Scholarly article on topic 'High Percentage Inorganic Arsenic Content of Mining Impacted and Nonimpacted Chinese Rice'

High Percentage Inorganic Arsenic Content of Mining Impacted and Nonimpacted Chinese Rice Academic research paper on "Environmental engineering"

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Academic research paper on topic "High Percentage Inorganic Arsenic Content of Mining Impacted and Nonimpacted Chinese Rice"

Environ. Sci. Technol. 2008, 42, 5008-5013

High Percentage Inorganic Arsenic Content of Mining Impacted and Nonimpacted Chinese Rice

Y.-G. ZHU,* + G.-X SUN,1 M. LEI,1 M. T E N G ,+ Y.-X. LI U , + N.-C. CHEN,* L.-H. WA N G , + A. M. CAREY,§ C. DEACON,§ A. RAAB,11 A. A. M E H AR G, § AND P. N. WILLIAMS* + § Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, China, Guangdong Institute of Eco-Environment and Soil Science, Guangzhou, 510650, China, Department of Plant and Soil Science, School of Biological Sciences, University of Aberdeen, Aberdeen AB24 3UU, U.K., and Department of Chemistry, University of Aberdeen, Aberdeen AB24 3UE, U.K.

Received January 12, 2008. Revised manuscript received March 28, 2008. Accepted April 9, 2008.

Two approaches were undertaken to characterize the arsenic (As) content of Chinese rice. First, a national market basket survey (n = 240) was conducted in provincial capitals, sourcing grain from China's premier rice production areas. Second, to reflect rural diets, paddy rice (n = 195) directly from farmers fields were collected from three regions in Hunan, a key rice producing province located in southern China. Two of the sites were within mining and smeltery districts, and the third was devoid of large-scale metal processing industries. Arsenic levels were determined in all the samples while a subset (n = 33) were characterized for As species, using a new simple and rapid extraction method suitable for use with Hamilton PRP-X100 anion exchange columns and HPLC-ICP-MS. The vast majority (85%) of the market rice grains possessed total As levels < 150 ng g-1. The rice collected from mine-impacted regions, however, were found to be highly enriched in As, reaching concentrations of up to 624 ng g-1. Inorganic As (Asi) was the predominant species detected in all of the speciated grain, with Asi levels in some samples exceeding 300 ng g-1. The Asi concentration in polished and unpolished Chinese rice was successfully predicted from total As levels. The mean baseline concentrations for Asi in Chinese market rice based on this survey were estimated to be 96 ng g-1 while levels in mine-impacted areas were higher with ca. 50% of the rice in one region predicted to fail the national standard.

Introduction

Inorganic arsenic (Asi) is classified as a nonthreshold carcinogen with a linear dose-response for chronic low level exposure (i). Only relatively recently has rice, and rice-derived products, been recognized as a significant dietary source of Asi (2-i5). In comparison to other cereal crops it exhibits a

* Corresponding author tel: +8610 62936940; fax: +8610 62936940; e-mail (P.N.W.): p.n.williams@abdn.ac.uk; (Y.-G.Z.) ygzhu@ rcees.ac.cn.

f Chinese Academy of Sciences.

* Guangdong Institute of Eco-Environment and Soil Science.

§ Department of Plant and Soil Science, University of Aberdeen. 11 Department of Chemistry, University of Aberdeen.

particular susceptibilityfor As accumulation in the grain (i0), levels which can be digested, absorbed, and subsequently metabolized in vivo (i5-i7). The problem is compounded by high consumption rates, typical in Asian, certain Latin American, and specific health related diets (9).

Farmers have been cultivating paddy rice (Oryza sativa L.) on China's east coast for approximately seven millennia (i8). Rice is the predominant dietary staple in southeast Asia with China being the world's foremost producer, accounting for over one-third of global supply. However the bulk of the grain is used to meet domestic needs (~1.3 billion mouths), making China only the sixth largest exporter (i9). Information on Asi concentrations in Chinese rice are sparse. Published studies exist regarding the As concentration of Chinese rice grain, mostly (20-23) focus on grains collected from land known to be As enriched, mainly resulting from mining or metal-processing industries requiring a posteriori knowledge of the sample area. Although of value, they do little to address the larger question concerning the safety of the average Chinese rice consumer.

