Depuration of polybrominated diphenyl ethers (PBDEs) and polychlorinated biphenyls (PCBs) in breast milk from California first-time mothers (primiparae).

Polybrominated diphenyl ethers (PBDEs) and polychlorinated biphenyls (PCBs) are persistent, lipophilic chemicals with different sources, pathways, and patterns of human exposure. Mixtures of PBDEs are present as noncovalently bound flame-retardant additives to synthetic fabrics, foams, or plastics in a variety of consumer products. Two mixtures, the pentaand octa-BDEs, are no longer used. Deca-BDE, the major PBDE with total production approaching that of PCBs at their peak, remains in use but is banned in Sweden, Maine, and Washington State. PBDE levels, where examined, have been increasing in humans and wildlife worldwide, doubling every 2-5 years (Hites 2004; Noren and Meironyte 2000; Sjodin et al. 2003a). PCBs have been banned for 30 years, but are present as dielectrics in telephone pole-mounted transformers common to urban areas. Exposure pathways for PCBs, as with most persistent organic pollutants, begin in the outdoor environment. Exposure pathways for PBDEs begin with indoor environments (Jones-Otazo et al. 2005; Schecter et al. 2005; Sjodin et al. 2003b; Stapleton et al. 2005; Thuresson et al. 2006; Wilford et al. 2005). Little is known regarding the rates of loss (depuration) of PBDEs and PCBs from mothers during lactation. It is of interest to examine depuration of PBDEs and PCBs in breastfeeding mothers to determine whether depuration patterns reflect differences in sources, pathways, and patterns of human exposures. In a review of published data on depuration of persistent chemicals via breast milk, LaKind et al. (2001) indicated that most studies were not specifically designed to measure depuration rates, and were limited by combinations of small sample size (n = 1-3), few intraindividual measurements, interindividual measurements, or pooled samples. Preliminary data from an ongoing study suggest low depuration rates for PBDEs and PCBs (Sjodin et al. 2005). Our studies, initiated in March 2003, were designed to assess depuration patterns for PBDEs and PCBs using serial samples of breast milk from lactating mothers.

Materials and Methods

Serial and precision studies. We examined depuration rates of 12 PBDE congeners and 80 PCB congeners by measuring concentrations in serial samples collected over extended periods from two groups (n = 9 for each)--short-term (ST) and long-term (LT)--each composed of volunteers from a cross-sectional study of California primiparae (CS CA) (n = 82) (Table 1). ST participants collected hand-expressed samples at fixed intervals (4, 6, 8, 12, 16, 20, and 24 weeks after infant birth) and a pumped sample at 6 weeks. Some ST mothers missed collections, and one ST mother collected 3 additional monthly samples, for a total of 10 samples over 38 weeks. LT participants collected two samples separated by varying intervals, with initial samples collected 1-6 weeks after birth and final samples 18 to > 85 weeks later.

Breast milk samples were run in batches of 12: nine experimental samples and one duplicate, one method blank, and one qualitycontrol sample from a breast milk pool (QCP). All samples from a participant were run in one batch.

Table 1. California breast milk studies.

Study             CS CA           ST           LT         0-28 DAB
                                                          (a)

No.                82              9            9         35

Study period      Mar 2003-Nov    Oct          Mar        Apr
                  2005            2004-Jun     2003-Sep   2003-Sep
                                  2005         2005       2005

Duration          130 weeks       24 weeks     Variable   4 weeks

Design            Primiparae;     CS CA        CS CA      CS CA
                  infant 2-8      subset; 5-7  subset; 2  subset;
                  weeks; samples  serial       samples:   samples
                  hand-expressed  samples 4,   first at   collected
                                  6, 8, 12,    1-6        0-28 DAB
                                  16, 20, 24   weeks;
                                  weeks; 6     second at
                                  weeks,       18-136
                                  concurrent   weeks
                                  HE/P

Demographics

Median age           31             29           32         32
(years)

Race (%)

