Human Body Burden and Dietary Methylmercury Intake: The

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Environmental Science & Technology

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Human body burden and dietary methylmercury intake: the relationship

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in a rice consuming population

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Ping Li,† Xinbin Feng,†,* Hing Man Chan,‡ Xiaofeng Zhang,§ Buyun Du†,‼

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†

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Chinese Academy of Sciences, Guiyang, 550081, China

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‡

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Ottawa, K1N 6N5, Canada

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§

State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry,

Centre for Advanced Research in Environmental Genomics, University of Ottawa,

Department of Toxicology, Public Health College, Harbin Medical University,

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Harbin, 150081, China

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‼

University of Chinese Academy of Sciences, Beijing, 100049, China

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*Corresponding Author (Feng X.)

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Phone: +86 851 85891356, Fax: +86 851 85891609, Email: [email protected]

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The authors declare they have no actual or potential competing financial interests.

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ACS Paragon Plus Environment


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ABSTRACT:

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Rice can be the main route of methylmercury (MeHg) exposure for rice consuming

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populations living in mercury (Hg) mining areas. However, the current risk

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assessment paradigm for MeHg exposure is based on epidemiological data collected

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from fish consuming populations. This study was designed to evaluate the relationship

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between dietary MeHg intake and human body burden in a rice consuming population

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from the Wanshan Hg mining area in China. Hair MeHg concentrations averaged

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2.07±1.79 µg/g, and the average blood MeHg concentration across the study area

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ranged from 2.20 to 9.36 µg/L. MeHg constituted 52.8±17.5% and 71.7±18.2% of

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total Hg (THg) on average in blood and hair samples, respectively. Blood and hair

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MeHg concentrations, rather than THg, can be used as proxy of human MeHg

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exposure. Hair MeHg levels showed no significant monthly variation, however hair

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THg can be impacted by inorganic Hg exposure. The toxicokinetic model of MeHg

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exposure based on fish consumption underestimated the human hair MeHg levels, and

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this may be a consequence of the high hair-to-blood MeHg ratio (361±105) in the

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studied rice consuming population. Use of risk assessment models based on fish

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consumption may not be appropriate for inland mining areas where rice is the staple

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food.


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INTRODUCTION

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The toxicity of methylmercury (MeHg) has been extensively documented by

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World Health Organization,1 United States Environmental Protection Agency,2

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Agency for Toxic Substances and Disease Registry,3 United States National Research

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Council4 and reviewed by Clarkson5, Clarkson and Magos6, Mergler et al.7 and

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Driscoll et al8. Exposure of the general population to MeHg occurs primarily through

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consumption of fish and marinemammals.7 MeHg contamination in fish poses a

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particular challenge to public health because of the documented nutrient benefits from

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fish for human health.7 MeHg in fish tissue has been identified as being bound to the

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thiol group of cysteine in fish protein,9 which is not removed and destroyed by the

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cooking or cleaning processes. The maximum level of MeHg in fish recommended by

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the Joint FAO/WHO Food Standards Programme CODEX Committee on

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Contaminants in Foods10 is 1.0 µg/g for predatory fish and 0.5 µg/g for other fishery

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products. This guideline has been adopted by most countries.

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The physiologically based pharmacokinetic (PBPK) model11 and the one

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compartment model12,13 have been used to link intake dose and blood concentration.

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Even though these relationships are well established, significant variability can be

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found in the reported parameters.14 A hair: blood mercury (Hg) concentration ratio of

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250:1 is frequent reported,1,2 but uncertainty also existed in this value with a range of

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140 to 370.1,15 Canuel et al.16 reported big differences between hair Hg concentration

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and the expected value based on calculated daily oral intake. Wang et al.17 and


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Goodrich et al.18 found an effect of genetic polymorphism on Hg concentrations in

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biomarkers even at low levels of Hg exposure.