Chinese standards for Asiin rice are probably the strictest in the world, which have been designed to protect a nation with high rice intakes (24). Maximum contaminant levels (MCL) are enforceable safety standards, derived from peer-reviewed data, they are not set specifically at levels known or anticipated to have no adverse health effects; rather they also account for technologicalfeasibility and costs associated with compliance (i, 24). In China, MCLs for Asi in rice grain were originally set at 700 ng g-1; after a review of Asi exposure in 2005. this was reduced to 150 ng g-1. At present, the proportion of Chinese rice that meets this requirement is undetermined.

In our study, the first of its kind, we screened market rice available in major cities for compliance with the MCL. As about half of the population in China still reside in rural settlements, we also sampled grainfromlocalfarmers in one of China's rice bowl regions (a prominent rice production zone), considering both mine-impacted and nonimpacted areas. New fast and convenient extraction and speciation methods were also developed and tested.

Material and Methods

Survey. Rice production in China is concentrated in southern and northeast regions (25). Polished (white) market rice was obtained from large cities in the southern provinces of Guangxi (n = 39), Guangdong (n = 30), Hong Kong (n = 19), Jiangxi (n = 30), Jiangsu-Shanghai (n = 34), Jiangsu (n = 25), and Guizhou (n = 31). Rice sourced from the northeast of China (n = 32) was obtained in Beijing markets and represented the provinces of Liaoning (n = 5), Heilongjiang (n = 11), Jilin (n = 4), and Hebei (n = 2) (Figure S3, Chinese map). In addition, three samples of US long grain rice were purchased in markets in Beijing as a comparison.

Hunan province is a major rice producer (26), with prolific ferrous and nonferrous metal industries that have been in operation, in some cases, for hundreds of years (27). Typical local daily rice consumption rates are among the highest in China at ca. 400 g/d wt per day (28). Two separate mining regions within Hunan and one in Daobanshan (Guangdong province, neighboring Hunan) were targeted for analysis (in total 157 samples), to determine background grain As concentrations. A region considered to be unaffected by prior mining activities was additionally sampled (n = 38).

Sample Preparation. Raw rice samples were washed with ultrapure water (18.2 Q), and then all grains (field and market)

10.1021/es8001103 CCC: $40.75 © 2008 American Chemical Society

Published on Web 05/22/2008

TABLE 1. Summary of Arsenic Species Characterisation and Extraction Recoveries of NIST Certified Reference Material (CRM) 1568a''

total As, As III, species sum,

ng g-1 ng g-1 As V, ng g-1 DMA, ng g 1 MMA, ng g-1 ng g-1 recovery, % n refs

ultra pure water 290 49 ± 1 30 ± 2 143 ± 2 8 ± 0 230 ± 4 79 ± 1 3

1% acetic acid 290 48 ± 7 34 ± 3 145 ± 11 9 ± 0 237 ± 22 82 ± 7 3

1% nitric acid 290 67 ± 5 36 ± 1 162 ± 1 5 ± 1 271 ( 3 93 ± 1 3

amylase and methanol:water 290 67 ± 4 39 ± 3 158 ± 5 13 ± 2 290 ± 2 96 *(29)

sonication probe 290 68 ± 3.7 21 ± 2 135 ± 4.1 8 ± 1 232 82 ± 4 *(30)

a Recovery = (species sum/total As) x 100.

were oven-dried (70 °C) until a constant weight was reached. All husks were removed and the grains powdered.

Chemicals. The list of chemicals and their suppliers used in this study are detailed in the Supporting Information, page S3.

