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Perchlorate in Indoor Dust and Human Urine in China: Contribution of Indoor Dust to Total Daily Intake Tao Zhang,*,†,‡,§ Xiaojia Chen,§ Dou Wang,§ Rudan Li,§ Yufang Ma,§ Weiwen Mo,§ Hongwen Sun,§,∥ and Kurunthachalam Kannan⊥ †

School of Environmental Science and Engineering, Sun Yat-Sen University, Guangzhou 510275, China Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, Sun Yat-Sen University, Guangzhou 510275, China § College of Environmental Sciences and Engineering, Nankai University, Tianjin 300071, China ∥ Ministry of Education Key Laboratory of Pollution Processes and Environmental Criteria, Nankai University, Tianjin 300071, China ⊥ Wadsworth Center, New York State Department of Health, and Department of Environmental Health Sciences, School of Public Health, State University of New York at Albany, Albany, New York 12201, United States ‡

S Supporting Information *

ABSTRACT: Perchlorate is used in fireworks and China is the largest fireworks producer and consumer in the world. Information regarding human exposure to perchlorate is scarce in China, and exposure via indoor dust ingestion (EDIindoor dust) has rarely been evaluated. In this study, perchlorate was found in indoor dust (detection rate: 100%, median: 47.4 μg/g), human urine (99%, 26.2 ng/mL), drinking water (100%, 3.99 ng/mL), and dairy milk (100%, 12.3 ng/mL) collected from cities that have fireworks manufacturing areas (Yueyang and Nanchang) and in cities that do not have fireworks manufacturing industries (Tianjin, Shijiazhuang, Yuxi and Guilin) in China. In comparison with perchlorate levels reported for other countries, perchlorate levels in urine samples from fireworks sites and nonfireworks sites in China were higher. Median indoor dust perchlorate concentrations were positively correlated (r = 0.964, p < 0.001) with outdoor dust perchlorate levels reported previously. The total daily intake (EDItoal) of perchlorate, estimated based on urinary levels, ranged from 0.090 to 27.72 μg/kg body weight (bw)/day for all studied participants; the percentage of donors who had EDItotal exceeding the reference dose (RfD) recommended by the United States Environmental Protection Agency (US EPA) was 79%, 48%, and 25% for toddlers (median: 1.829 μg/kg bw/day), adults (0.669 μg/kg bw/day), and children (median: 0.373 μg/kg bw/day), respectively. Toddlers (0.258 μg/kg bw/day) had the highest median EDIindoor dust, which was 2 to 5 times greater than the EDIindoor dust calculated for other age groups (the range of median values: 0.044 to 0.127 μg/kg bw/day). Contribution of indoor dust to EDItotal was 26%, 28%, and 7% for toddlers, children, and adults, respectively. Indoor dust contributed higher percentage to EDItotal than that by dairy milk (0.5−5%).

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neurodevelopment in children.15 Perchlorate was found in all human urine samples collected from the U.S. (median: 3.60 ng/mL; n = 2820), Greece (median: 4.10 ng/mL; n = 134), and Argentina (median: 13.5 ng/mL; n = 107).16−18 In China, high urinary perchlorate levels were reported among occupationally exposed (mean: 314 ng/mL) and general population (mean: 172 ng/mL) groups in a fireworks industry area.19 The mean exposure dose of perchlorate by adults (1.12 μg/kg body weight (bw)/day) from a fireworks manufacturing area was higher than the reference dose (RfD: 0.7 μg/kg bw/day)

INTRODUCTION Perchlorate is an emerging contaminant and it exists naturally in the environment.1,2 Perchlorate can also be derived from the use of perchlorate salts in military and industrial products such as solid rocket fuels, munitions, explosives and fireworks. Since the 1950s, the annual production of perchlorate in the U.S. has been estimated to be on the order of 4 × 108 kg.3 China produces 90% of the fireworks in the world, with thousands of tons produced every year. The combination of human activities and natural processes results in widespread presence of perchlorate in the environment, drinking water, and foods which ultimately leads to human exposure.1−13 Perchlorate exposure is a potential health concern because it interferes with hormone production by thyroid gland.14 Hypothyroidism can affect normal metabolism in adults and © XXXX American Chemical Society

