Probabilistic assessment of health risks of methylmercury from burning coal

This paper describes a probabilistic assessment of neurological risks incurred from consuming fish containing methylmercury (MeHg), focusing on the incremental effects of Hg deposited from local coal combustion. A Monte Carlo model is used to simulate a "worst case" scenario in which a population of 5000 fish eaters in the upper midwestern United States derive the freshwater fish portion of their diet from local waters near a hypothetical large coal-fired power plant. This population is characterized by distributions of body mass, half-life of MeHg, and the ratios of blood to body burden and hair to blood MeHg. Each person's diet consists of varying amounts of tuna fish, freshwater sportfish, and marine fish and shellfish, the MeHg contents of which are characterized by national distribution statistics, as are the consumption rates for marine fish. The consumption rates for freshwater fish are specific to the region. The fish portion size is linked to body mass by a variable correlation. Each meal is assumed to be an independent sample; thus, as metabolic equilibrium is approached, each person's body burden of MeHg tends to approach the value corresponding to the mean MeHg intake for the population. Predictions of MeHg levels in hair by this model compared well with an observed distribution of 1437 women. Two neurological endpoints were examined: adult paresthesia, as related to MeHg body burden, and congenital neurological effects, as associated with average concentrations of MeHg in maternal hair during pregnancy. Two exposure scenarios are considered: a "baseline" in which the source of the mercury in fish is from background atmospheric deposition, and an "impact" scenario, in which local Hg deposition and concentrations in fish are roughly doubled to represent additional deposition from the hypothetical nearby power plant. For both scenarios, the 99th percentile of MeHg body burden was more than an order of magnitude below the lowest level at which increased transient adult paresthesia was experienced in an acute MeHg poisoning incident in Iraq. We thus conclude that neurological risks to adults from MeHg resulting from atmospheric Hg deposition are trivial. Based on three epidemiological studies of congenital neurological risks, we find that fetal effects appear to be more critical and that there is a smaller margin of safety for pregnant consumers of freshwater sportfish. However, the margin of safety is still considerable and may have been diminished by uncertainties in the relationships between maternal hair Hg and the actual fetal exposures.