Total Digestion: Concentrated HNO3. For concentrations of total As, oven-dried milled subsamples (0.1-0.2 g) were weighed into quartz glass digestion tubes, steeped in 2.5 mL of nitric acid, and allowed to stand overnight at room temperature. The samples were randomized prior to digestion and divided into batches consisting of ~40 samples. Heating was achieved using a hot block at 120 °C, until extracts were clear, and then made up to a volume of 10 mL with ultrapure water. GBW CRM 10010 Chinese rice flour was used to validate the analyses. Samples were again randomized prior to analysis.

Arsenic Speciation Extraction: 1% HNO3. Three methods were evaluated, using NIST Certified Reference Material (CRM) 1568a US (Arkansas) long grain rice flour, for their suitability employed in conjunction with a microwave system, to extract As from rice grain while maintaining speciation integrity: 1%HNO3,1%acetic acid and ultra pure water (Table 1). On the basis of our findings, HNO3 was selected as an extractant and subsequently tested on rice bran, husk, shoot, and root matrixes.

Milled subsamples (0.2000 g) were weighed into 50 mL polypropylene digest tubes and steeped in 10 mL of 1% nitric acid. The mixture was allowed to stand overnight. Samples were randomized and then heated in a microwave-accelerated reaction system (CEM Microwave Technology Ltd.). The temperature was gently raised, first to 55 and then to 75 °C, with holding times of 10 min. Finally the digest was taken up to 95 °C; this was maintained for 30 min before cooling. Upon reaching room temperature, samples were centrifuged at 3214gfor 5 min. The supernatant was collected and passed through a 0.45 fm x 13 mm nylon filter (Membrana). To minimize any species transformation, samples were run within a few hours of filtration and kept in the dark and on ice.

NIST Certified Reference Material (CRM) 1568a US (Arkansas) long grain rice flour and GBW CRM 10010 Chinese rice flour were used to validate the analyses. Blank spikes (0.1 g of 1000 fig As g-1) and sample spikes (0.200 g of US market rice, plus 0.1 g of 1000 f g As g-1) for both As111 and AsV, and blanks were run with each extraction batch of ~35 samples. Samples were randomized prior to analysis. A subset of the speciated samples (~25%) were extracted and analyzed in duplicate to monitor extraction and analytical reproduci-bility.

Total Arsenic Detection for Rice Grain. Total As levels from the digest samples were derived using hydride generation atomic fluorescence spectrometry (HG-AFS) (modelAF-610A, Beijing Ruili Analytical Instruments Co., Beijing, China). Samples were reduced with thiourea and hydrochloric and ascorbic acid solution before mixing with potassium boro-hydride to form arsenic hydrides. Arsenic peaks were integrated, using manufacturer issued software (AFS 610),

and their concentration was determined using a five-point calibration (0, 5, 10, 15, and 20 ig of As L-1).

Arsenic Speciation and Total Arsenic Detection in Speciation Extracts. Arsenic speciation was assayed simultaneously by high performance liquid chromatography-inductively coupled plasma-mass spectrometry (HPLC-ICP-MS). Setup and procedures were adapted from Williams et al. (7). ICP-MS was used to determine the As content in a subset of speciation extracts. Further details are provided in the Supporting Information, pages S3-S5.

Analytical Quality Control Data. The recovery of As from Chinese rice flour CRM from total digestions were between 90-101%. The speciation recoveries ((species sum/totalAs) x 100) were as follows:

(i) Extraction Trial. NIST CRM 1568a using 1% HNO3 93 ± 1% (n = 3) (Table 1).

(ii) Rice Survey CRM Recovery. NIST CRM 1568a 96 ± 2% (n = 3), GBW CRM 10010 119 ± 5% (n = 3).

(iii) Rice Survey Spike Recover. (all n = 3): Asffl blank spike 78 ± 2%, AsV blank spike 89 ± 3%, Asm sample spike = 82 ± 3%, AsV sample spike = 81 ± 2%.

The average relative difference, plus/minus one standard deviation ((replicate 1 — replicate 2)/(mean of replicate 1 and replicate 2) x 100) of the speciated samples extracted and analyzed in duplicate was 7 ± 6%. Further quality control data are shown in the Supporting Information (Tables S1 and S2.). All data is reported on a dry weight basis unless stated otherwise.