African               9             10            5          9
American

Latina               12             10           10          9

Asian                 5             10            5          3

White                71             60           80         80

Education (%)

< High school         4             10            0          6
graduate

High school           8              0            0         11
graduate

Some college         19             30           15         14

College              70             60           85         69
graduate

Median BMI          22.7           21.9          24        23.6

Family income
(%)

< $18,000            15             10           10         11

$18,001-$36,000      12             70           25         17

$36,001-$60,000      12              0            5         20

[greater than or     43             20           50         51
equal to]
$60,001

Infant sex [%       60/40          56/44        56/44      63/37
(F/M)]

Median birth         20            20.5          20         20
length (in.)

Median birth         7.5            7.5         7.4         7.7
weight (lb)

Smoking status
(%)

Current smoker        3             10            5          3

Ex-smoker            43             40           10         37

Never-smoker         54             50           90         60

Breast-fed as       72/20          70/20        70/30      71/20
child [%
(yes/no)]

California
residence (%)

> 5 years              77             70           80         97

< 5 years              23             30           20          3

Study             29-56 DAB
                     (a)

No.                   35

Study period        Apr
                    2003-Nov
                    2005

Duration            4 weeks

Design              CS CA
                    subset;
                    samples
                    collected
                    29-56 DAB

Demographics

Median age            30
(years)

Race (%)

African                6
American

Latina                14

Asian                  6

White                 66

Education (%)

< High school          0
graduate

High school            6
graduate

Some college          31

College               63
graduate

Median BMI          22.5

Family income
(%)

< $18,000             20

$18,001-$36,000       34

$36,001-$60,000        6

[greater than or      29
equal to]
$60,001

Infant sex [%       54/46
(F/M)]

Median birth        20.5
length (in.)

Median birth           8
weight (lb)

Smoking status
(%)

Current smoker         3

Ex-smoker             46

Never-smoker          51

Breast-fed as      63/23
child [%
(yes/no)]

California
residence (%)

> 5 years             89

< 5 years             11

Abbreviations: BMI, body-mass index [weight (kg)/height (m)2]; DAB,
days after birth.
(a) Depuration rates early in lactation, (0-28 DAB) were compared with
rates later in lactation (29-56 DAB) using two groups of mothers
from the CS CA study, in which one group collected initial samples
3-28 DAB (n = 35), and a second group 29-56 DAB (n = 35).

To measure biologically based changes in PBDE or PCB levels in serial milk samples, good analytical precision is required to distinguish anticipated small changes from variations in laboratory background. To assess our precision, we made repeated measurements on identical samples. For ST participants, whose five to seven samples were analyzed as one batch, we analyzed eight identical QCP samples as one batch and calculated the relative SD (RSD; SD over the averaged eight observations, expressed as a percentage). For LT participants, whose two samples were analyzed in one batch, we analyzed duplicate samples (21 for PBDEs, 16 for PCBs) and calculated the RSDs (Table 2). For comparison, we also analyzed 16 pairs of concurrently collected hand-expressed and pumped samples (HE/P) (Table 2).

Table 2. Precision studies for breast milk.

                         [SIGMA]PBDEs         [SIGMA]PCBs
Study              No.        (ng/g lw)       (ng/g lw)

Concurrent QCPs       8             28-31            34-48
Duplicates(a)    21, 16          18-1,100           73-311
HE/P pairs           16            14-821           79-231

                         RSD (%)(a)
Study            BDE-47    BDE-153    PCB-153
Concurrent QCPs     3.5         1.3       3.9
Duplicates(a)       5.3         4.7       3.9
HE/P pairs          3.4         3.8       9.3

lw, lipid weight.
(a) Duplicate measurements were made in 21 samples for PBDEs, and in 16
samples for PCBs. RSDs for duplicates are means of the RSDs calculated
for each of the 21 or 16 pairs of duplicate measurements; RSDs for HE/P
pairs are means of the RSDs calculated for each of the 16 HE/P pairs of
samples.

