Methylmercury exposure and health effects in humans: a worldwide concern

In inland China, rice, rather than fish, is the major pathway for methylmercury exposure

BACKGROUND

Fish consumption is considered the primary pathway of methylmercury (MeHg) exposure for most people in the world. However, in the inland regions of China, most of the residents eat little fish, but they live in areas where a significant amount of mercury (Hg) is present in the environment.

OBJECTIVES

We assessed concentrations of total Hg and MeHg in samples of water, air, agricultural products, and other exposure media to determine the main exposure pathway of Hg in populations in inland China.

METHODS

We selected Guizhou Province for our study because it is highly contaminated with Hg and therefore is representative of other Hg-contaminated areas in China. We selected four study locations in Guizhou Province: three that represent typical environments with severe Hg pollution [due to Hg mining and smelting (Wanshan), traditional zinc smelting (recently closed; Weining), and heavy coal-based industry (Qingzhen)], and a village in a remote nature reserve (Leigong).

RESULTS

The probable daily intake (PDI) of MeHg for an adult population based on 60 kg body weight (bw) was considerably higher in Wanshan than in the other three locations. With an average PDI of 0.096 microg/kg bw/day (range, 0.015-0.45 microg/kg bw/day), approximately 34% of the inhabitants in Wanshan exceeded the reference dose of 0.1 microg/kg bw/day established by the U.S. Environmental Protection Agency. The PDI of MeHg for residents in the three other locations were all well below 0.1 microg/kg bw/day (averages from 0.017 to 0.023 microg/kg bw/day, with a maximum of 0.095 microg/kg bw/day). In all four areas, rice consumption accounted for 94-96% of the PDI of MeHg.

CONCLUSION

We found that rice consumption is by far the most important MeHg exposure route; however, most of the residents (except those in Hg-mining areas) have low PDIs of MeHg.

Degradation of methylmercury and its effects on mercury distribution and cycling in the Florida Everglades

Methylmercury (MeHg) is recognized as one of the major water quality concerns in the Florida Everglades. Degradation of MeHg in the water is thought to be one of the most important processes to the cycling of MeHg, but there is a lack of quantitative estimations of its effect on the distribution and cycling of MeHg in this ecosystem. Stable isotope (Me201Hg) addition method was implemented to investigate the degradation of MeHg in the Everglades. By combining these results with the field monitoring data, effects of photodegradation on MeHg distribution and its contribution to MeHg cycling were estimated. The results indicate that degradation of MeHg in Everglades water is mediated by sunlight and that UV-A and UV-B radiations are the principal driver. The spatial pattern of MeHg photodegradation potential (PPD) generally illustrated an increasing trend from north to south in the Everglades, which was opposite to the distribution of MeHg in water column. Correlation analysis shows that MeHg concentration in the water had a significant negative relation to PPD, suggesting that photodegradation could play an important role in controlling the distribution of MeHg in Everglades water. Furthermore, about 31.4% of MeHg input into the water body was removed by photodegradation, indicating its importance in the biogeochemical cycling of MeHg in the Everglades. This percent reduction is much lower than that reported for other ecosystems, which could be caused by the higher concentration of DOC in the Everglades. The relatively slower degradation of MeHg could be one of the main reasons for the high ratio of MeHg to total mercury (THg) in this ecosystem.

A functional matrix metalloproteinase (MMP)-9 polymorphism modifies plasma MMP-9 levels in subjects environmentally exposed to mercury

Mercury (Hg) exposure causes health problems including cardiovascular diseases. Although precise mechanisms have not been precisely defined yet, matrix metalloproteinases (MMPs) may be involved. The gene encoding MMP-9 presents genetic polymorphisms which affect the expression and activity level of this enzyme. Two polymorphisms in the promoter region [C(-1562)T and (CA)(n)] are functionally relevant, and are implicated in several diseases. This study aimed at examining how these polymorphisms affect the circulating MMP-9 levels and its endogenous inhibitor, the tissue inhibitor of metalloproteinase-1 (TIMP-1) in 266 subjects environmentally exposed to Hg. Blood and plasma Hg concentrations were determined by inductively coupled plasma-mass spectrometry (ICP-MS). MMP-9 and TIMP-1 concentrations were measured in plasma samples by gelatin zymography and ELISA, respectively. Genotypes for the C(-1562)T and the microsatellite (CA)(n) polymorphisms were determined. We found a positive association (P<0.05) between plasma Hg concentrations and MMP-9/TIMP-1 ratio (an index of net MMP-9 activity). When the subjects were divided into tertiles with basis on their plasma Hg concentrations, we found that the (CA)(n) polymorphism modified MMP-9 concentrations and MMP-9/TIMP-1 ratio in subjects with the lowest Hg concentrations (first tertile), with the highest MMP-9 levels being found in subjects with genotypes including alleles with 21 or more CA repeats (H alleles) (P<0.05). Conversely, this polymorphism had no effects on subjects with intermediate or high plasma Hg levels (second and third tertiles, respectively). The C(-1562)T polymorphism had no effects on MMP-9 levels. These findings suggest a significant interaction between the (CA)(n) polymorphism and low levels of Hg exposure, possibly increasing the risk of developing diseases in subjects with H alleles.

Mercury exposure and children's health

Acute or chronic mercury exposure can cause adverse effects during any period of development. Mercury is a highly toxic element; there is no known safe level of exposure. Ideally, neither children nor adults should have any mercury in their bodies because it provides no physiological benefit. Prenatal and postnatal mercury exposures occur frequently in many different ways. Pediatricians, nurses, and other health care providers should understand the scope of mercury exposures and health problems among children and be prepared to handle mercury exposures in medical practice. Prevention is the key to reducing mercury poisoning. Mercury exists in different chemical forms: elemental (or metallic), inorganic, and organic (methylmercury and ethyl mercury). Mercury exposure can cause acute and chronic intoxication at low levels of exposure. Mercury is neuro-, nephro-, and immunotoxic. The development of the child in utero and early in life is at particular risk. Mercury is ubiquitous and persistent. Mercury is a global pollutant, bio-accumulating, mainly through the aquatic food chain, resulting in a serious health hazard for children. This article provides an extensive review of mercury exposure and children's health.

