3b: Rice and other foods as sources of methylmercury exposure to humans

Consuming methylmercury (MeHg) contaminated rice is a more important MeHg exposure pathway to certain Chinese populations compared to exposure from fish. However, little is known about the sources and biogeochemical processes governing rice MeHg levels. We use modeling approaches to 1) identify locations in China where atmospheric mercury deposition to rice paddies are high, 2) understand biogeochemical processes governing MeHg sources and fate to rice, and 3) project future changes in rice MeHg levels in response to actions that implement the Minamata Convention on Mercury. We use GEOS-Chem, a global atmospheric-chemistry-transport model, to simulate present and future anthropogenic mercury emissions and deposition to China rice paddies. Future emission scenarios are implemented based on technologies and policies consistent with the Minamata Convention to regulate mercury emissions from Chinese coal-fired power plants. A rice paddy box model (flooded water, porewater, topsoil, subsoil, rice) is constructed to simulate sources, biogeochemical processes, and fate of inorganic mercury (IHg) and MeHg. Model simulations of present day atmospheric mercury deposition show that the locations where atmospheric mercury deposition to rice paddies are high are prevalent in central China (Anhui, Chongqing, Hunan, Hubei, Jiangxi). These regions demonstrate the highest rice production, consumption, and average rice total mercury concentration (49 ng/g) those that exceed the China national limit (20 ng/g). Using GEOS-Chem simulated atmospheric mercury deposition and irrigation as input, the box model adequately reproduces average China IHg and MeHg levels in the rice paddy reservoirs. Using the box model, we find that infiltration of atmospheric mercury and desorption of soil mercury into porewater are the most important processes governing the relative contribution of atmospheric mercury to rice. Sensitivity simulations show that rice IHg and MeHg levels are more sensitive to changes in infiltration, rice root absorption, and methylation compared to direct atmospheric deposition, suggesting that atmospheric deposition alone cannot explain the observed spatial variability in rice mercury levels across China. For MeHg, we estimate that a 10% change in methylation rate leads to a 9% change in rice MeHg levels. Using the future atmospheric mercury deposition scenarios, we estimate that rice MeHg levels can increase as much as 24% and decrease as much as 17% under absence and presence of stringent policy to regulate mercury emissions from Chinese coal-fired power plants, respectively. By combining these scientific measures, our future work will evaluate the present and future public health and socioeconomic costs of rice MeHg contamination in China.

Consumption of marine and freshwater fish and shellfish has long been regarded as the major pathway of human exposure to methylmercury (MeHg), as MeHg can be bioaccumulated along the aquatic food chain and elevated levels of MeHg may be present in fish and shellfish. However, recent studies have suggested that rice may contain considerable levels of mercury (Hg) including MeHg and that rice consumption could another pathway for human to be exposed to MeHg in some areas where fish consumption is limited, e.g., in Hg mining mountain areas and in certain inland areas. The occurrence of elevated levels of MeHg in rice may suggest that rice products, such as infant rice cereals, could contain the MeHg being transferred from the rice used during manufacture processes, resulting in rice cereals acting as a potential pathway of infant exposure to MeHg. In this study, more than one hundred samples of common brand infant cereals marketed in the United States and China were sampled and analyzed for the concentrations of total mercury (THg) and MeHg using cold vapor atomic fluorescence spectrometry (CVAFS) and gas chromatography (GC) coupled to AFS, respectively. The concentrations of THg and MeHg in the cereal samples ranged from 0.35 to 15.9 ng/g and from 0.07 to 13.9 ng/g, respectively, with the means being 2.86 ng/g for THg and 1.61 ng/g for MeHg. Comparisons between rice-based and non-rice cereals revealed that THg and MeHg concentrations in rice cereal samples were significantly higher than those in non-rice cereals. The results suggest that rice-based infant cereals do contain considerable levels of MeHg and rice is the primary source of MeHg in the cereals. The bioaccessibility of MeHg in the rice cereals was preliminarily assessed using physiologically based extraction methods and the potential exposure of infants to MeHg through consumption of rice cereals was evaluated. More studies are warranted to address the potential health risks of infant diet MeHg exposure through rice cereals and the regulatory implications on MeHg levels in infant rice products.

The U.S. Environmental Protection Agency (EPA) has conducted a series of national and regional screening-level studies of freshwater fish contamination since 1998. The studies are unique in that sampling locations are selected according to an unequal probability (random) sampling design, affording the first opportunity to develop statistically representative estimates of contaminants in U.S. fish relative to human health. The objectives of the probability-based studies are to: a) estimate the national distribution of selected persistent, bioaccumulative, and toxic (PBT) chemicals in fish from lakes and rivers in the conterminous U.S. and from U.S. nearshore waters of the Great Lakes; b) estimate the percentage of the sampled population of waters with contaminant concentrations above human health protection thresholds; and c) define national baselines for tracking temporal changes in PBT chemical concentrations in freshwater fish. Mercury is included as a PBT chemical because mercury fish consumption advisories are widespread in the U.S. EPA conducted the first national probabilistic study of freshwater fish contamination, i.e., the National Lake Fish Tissue Study, in 2000-2003. In 2008-2009 and again in 2013-2014, EPA conducted a comprehensive national characterization of river fish contamination during the Agencys National Rivers and Streams Assessment (NRSA). The Great Lakes Human Health Fish Tissue Study was included as a component of EPAs National Coastal Condition Assessments in 2010 and 2015.