Results

Recoveries from As Speciation Extractions. The extraction solution strongly influenced speciation recoveries ((species sum/totalAs) x 100), which varied from 79% in water to 93% in 1% HNO3. Little difference was observed between replicates (Table 1). Poor recoveries for 1% acetic acid and water could be attributed to the low levels of Asm, based onlevels reported for NIST CRM 1568a, by Kohlmeyer et al. (29) and Sanz et al. (30) (Table 1), suggesting that these methods fail to extract all the Asi content. A solution of 1% HNO3 proved successful in extracting As from rice shoot, bran, husk, and root, when exactly the same procedure usedfor the grain was employed on these plant sections, yielding recoveries of 94 ± 11 (n = 9), 91 ± 13 (n = 3), 81 ± 13 (n = 3), and 106 ± 4% (n = 6), respectively. Foster et al. (31) have successfully used 2% HNO3 to extract As from marine and terrestrial plant matrixes, showing that extraction efficiencies were high and Asffl and AsV species are maintained.

Market Rice Survey. Dietary Exposure to As in Chinese Cities. Differences in As levels between market rice from northern and southern China (Table 2) were not found to be significant (p = 0.077 Kruskal—Wallis). Mean concentrations between all the regions sampled were comparable, with averages only differing by a maximum of 31 ng g-1 (Table 2). The lowest provincial average grain As levels were found in Hong Kong, Jiangsu, and Guizhou while Guangxi and Jiangxi were the highest. Cheng et al. (32) surveyed grain from 35 rice paddies near Shanghai, polishing the rice prior to analysis.

TABLE 2. Market Basket Suivey of Arsenic Concentrations in Rice

total arsenic concentration of rice grain (ng g-1)

region

far southeastern China

upper southeastern China

southwestern China northeast China

percentage of samples predicted to exceed

province/city mean median min- -max n 150 (ng Asi

121 98 15- 586 88 6

Guangxi 155 127 41- 586 39 10

Guangzhou 102 97 59- 245 30 3

Hong Kong 80 83 15- 138 19 0

114 114 29- 250 89 1

Jiangxi 145 145 68- 250 30 3

Shanghai 105 107 51 - 163 34 0

Jiangsu 88 85 29- 137 25 0

90 92 19- 162 31 0

Guizhou 90 92 19- 162 31 0

120 115 67- 188 32 0

Liaoning 108 89 68- 162 5 0

Heilongjiang 124 109 73- 187 11 0

Jilin 129 134 91 - 158 4 0

Hebei 128 128 98- 158 2 0

northeast unknown 118 115 67- 188 10 0

province

a Estimated Asi concentration derived from a linear regression of total arsenic against Asi; calculated from polished rice

TABLE 3. Field Suivey of Arsenic Concentrations In Rice

total arsenic concentration of rice grain (ng g-1)

_a ' « « ' Percentage of samples

predicted to exceed 150 (ng Asi g-1)6

55 28 23

a Hunan areas b + and c + refer to Hunan mining regions I and II, respectively, while a - denotes the control. b Estimated Asi concentration derived from a linear regression of total arsenic against Asi, calculated from unpolished rice grain.

province/city area» mine impacted median mean min - max n

Hunan a - 159 63 9- 367 38

b + 184 215 74- 484 40

c + 255 303 157- 624 22

Daobanshan(Guangdong) + 186 190 74- 447 95

The average grain concentrations were 120 ng g-1 and ranged from 60-190 ng g-1. This is similar to the levels we report for this area in this study (Table 2).