Received: September 10, 2014 Revised: January 12, 2015 Accepted: January 14, 2015

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DOI: 10.1021/es504444e Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology recommended by the United States Environmental Protection Agency (U.S. EPA).20 Nevertheless, there is a general lack of reports on human biological monitoring of perchlorate in nonfireworks manufacturing areas in China. The display of fireworks (or firecrackers) prevails over the entire country at several festivals; therefore, it is necessary to obtain information on human exposure of perchlorate in the general population in China. Another significant issue is that the sources and pathways of human exposure to perchlorate are not well-understood. Drinking water can be an important source of perchlorate.7−10,21 The U.S. EPA included perchlorate on the list of Drinking Water Candidate Contaminant, and perchlorate is widespread in tap water samples collected from the U.S, India, and China.7,10,21 Dietary perchlorate exposure is also likely, because a few studies reported the occurrence of perchlorate in dairy milk, fruits, vegetables, and other foodstuffs.11−13,22−26 Perchlorate is produced globally and continuously in the Earth’s atmosphere;27−29 given these findings, one might hypothesize that as perchlorate produced in the atmosphere can be accumulated in dust. In China, perchlorate exists in outdoor dust and aerosol.4,30 However, information on perchlorate levels in indoor dust is limited. Dust ingestion (or dermal absorption) is an important pathway for human exposure in the indoor environment, and therefore, assessment of human exposure to perchlorate from indoor dust is crucial. In the present study, concentrations of perchlorate were measured in indoor dust and human urine samples collected from fireworks and nonfireworks manufacturing areas in China. In addition, for comparison of daily intake of perchlorate via indoor dust (EDIindoor dust) and other sources, drinking water, and dairy milk samples were also analyzed. The main objectives of this study were to (1) determine EDIindoor dust of perchlorate by infants, toddlers, children, adolescents, and adults; (2) estimate the total exposure levels of perchlorate (EDItotal) for residents from fireworks and nonfireworks manufacturing areas; and (3) elucidate the contributions of various sources to EDItotal of perchlorate in China.

Figure 1. Sampling locations of samples collected in China. Red dots represented the sampling sites located in fireworks manufacturing areas, green dots represented the sampling sites located in nonfireworks manufacturing areas.

(n = 9), buses (n = 10), student dormitories (n = 10), laboratories (n = 8), and classrooms (n = 8) in Tianjin (SI Table S1 and Figure 1). Sampling was done from surfaces at least one meter above the floor, such as on bookshelves, moldings, household appliances, and lampshades in order to eliminate dirt, gravel, and sand. The samples were collected by sweeping with a hand-held brush or by vacuum cleaners. All dust samples were then air-dried, transferred to clean polypropylene (PP) tube and stored at −20 °C until analysis. Human Urine Samples. A total of 178 urine samples were collected between November, 2012 and January, 2013 from nonadults (1−10 yrs) and adults (≥18 yrs) in seven Chinese cities where dust samples were collected (SI Table S1). Participants were recruited at local families in each city, all of them were healthy, and none of them reported occupational exposure to perchlorate. The demographic information on the participants including age, sex, and sampling location are shown in SI Table S1. First-morning void (fasting time: > 8 h) urine samples were obtained and were stored in PP containers at −20 °C until analysis. The Institutional Review Board of the School of Environmental Science and Engineering, Sun Yat-Sen University, approved this study and an informed consent was obtained from all participants. Drinking Water and Dairy Milk Samples. During December, 2012 to January, 2013, 61 drinking water (i.e., tap water) samples from local families, and 26 pooled dairy milk samples from local markets or grocery stores were collected in all target Chinese cities (SI Table S1 and Figure 1). In this

