, are also addressed.