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Based on the studies conducted in the Faroes Islands (cord blood of 58 µg/L),

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USEPA set the limit of 0.1 µg/kg/d as the reference dose (RfD) for MeHg.2 According

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to the calculations made on the Faroe Islands study and another from the Seychelles

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(12 µg/g in maternal hair), the Joint FAO/WHO Expert Committee on Food Additives

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(JECFA) established a provisional tolerable weekly intake (PTWI) for MeHg at 1.6

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µg/kg/week.19 Quantification of these intake values is important as the effect of low

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level MeHg exposure on neurological development has been quantified. Loss of 0.18

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Intelligence Quotient (IQ) points is thought to be associated for each part per million

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increase of maternal hair Hg.20,21

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Recent studies have highlighted that rice can accumulate high levels of MeHg in

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mercury mining areas in Guizhou, Southwestern China,22-24 and the dynamic

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mechanism

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beeninvestigated.25,26 Rice, rather than fish, is the main route of human MeHg

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exposure in the Wanshan Hg mining area,27 Guizhou Province28 and inland area of

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southern China29. Estimations of probable daily intake (PDI) of MeHg from rice

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consumption indicate that rice consumption is a potential human health risk in Hg

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mining areas.27,28,30

of

MeHg

translocation

and

accumulation

in

rice

plant

has

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Rice lacks many of the beneficial micronutrients associated with fish (e.g., n-3

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long chain polyunsaturated fatty acid (n-3 LCPUFA), selenium (Se), the essential

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amino acids),31 and therefore current RfD and PTWI from fish consumption may



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underestimate the health effects of MeHg exposure from rice consumption. Balancing

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the risks and benefits of fish consumption has become an increasingly important goal

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of fish consumption advisories. Ginsberg and Toal32 provided an integrated analysis

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for MeHg and n-3 LCPUFA, which evaluated the net effect of fish consumption on a

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species-by-species basis. Zhang et al.33 proposed a new criterion for assessing MeHg

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exposure and Se intake based on Se−Hg interactions and the physiology/toxicology of

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Se. In general, the interaction between MeHg and micronutrients is unknown. Such

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interactions may impact the dose-response relationship between MeHg intake and

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body burden in a rice consuming population.

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This study was designed to evaluate the relationship between MeHg intake and

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blood and hair MeHg levels in a rice consuming population at the Wanshan Hg

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mining area. It also aimed to develop a risk assessment paradigm for MeHg exposure

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among rice consumption populations. In a companion paper, inorganic Hg (IHg)

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exposure, renal effects, and possible pathway were evaluated in the same study

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population.34

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MATERIALS AND METHODS

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Study area. The Wanshan Hg mining area, located in the eastern part of

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Guizhou province, was the largest Hg mine in China. Large scale Hg mining activities

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operated in the area for over 50 years, leading to serious Hg contamination to the local

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environment.35-40 The Hg adits and historical retorts are situated at the upstream of

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four rivers in the region (Figure S1) and significant quantities of mine wastes are the


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primary Hg contamination source to rivers.36,37,41 Rice is widely cultured throughout

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the valleys in this region and the river water is the main source for irrigation.

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Wanshan County consists of 5 towns, namely Wanshan, Huangdao, Xiaxi,

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Aozhai, and Gaolouping and covers an area of approximately 338 km2 (Figure S1).

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The population in Wanshan County in 2012 was 68,000 and the rural population

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constituted about 80% according to the local government’s website. The local

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economy is undeveloped and the Per Capita Gross Domestic Product was 14,914

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RMB (US Dollar 2,400) in 2011,42 which was about half of the national average in

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China. Seven sites in Xiaxi and Aozhai Town were selected for survey in the current

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study, which were distributed along the Aozhai and Xiaxia River with different

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gradients from the Hg pollution source (Figure S1).

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Sample collection. Sampling was conducted in December 2012. Participants

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were recruited with the criteria that they had been a local resident for over 3 months.