Fillets from common sport fish species were prepared as composite samples and analyzed for total mercury, based on the conservative assumption that most mercury is present in fish fillets as methylmercury. Mercury was detected at quantifiable levels in every fish sample collected for each study. The mean weighted mercury concentrations in fish fillets from U.S. lakes in 2000-2003, rivers in 2008-2009, rivers in 2013-2014, and from the nearshore Great Lakes in 2010 were 352, 229, 230, and 171 ppb, respectively. Nearly 49% of the sampled population representing 76,559 lakes; 25-26% of the sampled population for the rivers studies (representing 51,663 river miles for the 2008-2009 study and 44,338 river miles for the 2013-2014 study); and 11% of the nearshore Great Lakes sampled population representing 4,282 square miles exceeded EPAs 300 ppb tissue-based water quality criterion for methylmercury. These exceedances indicate that the potential risk of human health impacts due to mercury exposure from fish consumption varies by waterbody type.

Global concerns about mercury (Hg) are highly related to Hg methylation and bioaccumulation, as well as its ecological and food safety risk. Compared to other systems, understandings about Hg dynamics and risk in contaminated soil-plant systems are far from clear, and thus techniques for mitigating risk of methylmercury in soils are generally lacking. Our recent studies focus on investigating effects of human activities (e.g., Se fertilization, straw return, and paddy field aquaculture) on the biogeochemsitry (e.g., mercury methylation) and risk (e.g., Hg bioaccumulation and human exposure) of Hg in soil-crop systems: (1) Underlying mechanisms of methylmercury-selenium antagonism in soil-plant systems, as well as multiple effects of ‘fresh organic matter’ on methylmercury dynamics in farming soils were revealed, with the application a suite of modern analytical techniques (e.g., XANES and isotope tracing) and soil microbial methodologies. (2) By developing methods to assess methylmercury bioavailability, ecological and food safety risk of methylmercury associated with farming activities (e.g., straw return and paddy field aquaculture) were assessed, suggesting avenues for accurately assessing risk of methylmercury. (3) Based on those research, techniques for in-situ remediation of moderately/mildly polluted soils were developed, and used in soil remediation projects.

Human exposure to methylmercury (MeHg) is often attributed to the consumption of fish. However, in our recent studies, rice, rather than fish, has been identified to be the primary pathway (>90%) for methylmercury exposure for local population in Guizhou Province, Southeastern China. Since then, several questions have been raised: Are the MeHg exposure pathways in other provinces/areas of China the same as in Guizhou? What is the level of exposure risk through consumption of fish and rice? Are there any age differences? Gender differences? Or urban-rural differences? and so on. To answer these questions and providing related advice and solutions to the environmental policymakers and the general public, in the present study, a comprehensive meta-analysis based on over 300 literatures has been conducted, covering >11,000 rice samples and >19,000 fish samples collected from 34 different administrative provinces/regions in China over the past decades from 1981 to 2015. Data analyses also included intake rates of rice and fish, body weights, and other related parameters for urban/rural (male/female) residents with different age spans. The preliminary conclusions are: 1) in the regions in inland areas of China, the main human exposure routes for methylmercury is rice (rather than fish), while in the coastal areas the major pathway is fish (instead of rice); 2) on the contribution rate of methylmercury exposure from rice, for the same province/region, the rural areas are generally higher than that in the cities (in contrast, on the contribution rate of methylmercury exposure from fish, rural areas are generally lower than that in cities) due to different dietary habits (no significant gender differences was observed); 3) The risk level of methyl mercury exposure in China has three significant characteristics: age differences (the younger, the greater the risk; and vice versa), regional differences (coastal higher than inland; cities higher than rural areas), and gender characteristics (male are generally higher than female); 4) The level of exposure to methyl mercury in China is generally low, but risk levels in coastal areas are significantly higher than in inland regions; especially, residents in Hongkong, Taiwan, Macao, and Hainan may face a certain health risks for methylmercury exposure through fish consumption.