The vast majority (85%) of the rice grains possessed As levels < 150 ng g-1 and therefore are compliant, irrespective of As speciation, with the Chinese MCL for Asi of 150 ng g-1. Only 2% exceeded 200 ng As g-1 while ~50% were <100 ng As g-1. The mean content of As was 114 ng g-1 (Table 2), although this is lower than averages obtained from other market basket surveys by Meharg et al. (i2) conducted for (non-As contaminated) Bangladesh (120 ng g-1 n = 83), Thailand (140 ng g-1 n = 50), Italy (160 ng g-1, n = 28), Spain (180 ng g-1, n = 50), Japan (190 ng g-1, n = 26), USA (270 ng g-1, n = 198), and France (280 ng g-1, n = 33); it is still over twice that reported for Egypt (50 ng g-1, n = 100), Pakistan (60 ng g-1, n = 13), and India (50 ng g-1, n = 100).

A study from Taiwan (33) currently provides the most information on the As content of Chinese rice; however, the survey was conducted for a specific subpopulation and therefore not necessarily applicable to other regions of China. Nearly 400 samples of Taiwanese origin were analyzed. Market (packaged) and polished rice collected from storage barns averaged 100 ng g-1 fresh weight (f wt) (n = 266) and 50 ng g-1 f wt (n = 137), respectively.

Field Rice Survey. Dietary Exposure to As in Hunan Province. Much of the field-collected unpolished rice was found to be considerably As enriched. The highest level was 624 ng As g-1 (Table 3.). There were differences between the sample areas, with both Hunan mining regions showing larger mean As concentrations than Daobanshan and the control

zone, respectively. Hunan mining region II exhibited mean grain levels (303 ng As g-1) ca. twice that of the local background (163 ng As g-1) (Table 3) despite the baseline levels being higher than reported for other provinces (34). Findings from comparable surveys reveal similar trends. Arable soils close to the Dongjeongmine (South Korea) were found to be elevated in As, resulting in average grain As levels of 200 ng As g-1 (n = 3) (35) while the local background was ~90 ng As g-1 (36). Rice grown in the vicinity of a thallium—mercury-arsenic mine in Guizhou showed a 2- to 3-fold increase in grain As compared to a control site (37).

Speciated As Concentrations in Chinese Rice. As111, AsV, DMA (dimethylarsinic acid), and MMA (monomethylarsonic acid) were the only species detected in the rice extracts. As111 was predominant in Chinese rice while in the US grown rice DMA prevailed (Table 4). The data was not adjusted for recoveries. In each sample measured, DMA was detected, and in three grains MMA was additionally found. NIST CRM 1568a contained MMA, but there was none in the US market rice. The most elevated samples speciated (three samples from Guangxi province) were calculated to possess Asi concentrations of 380, 329, and 353 ng As g-1, being twice that of the MCL.

Predicting Grain Inorganic As Concentrations. When biplots of total As and Asi were produced (Figure S1) from our data for polished and unpolished rice, linear regressions were statistically significant and explained 94% and 72% of total variation, respectively. The resulting models were then validated by using total and Asi data from a Taiwanese survey by Schoof et al. (2) combined with Chinese rice samples

