Changes in Concentrations of Perfluorinated Compounds

Nov 23, 2010 - Department of Analytical Chemistry, Division of Environmental Medicine, Norwegian Institute of Public Health, P.O. Box 4404 Nydalen, ...
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Environ. Sci. Technol. 2010, 44, 9550–9556

Changes in Concentrations of Perfluorinated Compounds, Polybrominated Diphenyl Ethers, and Polychlorinated Biphenyls in Norwegian Breast-Milk during Twelve Months of Lactation C A T H R I N E T H O M S E N , * ,† L I N E S . H A U G , † HEIN STIGUM,‡ MAY FRØSHAUG,† SHARON L. BROADWELL,† AND G E O R G B E C H E R †,§ Department of Analytical Chemistry, Division of Environmental Medicine, Norwegian Institute of Public Health, P.O. Box 4404 Nydalen, NO-0403 Oslo, Norway, Department of Genes and Environment, Division of Epidemiology, Norwegian Institute of Public Health, P.O. Box 4404 Nydalen, NO-0403 Oslo, Norway, and Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, NO-0315 Oslo, Norway

Received June 30, 2010. Revised manuscript received September 29, 2010. Accepted November 4, 2010.

At present, scientific knowledge on depuration rates of persistent organic pollutants (POPs) is limited and the previous assumptions of considerable reduction of body burdens through breast-feeding have recently been challenged. We therefore studied elimination rates of important POPs in nine Norwegian primiparous mothers and one mother breast-feeding her second child by collecting breast-milk samples (n ) 70) monthly from about two weeks to up to twelve months after birth. Perfluorinated compounds (PFCs), polybrominated diphenyl ethers (PBDEs), hexabromocyclododecane (HBCD), and polychlorinated biphenyls (PCBs) were determined in the breastmilk samples. Linear mixed effect models were established for selected compounds, and significant decreases in the range of 1.2-4.7% in breast-milk concentrations per month were observed for a wide range of PCBs and PBDEs. For the first time, depuration rates for perfluorooctylsulfonate (PFOS) and perfluorooctanoic acid (PFOA) are presented, being 3.8 and 7.8% per month, respectively (p < 0.05). The relative amount of the branched PFOS isomers in the breast-milk samples was 18% on average (range 6-36%, RSD 30%). There were no significant differences in isomer pattern between the mothers, or changes during the lactation period. After a year of nursing the breast-milk concentrations of PFCs, PBDEs, and PCBs were reduced by 15-94%.

Introduction Breast-milk is the natural and optimal food for infants and breast-feeding has practical advantages for the mother. The * Corresponding author tel: +47 21 07 65 46; fax: +47 21 07 66 86; e-mail: [email protected]. † Department of Analytical Chemistry, Division of Environmental Medicine, Norwegian Institute of Public Health. ‡ Department of Genes and Environment, Division of Epidemiology, Norwegian Institute of Public Health. § Department of Chemistry, University of Oslo. 9550

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World Health Organization (WHO) and thus several governmental authorities strongly recommend breast-feeding due to health benefits for the infant (1). However, questions have been raised for some time whether environmental pollutants in breast-milk might adversely affect infant development and health. Halogenated organic compounds such as polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), hexabromocyclododecane, and perfluorinated compounds (PFCs) have been shown to be ubiquitous environmental pollutants (2-8) that accumulate in food chains and have potential to cause negative effects on human health (6, 9-11). Further, persistent lipophilic pollutants like PCBs, PBDEs, and HBCD are transferred to breast-milk and excreted during lactation. Extensive information on concentrations of PBDEs and PCBs in breast-milk is available (3, 12), but there are at present few data on temporal trends during lactation. Depuration rates affect exposure of infants to chemicals during breast-feeding, and this knowledge is essential to conduct a realistic exposure assessment. Also, the depuration rates indicate the importance of nursing as pathway of maternal excretion. Past studies have shown that concentrations of persistent lipophilic chemicals such as dioxins and PCBs typically decline during the course of lactation (13), however, Hooper et al. (14) recently reported that the concentrations of PBDEs and PCBs in breast-milk from nine primiparous Californian mothers were not substantially reduced after six months of breast-feeding. Accordingly, two other studies from the U.S. showed highly variable concentrations of PBDE in breast-milk during the lactation period (15, 16). In contrast to PCBs and PBDEs, PFCs are not associated with lipids but rather with proteins (17) and the impact of breast-feeding on maternal body burdens and concentrations in breast-milk is presently unknown. In general, only limited data exist on lactation as an exposure source of PFC for children, although the two major PFCs, perfluorooctylsulfonate (PFOS) and perfluorooctanoic acid (PFOA), are known to be present in breast-milk at low concentrations (8). Norwegian mothers are among the most enthusiastic breast-feeders in the world. More than 80% of all babies are breast-fed at the age of six months, and it is not unusual to nurse until the child is 1.5 years. Thus, evaluation of the exposure of infants to environmental chemicals through breast-feeding is of particular interest in Norway. The aim of this study was to investigate the elimination rates for PFCs, PBDEs, HBCD, and PCBs during the lactation period by measuring concentrations in longitudinally collected breastmilk samples from Norwegian mothers. In this context, a method using column-switching HPLC coupled to a triple quadrupole mass spectrometer was developed and validated to study the elimination rates of PFCs.

