Mercury (Hg), and the organometallic compounds formed from it, are among the most toxic of substances to the global environment. Mercury is environmentally ubiquitous, and both wildlife and humans are exposed to the toxic effects of its environmental residues, primarily elemental mercury (Hg0), divalent mercury (Hg2+) and methylmercury (MeHg). Humans are exposed to different forms of Hg, and potential health risks have been reported from such exposures; examples of Hg exposure include mercury vapor from dental amalgams, occupational exposures and exposures during artisan and small-scale gold mining operations. Despite the significance of those foregoing Hg exposures, of particular concern is human and wildlife exposure to MeHg, a potent neurotoxicant. Once incorporated into the body, MeHg easily penetrates the blood-brain barrier and causes damage to the central nervous system, particularly in fetuses. It bioaacumulates and biomagnifies in the aquatic food chain; consequently, fish and seafood consumption is the major pathway by which humans are exposed to MeHg. MeHg is the focus of this review. It adversely affects humans and is currently the subject of intense public health interest and worldwide concern. In this review, I summarize the sources and cycling of global mercury in the environment, pathways of exposure, toxicity and exposure evaluation, toxicokinetics, the common biomarkers to evaluate exposure and effects in populations, and finally review the nutritional risks and benefits from fish consumption.

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Human Health Effects of Methylmercury Exposure

Mercury is a ubiquitous contaminant, and a range of chemical species is generated by human activity and natural environmental change. Elemental mercury and its inorganic and organic compounds have different toxic properties, but all them are considered hazardous in human exposure. In an equimolecular exposure basis, organomercurials with a short aliphatic chain are the most harmful compounds and they may cause irreversible damage to the nervous system. Methylmercury (CH(3)Hg(+)) is the most studied following the neurotoxic outbreaks identified as Minamata disease and the Iraq poisoning. The first description of the CNS pathology dates from 1954. Since then, the clinical neurology, the neuropathology and the mechanisms of neurotoxicity of organomercurials have been widely studied. The high thiol reactivity of CH(3)Hg(+), as well as all mercury compounds, has been suggested to be the basis of their harmful biological effects. However, there is clear selectivity of CH(3)Hg(+) for specific cell types and brain structures, which is not yet fully understood. The main mechanisms involved are inhibition of protein synthesis, microtubule disruption, increase of intracellular Ca(2+) with disturbance of neurotransmitter function, oxidative stress and triggering of excitotoxicity mechanisms. The effects are more damaging during CNS development, leading to alterations of the structure and functionality of the nervous system. The major source of CH(3)Hg(+) exposure is the consumption of fish and, therefore, its intake is practically unavoidable. The present concern is on the study of the effects of low level exposure to CH(3)Hg(+) on human neurodevelopment, with a view to establishing a safe daily intake. Recommendations are 0.4 micro g/kg body weight/day by the WHO and US FDA and, recently, 0.1 micro g/kg body weight/day by the US EPA. Unfortunately, these levels are easily attained with few meals of fish per week, depending on the source of the fish and its position in the food chain.

Neurotoxicity of organomercurial compounds

Artisanal and small-scale gold mining (ASM) is a poverty-driven activity that provides an important source of livelihood for rural communities. The amount of mercury lost by 10 to 15 million small-scale gold miners can reach as much as 1,000 tonnes a year, potentially contaminating 100 million people worldwide. This document brings simple, replicable and sustainable techniques to be used by environmental and health agencies, mining-related institutions and researchers for monitoring mercury pollution. These are badly needed decision-making tools to support mitigation of a serious problem.

Protocols for Environmental and Health Assessment of Mercury Released by Artisanal and Small-Scale Gold Miners

This paper examines issues of human mercury (Hg) exposure and adverse health effects throughout the Amazon region. An extensive review was conducted using bibliographic indexes as well as secondary sources. There are several sources of Hg (mining, deforestation, reservoirs), and exposure takes place through inhalation or from fish consumption. There is a wide range of exposure, with mean hair-Hg levels above 15µg/g in several Amazonian communities, placing them among the highest reported levels in the world today. Dietary Hg intake has been estimated in the vicinity of 1-2µg/kg/day, considerably higher than the USEPA RfD of 0.1µg/kg/day or the World Health Organization recommendation of 0.23µg/kg/day. Neurobehavioral deficits and, in some cases, clinical signs have been reported both for adults and children in relation to Hg exposure in several Amazonian countries. There is also some evidence of cytogenetic damage, immune alterations, and cardiovascular toxicity. Since fish provide a highly nutritious food source, there is an urgent need to find realistic and feasible solutions that will reduce exposure and toxic risk, while maintaining healthy traditional dietary habits and preserving this unique biodiversity.