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MATERIALS AND METHODS Details in regard to chemicals and devices, sample preparation and instrumental analysis are provided in the Supporting Information (SI). Briefly, dust samples were extracted by MilliQ water; urine and dairy milk samples were diluted with MilliQ water, then diluted samples were filtered by using Millipore centrifugal filter unit (0.5 mL, 3 kDa); drinking water samples were directly injected. We quantified perchlorate in studied samples by isotope dilution, and sample analysis was performed on an Agilent 1200 Series high-performance liquid chromatograph (HPLC) coupled with an Agilent 6410B Triple Quadrupole mass spectrometer (MS/MS). Isotopically labeled internal standard of perchlorate (Cl18O4¯) was spiked into samples prior to preparation. Indoor Dust Samples. During December, 2012 to January, 2013, indoor dust samples (n = 91) were collected from seven cities (i.e., Dongying, Guilin, Nanchang, Shijiazhuang, Yueyang, Yuxi, and Tianjin; Figure 1) of China. The sampling locations are distributed in northern, central, southern, and southwestern area of China. Nanchang and Yueyang are located in fireworks manufacturing areas, whereas the other locations do not have active fireworks manufacturing facilities. House dust samples were collected from local families in all sampling sites (n = 5− 10 for each site); dust samples were also collected from offices B


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Environmental Science & Technology

multiplication of perchlorate concentrations in drinking water or dairy milk by the amount consumed (equations are shown in SI Table S2). We obtained the daily consumption value of dairy milk from the Chinese Center for Disease Control and Prevention (CDC) dietary pattern and nutrition survey, and consumption data for drinking water was obtained from the U.S. EPA exposure factor handbook (SI Table S3).31,38 In humans, orally administered perchlorate is rapidly cleared from the bloodstream by elimination through urine.39,40 Thus, urinary levels can be used in the exposure dose calculations. We estimated EDItotal based on urinary perchlorate concentrations. The daily urine excretion rates were multiplied by the urinary perchlorate concentrations to obtain EDItotal (equation is shown in SI Table S2). Values for daily urine excretion rate were adopted from the reference values suggested by the International Commission on Radiological Protection (SI Table S3).41 Since body weights and consumption rates vary with age (SI Table S3),31−33,38,41−43 the calculation of EDIindoor dust and EDIdrinking water was performed for five age groups: infants (0−1 yrs), toddlers (2−5 yrs), children (6−10 yrs), adolescents (11− 17 yrs), and adults (≥18 yrs), as suggested by the Chinese CDC; EDIdairy milk was calculated for toddlers, children, adolescents, and adults due to the fact that dairy milk is rarely consumed by infants in China. EDItotal was calculated for toddlers, children, and adults because urine samples were not available for infants and adolescents in this study. All individual perchlorate concentrations in home dust, drinking water, dairy milk, and human urine were used to calculate the corresponding EDIindoor dust, EDIdrinking water, EDIdairy milk, and EDItotal, respectively. Median and range of EDI values obtained for each age group were discussed in the present study. Statistical Analysis. Data analysis was performed with SPSS, Version 19.0. Perchlorate concentrations in each matrix, EDItotal and EDI of perchlorate via various exposure sources (indoor dust, drinking water and dairy milk) by infants, toddlers, children, adolescents, and adults were tested for normality using the Kolmogorov−Smirnov test. Differences between groups were analyzed by one-way ANOVA when the two sets of data were distributed normally; otherwise, Mann− Whitney U-test was used. Pearson correlation coefficients were used for the analysis of relationship between two sets of data with normal distribution (or log-normal distribution); otherwise, the Spearman’s rank correlation coefficient was used. A value of p < 0.05 denoted significance. The concentrations of perchlorate are expressed as ng/mL for urine, drinking water, and dairy milk samples; and as μg/g dry weight for dust samples.