Iron-mediated photochemical decomposition of methylmercury in an arctic Alaskan lake

Sunlight-induced decomposition is the principal sink for methylmercury (CH(3)Hg(+)) in arctic Alaskan lakes and reduces its availability for accumulation in aquatic food webs. However, the mechanistic chemistry of this process in natural waters is unknown. We examined experimentally the mechanism of photochemical CH(3)Hg(+) decomposition in filter-sterilized epilimnetic waters of Toolik Lake in arctic Alaska (68 degrees 38'N, 149 degrees 36'W), a region illuminated by sunlight almost continuously during the summer. Results from in situ incubation tests indicate that CH(3)Hg(+) is not decomposed principally by either direct photolysis (i.e., no degradation in reagent-grade water) or primary photochemical reactions with dissolved organic material. The preeminent role of labile Fe and associated photochemically produced reactive oxygen species is implicated by tests that show 1) additions of Fe(III) to reagent-grade water enhance CH(3)Hg(+) photodecomposition, 2) strong complexation of ambient Fe(III) with desferrioxamine B inhibits the reaction in lake water, and 3) experimental additions of organic molecules that scavenge hydroxyl radicals specifically among reactive oxygen species (dimethylsulfoxide and formic acid) inhibit CH(3)Hg(+) degradation. Lake-water dilution and Fe(III) addition experiments indicate that Fe is not the limiting reactant for CH(3)Hg(+) photodecomposition in Toolik Lake, which is consistent with prior results indicating that photon flux is a major control. These results demonstrate that CH(3)Hg(+) is decomposed in natural surface water by oxidants, apparently hydroxyl radical, generated from the photo-Fenton reaction.

Mercury in the Tapajós River basin, Brazilian Amazon: a review

This paper presents a review about mercury contamination and human exposure in the Tapajós River basin (Brazil), one of the major tributaries of the Amazon impacted by traditional gold mining from the mid 1980s. The most recent review in this region was published more than ten years ago and since then many articles about environment and especially human populations have revealed new aspects of mercury toxicology. Additionally, new biomarkers of mercury exposure and toxicity have been studied in these populations. However, there are still many open, about both mercury's biogeochemical cycle and mercury health risks. Further environmental and human risk research directions are proposed.

Bioaccumulation syndrome: identifying factors that make some stream food webs prone to elevated mercury bioaccumulation

Mercury is a ubiquitous contaminant in aquatic ecosystems, posing a significant health risk to humans and wildlife that eat fish. Mercury accumulates in aquatic food webs as methylmercury (MeHg), a particularly toxic and persistent organic mercury compound. While mercury in the environment originates largely from anthropogenic activities, MeHg accumulation in freshwater aquatic food webs is not a simple function of local or regional mercury pollution inputs. Studies show that even sites with similar mercury inputs can produce fish with mercury concentrations ranging over an order of magnitude. While much of the foundational work to identify the drivers of variation in mercury accumulation has focused on freshwater lakes, mercury contamination in stream ecosystems is emerging as an important research area. Here, we review recent research on mercury accumulation in stream-dwelling organisms. Taking a hierarchical approach, we identify a suite of characteristics of individual consumers, food webs, streams, watersheds, and regions that are consistently associated with elevated MeHg concentrations in stream fish. We delineate a conceptual, mechanistic basis for explaining the ecological processes that underlie this vulnerability to MeHg. Key factors, including suppressed individual growth of consumers, low rates of primary and secondary production, hydrologic connection to methylation sites (e.g., wetlands), heavily forested catchments, and acidification are frequently associated with increased MeHg concentrations in fish across both streams and lakes. Hence, we propose that these interacting factors define a syndrome of characteristics that drive high MeHg production and bioaccumulation rates across these freshwater aquatic ecosystems. Finally, based on an understanding of the ecological drivers of MeHg accumulation, we identify situations when anthropogenic effects and management practices could significantly exacerbate or ameliorate MeHg accumulation in stream fish.

Photolytic degradation of methylmercury enhanced by binding to natural organic ligands

Monomethylmercury is a neurotoxin that poses significant risks to human health1 due to its bioaccumulation in food webs. Sunlight degradation to inorganic mercury is an important component of the mercury cycle that maintains methylmercury at low concentrations in natural waters. Rates of photodecomposition, however, can vary drastically between surface waters2-5 for reasons that are largely unknown. Here, we show that photodegradation occurs through singlet oxygen, a highly reactive form of dissolved oxygen generated by sunlight irradiation of dissolved natural organic matter. The kinetics of degradation, however, depended on water constituents that bind methylmercury cations. Relatively fast degradation rates (similar to observations in freshwater lakes) applied only to methylmercury species bound to organic sulfur-containing thiol ligands such as glutathione, mercaptoacetate, and humics. In contrast, methylmercury-chloride complexes, which are dominant in marine systems, were unreactive. Binding by thiols lowered the excitation energy of the carbon-mercury bond on the methylmercury molecule6-7 and subsequently increased reactivity towards bond breakage and decomposition. Our results explain methylmercury photodecomposition rates that are relatively rapid in freshwater lakes2-4 and slow in marine waters5.

Methylmercury exposure and health effects from rice and fish consumption: a review

Methylmercury (MeHg) is highly toxic, and its principal target tissue in humans is the nervous system, which has made MeHg intoxication a public health concern for many decades. The general population is primarily exposed to MeHg through consumption of contaminated fish and marine mammals, but recent studies have reported high levels of MeHg in rice and confirmed that in China the main human exposure to MeHg is related to frequent rice consumption in mercury (Hg) polluted areas. This article reviews the progress in the research on MeHg accumulation in rice, human exposure and health effects, and nutrient and co-contaminant interactions. Compared with fish, rice is of poor nutritional quality and lacks specific micronutrients identified as having health benefits (e.g., n-3 long chain polyunsaturated fatty acid, selenium, essential amino acids). The effects of these nutrients on the toxicity of MeHg should be better addressed in future epidemiologic and clinical studies. More emphasis should be given to assessing the health effects of low level MeHg exposure in the long term, with appropriate recommendations, as needed, to reduce MeHg exposure in the rice-eating population.