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Human mercury exposure and adverse health effects in the Amazon: a review.

In this study, we showed that selenium (Se) deposition from metal smelters in Sudbury, Ontario, greatly re- duces the bioassimilation of mercury (Hg) by aquatic biota throughout the food web. Concentrations of total and methyl mercury in tissues of zooplankton, mayflies (Stenonema femoratum), amphipods (Hyalella azteca), and young- of-the-year perch (Perca flavescens) were positively correlated with increasing distance from Sudbury smelters and inversely correlated with Se concentrations in lake water. Although concentrations of total Hg and total Se in tissues showed weak or no correlation in most of the studied aquatic species, methyl mercury and total Se showed much clearer inverse trends. Similar to the results of our previous study of muscle tissue from adult perch and walleye (Sander vitreus), these findings suggest that Se plays an important role in limiting the whole-body assimilation of Hg at lower levels of the aquatic food chain. High Se concentrations may force a preferential assimilation of the element over Hg through a competitive adsorption on binding sites. They may also restrict the solubility and availability of Hg to aquatic organisms or reduce the methylation of this metal in lakes. Resume : Nous montrons dans cette etude que la deposition de selenium (Se) provenant des hauts fourneaux de Sud- bury en Ontario reduit grandement l'assimilation du mercure (Hg) par le biote aquatique tout au long de la chaine alimentaire. Les concentrations en Hg total et methylmercure dans les tissus de zooplancton, ephemeres (Stenonema femoratum), amphipodes (Hyalella azteca) et jeunes perchaudes (Perca flavescens) de l'annee ont pu etre directement correlees avec la distance qui les separe des hauts fourneaux de Sudbury et inversement correlees avec les concentra- tions de Se dans l'eau de lac. Alors que les concentrations en Hg total et Se total dans les tissus ne montrent que des correlations faibles ou nulles entre elles chez la plupart des organismes etudies, le methylmercure et le Se total mon- trent des relations inverses beaucoup plus claires. De facon similaire aux resultats de notre etude precedente sur les tissus musculaires de perchaudes et de dores (Sander vitreus) adultes, ces donnees laissent croire que le Se joue un role important en limitant l'assimilation integrale du Hg aux niveaux inferieurs de la chaine alimentaire aquatique. Les fortes concentrations de Se pourraient conduire a une assimilation preferentielle de cet element sur celle du Hg a travers une adsorption competitive sur des sites de liaison. Elles pourraient aussi restreindre la solubilite et la disponi- bilite du mercure chez les organismes aquatiques ou reduire la methylation de ce metal dans les lacs.

The effect of selenium on mercury assimilation by freshwater organisms

Correlation between mercury and selenium concentrations in Indian hair from Rondônia State, Amazon region, Brazil

Ž. Total mercury and selenium concentrations were determined in hair samples collected from Wari Pacaas Novos ´ Indians living in Doutor Tanajura village, Gujara-Mirim city, Rondonia State. The mercury concentrations in some ´ˆ samples are much higher than the values determined in samples from individuals not exposed to mercury contamination, occupationally or environmentally. The selenium concentrations are in the normal range. A correlation was observed between the mercury and selenium concentration and the values of the molar ratio approach 1 at low Hg concentrations. This fact is related to the equimolar complex formed by �Ž . 4 HgSe Seleprotein P, which can decrease the bioavailable mercury in the organism. 2002 Elsevier Science nm

Monica Soares de Campos

Papers

Correlation between mercury and selenium concentrations in Indian hair from Rondônia State, Amazon region, Brazil

Correlation between mercury and selenium concentrations in Indian hair from Rondonia State, ˆ