Mercury (Hg) is a complex and multifaceted pollutant. Its presence in tuna as a result of bioaccumulation, transformation and biomagnification processes is ubiquitous. As international and local agencies have done efforts to address Hg contamination, and have provided concentrations limits for this metal, countries should guarantee canned tuna available in markets may respect the regulations. In Colombia there are few studies assessing the current situation regarding Hg content in canned tuna, which is widely consumed. The objective of this research was to assess total mercury (T-Hg) levels in selected brands of canned tuna marketed in Cartagena, Colombia, determining fish consumption-based risks for ingestion. For that purpose, 252 cans of tuna were collected, representing six brands (A-F) in two mediums (water and oil). The overall mean T-Hg levels for all samples analyzed were 0.66±0.05 and 0.61±0.05 ppm, fresh weight, for water and oil, respectively. High T-Hg concentrations were detected in brands B and D. Only brands E and F offer guarantee of low risk for Hg-related health problems, as their T-Hg level was below the detection limit (0.001 ppm). According to the Colombia’s Maximum Permitted Mercury Level in tuna (1.0 ppm), 15.5% of the samples exceeded this concentration, and 18.3% were greater than those recommended by the joint Food and Agriculture Organization of the United Nations/World Health Organization (FAO/WHO) (0.5 ppm). This work is the first to estimate the risk of eating canned tuna marketed in Cartagena city, and confirms the potential health effects to the general population; particularly vulnerable groups, such as children, pregnant women and people with heart-related problems. Government agencies should carry out tighter controls in the industry before the products hit the market, as well as random monitoring campaigns in retail chains, as a guarantee to protect human health from Hg exposure.

Mercury (Hg) is a toxic metal that is found in aquatic food webs and is hazardous to human health. Consumption of methyl mercury (MeHg)-contaminated fish is the primary source of MeHg to humans. Methyl mercury concentrations in fish vary across the landscape and fish from most water bodies in the U.S. are not monitored for MeHg contamination. Therefore, it is critical that we develop the ability to predict regions that may have fish with high concentrations of MeHg. We examined the relationship between mercury deposition, landscape cover and average MeHg concentrations in 35.6-cm total length (TL) largemouth bass (Micropterus salmoides) in 24 USEPA Level III ecoregions located within 15 states in the southeastern U.S. (from Texas to Maryland). The National Descriptive Model of Mercury in Fish was utilized to estimate concentrations of MeHg in largemouth bass. Mercury deposition was estimated using wet Hg deposition data from the Mercury Deposition Network. To determine landscape cover, we used the National Land Cover Database. We used ecoregions as the unit of analysis because they are well-suited for spatial studies. In 20 ecoregions, the average MeHg concentrations in largemouth bass were above 300 ng/g ww, the threshold concentration of MeHg recommended by the USEPA for the issuance of fish consumption advisories. Six ecoregions had average MeHg concentrations in largemouth bass 600 ng/g. Mercury deposition adjusted for conifers and wetland coverage accounted for 76% of the variance in average MeHg concentrations in largemouth bass in ecoregions. To estimate the risk to humans, we computed the percentage of water bodies in each ecoregion with MeHg in largemouth bass above 300 ng/g ww. The percentage of water bodies increased as a sigmoid function of average MeHg in largemouth bass in ecoregions, with 13 of 24 ecoregions having 50% of the waterbodies with 35.6 cm TL largemouth bass having MeHg concentrations > 300 ng/g ww. We estimate that 58.6% of the 3 million lentic waterbodies in the southeastern U.S. may contain largemouth bass with concentrations of MeHg that are hazardous to humans. Because it is not possible to sample all water bodies in ecoregions with hazardous levels of MeHg in gamefish, we recommend county-level fish advisories be issued to warn citizens about the hazards of consuming gamefish from public and private water bodies.

Tunas are apex predators in marine food webs that accumulate mercury (Hg) to higher concentrations than most other fish. High human consumption and moderate to high species-dependent Hg concentrations cause tunas to provide more Hg (~40%) to the U.S. population than any other source. Reduced Hg emissions in North America in recent years have decreased Hg concentrations in North Atlantic waters and investigating whether changes propagate up to top predators is a key to evaluating the benefits of emission reduction efforts. To investigate recent temporal Hg trends in the largest and longest-lived tuna species, we measured total Hg concentrations in 1,292 Atlantic bluefin tuna (ABFT, Thunnus thynnus) captured in the Northwest Atlantic from 2004 to 2012. Mean Hg concentrations in ABFT muscle tissue were 0.76 ± 0.33 ppm; concentrations increased nonlinearly with length, weight, and estimated age, ranging from 0.25 ppm in fish <100 cm in length to 3.15 ppm in fish >250 cm (14-15 years old). No significant differences in Hg concentrations were noted between male and female ABFT. For six size classes of ABFT corresponding to 9-14 year old fish, the Hg concentrations declined significantly over time of capture, at a rate of 0.018 ± 0.003 ppm per year or around 2% per year. The decline rate of 19% over an 8-year period from the 1990s to the early 2000s paralleled comparable declines in anthropogenic Hg emission rates in N. America, N. Atlantic atmospheric Hg0 concentrations, and N. Atlantic seawater concentrations during this period. Because of their pronounced migratory behavior, Hg concentrations in ABFT are presumed to reflect ocean basin-wide exposures (through forage fish) rather than local exposures at one particular location. It thus appears that reductions in atmospheric Hg loading have rapidly propagated up marine food webs to a commercially important species, suggesting that emission reduction efforts have resulted in lower Hg concentrations in large, long-lived fish.