Prenatal methylmercury exposure through maternal rice ingestion: insights from a feasibility pilot in Guizhou Province, China

Effects of low doses of methylmercury (MeHg) exposure on definitive endoderm cell differentiation in human embryonic stem cells

Fetal development is one of the most sensitive windows to methylmercury (MeHg) toxicity. Laboratory and epidemiological studies have shown a dose-response relationship between fetal MeHg exposure and neuro performance in different life stages from infants to adults. In addition, MeHg exposure has been reported to be associated with disorders in endoderm-derived organs, such as morphological changes in liver cells and pancreatic cell dysfunctions. However, the mechanisms of the effects of MeHg on non-neuronal organs or systems, especially during the early development of endoderm-derived organs, remain unclear. Here we determined the effects of low concentrations of MeHg exposure during the differentiation of definitive endoderm (DE) cells from human embryonic stem cells (hESCs). hESCs were exposed to MeHg (0, 10, 100, and 200 nM) that covers the range of Hg concentrations typically found in human maternal blood during DE cell induction. Transcriptomic analysis showed that sub-lethal doses of MeHg exposure could alter global gene expression patterns during hESC to DE cell differentiation, leading to increased expression of endodermal genes/proteins and the over-promotion of endodermal fate, mainly through disrupting calcium homeostasis and generating ROS. Bioinformatic analysis results suggested that MeHg exerts its developmental toxicity mainly by disrupting ribosome biogenesis during early cell lineage differentiation. This disruption could lead to aberrant growth or dysfunctions of the developing endoderm-derived organs, and it may be the underlying mechanism for the observed congenital diseases later in life. Based on the results, we proposed an adverse outcome pathway for the effects of MeHg exposure during human embryonic stem cells to definitive endoderm differentiation.

Human Methylmercury Exposure and Potential Impacts in Central Tibet: Food and Traditional Tibetan Medicine

Methylmercury presents potent neurotoxicity to humans. Fish consumption is the leading source of human exposure to methylmercury worldwide. However, the exposure source in Tibet remains poorly understood because of the scarcity of observational data on most Tibetan foods, although high mercury levels were recently detected in some traditional Tibetan medicines. Here, the results of field investigations show that the joint consumption of traditional Tibetan medicines (TTMs), fish, and rice constitutes a primary exposure pathway to methylmercury in Tibetans and that the probable daily intake of methylmercury is close to that for many coastal regions. People who are young and high-income may have higher methylmercury exposure levels mainly because of economic development and cultural exchanges among regions. Our analysis indicates that a large proportion of the Tibetan population are likely to face a high methylmercury exposure risk and that mercury-susceptible populations in Tibet should be attentive to consuming TTMs with fish.

Metal(loid)s (As, Hg, Se, Pb and Cd) in paddy soil: Bioavailability and potential risk to human health

Rice is one of the principal staple foods, essential for safeguarding the global food and nutritional security, but due to different natural and anthropogenic sources, it also acts as one of the biggest reservoirs of potentially toxic metal(loids) like As, Hg, Se, Pb and Cd. This review summarizes mobilization, translocation and speciation mechanism of these metal(loids) in soil-plant continuum as well as available cost-effective remediation measures and future research needs to eliminate the long-term risk to human health. High concentrations of these elements not only cause toxicity problems in plants, but also in animals that consume them and gradual deposition of these elements leads to the risk of bioaccumulation. The extensive occurrence of contaminated rice grains globally poses substantial public health risk and merits immediate action. People living in hotspots of contamination are exposed to higher health risks, however, rice import/export among different countries make the problem of global concern. Accumulation of As, Hg, Se, Pb and Cd in rice grains can be reduced by reducing their bioavailability, and controlling their uptake by rice plants. The contaminated soils can be reclaimed by phytoremediation, bioremediation, chemical amendments and mechanical measures; however these methods are either too expensive and/or too slow. Integration of innovative agronomic practices like crop establishment methods and improved irrigation and nutrient management practices are important steps to help mitigate the accumulation in soil as well as plant parts. Adoption of transgenic techniques for development of rice cultivars with low accumulation in edible plant parts could be a realistic option that would permit rice cultivation in soils with high bioavailability of these metal(loid)s.