TABLE 4. Arsenic Speciation of Chinese Rice3

■ ■ b As III, As V, DMA, MMA, species sum, total

origin ng g-1 ng g-1 ng g-1 ng g-1 ng g-1 As,c ng g-1 recovery/ %

Guangxi (market) 302 77 102 13 495 586 84

245 84 147 - 476 510 93

291 63 93 8 454 550 83

165 55 112 - 343 329 104

86 37 31 - 153 154 100

Hong Kong (market) 84 42 38 - 164 138 119

76 33 23 - 132 120 110

63 34 21 - 118 106 112

Jiangxi (market) 87 38 34 7 167 164 102

96 25 31 - 152 184 83

87 34 28 - 150 190 79

83 39 27 - 149 157 95

Shanghai (market) 131 42 38 - 226 187 121

65 49 85 - 199 145 138

51 28 31 - 109 129 85

58 27 17 - 102 152 67

Guizhou (market) 89 52 25 - 166 150 111

72 33 16 - 121 128 94

47 33 9 - 88 19 471

Hunan (mine impacted) 214 43 60 - 318 484 66

181 43 60 - 283 385 74

171 62 11 - 244 321 76

134 46 29 - 209 325 64

102 30 27 - 159 201 79

68 30 26 - 123 139 88

76 24 10 - 111 196 56

46 17 15 - 79 159 49

Guandong (Mine Impacted) 174 51 11 - 237 243 98

129 37 47 - 213 249 86

111 32 16 - 159 219 73

83 26 47 - 156 251 62

71 36 41 - 149 151 98

80 35 27 - 141 131 107

54 24 20 - 98 95 103

Chinese CRM 63 40 18 - 121 102 119

US (market) bought in Beijing 101 51 136 - 289 350 82

95 32 141 - 269 329 82

79 42 133 - 254 308 82

a A subsample (n = 10) was extracted and analyzed in duplicate. The average relative difference, plus/minus one standard deviation, ((replicate 1 - replicate 2)/(mean of replicate 1 and replicate 2) x 100) was 7 ± 6%. b All market rice is polished while all mine-impacted rice is unpolished. c Total As = concd HNO3 digestion. d Recovery = (species sum/total As) x 100.

characterized for total and Asi (Figure S2). The linear regressions of predicted and actual Asi for polished and unpolished grain were significant (p < 0.001) with R2 values of85% and99%, respectively, following 1:1 ratios, throughout the sample range (Figure S2).

The models were applied to the complete market basket and field surveys. Predicted means of Asi in market rice were 96 ng Asi g-1 (n = 240) while median values were 90 ng Asi g-1 with only 2% of the samples exceeding the Chinese MCL, meaning 98 were compliant. Only 2 samples collected from control sites in Hunan were predicted to exceed the Chinese MCL while in contrast 22% and 25% of the rice from Dabaoshan and Hunan mining region II, respectively, were above this level. Alarmingly ~50% of the rice from Hunan mining region I failed national standards based on our estimates (Table 3).

The average recoveries ((species sum/total As) x 100) for polished rice were 99% with a standard error of 4. Recoveries for unpolished rice, however, were ~80%. Arsenic is concentrated in the bran layers of rice grain; therefore, unpolished can be considered to be more elevated in As than polished rice (38). On the basis of average brown to white As ratios calculated from Rahman (39), it can be estimated that bran layers increase overall grain As levels by ~20%, and that nearly all of the bran As is inorganic in nature (data unpublished).

The presented data could equate to the Asi content of the Hunan and Guangdong rice that would be directly consumed, i.e. bran layer removed. When the predicted Asi levels in market rice and mine-impacted grain are compared, considerable disparity or inequality in potential dietary exposure to a recognized carcinogen is apparent (Tables 1 and 2, Figure S4).

Discussion

Arsenic species extraction from rice grain, bran, husk, shoot, and root matrixes can be achieved rapidly using 1% HNO3 in combination with microwave heating. Our method is advantageous to other techniques such as methanol:water and TFA extraction, being more cost-effective, safer to handle, and allowing for easier disposal. Fast and effective characterization of rice As speciation is currently required, as many fundamental aspects of in planta As dynamics lay unresolved; for example, whether DMA and MMA grain content is controlled by cultivars, environment, or their interaction has yet to be ascertained. Figure S5. shows no obvious influence of total As on the proportion of organic As in Chinese rice, and regressions were nonsignificant. Two studies report Chinese rice as having a high proportion of total As in the organic form (8, 40). However, in light of this more extensive

survey, the proportion of Asi in the majority of Chinese rice is high (Table 4 and Figure S5) even when considerably arsenic elevated.

Based on speciation and concentration results, mean and median baseline levels of Asi in Chinese rice were determined to be 96 and 90 ng Asi g-1 or at ~2/3 of the Chinese MCL. US rice bought in markets in Beijing, despite exhibiting high proportions of organic As, were found to be have Asi levels averaging 132 ± 17 ng Asi g-1 (not adjusted for recovery), which is marginally higher than the Chinese rice. Other work found mean levels in US rice to vary from 74 ng Asi g-1 f wt to 112ngAsig-1fwt(3, 7, 41); however, a cautionary approach is required when using generalized average arsenic levels for US rice, as there is considerable nationwide variation in grain arsenic concentrations (9).

