HEURISTICS FOR PRACTICAL MEDICAL GEOLOGY: GEOAVAILABILITY AND BIOAVAILABILITY

Medical geologists deal with extrapolations that underlie both geological and biomedical sciences. Predictions of the environmental transport of contaminants rely on inferences drawn from simple laboratory studies that are applied to complex field systems. Assessments of health effects rely on in vitro or in vivo studies of simple systems that are applied to epidemiological studies of complex populations. These extrapolations lead to uncertainties in predictions of health effects from environmental contamination, however, studies in these simple systems provide a reasonable basis for initial risk assessments.

Heuristics are simplifying frameworks or “rules of thumb” to find intermediate or short-term solutions to complex problems through loosely defined rules. In Medical Geology, the complex problem is the assessment of health risks posed by environmental hazards and the solutions obtained are not guaranteed to be the optimal final solutions but instead are sufficient for screening purposes in risk management.

The heuristic of geoavailability is the amount of a contaminant that could reach a potentially exposed population. This is the net result of a number of processes including the release of a contaminant into the environment, and its transport and transformation by hydrogeochemical processes that lead to exposures. If the geoavailability of a contaminant is very low in site-specific conditions, then it is reasonable to propose that the environmental risk is relatively low, and exposure is not significant. To a first approximation, the geoavailability can be estimated by a geoavailability factor (GAF), which is the reciprocal of the retardation factor commonly used in contaminant transport calculations.

The second heuristic, bioavailability, is the fraction of an ingested toxin that would reach the biological site of damage. If the bioavailability is low under specific conditions, then it is reasonable to propose that the health risk due to the exposure is low. In many cases, this can be correlated to the bioaccessibility, which is the measure of the fractional dissolution of the chemical from environmental media such as soil or dust into a simulated gastric fluid or pulmonary fluid. Measurements of the geoavailability factor and the bioaccessibility use a common set of computational and experimental techniques in geochemistry. Examples of calculations, limitations and the utility of these simplifying heuristics for screening risk and health assessment are discussed in this presentation.