Materials and Methods Study Subjects and Sampling. This study was conducted on breast-milk samples from nine mothers living in the Oslo area. The mothers fulfilled the selection criteria used in the WHO-coordinated surveys of human milk for POPs, i.e., they were single child primiparous mothers, their pregnancies were normal, both mother and child were healthy, the children were primarily breast-fed until they were six months old, and the mothers were born and had resided in Norway for the previous five years (with the exception of one mother who was born abroad, but had lived the five last years in western Europe). In addition, one mother also donated samples when nursing her second child. We made one 10.1021/es1021922

 2010 American Chemical Society

Published on Web 11/23/2010

FIGURE 1. Information on the collection of individual breast-milk samples. exception to the WHO guidelines by including mothers above 30 years of age. Informed consent was obtained from all the mothers. The mothers were provided precleaned screw cap bottles and they collected breast-milk approximately once a month from about two weeks after birth up to twelve months. The mothers were encouraged to collect the samples in the morning. The breast-milk was obtained by manual expression to avoid potential contamination by breast pumps and were immediately frozen (∼ -18 °C). The number of samples collected by each mother was from n ) 3 to n ) 10 (n ) 70 in total) as depicted in Figure 1. Three mothers sampled breast-milk during 2001 and 2003, while six sampled during 2005 and 2006, and one sampled in 2008 and 2009. The mothers completed a questionnaire regarding sociodemographic data. The median age, weight, and BMI of the participants at the first sampling were 29 years, 71 kg, and 25.5 kg/m2, respectively. Analysis of Breast-Milk. Determination of PBDEs, HBCD, and PCBs. The breastmilk samples were extracted and analyzed according to the methods described by Thomsen et al. (18, 19). Details regarding the analyses can be found in the Supporting Information. As small changes in the concentrations of PBDEs and PCBs were expected, good analytical precision was required. Thus the repeatability of these determinations was assessed for this particular study by extracting five replicates of a quality control breast-milk sample within a day. The subsequent analyses were performed in one batch. Quality Control. The laboratory participated in 2006 in the annual Interlaboratory Comparison on Dioxins in Food (20) and obtained concentrations within (1 SD of the consensus value for all PCBs and PBDEs in the breast-milk sample except BDE-28, which was within (2 SD. BDE-209 was measured later, and the concentration found was within (1 SD. No ring test was available for determination of HBCD in human samples, however, our laboratory obtained concentrations within (1 SD of the consensus value in an interlaboratory comparison study on HBCD in samples of herring and cod liver oil (21). Determination of PFCs. Chemicals. PFOA, perfluorononanoic acid (PFNA), perfluorodecanoic acid (PFDA), perfluoroundecanoic acid (PFUnDA), perfluorohexane sulfonic acid (PFHxS), perfluoroheptane sulfonic acid (PFHpS), PFOS, perfluoro-n-[1,2,3,4-13C4]octanoic acid (MPFOA), perfluoro-n-[1,2,3,4,5-13C5]nonanoic acid (MPF-