Mercury contents in rice and potential health risks across China

Rice samples were collected at 560 sites in 15 provinces across China in areas without known point mercury (Hg) sources. Total mercury (THg) and methylmercury (MeHg) concentrations were analyzed in these rice samples for risk assessment. Relatively low THg and MeHg concentrations were found in the majority of the white rice samples with an overall mean of 4.74 (1.06-22.7) μg kg^-1 and 0.682 (0.03-8.71) μg kg^-1, respectively. The means (range of) THg concentration of rice in each geographical region were 5.23 (1.07-19.5) μg kg^-1, 5.14 (1.06-17.2) μg kg^-1, 4.45 (1.41-17.2) μg kg^-1, 4.20 (1.48-19.4) μg kg^-1, 3.49 (1.49-10.7) μg kg^-1, and 4.53 (1.30-19.4) μg kg^-1 in east, centre, south, southwest, northwest and northeast, China, respectively, and the corresponding values for MeHg concentrations were 0.898 (0.127-8.35) μg kg^-1, 0.603 (0.207-2.48) μg kg^-1, 0.516 (0.032-1.50) μg kg^-1, 0.615 (0.050-5.03) μg kg^-1, 0.704 (0.148-2.41) μg kg^-1 and 0.565 (0.035-8.71) μg kg^-1, respectively. Hg contents in rice across China were found to be at background levels. Both the probable daily intakes (PDIs) of inorganic Hg (IHg) and MeHg from rice consumption showed low risks for general population in the investigated regions.

Traditional Tibetan Medicine Induced High Methylmercury Exposure Level and Environmental Mercury Burden in Tibet, China

Highly elevated concentrations of total mercury (THg) and methylmercury (MeHg) were found in the municipal sewage in Tibet. Material flow analysis supports the hypothesis that these elevated concentrations are related to regular ingestion of Hg-containing Traditional Tibetan Medicine (TTM). In Tibet in 2015, a total of 3600 kg of THg was released from human body into the terrestrial environment as a result of TTM ingestion, amounting to 45% of the total THg release into the terrestrial environment in Tibet, hence substantially enhancing the environmental Hg burden. Regular ingestion of TTM leads to chronic exposure of Tibetans to inorganic Hg (IHg) and MeHg, which is 34 to 3000-fold and 0-12-fold higher than from any other known dietary sources, respectively. Application of a human physiology model demonstrated that ingestion of TTM can induce high blood IHg and MeHg levels in the human body. Moreover, 180 days would be required for the MeHg to be cleared out of the human body and return to the initial concentration i.e. prior to the ingestion of 1 TTM pill. Our analysis suggests that high Hg level contained in TTM could be harmful to human health and elevate the environmental Hg burden in Tibet.

Effects of soil properties on production and bioaccumulation of methylmercury in rice paddies at a mercury mining area, China

Rice paddy soil is recognized as the hotspot of mercury (Hg) methylation, which is mainly a biotic process mediated by many abiotic factors. In this study, effects of key soil properties on the production and bioaccumulation of Hg and methylmercury (MeHg) in Hg-contaminated rice paddies were investigated. Rice and soil samples were collected from the active Hg smelting site and abandoned Hg mining sites (a total of 124 paddy fields) in the Wanshan Mercury Mine, China. Total Hg (THg) and MeHg in soils and rice grains, together with sulfur (S), selenium (Se), organic matter (OM), nitrogen (N), phosphorus (P), mineral compositions (e.g., SiO₂, Al₂O₃ and Fe₂O₃) and pH in soils were quantified. The results showed that long-term Hg mining activities had resulted in THg and MeHg contaminations in soil-rice system. The newly-deposited atmospheric Hg was more readily methylated relative to the native Hg already in soils, which could be responsible for the elevated MeHg levels in soils and rice grains around the active artificial Hg smelting site. The MeHg concentrations in soils and rice grains showed a significantly negative relationship with soil N/Hg, S/Hg and OM/Hg ratio possibly due to the formation of low-bioavailability Hg-S(N)-OM complexes in rhizosphere. The Hg-Se antagonism undoubtedly occurred in soil-rice system, while its role in bioaccumulation of MeHg in the MeHg-contaminated rice paddies was minor. However, other soil properties showed less influence on the production and bioaccumulation of MeHg in rice paddies located at the Wanshan Mercury Mine zone.