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The recruitment strategy in the survey was to recruit 10-20% of whole population at

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each site. All the participants participated voluntarily. The recruitment period lasted

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two days. A questionnaire was used to obtain the basic information on age, body

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weight, profession, history of involvement in artisanal Hg mining activity, dental

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fillings, smoking and alcohol drinking habits, illness, and the amount of daily rice

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consumption. Daily rice intake (g/day) was quantified by participants according to

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eating habits and whole family monthly and/or annual rice consumption figures

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reported by housewives.



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Residents of the Wanshan mining area seldom eat fish and consumption is

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similar to the average daily fish intake of 1.2 g/day for Guizhou rural residents42. A

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previous study has shown that the MeHg concentrations in fish samples collected

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from Wanshan Hg mine were relatively low, with a mean of 0.060 µg/g.43 Fish

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samples were not collected in the current study as these figures indicate that fish is not

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a significant dietary source of MeHg in the population in Wanshan.28,29,44

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Hair samples were cut with stainless steel scissors from the occipital region of

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the scalp of each participant, bundled together in polyethylene bags and then

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transferred to the laboratory. Eleven female subjects with long hair were selected from

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across the study area for sequential Hg analysis. Hair strands were cut into sequential

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12 centimeters sections from the scalp, in order to examine monthly variations of hair

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Hg levels in the past 1 year (the growth rate of hair was estimated to be 1 cm per

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month4). Venous blood samples (5 mL) were collected from each participant in

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prepared EDTA vacuum tubes. Blood samples were stored on ice during the fieldwork

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and transferred to -20 °C upon receipt at the laboratory. Raw rice samples were

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collected from each participating household.

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The present study obtained ethics approval from the Institute of Geochemistry,

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Chinese Academy of Sciences. All participants signed an informed consent before

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engaging in the survey.

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Analytical methods. The portion of hair within 3 cm from the scalp was selected

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for total Hg (THg) and MeHg analysis. Hair samples were washed with nonionic


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detergent, distilled water, and acetone according to Li et al.45 and dried in an oven at

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60 ℃ overnight. Rice samples were air-dried, crushed, and sieved to 150 meshes. The

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THg concentrations in the hair and rice samples were analyzed by the RA-915+ Hg

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analyzer coupled with PYRO-915+ attachment (Lumex, Russia) using thermal

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decomposition and Zeeman Atomic Absorption Spectrometry. Whole blood samples

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(0.1 mL) were digested with a mixture of nitric and sulphuric acid (v/v 4:1) at 95 ℃

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for 3 h. The digests were analyzed for THg concentration following the sequence of

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BrCl oxidation, SnCl2 reduction, purge, gold trap, and cold vapor atomic fluorescence

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spectrometry (CVAFS).46

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Hair, rice, and blood samples were digested using the KOH-methanol/solvent

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extraction technique.47 MeHg concentrations were measured using aqueous

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ethylation, purge, trap, and GC-CVAFS detection (Brooks Rand MERX). Detailed

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information on the quality control systems adopted in this work is listed in Table S1

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in the Supporting information.

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Calculation of Probable Daily Intake (PDI). To estimate daily MeHg intake

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from rice consumption, we calculated PDI values for each participant at the 7 sites

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using equation 1:

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PDI= (C× IR× 10-3)/bw

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Where PDI is given in micrograms per kilogram of body weight per day

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(µg/kg/d); bw=body weight (kg); C is the MeHg concentration in rice (ng/g); IR is

(equation 1)



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daily intake rate of rice (g/d), which was obtained from the questionnaire of each

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participant.

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Data Analysis. All data were analyzed using SPSS 19.0 for windows. The

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characteristics of Hg concentration were described by Mean±Standard Deviation (SD)

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and examined using descriptive statistics. Mean Hg concentrations in rice, hair and

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blood collected from different sites were compared using ANOVA. Relationships

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between rice, hair, blood Hg concentrations, and PDI of MeHg was analyzed using

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the Pearson correlation analysis. Results of the statistical tests were considered as

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statistically significant when p

Human Body Burden and Dietary Methylmercury Intake: The

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