Bioaccumulation of methylmercury versus inorganic mercury in rice (Oryza sativa L.) grain

Mercury (Hg) bioaccumulation in aquatic food webs has been much studied, motivated from high Hg levels found in many fish species important for human consumption. Hg bioaccumulation in terrestrial food chains have received little attention and assumed to be of minor importance. However, recent studies showed that rice can be an important pathway of methylmercury (MeHg) exposure to inhabitants in Hg mining areas in China. In this study, 59 sampling sites (including 32 sites from "heavily polluted area", 19 from "less-impacted area" and 8 from "control sites") were selected in a Hg mining area in China and both inorganic Hg (IHg) and MeHg were determined in rice grain (brown rice) and soil samples to evaluate Hg bioaccumulation in rice grain. Bio-Accumulation Factors (BAFs) for IHg ranged from 0.00014 to 0.51 and from 0.71 to 50 for MeHg. BAFs for MeHg were on average more than 800 times higher than those for IHg (maximum: 40,000 times). This study, for the first time, showed that rice grain is an intensive bioaccumulator of MeHg, but not of IHg, which may be trapped by the roots.

Something fishy? News media presentation of complex health issues related to fish consumption guidelines

OBJECTIVE

The news media are an important source of dietary information. Understanding news content, particularly the portrayal of risks and benefits of certain foods, is relevant for effective public health communication. Fish consumption may reduce risk for CVD and aid neonatal development, but recent work shows public confusion about the benefits of fish, challenged by the evidence of mercury and other contaminants in fish. We present an analysis of the messages about fish in US news media over 15 years, identifying trends in coverage and highlighting implications of current messaging.

DESIGN

We conducted a descriptive text analysis and coded for manifest content: locality of focus, story frame, reference to studies, inclusion of government guidelines and portrayal of uncertainty. We identified chronological patterns and analysed the data for statistically significant relationships between media source and content.

SETTING

News stories were selected from five daily newspapers and five television networks (1993-2007).

SUBJECTS

We analysed 310 health-related news stories on fish.

RESULTS

Risk messages outweighed benefit messages four to one, and health benefits only became prominent after 2002. No difference existed in coverage topic by news source. Fish consumption has increasingly become a national issue.

CONCLUSIONS

With the bulk of messages about fish consumption focused on risk, the benefits may be lost to consumers. This gap creates a need for public health to work with news media to more effectively communicate benefits and risks around fish consumption and health and to consider options for communicating tailored information where it can be more readily utilised.

Ecological, groundwater, and human health risk assessment in a mining region of Nicaragua

The objective of the present study was to integrate the relative risk from mercury exposure to stream biota, groundwater, and humans in the Río Artiguas (Sucio) river basin, Nicaragua, where local gold mining occurs. A hazard quotient was used as a common exchange rate in probabilistic estimations of exposure and effects by means of Monte Carlo simulations. The endpoint for stream organisms was the lethal no-observed-effect concentration (NOECs), for groundwater the WHO guideline and the inhibitory Hg concentrations in bacteria (IC), and for humans the tolerable daily intake (TDI) and the benchmark dose level with an uncertainty factor of 10 (BMDLs(0.1)). Macroinvertebrates and fish in the contaminated river are faced with a higher risk to suffer from exposure to Hg than humans eating contaminated fish and bacteria living in the groundwater. The river sediment is the most hazardous source for the macroinvertebrates, and macroinvertebrates make up the highest risk for fish. The distribution of body concentrations of Hg in fish in the mining areas of the basin may exceed the distribution of endpoint values with close to 100% probability. Similarly, the Hg concentration in cord blood of humans feeding on fish from the river was predicted to exceed the BMDLs(0.1) with about 10% probability. Most of the risk to the groundwater quality is confined to the vicinity of the gold refining plants and along the river, with a probability of about 20% to exceed the guideline value.

A systems-based approach to investigate dose- and time-dependent methylmercury-induced gene expression response in C57BL/6 mouse embryos undergoing neurulation

BACKGROUND

Aberrations during neurulation due to genetic and/or environmental factors underlie a variety of adverse developmental outcomes, including neural tube defects (NTDs). Methylmercury (MeHg) is a developmental neurotoxicant and teratogen that perturbs a wide range of biological processes/pathways in animal models, including those involved in early gestation (e.g., cell cycle, cell differentiation). Yet, the relationship between these MeHg-linked effects and changes in gestational development remains unresolved. Specifically, current information lacks mechanistic comparisons across dose or time for MeHg exposure during neurulation. These detailed investigations are crucial for identifying sensitive indicators of toxicity and for risk assessment applications.

METHODS

Using a systems-based toxicogenomic approach, we examined dose- and time-dependent effects of MeHg on gene expression in C57BL/6 mouse embryos during cranial neural tube closure, assessing for significantly altered genes and associated Gene Ontology (GO) biological processes. Using the GO-based application GO-Quant, we quantitatively assessed dose- and time-dependent effects on gene expression within enriched GO biological processes impacted by MeHg.

RESULTS

We observed MeHg to significantly alter expression of 883 genes, including several genes (e.g., Vangl2, Celsr1, Ptk7, Twist, Tcf7) previously characterized to be crucial for neural tube development. Significantly altered genes were associated with development cell adhesion, cell cycle, and cell differentiation-related GO biological processes.

CONCLUSIONS

Our results suggest that MeHg-induced impacts within these biological processes during gestational development may underlie MeHg-induced teratogenic and neurodevelopmental toxicity outcomes.

Mercury in South Carolina fishes, USA

The South Carolina Department of Health and Environmental Control has collected, processed, and analyzed fish tissue total mercury (Hg) since 1976. For this study, skin-on-filet data from 1993 to 2007 were examined to determine biotic, spatial and temporal trends in tissue Hg levels for SC fishes. Because of the relatively high number of tissue Hg values below the analytical detection limits interval censored regression and censored least absolute deviations were used to construct several models to characterize trends. Large pelagic, piscivorous fish species, such as bowfin (Amia calva Linnaeus 1766), had higher levels of tissue Hg than smaller omnivorous species. Estuarine species had relatively low levels of tissue Hg compared to freshwater species, while two large open ocean species, king mackerel (Scomberomorus cavalla Cuvier 1829) and swordfish (Xiphias gladius Linnaeus 1758), had higher tissue Hg readings. For a given fish species, length was an important predictor of tissue Hg with larger individuals having higher levels than smaller individuals. The USEPA Level III ecoregion and water body type from where the fishes were collected were important in predicting the levels of tissue Hg. The Middle Atlantic Coastal Plain ecoregion had fishes with the highest levels of tissue Hg, while the Piedmont and Southern Coastal Plain ecoregions had the lowest. For a given ecoregion, large reservoirs and regulated rivers had fish with lower levels of tissue Hg than unregulated rivers. For reservoirs, the size of the impoundment was a significant predictor of tissue mercury with small reservoirs having higher levels of tissue mercury than large reservoirs. Landuse and water chemistry accounted for differences seen in fish of various ecoregions and waterbody types. Sampling locations associated with a high percentage of wetland area had fish with high levels of tissue Hg. Correlation analysis showed a strong positive relationship between tissue Hg levels and water column iron, total organic carbon, ammonia, and total kjedahl nitrogen, and a negative relationship with alkalinity, dissolved oxygen and pH. Results from principle component analysis revealed patterns between waterbody type and water chemistry variables that suggests hydrologic modification can have profound effects on the levels of fish tissue Hg in riverine systems. From 1993 to 2007, fish tissue Hg levels have trended lower. A spike in tissue Hg levels was observed in 2003-2005. The drying and rewetting of the landscape after the 2002 drought is hypothesized to have caused an increase in the methylation efficiencies of the system.