Participants were healthy, first-time (primiparous) breast-feeding mothers with healthy, singleton, 1to 8-week-old infants and abundant milk supplies (Table 1). We followed institutional review board guidelines, and participants signed approved consent forms.

Mothers collected milk samples (~ 100 mL each) into chemically clean, 120-mL foilwrapped or amber glass jars over a 2to 3-day period. Jars were refrigerated between collections, frozen at completion, transported to the laboratory, and stored at -20[degrees]C. Mothers followed written protocols to minimize contamination by skin, hair, or dust. Samples were collected between March 2003 and October 2005.

Sample analysis. Samples were prepared in four steps (lyophilization, extraction and fat determination, and mixed silica gel column cleanup, and gel permeation chromatography column cleanup), and were analyzed by highresolution gas chromatography-mass spectrometry. Methods are described elsewhere (She et al. 2007).

We examined patterns of mean PBDE and PCB levels over time using mixed-effects linear regression models (xtmixed program in Stata 9.0; Stata Corporation, College Station, TX) for log values (levels were log-normally distributed). Mixed-effects models allowed for random intercepts and random slopes for the regression of a (log) level against time since birth (Rabe-Hesketh and Skrondal 2005). This allows a unique linear regression relationship between log PBDE measurements and the time since birth for each mother, while assuming that intercepts and slopes for the sampled mothers arise from a population distribution of intercepts and slopes. The variances of this latter distribution indicate quantitatively how much mothers differ in either their intercept or slope, or both. The intercept is the log exposure value at time zero (i.e., birth).

Results

Demographics of study populations. Mothers and infants in the ST, LT, and CS CA studies were similar in several demographic, physical, and lifestyle indices (age, race, education, body mass index, smoking status, years residency in California, and breast-fed as child, with infants similar in sex ratio as well as birth length and weight) and dissimilar in family income (Table 1).

PBDE and PCB levels. [SIGMA]PBDEs were calculated by summing values for BDEs 32, 28/33, 47, 66, 71, 85, 99, 100, 153, 154, and 183. [SIGMA]PCBs summed values for PCBs 110, 114, 118, 105/127, 132/153, 138, 156, 157, 180, and 170/190. [SIGMA]PBDE and [SIGMA]PCB levels and distributions in the ST, LT, and CS CA populations were similar (Figure 1). BDE-47 and PCB-153 were the major congeners in all three studies and correlated with [SIGMA]PBDEs (r = 0.99) and [SIGMA]PCBs (r = 0.89), respectively. Consequently, we have used BDE-47 and PCB153 values to illustrate depuration profiles.

Analytical precision. BDE-47 and PCB-153 values are plotted in Figures 2 and 3 for ST and LT mothers, respectively. Values for the eight identical QCP samples are plotted at ST collection points as if they were samples from an ST mother, with dotted lines indicating 1 SD from mean QCP. RSDs for the eight identical QCP and duplicate samples are given in Table 2. Based on analyses of identical samples (37 duplicate pairs and 8 concurrent QCP samples) for BDE-47 and PCB-153, respectively, mean duplicate RSDs are 5.3% and 3.9%, and RSDs for 8 QCP samples are 3.5% and 3.9%.

Regression analysis of ST study. Results of fitting various statistical models are summarized in Table 3.

Table 3. Changes in levels of PBDEs and PCBs in ST and LT studies.

