Ingestion is another route of entry for potentially hazardous minerals. The risk of cancers due to ingestion of asbestos and other fibrous minerals has not been demonstrated unequivocally. However, minerals can also be sources for toxic elements that are released during leaching or dissolution in the digestive tract. Most estimates of risk from toxic materials are concentration based, which is valid only if the entirety of a toxic material is available to the biological system. Recent work by several groups has demonstrated that this assumption is not valid. Rather, the bioavailability of a toxic element is determined by the dissolution characteristics of the mineral in which it is found.
Numerous approaches can be used to evaluate the bioavailability of toxic elements, including in vitro assays, in vivo assays, and mineralogical/geochemical analysis. Freeman et al. [ Freeman et al., 1993] used an in vivo model (in rabbits) to assess the bioavailability of arsenic in soil adjacent to smelter activities. They found that most of the arsenic was not ``bioavailable'' within the time frame of the digestive cycle. For example, 80% of the administered dose was eliminated in the feces following oral administration. Dieter et al. [ Dieter et al., 1993] used a rat model to evaluate the bioavailability of lead from various sources. They showed that in a sulfide lead is much less bioavailable than when in an oxide. Davis et al. [ Davis et al., 1993] found somewhat similar results using mineralogical analysis and geochemical modeling. They found that the primary lead mineral assemblage (galena and lead oxides) weathers to a less soluble assemblage, which forms a protective rind around the more soluble primary assemblage; hence, the bioavailability of lead will be determined by the solubilities of the weathering products. Borch and coworkers [ Borch et al., 1994] are developing an in vitro system for evaluating lead and arsenic bioavailability. Their assay is based on accepted techniques in nutrition [ Crews et al., 1983; Crews et al., 1985; Miller et al., 1981] and involves sequential leaching in simulated gastrointestinal environments. They have successfully demonstrated that the bioavailability of arsenic from two mine tailings is very low (e.g., 1.3% from the Ruth Mine). Similarly, using a mineral leaching assay, Rabinowitz [ Rabinowitz, 1993] demonstrated that the bioavailability of lead was lowered by a phosphate treatment.
Each of the above studies demonstrates that geochemical and mineralogical considerations are essential for the successful evaluation of the bioavailability of toxic elements from minerals. As emphasized above under Inhaled Minerals, data on the dissolution behavior of minerals in physiological environments are urgently needed to aid research on the health effects of minerals.