Mercury concentrations in landlocked Arctic char (Salvelinus alpinus) from the Canadian Arctic. Part II: influence of lake biotic and abiotic characteristics on geographic trends in 27 populations

Among-lake variation in mercury (Hg) concentrations in landlocked Arctic char was examined in 27 char populations from remote lakes across the Canadian Arctic. A total of 520 landlocked Arctic char were collected from 27 lakes, as well as sediments and surface water from a subset of lakes in 1999, 2002, and 2005 to 2007. Size, length, age, and trophic position (delta(15)N) of individual char were determined and relationships with total Hg (THg) concentrations investigated, to identify a common covariate for adjustment using analysis of covariance (ANCOVA). A subset of 216 char from 24 populations was used for spatial comparison, after length-adjustment. The influence of trophic position and food web length and abiotic characteristics such as location, geomorphology, lake area, catchment area, catchment-to-lake area ratio of the lakes on adjusted THg concentrations in char muscle tissue were then evaluated. Arctic char from Amituk Lake (Cornwallis Island) had the highest Hg concentrations (1.31 microg/g wet wt), while Tessisoak Lake (Labrador, 0.07 microg/g wet wt) had the lowest. Concentrations of THg were positively correlated with size, delta(15)N, and age, respectively, in 88, 71, and 58% of 24 char populations. Length and delta(15)N were correlated in 67% of 24 char populations. Food chain length did not explain the differences in length-adjusted THg concentrations in char. No relationships between adjusted THg concentrations in char and latitude or longitude were found, however, THg concentrations in char showed a positive correlation with catchment-to-lake area ratio. Furthermore, we conclude that inputs from the surrounding environment may influence THg concentrations, and will ultimately affect THg concentrations in char as a result of predicted climate-driven changes that may occur in Arctic lake watersheds.

Stable isotope (N, C, Hg) study of methylmercury sources and trophic transfer in the northern gulf of Mexico

We combined N, C, and Hg stable isotope measurements to identify the most important factors that influence MeHg accumulation in fish from the northern Gulf of Mexico (nGOM), and to determine if coastal species residing in the Mississippi River (MR) plume and migratory oceanic species derive their MeHg from the same, or different, sources. In six coastal species and two oceanic species (blackfin and yellowfin tuna), trophic position as measured by delta(15)N explained most of the variance in log[MeHg] (r(2) approximately 0.8), but coastal species and tuna fell along distinct, nearly parallel lines with significantly different intercepts. The tuna also had significantly higher delta(202)Hg (0.2-0.5 per thousand) and Delta(201)Hg ( approximately 1.5 per thousand) than the coastal fish (delta(202)Hg = 0 to -1.0 per thousand; Delta(201)Hg approximately 0.4 per thousand). The observations can be best explained by largely disconnected food webs rooted in different baseline delta(15)N signatures (MR-plume vs oceanic) and isotopically distinct MeHg sources, with oceanic MeHg having undergone substantial photodegradation ( approximately 50%) before entering the base of the food web. Given the MR's large, productive footprint in the nGOM and the potential for exporting prey and MeHg to the adjacent oligotrophic GOM, the disconnected food webs and different MeHg sources are consistent with recent evidence in other systems of important oceanic MeHg sources.

Mercury concentrations in lentic fish populations related to ecosystem and watershed characteristics

Predicting mercury (Hg) concentrations of fishes at large spatial scales is a fundamental environmental challenge with the potential to improve human health. In this study, mercury concentrations were examined for five species across 161 lakes and ecosystem, and watershed parameters were investigated as explanatory variables in statistical models. For all species, Hg concentrations were significantly, positively related to wetland coverage. For three species (largemouth bass, pike, and walleye), Hg concentrations were significantly, negatively related to lake trophic state index (TSI), suggestive of growth biodilution. There were no significant relationships between ecosystem size and mercury concentrations. However, Hg concentrations were strongly, positively related to ecosystem size across species. Scores of small or remote lakes that have never been tested could be prioritized for testing using models akin to those presented in this article. Such an approach could also be useful for exploring how Hg concentrations of fishes might respond to natural or anthropogenic changes to ecosystems over time.

Mercury exposure and oxidative stress in communities of the Brazilian Amazon

This study was designed to assess possible associations between biomarkers of mercury (Hg) exposure and oxidative stress in fish-eating Amazonian communities. Clinical samples were obtained from riparians living in the Brazilian Amazon. Biomarkers of oxidative stress (glutathione - GSH, glutathione peroxidase - GSH-Px, catalase - CAT, activity and reactivation index of delta-aminolevulinate dehydratase - ALA-D (R%) were determined in blood. Total Hg was measured in whole blood (B-Hg), plasma (P-Hg) and hair (H-Hg). Association between biomarkers of Hg exposure and oxidative stress were examined using multiple regression models, including age, gender, alcohol consumption, smoking status, fish consumption and then stratified for gender. Significant inverse relations were observed between GSH-Px, GSH, CAT, ALA-D activity and B-Hg or H-Hg (p<0.05). ALA-D reactivation index was positively related to B-Hg (p<0.0001). P-Hg was directly related to ALA-D reactivation index and inversely associated with GSH-Px, GSH, and ALA-D activity (p<0.05). When stratified for gender, women showed significant inverse associations between all biomarkers of Hg exposure and CAT (p<0.05) or GSH (p<0.05), while for men only P-Hg showed a significant inverse relation with GSH (p<0.001). Our results clearly demonstrated an association between Hg exposure and oxidative stress. Moreover, for B-Hg, P-Hg and H-Hg gender differences were present.