Heavy metals (lead, cadmium, methylmercury, arsenic) in commonly imported rice grains (Oryza sativa) sold in Saudi Arabia and their potential health risk

The levels of heavy metals (lead, cadmium, methylmercury and arsenic) were determined in 37 brands of imported rice commonly consumed in Saudi Arabia after soaking and rinsing with water, and their potential health risks to residents were estimated by three indices: hazard quotient (HQ), hazard index (HI) and cancer risk (CR). The mean levels of lead, cadmium, methylmercury and total arsenic in soaked (rinsed) rice grains were 0.034 (0.057), 0.015 (0.027), 0.004 (0.007) and 0.202 (0.183) μg/g dry weight, respectively. Soaking or rinsing rice grains with water decreased lead and cadmium levels in all brands to safe levels. All brands had total arsenic above the acceptable regulatory limits, irrespective of soaking or rinsing, and eight soaked and 12 rinsed brands contained methylmercury. The levels of all heavy metals except cadmium were above the acceptable regulatory limits when the rice was neither rinsed nor soaked. Weekly intakes of lead, cadmium, methylmercury and total arsenic from soaked (rinsed) grains were 0.638 (1.068), 0.279 (0.503), 0.271 (0.309) and 3.769 (3.407) μg/kg body weight (bw). The weekly intakes of lead and methylmercury from the consumption of one rinsed and two soaked rice brands respectively, exceeded the Provisional Tolerance Weekly Intake set by the Food and Agriculture Organization and the World Health Organization. The weekly intake of total arsenic for all brands was above the lowest benchmark dose lower confidence limit (BMDL₀₁) level of 0.3μg/kg bw/d for an increased cancer risk set by European Food Safety Authority. Either soaking or rinsing grains before consumption can minimize the non-carcinogenic health risks to residents from cadmium and lead (HQ<1). Our local consumers, though, may experience health consequences from rice contaminated mainly with arsenic (HQ>1 all brands) and to a lesser extent with methylmercury (HQ>1 in 4 brands), even when soaked or rinsed with water before consumption. The combined non-carcinogenic effect of all metals expressed as HI was >1, including soaked or rinsed rice, with total arsenic the major contributor followed by methylmercury. CR for total arsenic, whether consuming soaked, rinsed, un-soaked or unrinsed grains, exceeded the acceptable level of 10^-4. Long-term consumption of rice contaminated with heavy metals, particularly arsenic, can pose potential health risks to the local population, especially vulnerable groups (pregnant women, children, elderly and patients). More attention should thus be given to contaminated rice and preventive measures should be taken.

Stable Mercury Isotopes in Polished Rice (Oryza sativa L.) and Hair from Rice Consumers

Mercury (Hg) isotopic signatures were characterized in polished rice samples from China, U.S., and Indonesia (n = 45). Hg isotopes were also analyzed in paired hair samples for participants from China (n = 21). For the latter, we also quantified the proportion of methylmercury intake through rice (range: 31-100%), and the weekly servings of fish meals (range: 0-5.6 servings/weekly). For these participants, 29% (n = 6) never ingested fish, 52% (n = 11) ingested fish < twice/weekly, and 19% (n = 4) ingested fish ≥ twice/weekly. In rice and hair, both mass-dependent fractionation (MDF, reported as δ²⁰²Hg) and mass-independent fractionation (MIF, reported as Δ¹⁹⁹Hg) of Hg isotopes were observed. Compared to rice, hair δ²⁰²Hg values were enriched on average (±1 standard deviation) by 1.9 ± 0.61‰, although the range was wide (range: 0.45‰, 3.0‰). Hair Δ¹⁹⁹Hg was significantly inversely associated with %methylmercury intake from rice (Spearman's rho = -0.61, p < 0.01, n = 21), i.e., as the proportion of methylmercury intake from rice increased, MIF decreased. Additionally, hair Δ¹⁹⁹Hg was significantly higher for participants ingesting fish ≥ twice/weekly compared to those who did not ingest fish or ingested fish < twice/weekly (ANOVA, p < 0.05, n = 21); Overall, results suggest that Hg isotopes (especially MIF) in human hair can be used to distinguish methylmercury intake from rice versus fish.