Study   Congener      Change       95% CI (a)
                    per month
                      (%) (a)

ST         BDE-47          -3       -2 to -4
           BDE-99          -2   -0.2 to -3.5
          BDE-100          -2       -1 to -3
          BDE-153          -2       -1 to -3
       PCB-153 (d)        0.3        -2 to 2
       PCB-153 (e)       -0.6        -3 to 2
LT         BDE-47          -1     -0.1 to -2
          PCB-153          -1   -0.5 to -1.6

Study   p-Value (b)     p-Value (c)

ST     [10.sup.-11]           0.89
               0.03           0.11
             0.0003              1
              0.001           0.36
               0.97             NA
               0.40         0.0001
LT             0.03       < 0.0001
             0.0001           0.07

NA, not applicable.
(a) Declines per month and 95% CIs are fixed slope estimates. With a
random slope model, the estimate of the mean slope is identical to this
fixed slope estimate (using the reported significant figures), except
for PCB-153. For PCB-153, fixed slope estimates and mean random slope
estimates are given separately. bp-Value for test of zero slope using
fixed slope/random intercept model. cp-Value for the test of
homogeneity of slopes in random intercept/random slope model. These
p-values are conservative; Snijders & Bosker (1999) suggest dividing
the p-value by 2 to address this conservativeness. d Fixed slope
estimate. eRandom slope estimate.

A simple mixed-effects model with only a random intercept and a fixed slope for the linear change in (log) BDE-47 over time since birth yields an estimated 3% decline in BDE-47 per month, with a 95% confidence interval (CI) of 2-4% decline per month (defined as 30 days; p < 0.001), using a robust method to calculate the variance of the estimated slope. The random effects model indicates that 99.6% of measurement variability arises from between-mother variation (i.e., within-mother correlation is 0.996). Thus, BDE-47 values differ between mothers, but repeated measures on the same mother are similar (Figure 2).

Extending the model to allow for random slopes of (log) BDE-47 against time since birth (i.e., a different slope for each mother) yields an average slope of -3% per month, essentially the same as the fixed slope result, with little variation in individual slopes (Figure 2).

Monthly declines for BDE-99 and BDE-100 were similar (2%; Table 3), and between-mother variation accounted for 99% and 99.3% of the total variation, respectively. Results using random slopes also mimic BDE-47, with slightly more variation in mothers' slopes for BDE-99.

PCB-153 increased 0.3% per month (95% CI, -2 to 2% per month; p > 0.9) when only random intercepts were included, and between-mother variation accounted for 97% of total variation. With random slopes across mothers, the slopes averaged -0.6% per month, with 95% of the slopes estimated to lie between -3% and 2% per month (Table 3). The likelihood ratio test statistic for examining the evidence for random slopes is 13.7 with a nominal p-value of 0.001, despite the wellknown conservativeness of this test (Snijders and Bosker 1999).

Regression analysis of LT study. Statistical analyses of combined data for all LT study participants showed BDE-47 and PCB-153 levels declining, on average, 1% per month (Table 3), using random intercepts only. Betweenmother variation accounted for 95% and 91% of total variation for BDE-47 and PCB-153, respectively, over these longer periods.

With random slopes, BDE-47 exhibited varying depuration slopes across mothers, with an average slope again corresponding to a decline of 1% per month (p = 0.035), with 95% of mothers' slopes estimated to lie between -5% and 2% per month (only one of nine mothers had a positive slope). PCBs had less evidence for random slopes, with 95% of slopes estimated to lie between -4% and 0.8% per month, again averaging -1% per month.

Depuration rates earlier in lactation (0-4 weeks after birth). Using two groups of mothers from the CS CA study, we also assessed depuration rates for the 28-day period before initial breast milk samples were collected by ST mothers. One group had collected their initial samples early in lactation, 3-28 days after birth (DAB; n = 35), and the other group later, 29-56 DAB (n = 35). PBDE and PCB levels in the two groups were roughly similar (Table 1), as were levels in earlier versus later samples collected in either of the two 28-day periods (0-28 or 29-56 DAB). In plots of chemical levels compared with DAB, we found no evidence that samples collected earlier in either of the two 28-day periods (0-28 or 29-56 DAB) had higher PBDE or PCB levels. We examined and ruled out confounding by biases in collection date and region. Thus, we found no compelling evidence that depuration rates early in lactation (< 28 DAB) were higher than those we measured in the ST and LT studies later in lactation. This contrasts with a 50% decrease in polychlorinated dibenzo-pdioxin (PCDD) levels in one mother between 7 and 35 DAB (Furst et al. 1989) and a "main decrease in the first [6] weeks" in 15 mothers (Beck et al. 1994).