NA), perfluoro-n-[1,2-13C2]decanoic acid (MPFDA), sodium perfluoro-1-hexane [18O2] sulfonate (MPFHxS), and sodium perfluoro-1-[1,2,3,4-13C4]octanesulfonate (MPFOS) were purchased from Wellington Laboratories (Guelph, Ontario, Canada). The other chemicals and glassware used are described elsewhere (22). Sample Preparation. Our previously described method for determination of PFCs in serum (22) was slightly modified for the analyses of breast-milk. After thawing and homogenization in a thermoshake incubator at 37 °C, 200 µL of either breast-milk or unprocessed cow’s milk was transferred to a centrifugation tube, to which were added internal standards (0.3 ng MPFOA, MPFNA, MPFDA, MPFHxS, and MPFOS in 30 µL methanol), native PFCs (only calibration and validation solutions, 30-50 µL), and acetonitrile to make up a total volume of 600 µL for thorough precipitation of proteins, and then mixed using a whirl mixer. The samples were further centrifuged at 14 000 rpm for 20 min and the supernatant was transferred to a glass autosampler vial, to which was added 500 µL of 0.1 M formic acid and then mixed on a whirl mixer. Method Validation. To investigate the linearity of the method, 31 calibration solutions of unprocessed cow’s milk at nine different concentrations from 0.010 to 1.5 ng PFC/ mL of milk were prepared in 3-5 replicates. Six replicates of cow’s milk spiked at three concentrations (0.025, 0.15, and 1.5 ng PFC/mL milk) were prepared and analyzed to assess the repeatability and accuracy of the method (n ) 18 in total). The cow’s milk did not contain any PFCs (n ) 5). In addition, six replicates of both a spiked (0.15 ng PFC/mL) and nonspiked breast-milk quality control sample were analyzed. The signal/noise (S/N) ratio of the calibration solutions was used to estimate the limit of detection (LOD, S/N ) 3) and limit of quantification (LOQ, S/N ) 10). Instrumentation, Analysis and Quantification. All extracts were analyzed by injection of 400 µL on a columnswitching LC system coupled to a triple quadrupole mass spectrometer described in detail previously (22). Sixty eight of the 70 samples from the lactation study were available for PFC analyses, and all samples were analyzed in one batch. Calibration solutions (n ) 27) were prepared in unprocessed cow’s milk covering 0.01 to 1.5 ng PFC/mL milk, and linear (PFOA, PFNA, PFHxS, and PFHpS) or quadratic (PFDA, PFUnDA, and PFOS) calibration curves were established using weighing (1/concentration). Procedural blanks of purified VOL. 44, NO. 24, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Results from Validation of the Analytical Method for PFCsa spiked cow’s milk 0.025 ng/mL

PFOA PFNA PFDA PFUnDA PFHxS PFHpS PFOS a

0.15 ng/mL

1.5 ng/mL

estimated

0.15 ng/mL

LOD

LOQ

accuracy

RSD

accuracy

RSD

accuracy

RSD

RSD

ng/mL

ng/mL

66 71 70 85 80 72

45 31 34 19 19 20

92 95 106 109 104 100 97

10 17 10 18 7.3 5.4 8.6

97 104 103 106 102 102 104

3.1 5.2 4.8 4.6 1.8 3.0 2.4

4.6 5.3 7.7 13 5.3 5.6 4.4

0.008 0.008 0.003 0.008 0.003 0.003 0.003

0.025 0.025 0.010 0.025 0.010 0.010 0.010

The accuracy and RSD are given in percent.