The reduction of US drinking water MCLs from 50 ug L-1 to the WHO endorsed level of 10 ug L-1 followed the largest and most comprehensive review of chronic As exposure and associated cancer risks to date (1, 42). Cancer risk models used in the assessment were specifically adjusted to consider Asi from food (1); therefore, the drinking water standards afford no direct protection against other dietary sources of Asi. Once Asi has been absorbed through intestinal membranes and enters the circulatory system, its source is immaterial. This thereby enables the US drinking water MCL for As, with its associated excess cancer risk, to be used as an appropriate measure of other dietary Asi exposures, if those sources are readily bioavailable.

US cancer risks associated with exposure to Asi in drinking water were initially modeled with a US water consumption rate of 2 L per day (42, 43); however, it was subsequently adjusted after review because it did not address differences in US water consumption patterns. National dietary surveys (CSFII) were used in simulations of entire lifetime exposure that encompassed population consumption variability, in addition to differences in gender and age; based on this, the average daily intake was determined to be1L(1). Therefore the daily lifetime exposure for the average US citizen from drinking water at the statutory safety limit would be 10 ug. Exposure to Asi from the daily consumption of 400 g/d wt (average rice consumption in rural Hunan) (28) of rice or rice product with a concentration of 150 ng Asi g-1 could result in a dietary intake from rice alone of 60 ug, or 6 times the exposure for US citizens from drinking water at their MCL. So although China is the first and only country to presently endorse a maximum limit in grain of 150 ng Asi g-1, this should not be considered an act of unnecessarily restrictive legislating. When placed in context, this standard is comparable to the now largely obsolete drinking water MCLs of 50 ugL-1 once adopted by many countries, including the US, EU member states, and China.

In accordance with the US EPA who set maximum contaminant level goals (MCLG) of zero for Asi in drinking water (1), we propose the setting of an MCLG for As in rice, especially the rice intended for populations with high rice consumptions. Although complete removal of Asi from rice is unfeasible, a 5-fold reduction in the background Asi concentration of Chinese rice would result in grain levels comparable to those found in other cereal crops (10, 44). Exposure to Asi from the daily consumption of 400 g of rice or rice product with a concentration of 20 ng g-1 could result in a dietary intake from rice alone of 8 ug (lower than average predicted Asi dietary exposures for the US population estimated at 10 ug per day (1)).

The risk posed to subpopulations who are consuming large amounts of rice with elevated Asi contents needs to be further addressed. In areas either naturally or anthropo-genically enriched in As, exposure is already likely to be high. Wang et al. (45) found elevated levels of As in the hair and blood of a Chinese community that resided close to a smeltery

district. This concurs with other studies in the southwest of England, an area renowned for metal mining (46, 47). In a recent study by Lee etal. (11) they concluded that rice, grown on contaminated soils, was the primary source of As exposure for local villagers that lived in the vicinity of an abandoned gold and silver mine (Myungbong, South Korea).

In conclusion, the vast majority of polished Chinese market rice falls well within nationally set safety standards for grain Asi, exhibiting average levels lower than a number of other major rice producing countries. However, in contrast to this, localized contamination (especially in mining regions) of rice is at present a poorly characterized exposure pathway for Asi.

Acknowledgments

The project was supported by the Ministry of Science and Technology, China (2002CB410808), and Chinese Academy of Sciences (KZCX1-YW-06-03). In addition we recognize the support in the form of a Chinese Academy of Sciences's "Research Fellowship for International Young Researchers" andfunds from the Royal Society of Edinburgh's International Exchange programme and Carnegie Scholarship. We would also like to especially thank Yuhong Chen (Chinese Life Science and ChemicalAnalysis Group, Agilent Technologies) and Jutta Frank (Hamilton, Bonaduz AG) for their continued support and technical expertise.

Supporting Information Available

Chemical lists, analytical methodologies, sampling maps, grain As models, summaries of speciation data, and quality control and method development data. This material is available free of charge via the Internet at http://pubs.acs.org.

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