Discussion

Power of the depuration study: analytical precision and sample size. The low RSD and tight SD lines for the 8 QCP samples (Figures 2-3) and the low RSDs for the 16 and 21 duplicates indicate good precision in PBDE and PCB measurements, The point-to-point variations seen in depuration profiles at the different ST sampling points have biological origins, as the variations are much larger than those seen with the 8 QCP samples (Figure 2).

Sample size is limited in both ST and LT studies. However, the precision and regularity of the data, and the collection of serial samples from each ST mother, argue that the results give accurate estimates of monthly percentage declines. The repeated measures on all 18 mothers provide considerably more reliable estimates of per-person monthly declines than an equivalently sized cross-sectional design, especially because observed rates of decline are consistent both across mothers and a wide range of contaminant levels.

ST study (4-24 weeks after birth). PBDE congener levels generally decreased 2-3% per month, with BDE-47 decreasing 3% per month with homogeneous slopes. These decreases held true over a 50-fold range of initial [SIGMA]PBDE levels (21-1,330 ng/g lipid weight) in the mothers. Slopes for PCB-153 were more heterogeneous than for PBDE-47, and far exceeded the small variations seen in the plot of 8 QCP samples. The random slope estimate for PCB-153 averaged -0.6% per month (95% CI, -3% to 2%; p = 0.40 (Table 3). The wide 95% CIs reflect variable depuration slopes and variation in individual slopes. Similar variations in PCBs have been reported for serial samples from three separate mothers (Gonzalez et al. 1995; Skaare and Polder 1990; Yakushiji et al.1978) and in PBBs from a fourth (Brilliant et al. 1978).

The larger variations in PCBs compared with PBDEs seen in some ST participants remain unexplained, but they could be due to laboratory, chance, or biology. If biology, respective half-lives seem unlikely factors, because estimated half-lives in humans for PCBs (2-5 years) and the lower brominated PBDEs (1.6-6.5 years; Geyer et al. 2004) are similar. However, differences between PBDEs and PCBs in their sources, pathways, and patterns (continuous vs. intermittent) of human exposures may contribute. Major PCB exposures are dietary (from consumption of contaminated fish or animal fats), and these exposures can be intermittent. For PBDEs, exposures may be more continuous; a major contributor to human exposures may be inhalation of indoor dusts in homes, offices, or cars, for example (Jones-Otazo et al. 2005; Schecter et al. 2005; Sjodin et al. 2003b; Stapleton et al. 2005; Thuresson et al. 2006; Wilford et al. 2005).

Diet might explain the intraindividual variations seen in PCB depuration slopes seen here and in the serial breast milk samples from a mother in Norway (Skaare and Polder 1990) and in PBBs in a Michigan mother after the Firemaster accident (Brilliant et al. 1978). PCBs in milk could reflect fluctuations in dietary PCBs, for example, an occasional meal of PCB-laden fish. However, for this to occur, dietary fat (and PCBs) must preferentially enter breast milk either directly, bypassing body fat, or preferentially from body fat. They also must comprise a significant portion of milk fat/PCBs.

LT study (up to 136 weeks after birth). Results of the LT study were similar to those of the ST study, although BDE-47 slopes were less homogeneous. The smaller number of observations per mother in the LT series and the longer time periods may contribute to greater variation in slopes. Concentrations of BDE-47 and PCB-153 declined in 8 and 7 of the 9 LT participants, respectively, and averaged 1% per month. These decreases are greater than the variations seen in the QCP samples (Figure 3) and suggest biological origins.