study, drinking water denotes water that was distributed through publicly owned water treatment plants and that was intended for drinking or food processing. Each pooled dairy milk sample represents a milk brand of multiple samples (n = 3−6) that were well mixed for analysis as one dairy milk sample. At least one most popular local brand and one nationally well-known brand were collected in each location. Drinking water and dairy milk were stored in PP containers at −20 °C until analysis. Quality Assurance and Quality Control. Milli-Q water was injected after every batch of 5−10 samples as a check for instrumental blank and memory effects. Procedural blanks were analyzed for every 20 samples to monitor for contamination in reagents and centrifugal filter unit. Sampling materials such as hand-held brush, membrane used in vacuum cleaners, and PP tube were also checked for the presence of perchlorate. All blanks were free of detectable levels of perchlorate. In order to avoid cross-contamination, all hand-held brushes were disposable and used once, and membranes used in vacuum cleaners were cleaned by Milli-Q water and air-dried prior to sampling. Recovery of perchlorate spiked onto sample matrixes and through the analytical procedure was determined for each sample type (n = 3 for each type). Perchlorate was spiked into urine, drinking water, and dairy milk at 10 ng/mL (100 μL; 0.1 ng/μL) and into dust at 10 μg/mL (100 μL; 0.1 μg/μL). Recoveries (±standard deviation, SD) of perchlorate spiked into urine, drinking water, dairy milk, and dust were 99 ± 11%, 100 ± 5%, 109 ± 13%, and 124 ± 17%, respectively. The high recoveries of perchlorate spiked into indoor dust might be due to the matrix effect caused by sample background. Mean recoveries of internal standard (10 ng/mL) among sample matrices ranged from 80 ± 12% to 118 ± 18%. A 10-point calibration standard (in Milli-Q water) encompassing concentrations ranging from 0.1 to 500 ng/mL, was injected before each batch of 20 samples. The regression coefficient for calibration curves was >99%. Internal standard was also spiked into each calibration standard at 10 ng/mL. The limit of quantitation (LOQ) for urine, drinking water, and dairy milk was 1.00, 0.20, and 0.20 ng/mL, respectively; the LOQ was 50 ng/g for indoor dust. Daily Intake Estimation. Human exposure to perchlorate from indoor dust can occur through oral ingestion, dermal absorption, and inhalation. In this study, exposure to perchlorate via ingestion (EDIingestion) and dermal absorption (EDIabsorption) of indoor dust was estimated from perchlorate concentrations determined in indoor dust, through the application of exposure/ingestion factors recommended by the US EPA (equations are shown in SI Table S2, parameters are shown in SI Table S3).31−33 Data on indoor air perchlorate levels are not available, and inhalation is thought to be a minor contributor to overall exposures ( RfD

median

min

max

%> RfD

median

min

max

%> RfD

infants toddlers children adolescents adults

0.127 0.258 0.104 0.056 0.044

0.004 0.009 0.004 0.002 0.002

2.208 4.465 1.808 0.963 0.770

6.4 19 6.4 2.1 2.1

0.155 0.078 0.078 0.048 0.070

0.010 0.005 0.005 0.003 0.005

10.86 5.474 5.494 3.366 4.926

18 3.3 3.3 1.6 1.6

NAe 0.033 0.019 0.007 0.003

NA 0.005 0.003 0.001 Nanchang > Shijiazhuang > Tianjin > Guilin > Dongying > Yuxi (Table 1). Shijiazhuang, had the third highest median urinary perchlorate level although dust samples from this city had the lowest concentration among all sampling locations. The reason for this inconsistent trend between dust and urine in Shijiazhuang is not known. However, it is worthwhile to note that Yueyang and Nanchang had the highest indoor dust perchlorate concentrations (Table 1); similarly, Dongying and Yuxi concurrently had the lowest indoor dust perchlorate levels (Table 1). Furthermore, when the data for Shijiazhuang were excluded, a significantly positive correlation (r = 0.943, p < 0.01) was found for perchlorate concentrations between indoor dust and urine among the sampling sites (SI Figure S2). Our results suggested that indoor dust may be a predictor of human exposure to perchlorate in China. In our previous study, perchlorate was detected in all human whole blood samples in Nanchang where the second largest fireworks manufacturing operations exist in China.20 In the present study, however, perchlorate was widely detected in urine samples not only from fireworks manufacturing sites (Nanchang and Yueyang) but also from nonfireworks manufacturing sites (Shijiazhuang, Tianjin, Guilin, Dongying and Yuxi), which indicated widespread exposure to perchlorate by the Chinese population. Another previous study from China reported urinary perchlorate concentrations in general population (mean: 172 ng/mL) from a fireworks production area (Liuyang, Hunan provinces);19 the city of Yueyang (mean: 226 ng/mL) investigated in this study and Liuyang both belong