water showed negligible contamination of PFCs. For quantification of PFOS, the total area of the linear and branched isomers was integrated. The relative amount of the branched isomers was also quantified. Similar to HBCD, PBDEs, and PCBs, compounds below the LOQ or compounds not detected have been left out of the calculations. Quality Control. The laboratory participated in 2010 in a proficiency test on determination of PFCs in breast-milk and obtained concentrations within (1 SD of the consensus value for all PFCs found above the LOQ (B. van Bavel, personal communication). Our laboratory has also participated successfully in several ring trials for determination of PFCs in serum (22). Statistical Analysis. The data represent a selection of mothers with repeated measurements of concentrations of PBDEs, PCBs, and PFCs in longitudinally collected breastmilk samples. First, the concentrations by day of sample collection for each mother were plotted, and linear or nonlinear trends as well as mothers with diverging patterns were investigated. One mother (Mother 3) had higher PBDE levels and a much steeper downward slope than the other mothers. Reanalysis of this mother’s breast-milk samples confirmed the diverging results, and the data set was statistically treated both with and without this mother. To estimate the slope of the concentrations over time, we used linear mixed models. Because of the small data set, models with random intercept were explored. No confounders were used in the models. To account for possible skewness in the residuals, both normal based and robust variance estimation was used. The variances were almost identical, thus only the results from the robust regression are reported. Because the small number of mothers may imply that asymptotic p-values may not be trusted, we also did bootstrap estimations of the random intercept models at the level of the mothers, i.e., the data for the mothers were bootstrapped, leaving the measurements for each mother unchanged. Bias corrected and accelerated (bca) confidence intervals for the concentrations slopes were reported based on the bootstrap results. The analysis was restricted to compounds determined with a precision e11% RSD, that were present in more than 70% of the samples; i.e., BDE-28, 47, 99, 100, 153, 154, CB-101, 118, 138, 153, 156, 170, 180, 183, 187, 194, PFOS, and PFOA.

Results and Discussion Validation of the PFC Method. As breast-milk contains more proteins than serum, the major modification of the serum method comprised precipitation with acetonitrile, a stronger precipitation agent than methanol, which resulted in sufficient removal of milk proteins. The following evaluation of the validation experiment has been limited to the seven PFCs most frequently detected in blood; i.e., PFOA, PFNA, PFDA, PFUnDA, PFHxS, PFHpS, and PFOS (11, 23-25). The method was proven to be linear through 0.010-1.5 ng/mL milk for PFDA, PFHxS, PFHpS, and PFOS and 0.025-1.5 ng/mL milk 9552

spiked breast-milk

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for PFOA, PFNA, and PFUnDA. Due to the use of matrix matched calibration solutions, the linearity of the calibration curve expresses the linearity of the whole method. The R2 of the calibration curve was >0.99 for all the PFCs, except PFOA, for which it was 0.988. The LOD of the method (Table 1) was estimated from the calibration curve and was 0.003 or 0.008 ng/mL milk, which is comparable to LODs reported in other studies (23, 25-27). The accuracy determined as measured concentration relative to the spiked amount, ranged from 66 to 109%, the poorest seen at the lowest spike level which is close to the LOQ for PFOA, PFNA, and PFUnDA (Table 1). A satisfying repeatability in the range of 2-18% RSD was demonstrated at 0.15 and 1.5 ng/mL, while up to 45% RSD was observed at the lowest concentration (0.025 ng/mL) (Table 1). Six replicate analyses of a spiked breast-milk sample showed a similar repeatability for all seven compounds in human milk (Table 1). Concentrations of BFRs, PCBs, and PFCs in the BreastMilk. Up to 11 BFRs (BDE-28, 37, 47, 85, 99, 100, 153, 154, 183, 209, and HBCD), 18 PCBs (CB-101, 105, 114, 118, 123, 128, 138, 153, 156, 157, 167, 170, 180, 183, 187, 189, 194, and 209), and 2 PFCs were found above the LOQ in the breastmilk samples. The median concentration and range of these compounds are presented in the Supporting Information (Table S1). The concentrations of the BFRs are within the ranges that were found in a large study on breast-milk from background exposed Norwegian mothers (n ) 393) (28). On a ng/g lipid basis, BDE-47 was dominant in all but nine samples (five of which were from the same mother) in which BDE-209 was found at highest concentration. The concentrations of the PCBs are similar to the concentrations reported in a recent paper on Norwegian breast-milk (29). The highest concentration was observed for CB-153, followed by CB180, 138, 118, 170, 156, and 183. The concentrations of PFOS and PFOA were within the ranges previously reported in breast-milk in Europe and Asia (8). Based on the observed concentration levels and the selection criteria used, we consider these breast-milk samples representative for background exposed Norwegian mothers. Changes through the Lactation Period. The breast-milk concentrations of the compounds varied differently throughout the lactation period for the individual mothers as can be seen from Figure 2, where the concentrations of BDE-47, BDE-209, HBCD, CB-153, PFOA, and PFOS are shown as examples. Similar temporal patterns were observed for the other PBDEs and PCBs. If comparing the concentrations in the first sample collected after birth with the sample taken about six months later, the tri- to hexabrominated PBDEs had decreased in eight and increased in two of the ten sample sets. For one additional mother BDE-100 and 153 increased between the first and last sample. Large variation was observed for HBCD and BDE-209, and the concentrations were below LOQ in several samples (Table S1 and Figure 2). A decreasing trend was in general seen for all PCBs, but a slight increase was observed occasionally for