Health assessment of artisanal gold miners in Tanzania

In 2003 UNIDO (United Nations Industrial Development Organization) conducted an environmental and health assessment in a small-scale mining area in Tanzania. BGS (British Geological Survey) performed the environmental assessment. The Institute of Forensic Medicine - University of Munich performed the health assessment. The results of the medical, neurological and neuro-psychological examination of 180 participants from the affected area of Rwamagasa and 31 controls were analyzed. Urine, blood and hair samples were analyzed to detect the level of mercury body burden. Mercury concentrations in the bio-monitors urine, blood and hair were statistically significantly higher in the exposed population from Rwamagasa compared to the control group from Katoro. Only amalgam burners showed mercury levels above the toxicological threshold limits. A speciation of mercury in hair indicated that mainly elemental mercury vapor contributed to the high body burden of the artisanal miners. 104 amalgam-burners, the most exposed population group, were examined. 25 of these workers were found to be intoxicated. Small-scale mining is a serious health hazard for amalgam burners. Reduction of the exposure is essential to prevent further damage.

Rapid, efficient growth reduces mercury concentrations in stream-dwelling Atlantic salmon

Mercury (Hg) is a potent toxin that biomagnifies in aquatic food webs. Large fish generally have higher Hg concentrations than small fish of the same species. However, models predict that fish that grow large faster should have lower Hg concentrations than small, slow-growing fish due to somatic growth dilution (SGD). We examined the relationship between Hg concentrations and growth rate in fish using a large-scale field experiment. Atlantic salmon (Salmo salar) fry hatched under uniform initial conditions were released at eighteen sites in natural streams, collected after one growing season, and Hg concentration and growth measured. As expected for Hg accumulation from food, mercury concentrations in salmon tracked Hg concentrations in their prey. Nonetheless, large, fast-growing salmon had lower Hg concentrations than small, slow-growing salmon, consistent with SGD. While prey Hg concentration accounted for 59% of the explained variation in salmon Hg concentration across sites, salmon growth rate accounted for 38% of the explained variation independent of prey Hg concentration. A mass-balance Hg accumulation model shows that such SGD occurs when fast growth is associated with high growth efficiency. Fish growth is tremendously variable and sensitive to anthropogenic impacts, so SGD of Hg has important implications for fisheries management.

Assessment of toxic elements' content in swine kidneys: Pathomorphological analysis

Background: In order to ensure the safety of consumers in Serbia, the toxic elements' (Cd, Hg, As, Pb) content in swine kidneys collected from three different sites in Serbia (n=90), were determined by atomic absorption spectrometry. Also, in order to find information on the effects of accumulation of toxic elements in swine kidneys, histopathological examination of kidneys was carried out. Methods: Determination of toxic elements (Cd, Hg, As, Pb) in swine kidneys was performed by atomic absorption spectrometry. For microscopic examination, kidney samples were fixed in 10% neutral buffered formalin and absolute alcohol for 5 to 7 days, processed by routine methods, sectioned at 5-8 ?m, and stained with hematoxylin and eosin (HE) for light microscopy. Results: The presence of mercury was found in 33.3% of kidney samples in the range of 0.005-0.055 mg/kg, while presence of cadmium was found in less degree (27.7%) but in higher content (0.05-1,23 mg/kg). The presence of arsenic was found only in one sample, while presence of lead was not found. The metal-to-metal correlation analysis supported the theory that there were different sources of contamination. Histopathological examination of kidneys confirms tubulopathies with edema and cell vacuolization. In addition, hemorrhages and necrosis of proximal kidney tubules' cells were found. Conclusion: This study shows the presence of toxic elements in pigs butchered in Serbia at levels comparable to those reported in other countries., and consequently do not represent any concern from a consumer safety point of view. The lack of strong correlation between histopathological changes and incidence of toxic elements found in our trial may explain the possible synergism among toxic elements and other nephrotoxic compounds, which enhances the toxicity, especially in cases of low contamination.

Mercury distribution and bioaccumulation up the soil-plant-grasshopper-spider food chain in Huludao City, China

The purpose of this study is to investigate total mercury (THg) distribution and its bioaccumulation up the soil-plant-grasshopper-spider in the Huludao City, which is polluted seriously by chlor-alkali and zinc smelting industry in Northeast of China. Results indicated that average THg concentrations in soil, plant leaves, grasshopper Locusta migratoria manilensis and Acrida chinensis, and spider were 0.151, 0.119, 0.167 and 0.134 mg/kg, respectively. THg spatial distribution suggested that most of mercury came from the chlor-alkali plant and the two zinc smelteries. The highest mercury concentration was found in the wings among different grasshoppers' organs. Although spiders are the predatory, THg concentrations in their bodies were not high, and only on the same level as in grasshoppers, which might be due to spiders' special living habits. In the light of the mercury transportation at every stage of the soil-plant-grasshopper-spider food chain, the bioaccumulation factors were 0.03, 0.79-1.11 and 0.80-1.13 respectively. It suggested that mercury biomagnification up terrestrial food chains was not so large and obvious as it was in the aquatic food chain.

Sources of mercury exposure for U.S. seafood consumers: implications for policy

BACKGROUND

Recent policies attempting to reduce adverse effects of methylmercury exposure from fish consumption in the United States have targeted reductions in anthropogenic emissions from U.S. sources.

OBJECTIVES

To analyze the prospects for future North American and international emissions controls, we assessed the potential contributions of anthropogenic, historical, and natural mercury to exposure trajectories in the U.S. population over a 40-year time horizon.

METHODS

We used models that simulate global atmospheric chemistry (GEOS-Chem); the fate, transport, and bioaccumulation of mercury in four types of freshwater ecosystems; and mercury cycling among different ocean basins. We considered effects on mercury exposures in the U.S. population based on dietary survey information and consumption data from the sale of commercial market fish.

RESULTS

Although North American emissions controls may reduce mercury exposure by up to 50% for certain highly exposed groups such as indigenous peoples in the Northeast, the potential effects of emissions controls on populations consuming marine fish from the commercial market are less certain because of limited measurements.

CONCLUSIONS

Despite uncertainties in the exposure pathway, results indicate that a combination of North American and international emissions controls with adaptation strategies is necessary to manage methylmercury risks across various demographic groups in the United States.

Hair mercury levels in Amazonian populations: spatial distribution and trends

BACKGROUND

Mercury is present in the Amazonian aquatic environments from both natural and anthropogenic sources. As a consequence, many riverside populations are exposed to methylmercury, a highly toxic organic form of mercury, because of their intense fish consumption. Many studies have analysed this exposure from different approaches since the early nineties. This review aims to systematize the information in spatial distribution, comparing hair mercury levels by studied population and Amazonian river basin, looking for exposure trends.