to Hunan province, and comparable urinary perchlorate concentrations were found in partcipants from both cities. However, our urinary perchlorate concentrations (general population) were much lower than those reported for occupationally exposed Chinese fireworks workers (mean: 314 ng/mL).19 In comparison with the contemporaneous studies from other countries, the concentrations of perchlorate in urine from Chinese people were approximately 5.0 times greater than the concentrations reported in the U.S. (median: 4.4−5.2 ng/mL for children, 3.5 ng/mL for adults) and Greece (median: 4.1 ng/mL for pregnant women), and 2 times higher than those found in Argentina (median: 13.5 ng/mL for adult females).16−18 Concentrations in Drinking Water and Dairy Milk. All drinking water and dairy milk samples conatined measurable levels of perchlorate. The perchlorate concentrations ranged from 0.26 to 280 ng/mL with a median of 3.99 ng/mL in drinking water, and ranged from 1.74 to 22.0 ng/mL with a median of 12.3 ng/mL in dairy milk. Among all sampling locations studied, median perchlorate concentrations varied in drinking water by 20-fold (1.11 to 24.4 ng/mL), whereas comparable levels were found in dairy milk (6.76 to 13.9 ng/ mL) (Table 1). The sequence of perchlorate concentrations was found as Shijiazhuang > Nanchang > Guilin > Yuxi > Yueyang > Dongying > Tianjin in drinking water, and Yueyang ≈ Guilin ≈ Nanchang > Yuxi ≈ Shijiazhuang > Dongying > Tianjin in dairy milk (Table 1). No statistically significant correlations of perchlorate concentrations between human urine and drinking water (r = 0.250, p = 0.589) or dairy milk (r = 0.429, p = 0.337) were found (SI Figure S3) across all locations. Perchlorate Exposure from Indoor Dust. The median (range) values for EDIindoor dust of perchlorate stratified by age (i.e., infants, toddlers, children, adolescents, and adults) are listed in Table 2. The EDIindoor dust of perchlorate ranged from 0.002 to 4.465 μg/kg bw/day for all age groups (Table 2). Overall, toddlers (0.258 μg/kg bw/day) had the highest median EDIindoor dust of perchlorate, which was 2−5 times greater than the EDIindoor dust estimated for the other age groups; this may be attributed to higher dust ingestion rate observed for toddlers (SI Table S3). No significant gender-related differences in EDIindoor dust were found in this study when the entire EDIindoor dust data set for the seven sampling locations were analyzed collectively or when EDIindoor dust for individual sites were analyzed separately (data not shown). E


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include fireworks manufacturing areas (Nanchang and Yueyang), and high levels of perchlorate are expected in these locations. Although the manufacture of fireworks is rare in other sampling locations studied, the display of fireworks and firecrackers are popular during family celebrations throughout this country. The manufacture and exhibition of fireworks can increase the perchlorate exposure levels among the Chinese population.4,19,20 Contribution of Indoor Dust to Total Daily Intake. The contribution of various exposure sources to EDItotal was examined as estimated daily intake via each source divided by the EDItotal. Our EDItotal values were calculated for toddlers, children, and adults due to the fact that no urine samples from infants and adolescents were analyzed in this study. To quantify the contribution of EDIindoor dust to EDItotal for toddlers, children, and adults (Figure 3), the median EDIindoor dust value