FIGURE 2. Breast-milk concentrations (PBDEs and PCB in ng/g lipids, PFOS and PFOA in ng/mL) of selected compounds during the lactation period for the individual mothers. some of the congeners in one to two sample sets. A similar inconsistency in the direction of change for PCBs and PBDEs has recently been shown in breast-milk samples collected during the first three months of nursing (15), as well as for PBDEs in breast-milk sampled three and twelve months postpartum (16). In contrast, a consistent decrease was observed in the breastmilk concentrations of PFOS and PFOA, except for PFOS in one sample set. With the exception of PBDEs for Mother 3, the concentrations of PBDEs and PCBs seem quite stable during the individual sampling periods (examples in Figure 2). However, if the concentrations are normalized to the first sample, a downward trend is observed as presented in Figure 3 where

BDE-47 and CB-153 are shown as examples. An even more pronounced downward trend is seen for the normalized PFOS and PFOA levels (Figure 3). Analytical Repeatability. The precision of the analyses was assessed by the repeatability of the PFCs, PBDEs, and PCBs determined in six, five, and five replicates, respectively, of an unfortified human breast-milk sample. As can be seen in Table 2, repeatability was better than ∼11% RSD for six PBDEs and ten PCBs as well as for PFOS and PFOA. This precision was considered sufficient to study the small changes in concentration expected in breast-milk samples collected monthly, and these compounds were thus selected for further statistical evaluation. HBCD and BDE-183 had a concentra-

FIGURE 3. Concentrations of BDE-47, CB-153, PFOA, and PFOS normalized to the first sample. VOL. 44, NO. 24, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 2. Repeatability of the Determinations Using an Unfortified Sample of Breast-Milk compounds

n

concentration (in ng/g lipids)

repeatability (RSD in %)

BDE-28 BDE-47 BDE-99 BDE-100 BDE-153 BDE-154 CB-101 CB-118 CB-138 CB-153 CB-156 CB-170 CB-180 CB-183 CB-187 CB-194 PFOA PFOS lipid content

5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 6 6 5

0.075 1.0 0.25 0.24 0.79 0.64 7.0 12 28 61 8.1 17 42 3.1 11 4.7 0.062a 0.15a 3.6b

11 4.7 4.4 3.9 3.8 2.7 6.5 6.8 1.9 1.8 8.4 2.6 1.5 11 3.4 7.4 6.1 3.1 0.4

a

In ng/mL.

b

In percent.

tion lower than LOQ in this sample, and their precision could not be assessed. The lipid content of this sample was 3.6% and the RSD of the five lipid determinations was 0.4%. Depuration Rates. PBDEs and PCBs. To investigate distinct patterns in the concentrations of PFCs, PBDEs, and PCBs over time, linear mixed effects models were established. This allows establishing unique linear relationships between measured concentrations and the days after birth (DAB) for each mother. Because of the limited data, only random intercept models were used. This set the same trend, but allows different concentrations for each mother. Despite the varying trends