METHODS

The reviewed papers were selected from scientific databases and online libraries. We included studies with a direct measure of hair mercury concentrations in a sample size larger than 10 people, without considering the objectives, approach of the study or mercury speciation. The results are presented in tables and maps by river basin, displaying hair mercury levels and specifying the studied population and health impact, if any.

RESULTS

The majority of the studies have been carried out in communities from the central Amazonian regions, particularly on the Tapajós River basin. The results seem quite variable; hair mercury means range from 1.1 to 34.2 microg/g. Most studies did not show any significant difference in hair mercury levels by gender or age. Overall, authors emphasized fish consumption frequency as the main risk factor of exposure. The most studied adverse health effect is by far the neurological performance, especially motricity. However, it is not possible to conclude on the relation between hair mercury levels and health impact in the Amazonian situation because of the relatively small number of studies.

CONCLUSIONS

Hair mercury levels in the Amazonian regions seem to be very heterogenic, depending on several factors. There is no obvious spatial trend and there are many areas that have never been studied. Taking into account the low mercury levels currently handled as acceptable, the majority of the Amazonian populations can be considered exposed to methylmercury contamination. The situation for many of these traditional communities is very complex because of their high dependence on fish nutrients. It remains difficult to conclude on the Public Health implication of mercury exposure in this context.

Human hair mercury levels in the Wanshan mercury mining area, Guizhou Province, China

The total mercury (T-Hg) and methyl mercury (Me-Hg) concentrations in the hair were measured to evaluate mercury (Hg) exposure for the residents in Da-shui-xi Village (DSX) and Xia-chang-xi Village (XCX) in the Wanshan Hg mining area, Guizhou Province, Southwestern China. The mean concentrations in the hair of DSX residents were 5.5 ± 2.7 μg/g and 1.9 ± 0.9 μg/g for T-Hg and Me-Hg, respectively. The concentrations in the hair of XCX residents were 3.3 ± 1.4 μg/g and 1.2 ± 0.5 μg/g for T-Hg and Me-Hg, respectively. Hair Me-Hg concentrations were significantly correlated to T-Hg (r = 0.42, P < 0.01) in the two sites; on average, hair Me-Hg concentration accounted for 40 and 44% of T-Hg for DSX and XCX residents, respectively. Age has no obvious correlation with hair Hg and the hair Hg levels showed a significant gender difference, with higher T-Hg and Me-Hg concentrations in the hair from males than females. The rice collected from the two sites showed high levels of T-Hg and Me-Hg concentration. The results indicated a certain Hg exposure for the residents in DSX and XCX in the Wanshan Hg mining area.

Mercury concentrations in surface water and harvested waterfowl from the prairie pothole region of Saskatchewan

Mercury cycling in prairie ecosystems is poorly understood. We examined methylmercury (MeHg) concentrations in whole water from 49 diverse prairie wetlands and lakes in Saskatchewan. We also determined total Hg (THg) concentrations in waterfowl harvested by hunters for consumption. Average whole water MeHg concentrations ranged from 0.02 to over 4 ng L(-1) and were higher in water from wetland ponds compared to those in lakes. High MeHg concentrations in prairie wetlands present the possibility of increased Hg concentrations in biota inhabiting these and other similar systems. We therefore measured THg in 72 birds representing 13 species of waterfowl that commonly use prairie aquatic habitats. A large range in THg concentrations was observed among individual birds, with values ranging from below the detection limit to over 435 ng g(-1). When waterfowl were classified according to diet, we observed clear evidence of THg biomagnification with increasing proportion of animal prey consumed. THg concentrations in waterfowl collected by hunters did not exceed consumption guidelines of 0.5 mg kg(-1) developed for fish. This is the first study that has reported MeHg concentrations in water from the prairie pothole region of southern Saskatchewan.

Probing bioinorganic chemistry processes in the bloodstream to gain new insights into the origin of human diseases

In the context of elucidating the origin of human diseases, past poisoning epidemics have revealed that exceedingly small doses of inorganic environmental pollutants can result in dramatic effects on human health. Today, numerous organic and inorganic pollutants have been quantified in human blood, but the interpretation of these concentrations remains--from a public health point of view--problematic. Conversely, the biomolecular origin for several grievous human diseases is essentially unknown. Taken together and viewed in the context of recent bioinorganic research findings, the established human blood concentrations of toxic metals and metalloids may be functionally connected with the etiology of specific human diseases. To unravel the underlying biomolecular mechanisms, and taking into account the basic flow of dietary matter through mammalian organisms, a better understanding of the bioinorganic chemistry of toxic metals and metalloid compounds in the bloodstream is emerging as a promising avenue for future research. To this end, the concerted application of modern proteomic methodologies, synchrotron-based X-ray absorption spectroscopy and established spectroscopic techniques will contribute to better define the role that blood-based bioinorganic chemistry-related processes play in the origin of human diseases. The application of this and other modern proteomic methodologies could contribute to a better understanding of the role that blood-based bioinorganic chemistry-related processes play in the origin and etiology of human diseases.

Spatial trends and impairment assessment of mercury in sport fish in the Sacramento-San Joaquin Delta watershed

A three-year study was conducted to examine mercury in sport fish from the Sacramento-San Joaquin Delta. More than 4000 fish from 31 species were collected and analyzed for total mercury in individual muscle filets. Largemouth bass and striped bass were the most contaminated, averaging 0.40 microg/g, while redear sunfish, bluegill and rainbow trout exhibited the lowest (<0.15 microg/g) concentrations. Spatial variation in mercury was evaluated with an analysis of covariance model, which accounted for variability due to fish size and regional hydrology. Significant regional differences in mercury were apparent in size-standardized largemouth bass, with concentrations on the Cosumnes and Mokelumne rivers significantly higher than the central and western Delta. Significant prey-predator mercury correlations were also apparent, which may explain a significant proportion of the spatial variation in the watershed.