In 2005, the National Academy of Sciences recommended to the U.S. EPA that an RfD of 0.7 μg/kg bw/day be applied, based on a no observed effect level (7.0 μg/kg bw/day) for inhibition of iodine uptake.14 Although median EDIindoor dust values for all age groups (Table 2) were well below the RfD, all maximum values of EDIindoor dust for each age groups (0.770 to 4.465 μg/kg bw/day) exceeded the RfD (Table 2). It is worthwhile to note that perchlorate exposure in humans is not only from indoor dust but also from other sources such as diet, drinking water, and outdoor dust;7−13,30 however, we found that exposure for dust alone resulted in 2−20% of EDIindoor dust values exceeding the RfD (Table 2). Furthermore, only home dust samples were used for the calculation of EDIindoor dust, and higher concentrations were found in other categories of microenvironment (such as office and bus) in Tianjin (Table 1), and therefore the data used here to estimate the EDIindoor dust may be slightly underestimated. Further, it should also be borne in mind that specific exposure conditions (high dust ingestion and occupational exposure) can contribute to high perchlorate exposure via dust ingestion. Total Daily Intake of Perchlorate. Among all sampling locations studied, the EDItotal values for perchlorate calculated based on urinary levels ranged from 0.291 to 27.72 μg/kg bw/ day with a median of 1.829 μg/kg bw/day for toddlers, 0.028 to 11.52 μg/kg bw/day with a median of 0.373 μg/kg bw/day for children, and 0.090 to 23.45 μg/kg bw/day with a median of 0.669 μg/kg bw/day for adults. The EDItotal value for toddlers was significantly higher (p < 0.01) than those for children and adults. In comparison with the RfD value, 47% of study participants (total: n = 178) had EDItotal exceeding the RfD. Among each age group, percentage of donors who had EDItotal exceeding the RfD was 79%, 48%, and 25% for toddlers, adults, and children, respectively. When we compared the EDItotal with the provisional maximum tolerable daily intake (PMTDI: 10 μg/kg bw/day) recommended by the World Health Organization,46 percentage of participants who had EDItotal exceeding the PMTDI was 18%, 6%, and 2% for toddlers, adults, and children, respectively. Our results suggest that more attention should be paid to perchlorate exposure by the Chinese population. Blount et al. assessed perchlorate exposure in a nationally representative population of 2820 U.S. residents, as part of the National Health and Nutrition Examination Survey, and the median (95th percentile) EDItotal of perchlorate was estimated to be 0.064 (0.234) μg/kg bw/day for American adults.16 Our values for EDItotal for adults in China (all locations: 0.651 μg/kg bw/day; fireworks manufacturing areas: 1.918 μg/kg bw/day; nonfireworks manufacturing areas: 0.501 μg/kg bw/day) were an order of magnitude greater than the value reported in the US.16 Although the EDItotal estimates are not available for other countries, we can deduce that Chinese people are exposed to a higher level of perchlorate than those of residents from Greece and Argentina, based on the urinary perchlorate levels reported in these two countries (Greece: median = 4.1 ng/mL; Argentina: median = 13.5 ng/mL).17,18 In addition, the median EDItotal of perchlorate for toddlers and children were much greater than the estimated dietary exposure dose (EDIdiet) for toddlers and children from other countries such as Canada (toddlers: 0.037 μg/kg bw/day; children: 0.041 μg/kg bw/ day), Korea (toddlers: 0.14 μg/kg bw/day; children: 0.07 μg/ kg bw/day) and the U.S. (toddlers: 0.25−0.28 μg/kg bw/day; children: 0.17−0.20 μg/kg bw/day).11−13 China is the largest fireworks producer in the world, and our sampling locations

Figure 3. Contributions to total daily intake and estimated daily intake from various exposure sources (indoor dust, drinking water and dairy milk) by select age groups (toddlers, children and adults) in China. The assessment of sources of human exposure to perchlorate by toddlers (1−5 yrs) was based on the results observed for Yueyang, Nanchang and Shijiazhuang due to the fact that samples for toddlers were collected only from these locations; the assessment of sources of human exposure to perchlorate by children (6−10 yrs) was based on the results observed for Yuxi, Guilin and Dongying due to the fact that samples for children were collected only from the three locations; the assessment of sources of human exposure to perchlorate by adults (≥18 yrs) was based on results observed for all sampling locations.

(0.468 μg/kg bw/day) was calculated for toddlers from Yueyang, Nanchang, and Shijiazhuang (sampling locations of urine samples from toddlers), and the median EDIindoor dust value (0.105 μg/kg bw/day) was calculated for children from Yuxi, Guilin, and Dongying (sampling locations of urine samples from children). Our estimates indicated that EDIindoor dust contributed 7−28% of EDItotal for toddlers, children, and adults in China. The contributions of indoor dust to EDItotal for toddlers (26%), and children (28%) were much higher than that for adults (7%) (Figure 3). For the comparison of EDIindoor dust and other sources of perchlorate exposure in toddlers, children, and adults, the EDIdrinking water and EDIdairy milk values were also calculated for those locations where urine samples from toddlers, children or adults had been collected. The median EDIdrinking water values for toddlers, children, and adults were 0.145, 0.113, and 0.070 μg/ kg bw/day, respectively, and the median EDIdairy milk values were much lower than the EDIdrinking water, with a median value of 0.034 μg/kg bw/day for toddlers, 0.017 μg/kg bw/day for children, and 0.003 μg/kg bw/day for adults. For toddlers, indoor dust (26%) made a more appreciable contribution to EDItotal than drinking water (8%) did (Figure 3). The contribution of indoor dust (28%) to EDItotal was comparable to that of drinking water (30%) in children (Figure 3). F