in the breast-milk levels described above, the slopes (coefficient for DAB) of the concentrations over time were significantly different from zero and negative for most of the compounds (Table 3). For the PBDEs and the PCBs (except CB-101) the intraclass correlation was >∼0.8, indicating that >80% of the measurement variability arises from betweenmother variation. Due to the low number of mothers in the study, the variation was also assessed using bootstrapping, estimating bca confidence intervals. The bootstrap results agreed with the asymptotic p values in the sense that all significant p values corresponded with confidence intervals below zero. The model estimates show that concentrations of BDE-28, 47, 100, and 153 as well as all the PCBs except CB-101 and 194, PFOA, and PFOS are significantly reduced throughout the lactation period. The CB-101 determination was subjected to occasional blank contamination, and CB194 and BDE-154 were found in low concentrations. This may explain the nonsignificant p value seen for these compounds. A steeper decrease in concentrations of PBDE through the lactation period was observed for Mother 3 and was confirmed by reanalysis of the samples. The reason for this is not known. Certainly, the highest breast-milk concentrations of PBDEs were observed for this mother (Figure 2), but Hooper et al. (14) reported that depuration rates for PBDEs were independent of the initial concentration in the breastmilk. When excluding Mother 3 from the mixed effect models, the intercepts and coefficients were slightly reduced for the PBDEs, and the models for BDE-100 and 153 did not reach statistical significance. However, treating one out of nine observations as outlier is problematic and there is no way of assessing whether this observation was a true outlier or just a part of the normal variation. Of the nine mothers, one also provided breast-milk while nursing her second child. However, excluding this sample set did not alter the models notably. To ease the interpretation of the mixed effects model, concentration changes in % per month (30 days), half year

TABLE 3. Change in Concentrations over Time Based on Linear Mixed Modelsa

n

intercept

DAB coef.

p value

% change per 30 days

% change per 180 days

% change per 365 days

intraclass correlation

95% bca confidence interval -0.00008) -0.00093) 0.00178) 0) -0.00004) 0.00026)

significance

BDE-28 BDE-47 BDE-99 BDE-100 BDE-153 BDE-154 sum 6 PBDE

59 70 70 69 70 51

0.19 2.90 0.88 0.58 0.90 0.06

-0.00029 -0.00265 -0.00067 -0.00046 -0.00052 0.00009

0.001 0.005 0.299 0.020 0.034 0.091

-4.7 -2.7 -2.3 -2.3 -1.7 4.7

-27.9 -16.4 -13.7 -14.0 -10.3 28.1

-56.7 -33.3 -27.8 -28.4 -20.9 57.0

0.78 0.90 0.78 0.90 0.85 0.72

(-0.00069, (-0.01069, (-0.00357, (-0.00151, (-0.00152, (-0.00008,

70

5.50

-0.00459

0.012

-2.5

-15.0

-30.5

0.89

(-0.01363, 0.00020)

*

CB-101 CB-118 CB-138 CB-153 CB-156 CB-170 CB-180 CB-183 CB-187 CB-194 sum 10 PCB

70 70 70 70 70 70 70 70 70 70

9.10 10.38 17.73 41.67 3.45 7.81 18.90 2.17 6.09 1.39

-0.00475 -0.00803 -0.01323 -0.02664 -0.00238 -0.00427 -0.00892 -0.00090 -0.00259 -0.00024

0.576 0.000 0.000 0.000 0.000 0.000 0.003 0.028 0.009 0.358

-1.6 -2.3 -2.2 -1.9 -2.1 -1.6 -1.4 -1.2 -1.3 -0.5

-9.4 -13.9 -13.4 -11.5 -12.4 -9.8 -8.5 -7.5 -7.7 -3.1

-19.1 -28.2 -27.2 -23.3 -25.2 -20.0 -17.2 -15.2 -15.5 -6.4

0.29 0.83 0.86 0.86 0.92 0.88 0.87 0.82 0.88 0.85

(-0.02961, (-0.01155, (-0.02708, (-0.04291, (-0.00393, (-0.00759, (-0.01682, (-0.00205, (-0.00517, (-0.00086,

* * * * * * * *

70

118.37

-0.06926

0.001

-1.8

-10.5

-21.4

0.81

(0.1210, -0.03144)

*

PFOA PFOS

66 68

87.76 159.44

-0.22499 -0.16291

0.000 0.000

-7.7 -3.1

-46.1 -18.4

-93.6 -37.3

0.48 0.85

(-0.31654, -0.16319) (-0.23663, -0.07879)

* *

0.01009) -0.0042) -00.701) -0.01265) -0.00121) -0.00146) -0.00274) -0.00032) -0.001) 0.00038)

* * * *

a Random slope models with robust variance estimation. Intercept ) expected concentration at birth (ng/g lipids for PBDEs and PCBs, pg/mL for PFCs). The indicator for significance is based on bootstrap estimates from the models.