Effects of methyl mercury in combination with polychlorinated biphenyls and brominated flame retardants on the uptake of glutamate in rat brain synaptosomes: a mathematical approach for the study of mixtures

Regulatory limit values for toxicants are in general determined by the toxicology of the single compounds. However, little is known about their combined effects. Methyl mercury (MeHg), polychlorinated biphenyls (PCBs), and brominated flame retardants (BFRs) are dominant contaminants in the environment and food. MeHg is a well known neurotoxicant, especially affecting the developing brain. There is increasing evidence that PCB and BFRs also have neurotoxic effects. An enhanced effect of these toxicants, due to either synergistic or additive effects, would be considered as a risk for the fetal development. Here we studied the combinatorial effects of MeHg in combination with PCB or BFR on the reuptake of glutamate in synaptosomes. To provide the optimal conclusion regarding type of interaction, we have analyzed the data using two mathematical models, the Löewe model of additivity and Bliss' model of independent action. Binary and ternary mixtures in different proportions were made. The toxicants had primarily additive effects, as shown with both models, although tendencies towards synergism were observed. MeHg was by far the most potent inhibitor of uptake with an EC(50) value of 0.33 microM. A reconstituted mixture from a relevant fish sample was made in order to elucidate which chemical was responsible for the observed effect. Some interaction was experienced between PCB and MeHg, but in general MeHg seemed to explain the observed effect. We also show that mixture effects should not be assessed by effect addition.

Testing and application of surrogate surfaces for understanding potential gaseous oxidized mercury dry deposition

Currently there is no standard method for measurement of atmospheric mercury dry deposition. While all operationally defined forms of atmospheric mercury (elemental, oxidized, and particulate) can be dry deposited, oxidized forms are of concern due to high deposition velocities, water solubility, and reactivity. This paper describes the development of a surrogate surface for characterizing potential dry deposition of gaseous oxidized mercury (GOM). Laboratory tests showed that the surface collected HgCl2, HgBr2, and HgO with equal efficiency, and deposition was not significantly influenced by temperature, humidity, or ozone concentrations. Deposition of mercury to surfaces in field deployments was correlated with GOM concentrations (r2 = 0.84, p < 0.01, n = 326. Weekly mean GOM deposition velocities from surface deployments (1.1 +/- 0.6 cm s(-1)) were higher than modeled values (0.4 +/- 0.2 cm s(-1)) at four field sites, but were within the range reported for direct measurements. Although the surfaces do not simulate the heterogeneity of natural surfaces and need to be validated by direct measurements, they do provide a physical means for estimating temporal trends and spatial variability of dry deposition of GOM.

Importance of ultraviolet radiation in the photodemethylation of methylmercury in freshwater ecosystems

Photodemethylation (PD) is thought to be the most important biogeochemical sink of methylmercury (MeHg) in freshwater lakes. However, we possess little mechanistic knowledge of this important biogeochemical process with regard to, for instance, the role of ultraviolet (UV) radiation versus visible light in mediating MeHg PD. This information is critical to correctly model MeHg PD at the whole-lake level, since wavelengths in the UV and visible regions of the solar spectrum are attenuated at very different rates in the water column of lakes. Furthermore, the established methodology for quantifying MeHg PD requires the addition of a MeHg spike, which often increases the concentration of ambient MeHg by 1 to 2 orders of magnitude; however, the assumption that the MeHg spike behaves like ambient MeHg has never been verified. We quantified MeHg PD rates using an isotopically enriched Me199Hg tracer added to lake waters already containing high concentrations of ambient MeHg, allowing us to simultaneously monitor the decomposition rate of the spike and ambient MeHg. Experiments were conducted at the Experimental Lakes Area to quantify the first-order rate constant (k(pd)) of MeHg PD in samples exposed to (1) full solar radiation, (2) UV-A and visible light (i.e., with UV-B blocked), or (3) visible light only. We demonstrate for the first time that the use of a MeHg spike to quantify PD rates is appropriate since spike and ambient MeHg-both in samples with and without a spike of Me199Hg--are photodemethylated at the same rate. We also show that rates of MeHg PD are reduced by an order of magnitude in the absence of UV radiation and that to correctly model MeHg PD at the whole-lake scale, both UV and visible light mediated MeHg PD rates must be independently calculated using the light-specific rate constants (k(pd-UWB), k(pd.UVA), k(pd-VIS)). By examining modeled a real MeHg PD fluxes, we observed that UV radiation accounts for 58% and 79% of MeHg PD activity in a clear and colored lake, respectively. Finally, we demonstrate that correcting k(pd-overall) for the attenuation of solar radiation by Teflon bottles, which are normally used for MeHg PD experiments, increases the measured value of 3.69 x 10(-3) m2 E(-1) to 4.41 x 10(-3) m2 E(-1).

Submarine groundwater discharge of total mercury and monomethylmercury to central California coastal waters

Fluxes of total mercury (Hg(T)) and monomethylmercury (MMHg) associated with submarine groundwater discharge (SGD) at two sites onthe central California coast were estimated by combining measurements of Hg(T) and MMHg in groundwater with the use of short-lived, naturally occurring radium isotopes as tracers of groundwater inputs. Concentrations of Hg(T) were relatively low, ranging from 1.2 to 28.3 pM in filtered groundwater, 0.8 to 11.6 pM in filtered surface waters, and 2.5 to 12.9 pM in unfiltered surface waters. Concentrations of MMHg ranged from < 0.04 to 3.1 pM in filtered groundwater, < 0.04 to 0.53 pM in filtered surface waters, and 0.07 to 1.2 pM in unfiltered surface waters. Multiple linear regression analysis identified significant (p < 0.05) positive correlations between dissolved groundwater concentrations of Hg(T) and those of NH4+ and SiO2, and between dissolved groundwater concentrations of MMHg and those of Hg(T) and NH4+. However, such relationships did not account for the majority of the variability in concentration data for either mercury species in groundwater. Fluxes of Hg(T) via SGD were estimated to be 250 +/- 160 nmol day m(-1) of shoreline at Stinson Beach and 3.0 +/- 2.0 nmol m(-2) day(-1) at Elkhorn Slough. These Hg(T) fluxes are substantially greater than net atmospheric inputs of Hg(T) reported for waters in nearby San Francisco Bay. Calculated fluxes of MMHg to coastal waters via SGD were 10 +/- 12 nmol day(-1) m(-1) of shoreline at Stinson Beach and 0.24 +/- 0.21 nmol m(-2) day at Elkhorn Slough. These MMHg fluxes are similar to benthic fluxes of MMHg out of surface sediments commonly reported for estuarine and coastal environments. Consequently, this work demonstrates that SGD is an important source of both Hg(T) and MMHg to coastal waters along the central California coast.

