THE TOXICOLOGICAL GEOCHEMISTRY OF DUSTS, SOILS, AND OTHER EARTH MATERIALS - AN OVERVIEW OF PROCESSES AND THE INTERDISCIPLINARY RESEARCH METHODS USED TO UNDERSTAND THEM

Adverse health effects resulting from exposure to dusts, soils, and other earth materials are closely linked to chemical reactions between the materials and various water-based body fluids (respiratory, gastrointestinal, perspiration, blood serum, interstitial, and intracellular fluids) encountered along exposure routes. Toxicological geochemistry (TG) is the study of chemical interactions between body fluids and earth materials, and their influences on toxicity. These chemical interactions are controlled by: 1) particle mineralogy, size, shape, chemical composition, and solubility; 2) chemical characteristics of the body fluid (pH, inorganic electrolytes, organic ligands, redox-active species); 3) exposure dose, and; 4) mineral dissolution and redox kinetics. Earth materials can contain bioaccessible (readily dissolved) heavy metals or metalloids that, if absorbed in sufficient doses, can trigger toxicity. For example, ingested Pb-carbonates are soluble in acidic gastric fluids and the Pb is readily absorbed in the intestines. In contrast, chromate from soluble salts is more readily absorbed via near-neutral pH fluids lining the lungs. Bioreactive materials (alkaline dusts, acidic volcanic gases) can modify body fluid chemistry and trigger tissue irritation or damage. Biodurable asbestos fibers are poorly cleared by lung macrophages, do not readily dissolve in the lung fluids, and can therefore persist in the lungs for years; in sufficient dose, asbestosis can result, and long-term fluid-mineral reactions (redox cycling, resulting generation of free radicals) are thought to help promote lung cancer and mesothelioma. Useful TG research methods, which are best accomplished collaboratively by earth and health scientists, include: characterization of bulk materials and of particles in pathologic tissue samples to determine their mineralogy, shape, size, and content and redox state of potentially toxic metals; in vitro leach tests to determine particle biodurability and bioreactivity, and the bioaccessibility and redox kinetics of contained toxins in simulated body fluids; chemical modeling of fluid-mineral reactions; in vitro and in vivo toxicity testing of well characterized earth materials; and exposure/uptake assessments following exposure to earth materials.