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(180 days), and year (365 days) are also presented in Table 3. A monthly decline in the range 1.7-4.7% was estimated for the PBDEs and 1.2-2.3% for the PCBs. This is in close agreement with depuration rates for PBDEs and PCBs recently published by Hooper et al. where the monthly reduction was 1-3% on average (14). In contradiction to this, trends during lactation were not significant and less consistent in two other studies from the U.S. (15, 16). PFCs. PFCs do not accumulate in lipids, but rather bind to serum proteins (17). Thus these compounds must be considered differently than PBDEs and PCBs. Whereas the concentrations of PCBs and PBDEs on a lipid basis are expected to be almost the same in breast-milk and serum, concentrations of PFOA and PFOS in breast-milk have been found to be 3.8 and 1.4% of the serum concentrations on volume basis, respectively (L. Haug, personal communication). The relationships between breast-milk and concentrations in serum were linear, with correlation coefficients of 0.63 (n ) 19) for PFOS and 0.99 (n ) 10) for PFOA. This is in accordance with the partitioning of PFOS between breastmilk and serum described by Ka¨rrman et al. (25). In light of this, the twice as high depuration rate of PFOA compared to the rate observed for PFOS is reasonable; 7.7 and 3.1% reduction per month for PFOA and PFOS, respectively (Table 3). To estimate exposure to PFCs through breast-milk, a consumption of 700 mL per day may be assumed. This leads to an intake of 61 and 112 ng/day for the infants at the beginning of the breast-feeding. This is similar to the dietary intake recently established for Norwegian adults, i.e., 42 and 105 ng/day for PFOA and PFOS, respectively (30). Thus, even though the concentrations of PFCs in breast-milk are about 2 orders of magnitude lower than those in serum, this study shows that lactation is a significant exposure source for the infant. The Scientific Panel on Contaminants in the Food Chain within the European Food Safety Authority (EFSA) has established tolerable daily intakes (TDI) for PFOS and PFOA of 150 ng/kg body weight/day and 1.5 µg/kg body weight/day, respectively (31). The estimated dietary intakes of PFOS and PFOA for an exclusively breast-fed infant of 4 kg are about 5 and 100 times below this TDI. However, it has to be taken into account that TDI are established for lifelong exposure and cannot be applied to the relatively short period of breast-feeding. As the PFCs are mainly bound to serum proteins and the liver (17), the body burden of PFCs in the mothers is expected to be more influenced by breast-feeding than the body burdens of lipid soluble compounds. Table 3 shows that during one year of breast-feeding, the concentrations of PFOA and PFOS in breast-milk were reduced by about 94 and 37%, respectively, and that lactation is an important route of excretion in mothers. This is also supported by a recent study showing that breast-feeding history is one of the major determinants for concentrations of PFCs in serum (30). The relative amount of the branched PFOS isomers in the breast-milk samples was 18% on average (range 6-36%, RSD 30%). There were no significant differences in isomer pattern among the mothers, or changes during the lactation period. Isomer patterns of PFOS in breast-milk have to our knowledge not been published previously, and may shed light on the pharmacokinetic properties of PFOS. The strength of this study is the relatively frequent sampling and the large number of breast-milk samples obtained from each mother, making it possible to establish significant relationships between breast-milk concentrations and time postpartum. However, information on changes in body weight and diet during the sampling period was not available, and other factors likely to influence depuration rates could not be assessed. This study showed that concentrations of most PBDEs, PCBs, and PFOS in breast-milk were reduced by about 15-30%, and for PFOA by more than

90%, during one year of breast-feeding. Lactation will thus have implications for the body burden of the mothers of these compounds and information on breast-feeding history is crucial in studies investigating factors that may influence the body burden.

Acknowledgments We greatly acknowledge all the mothers who voluntarily donated breast-milk over a long period of time, and the Research Council of Norway for financial support.

Supporting Information Available Details regarding the determination of PBDEs, HBCD, and PCBs, and the median and concentration range of all detected compounds. This information is available free of charge via the Internet at http://pubs.acs.org/.

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