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

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 15 microg/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 1-2 microg/kg/day, considerably higher than the USEPA RfD of 0.1 microg/kg/day or the World Health Organization recommendation of 0.23 microg/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.

Marine fish food in the United States and methylmercury risk

This paper takes a market-oriented approach to study potential exposure of methylmercury (MeHg) risk to human health from US domestic commercial ocean fish landings (consumption). Information is assembled on MeHg concentration levels of marine species. Landings were examined for 1995-2005. Confined to this seafood source, trends of landings indicating high concentration species (above 0.7 ppm, tilefish, shark, king mackerel and swordfish) were significantly decreased. People bought stable amount of medium MeHg level species (0.3-0.7 ppm, grouper, Spanish mackerel) but less amount of low concentration level species (below 0.3 ppm, catfish, tuna and southern flounder). Based on estimated prices of species and quantities taken it is found that consumers, one crucial link of the entire MeHg risk assessment process, had exhibited awareness of potential MeHg risk in fish and demonstrated in household fish consumption. Information assembled in this paper is insufficient to draw further inference on specific population cohorts more susceptible to potential exposure of MeHg risk. This inquiry may be extended to imported fish in the US for comparison.

Trend reversal of mercury concentrations in piscivorous fish from Minnesota lakes: 1982-2006

The trend of mercury concentrations in standardized length northern pike (NP55) and walleye (WE40) was evaluated for a 25-year period, 1982-2006, based on a data set of 1707 cases from 845 lakes throughout Minnesota. Two lines of evidence--changes within individual lakes and regression analyses for all lakes together--indicate a downward trend before the mid-1990s and an upward trend thereafter. Within lakes,the evidence is based on the difference between two years of data at least 5 years apart. Before 1995, 64% decreased and 31% increased; after 1995,35% decreased and 60% increased. Three regression models-linear, quadratic, and two-segment linear piecewise-were evaluated for best fit using the Akaike Information Criteria (AIC). The two-segment linear piecewise regression model, with a breakpoint of 1992, was the best fit, while the quadratic model, with an inflection point of 1995, also had substantial support. The linear model was not supported (deltaAIC > 10). Based on least-squares linear regressions applied separately to 1982-1992 and 1992-2006, mercury concentrations in NP55 and WE40 decreased 4.6 +/- 1.3% (95% CI) per year from 1982 to 1992 and increased 1.4 +/- 0.8% per year from 1992 to 2006.

Feeding habits and habitats preferences affecting mercury bioaccumulation in 37 subtropical fish species from Wujiang River, China

The present study is the first to report the total mercury concentration of 37 fish species collected from Wujiang River, which is the largest branch on the southern bank of Yangtze River, China and proposed for hydropower development. Total mercury concentrations varied among the 37 subtropical species examined. We found higher mercury concentrations in carnivorous species demonstrating greater mercury bioaccumulation in species with more predatory feeding habits. There is no significant difference between fish grouped by habitat preference and feeding habit. However, carnivorous species preferring benthic positions had higher total mercury concentrations than others suggesting that mercury accumulation is related to the interaction of feeding habit and habitat preference. In our study, fish that are bottom living and feed on other fish or aquatic animals are more likely at high risk of mercury exposure. Additional mercury contamination and future impoundment may raise mercury concentration in fish in the Wujiang causing concern for human health and ecological impacts.

Adult women's blood mercury concentrations vary regionally in the United States: association with patterns of fish consumption (NHANES 1999-2004)

BACKGROUND

The current, continuous National Health and Nutrition Examination Survey (NHANES) has included blood mercury (BHg) and fish/shellfish consumption since it began in 1999. NHANES 1999-2004 data form the basis for these analyses.

OBJECTIVES

This study was designed to determine BHg distributions within U.S. Census regions and within coastal and noncoastal areas among women of childbearing age, their association with patterns of fish consumption, and changes from 1999 through 2004.

METHODS

We performed univariate and bivariate analyses to determine the distribution of BHg and fish consumption in the population and to investigate differences by geography, race/ethnicity, and income. We used multivariate analysis (regression) to determine the strongest predictors of BHg among geography, demographic factors, and fish consumption.

RESULTS

Elevated BHg occurred more commonly among women of childbearing age living in coastal areas of the United States (approximately one in six women). Regionally, exposures differ across the United States: Northeast > South and West > Midwest. Asian women and women with higher income ate more fish and had higher BHg. Time-trend analyses identified reduced BHg and reduced intake of Hg in the upper percentiles without an overall reduction of fish consumption.

CONCLUSIONS

BHg is associated with income, ethnicity, residence (census region and coastal proximity). From 1999 through 2004, BHg decreased without a concomitant decrease in fish consumption. Data are consistent with a shift over this time period in fish species in women's diets.

The legacy of mercury cycling from mining sources in an aquatic ecosystem: from ore to organism

Clear Lake is the site of an abandoned mercury (Hg) mine (active intermittently from 1873 to 1957), now a U.S. Environmental Protection Agency Superfund Site. Mining activities, including bulldozing waste rock and tailings into the lake, resulted in approximately 100 Mg of Hg entering the lake's ecosystem. This series of papers represents the culmination of approximately 15 years of Hg-related studies on this ecosystem, following Hg from the ore body to the highest trophic levels. A series of physical, chemical, biological, and limnological studies elucidate how ongoing Hg loading to the lake is influenced by acid mine drainage and how wind-driven currents and baroclinic circulation patterns redistribute Hg throughout the lake. Methylmercury (MeHg) production in this system is controlled by both sulfate-reducing bacteria as well as newly identified iron-reducing bacteria. Sediment cores (dated with dichlorodiphenyldichlorethane [DDD], 210pb, and 14C) to approximately 250 cm depth (representing up to approximately 3000 years before present) elucidate a record of total Hg (TotHg) loading to the lake from natural sources and mining and demonstrate how MeHg remains stable at depth within the sediment column for decades to millenia. Core data also identify other stresses that have influenced the Clear Lake Basin especially over the past 150 years. Although Clear Lake is one of the most Hg-contaminated lakes in the world, biota do not exhibit MeHg concentrations as high as would be predicted based on the gross level of Hg loading. We compare Clear Lake's TotHg and MeHg concentrations with other sites worldwide and suggest several hypotheses to explain why this discrepancy exists. Based on our data, together with state and federal water and sediment quality criteria, we predict potential resulting environmental and human health effects and provide data that can assist remediation efforts.