Low-level methylmercury exposure through rice ingestion in a cohort of pregnant mothers in rural China

BACKGROUND

Rice ingestion is an important dietary exposure pathway for methylmercury. There are few studies concerning prenatal methylmercury exposure through rice ingestion, yet the health risks are greatest to the developing fetus, and thus should be investigated.

OBJECTIVES

Our main objective was to quantify dietary methylmercury intake through rice and fish/shellfish ingestion among pregnant mothers living in southern China, where rice was a staple food and mercury contamination was considered minimal.

METHODS

A total of 398 mothers were recruited at parturition, who donated scalp hair and blood samples. Total mercury and/or methylmercury concentrations were measured in biomarkers, in rice samples from each participant's home, and in fish tissue purchased from local markets. Additional fish/shellfish mercury concentrations were obtained from a literature search. Dietary methylmercury intake during the third trimester was equivalent to the ingestion rate for rice (or fish/shellfish)×the respective methylmercury concentration.

RESULTS

Dietary methylmercury intake from both rice and fish/shellfish ingestion averaged 1.2±1.8µg/day (median=0.79µg/day, range=0-22µg/day), including on average 71% from rice ingestion (median: 87%, range: 0-100%), and 29% from fish/shellfish consumption (median 13%, range: 0-100%). Median concentrations of hair total mercury, hair methylmercury, and blood total mercury were 0.40µg/g (range: 0.08-1.7µg/g), 0.28µg/g (range: 0.01-1.4µg/g), and 1.2µg/L (range: 0.29-8.6µg/L), respectively, and all three biomarkers were positively correlated with dietary methylmercury intake through rice ingestion (Spearman's rho=0.18-0.21, p≤0.0005), although the correlations were weak. In contrast, biomarkers were not correlated with fish/shellfish methylmercury intake (Spearman's rho=0.04-0.08, p=0.11-0.46).

CONCLUSIONS

Among pregnant mothers living in rural inland China, rice ingestion contributed to prenatal methylmercury exposure, more so than fish/shellfish ingestion.

Maternal methylmercury exposure through rice ingestion and offspring neurodevelopment: A prospective cohort study

BACKGROUND

Dietary methylmercury intake can occur not only through fish ingestion but also through rice ingestion; however, rice does not contain the same beneficial micronutrients as fish.

OBJECTIVES

In rural China, where rice is a staple food, associations between prenatal methylmercury exposure (assessed using maternal hair mercury) and impacts on offspring neurodevelopment were investigated.

METHODS

A total of 398 mothers were recruited at parturition at which time a sample of scalp hair was collected. Offspring (n=270, 68%) were assessed at 12 months using the Bayley Scales of Infant Development-II, yielding age-adjusted scores for the Mental Developmental Index (MDI) and Psychomotor Developmental Index (PDI).

RESULTS

Among 270 mothers, 85% ingested rice daily, 41% never or rarely ingested fish/shellfish and 11% ingested fish/shellfish at least twice/weekly. Maternal hair mercury averaged 0.41μg/g (median: 0.39μg/g, range: 0.079-1.7μg/g). In unadjusted models, offspring neurodevelopment (both MDI and PDI) was inversely correlated with hair mercury. Associations were strengthened after adjustment for fish/shellfish ingestion, rice ingestion, total energy intake (kcal), and maternal/offspring characteristics for both the MDI [Beta: -4.9, 95% Confidence Interval (CI): -9.7, -0.12] and the PDI (Beta: -2.7, 95% CI: -8.3, 2.9), although confidence intervals remained wide for the latter.

CONCLUSIONS

For 12-month old offspring living in rural China, prenatal methylmercury exposure was associated with statistically significant decrements in offspring cognition, but not psychomotor development. Results expose potential new vulnerabilities for communities depending on rice as a staple food.