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However, contribution of drinking water (11%) to perchlorate exposure was higher than that of indoor dust (7%) for adults (Figure 3). The differences in the relative significance of indoor dust and drinking water among toddlers, children and adults may be explained by age-related consumption or ingestion rates of dust and drinking water. The dust ingestion rate is the highest for toddlers (0.10 g/day), followed by children (0.05 g/ day) and adults (0.05 g/day) while drinking water consumption rate increased with increasing age (SI Table S3). Dairy milk was a minor contributor to EDItotal of perchlorate, accounting for 2% for toddlers, 5% for children, and 0.5% for adults (Figure 3). The contribution of indoor dust to EDItotal was markedly higher than that of dairy milk in all three age groups. Relatively low contribution of dairy milk to perchlorate intake found in this study is due to the fact that consumption rates of dairy milk are low (15.7−40.3 mL) in China; however, the consumption rates of dairy milk ranged from 55 to 224 g (whole milk) in the U.S.31,38 Although the contribution of dairy milk to perchlorate intake is low, breast milk may be a dominant exposure pathway of perchlorate in infants; in the US, perchlorate is widely detected in human milk at a concentration range of 0.01 to 204 ng/mL, resulting in higher exposure in breast-fed infants (0.220 μg/kg bw/day) than in infants consuming milk-based formula (0.027−0.103 μg/kg bw/day).47−50 Apart from indoor dust, drinking water and milk, other potential sources have contributed 41% and 76% of the EDItotal of perchlorate in toddlers and adults, respectively (Figure 3). We calculated daily intake of perchlorate by toddlers and adults via several other sources including rice (EDIrice), vegetables (EDIvegetable), and fruits (EDIfruit) based on the reported concentrations [median: 0.55 ng/g in rice, 20.6 ng/g fresh weight (fw) in vegetable and 7.72 ng/g fw in fruit] of perchlorate in these food groups in China and the daily consumption rates published by the Chinese CDC.25,26,38 The respective EDIrice, EDIvegetable, and EDIfruit values were 0.004, 0.193, and 0.012 μg/kg bw/day for toddlers, and 0.003, 0.123, and 0.003 μg/kg bw/day for adults in China. The EDIindoor dust values of perchlorate by toddlers were higher than the daily intake from rice, vegetables, and fruits. For adults, although vegetables made a more appreciable contribution to EDItotal than indoor dust did, indoor dust ingestion was a more important source of perchlorate in humans compared with rice and fruit consumption. Our results indicate that indoor dust is an important source of human perchlorate exposure in China. In summary, the results obtained in this study indicate widespread human exposure to perchlorate by the Chinese population. Residents in China are exposed to a higher level of perchlorate than those previously reported in other countries. Indoor dust is an important exposure route that could account for 26%, 28%, and 7% of EDItotal for toddlers, children, and adults, respectively, in China. This study not only deepen the knowledge on human exposure to perchlorate by the Chinese population, but also help to understand the exposure pathways of perchlorate in humans.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 86-20-84113454; fax: 86-20-84113454; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Natural Science Foundation of China (No. 21207071 and No. 41225014) and Fundamental Research Funds for the Central Universities are acknowledged for their partial research supports. The Shanghai Tongji Gao Tingyao Environmental Science & Technology Development Foundation of China are acknowledged for scholarship support. A part of this study (analysis was performed at Wadsworth Center) was funded by a grant (1U38EH000464-01) from the US CDC (Atlanta, GA) to Wadsworth Center, New York State Department of Health. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the CDC. We gratefully acknowledge the donors who contributed the urine samples for this study.

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