Alterations in biochemical markers due to mercury (Hg) exposure and its influence on infant's neurodevelopment

This study examined the role of oxidative stress due to mercury (Hg) exposure on infant's neurodevelopmental performance. A total of 944 healthy Saudi mothers and their respective infants (aged 3-12 months) were recruited from 57 Primary Health Care Centers in Riyadh City. Total mercury (Hg) was measured in mothers and infants urine and hair samples, as well as mother's blood and breast milk. Methylmercury (MeHg) was determined in the mothers and infants' hair and mother's blood. Urinary 8-hydroxy-2'-deoxyguanosine (8-OHdG), malondialdehyde (MDA), and porphyrins were used to assess oxidative stress. The infant's neurodevelopment was evaluated using Denver Developmental Screening Test II (DDST-II) and Parents' Evaluation of Developmental Status. The median total Hg levels in mother's urine, infant's urine, mother's hair, infant's hair, and mother's blood and breast milk were 0.995μg/l, 0.716μg/l, 0.118μg/g dw, 0.101μg/g dw, 0.635μg/l, and 0.884μg/l respectively. The median MeHg levels in mother's hair, infant's hair, and mother's blood were 0.132μg/g dw, 0.091μg/g dw, and 2.341μg/l respectively. A significant interrelationship between mothers and infants Hg measures in various matrices was noted. This suggests that mother's exposure to different forms of Hg (total and/or MeHg) from various sources contributed significantly to the metal body burden of their respective infants. Even though Hg exposure was low, it induced high oxidative stress in mothers and infants. The influence of multiplicative interaction terms between Hg measures and oxidative stress biomarkers was tested using multiple regression analysis. Significant interactions between the urinary Hg levels in mothers and infants and oxidative stress biomarkers (8-OHdG and MDA) were noted. The MeHg levels in mother-infant hair revealed similar interaction patterns. The p-values for both were below 0.001. These observations suggest that the exposure of our infants to Hg via mothers either during pregnancy and/or neonatal life, promoted oxidative stress that might have played a role in infant neurodevelopmental delays that we reported previously. The results confirmed that the interaction between infant's MeHg in hair and 8-OHdG and MDA levels was significantly associated with a delay in DDST-II performance (ß=-0.188, p=0.028). This finding provides an insight into the potential consequences of Hg-induced oxidative stress to infant's cognitive neurodevelopment for the first time. This observation still needs future studies to be validated. Given the low MeHg levels in our population, these findings are of particular importance.

Investigation of biogeochemical controls on the formation, uptake and accumulation of methylmercury in rice paddies in the vicinity of a coal-fired power plant and a municipal solid waste incinerator in Taiwan

Recent studies have shown that rice consumption is another critical route of human exposure to methylmercury (MeHg), the most toxic and accumulative form of mercury (Hg) in the food web. Yet, the mechanisms that underlie the production and accumulation of MeHg in the paddy ecosystem are still poorly understood. In 2013 and 2014, we conducted field campaigns and laboratory experiments over a rice growing season to examine Hg and MeHg cycling, as well as associated biogeochemistry in a suite of paddies close to a municipal solid waste incinerator and a coal-fired power plant station in Taiwan. Concentrations of total Hg and MeHg in paddy soil and rice grain at both sites were low and found not to exceed the control standards for farmland soil and edible rice in Taiwan. However, seasonal variations of MeHg concentrations observed in pore water samples indicate that the in situ bioavailability of inorganic Hg and activity of Hg-methylating microbes in the rhizosphere increased from the early-season and peaked at the mid-season, presumably due to the anoxia created under flooded conditions and root exudation of organic compounds. The presence of Hg-methylators was also confirmed by the hgcA gene detected in all root soil samples. Subsequent methylation tests performed by incubating the root soil with inorganic Hg and an inhibitor or stimulant specific for certain microbes further revealed that sulfate-reducers might have been the principal Hg-methylting guild at the study sites. Interestingly, results of hydroponic experiments conducted by cultivating rice in a defined nutrient solution amended with fixed MeHg and varying levels of MeHg-binding ligands suggested that chemical speciation in soil pore water may play a key role in controlling MeHg accumulation in rice, and both passive and active transport pathways seem to take place in the uptake of MeHg in rice roots.

Human Body Burden and Dietary Methylmercury Intake: The Relationship in a Rice-Consuming Population

Rice can be the main route of methylmercury (MeHg) exposure for rice-consuming populations living in area where mercury (Hg) is mined. However, the current risk assessment paradigm for MeHg exposure is based on epidemiological data collected from fish-consuming populations. This study was designed to evaluate the relationship between dietary MeHg intake and human body burden in a rice -consuming population from the Wanshan Hg mining area in China. Hair MeHg concentrations averaged 2.07 ± 1.79 μg/g, and the average blood MeHg concentration across the study area ranged from 2.20 to 9.36 μg/L. MeHg constituted 52.8 ± 17.5% and 71.7 ± 18.2% of total Hg (THg) on average in blood and hair samples, respectively. Blood and hair MeHg concentrations, rather than THg, can be used as a proxy of human MeHg exposure. Hair MeHg levels showed no significant monthly variation; however, hair THg can be impacted by inorganic Hg exposure. The toxicokinetic model of MeHg exposure based on fish consumption underestimated the human hair MeHg levels, and this may be a consequence of the high hair-to-blood MeHg ratio (361 ± 105) in the studied rice-consuming population. The use of risk assessment models based on fish consumption may not be appropriate for inland mining areas where rice is the staple food.

Modeled methylmercury exposure and risk from rice consumption for vulnerable populations in a traditional fish-eating area in China

The circulation of rice from contaminated areas could escalate exposure risk from a local problem to a national issue and affect a wider population beyond the region of origin, as confirmed by the "Poison Rice Incident" in May 2013 in Guangzhou, China. In the present study, the authors established a food chain model based on the aquivalence method to identify major sources of methylmercury (MeHg), estimate the levels of MeHg, and quantify exposure to MeHg via rice and aquatic food consumption. Different types of organism samples from the Haihe River also were collected to verify the calculated values. The MeHg intake in pregnant women was 1529.1 ng/d from the aquatic food chain and as high as 2804.0 ng/d from rice, although the intake varied among scenarios. The maximum possible MeHg concentration in the blood of pregnant women was 5.21 µg/L, higher than the threshold value of MeHg recommended by the US Environmental Protection Agency (4.4 µg/L), which indicated that pregnant women could face risk from MeHg exposure. The authors also assessed the risk of MeHg exposure in pregnant women and their breastfed infants using a new index, HQEquivalent . In 4 scenarios, the HQEquivalent indices ranged from 0.42 to 1.18 for pregnant women and from 0.29 to 0.83 for breastfed infants.

Interpretation of the source-specific substantive control measures of the Minamata Convention on Mercury

Being persistent, toxic, and bio-accumulative, Mercury (Hg) seriously affects the environment and human health. Due to Hg's attribute of long-range environmental transport across national borders, especially through atmospheric transport, no country can fully protect its environment and human health with its own efforts, without global cooperation. The Minamata Convention on Mercury, which was formally adopted and opened for signature in October 2013, is the only global environmental regime on the control of Hg pollution. Its main substantive control measures are source-specific: its phasing-out, phasing-down, and other main substantive requirements all direct to specific categories of pollution sources through the regulation of specific sectors of the economy and social life. This Convention does not take a national quota approach to quantify the Parties' nationwide total allowable consumption or discharge of Hg or Hg compounds, nor does it quantify their nationwide total reduction requirements. This paper attempts to find the underlying reasons for this source-specific approach and offers two interpretations. One possible interpretation is that Hg might be a non-threshold pollutant, i.e., a pollutant without a risk-free value of concentration. The existence of a reference dose (RfD), reference concentration (RfC), provisional tolerable weekly intake (PTWI), minimal risk level (MRL) or other similar reference values of Hg does not necessarily mean that Hg cannot be regarded as non-threshold because such reference values have scientific uncertainties and may also involve policy considerations. Another interpretation is that Hg lacks a feasibly determinable total allowable quantity. There is evidence that negotiators might have treated Hg as non-threshold, or at least accepted that Hg lacks a feasibly determinable total allowable quantity: (1) The negotiators were informed about the serious situations of the current emissions, releases, and legacy deposition; (2) the UNEP Secretariat took the position that Hg is non-threshold and should be eliminated to the maximum; (3) European countries, the USA and other western countries were in a better position to regard Hg as non-threshold and push forward a global reduction of Hg emissions and releases to the minimum; (4) the negotiators took the Stockholm Convention on Persistent Organic Pollutants (POPs) as a model; and (5) a fairly large number of non-governmental organizations (NGOs) were organized under umbrella NGO networks such as the Zero Mercury Working Group (ZMWG) and the International POPs Elimination Network (IPEN) and made a significant contribution to the negotiation process. The interpretations for the Minamata Convention might similarly be used to interpret the source-specific approach of the Stockholm Convention on POPs and the national quota approach of global environmental regimes on stratospheric ozone and climate mitigation. These two interpretations focus on the features of the pollutants and for this reason may be useful for future negotiators of other international environmental treaties to select appropriate models. They also suggest that the source-specific approach may be adopted in the future for pollutants with similar features of being possibly non-threshold and without a feasibly determinable total allowable quantity.

Retrospective study of methylmercury and other metal(loid)s in Madagascar unpolished rice (Oryza sativa L.)

The rice ingestion rate in Madagascar is among the highest globally; however studies concerning metal(loid) concentrations in Madagascar rice are lacking. For Madagascar unpolished rice (n = 51 landraces), levels of toxic elements (e.g., total mercury, methylmercury, arsenic and cadmium) as well as essential micronutrients (e.g., zinc and selenium) were uniformly low, indicating potentially both positive and negative health effects. Aside from manganese (Wilcoxon rank sum, p < 0.01), no significant differences in concentrations for all trace elements were observed between rice with red bran (n = 20)and brown bran (n = 31) (Wilcoxon rank sum, p = 0.06-0.91). Compared to all elements in rice,rubidium (i.e., tracer for phloem transport) was most positively correlated with methylmercury (Pearson'sr = 0.33, p < 0.05) and total mercury (r = 0.44, p < 0.05), while strontium (i.e., tracer for xylemtransport) was least correlated with total mercury and methylmercury (r < 0.01 for both), suggesting inorganic mercury and methylmercury were possibly more mobile in phloem compared to xylem.

A dietary-wide association study (DWAS) of environmental metal exposure in US children and adults

BACKGROUND

A growing body of evidence suggests that exposure to toxic metals occurs through diet but few studies have comprehensively examined dietary sources of exposure in US populations.

PURPOSE

Our goal was to perform a novel dietary-wide association study (DWAS) to identify specific dietary sources of lead, cadmium, mercury, and arsenic exposure in US children and adults.

METHODS

We combined data from the National Health and Nutrition Examination Survey with data from the US Department of Agriculture's Food Intakes Converted to Retail Commodities Database to examine associations between 49 different foods and environmental metal exposure. Using blood and urinary biomarkers for lead, cadmium, mercury, and arsenic, we compared sources of dietary exposure among children to that of adults.

RESULTS

Diet accounted for more of the variation in mercury and arsenic than lead and cadmium. For instance we estimate 4.5% of the variation of mercury among children and 10.5% among adults is explained by diet. We identified a previously unrecognized association between rice consumption and mercury in a US study population--adjusted for other dietary sources such as seafood, an increase of 10 g/day of rice consumption was associated with a 4.8% (95% CI: 3.6, 5.2) increase in blood mercury concentration. Associations between diet and metal exposure were similar among children and adults, and we recapitulated other known dietary sources of exposure.

CONCLUSION

Utilizing this combination of data sources, this approach has the potential to identify and monitor dietary sources of metal exposure in the US population.

Rice methylmercury exposure and mitigation: a comprehensive review

Rice cultivation practices from field preparation to post-harvest transform rice paddies into hot spots for microbial mercury methylation, converting less-toxic inorganic mercury to more-toxic methylmercury, which is likely translocated to rice grain. This review includes 51 studies reporting rice total mercury and/or methylmercury concentrations, based on rice (Orzya sativa) cultivated or purchased in 15 countries. Not surprisingly, both rice total mercury and methylmercury levels were significantly higher in polluted sites compared to non-polluted sites (Wilcoxon rank sum, p<0.001). However, rice percent methylmercury (of total mercury) did not differ statistically between polluted and non-polluted sites (Wilcoxon rank sum, p=0.35), suggesting comparable mercury methylation rates in paddy soil across these sites and/or similar accumulation of mercury species for these rice cultivars. Studies characterizing the effects of rice cultivation under more aerobic conditions were reviewed to determine the mitigation potential of this practice. Rice management practices utilizing alternating wetting and drying (instead of continuous flooding) caused soil methylmercury levels to spike, resulting in a strong methylmercury pulse after fields were dried and reflooded; however, it is uncertain whether this led to increased translocation of methylmercury from paddy soil to rice grain. Due to the potential health risks, it is advisable to investigate this issue further, and to develop separate water management strategies for mercury polluted and non-polluted sites, in order to minimize methylmercury exposure through rice ingestion.