Other depuration studies in breast milk. In a review of the limited published data on depuration of persistent chemicals via breast milk, LaKind et al. (2001) cited studies showing decreases for dioxins/furans ranging from 2-14% per month (Beck et al. 1994; Dahl et al. 1995; Furst et al. 1989, 1992; Hori 1993; Schecter et al. 1996), and 2-8% per month for PCBs (Hori 1993; Rogan et al. 1986; Schecter et al. 1998; Yakushiji et al. 1978). The decreases were generally higher than those we found in the present study for PBDEs (1-3%) and PCBs (0.6-1%) using serial samples from 18 primiparae. Preliminary data (Sjodin et al. 2005) suggest lower depuration rates for PBDEs, resembling those reported here.

Hand-expressed versus pumped samples. Mean RSDs for BDE-47 and PCB-153 for the HE/P samples were 3.4% and 9.3%, respectively (Table 2). PCBs were 5-10% lower in 11 of 16 pumped samples compared with hand-expressed samples, suggesting that PCBs may stick to the plastic pump. The reductions were small, however, and breast pumps seem to be acceptable collection methods for breast milk.

Design of breast milk studies. To assess the mother's body burden of persistent, lipophilic chemicals such as PBDEs and PCBs, current designs recommend handexpressing samples and collecting early (2-8 weeks) in lactation, which is challenging for first-time mothers with days-old infants. Our results may help simplify future breast milk studies in several ways. First, HE/P data show that breast pumps can be used for sample collection without much loss in accuracy for PBDEs and PCBs. Second, ST data show that PBDE and PCB levels change slowly over time, so that samples collected later in lactation (3-4 months) still give reasonable estimates of the mother's body burden during pregnancy and presumed fetal exposures. Later collections would be easier on mother and infant and would make recruitment easier. Third, our precision data indicate that a single breast milk sample, properly analyzed, can accurately assess PBDE and PCB levels.

Conclusions

Our time-series data indicate that body burdens of PBDEs and PCBs are lowered by lactation, but only slowly, averaging 1-3% per month. This was true for mothers with a 50-fold range of initial values (e.g., [SIGMA]PBDEs of 21-1,330 ng/g lipid weight). The similarities between mothers in the serial studies (ST and LT; n = 18) and the cross-sectional study (CS CA: n = 82) in demographic, physical, and lifestyle characteristics, as well as in levels and patterns of PBDEs and PCBs, suggest that depuration rates observed here may have broader applicability.

Our data indicate that 6 months of breastfeeding decreases PBDE levels in mothers by 12-18% and PCB 153 levels by approximately 4%. These rates of decrease are not higher earlier in lactation (< 28 DAB), but they may be lower later: declines in BDE-47 and PCB-153 averaged 1% per month over longer lactation periods. Consequently, 6-12 months of breastfeeding would not greatly reduce a mother's body burden of these chemicals. If 6 months of breast-feeding reduced the levels of PBDEs and PCBs in primiparae by only 4-18%, fetal and lactational exposures for a second child would not be markedly lower than those for the first child. Women, like men, seem to have no easy way to reduce levels of PBDE and PCB contaminants. For lactating women, "pump and dump" strategies do not much reduce body burdens. Finally, PBDEs are ubiquitous indoor contaminants (homes, offices, cars, etc.). Many of us spend > 90% of our time indoors. Effective primary prevention measures to reduce exposures to PBDEs are unlikely to come from changes in "lifestyle" but rather through decreasing the use of these chemicals in consumer products.

REFERENCES

Beck H, Dross A, Mathar W. 1994. PCDD and PCDF exposure and levels in humans in Germany. Environ Health Perspect 102(suppl 1):173-185.

Brilliant L, Wilcox K, Van Amburg G, Eyster J, Isbisper J, Bloomer A, et al. 1978. Breast-milk monitoring to measure Michigan's contamination with polybrominated biphenyls. Lancet 2(8091):643-646.

Dahl P, Lindstrom G, Wiberg K, Rappe C. 1995. Absorption of polychlorinated biphenyls, dibenzo-p-dioxins and dibenzofurans by breast-fed infants. Chemosphere 30(12):2297-2306.

Furst P, Furst C, Wilmers K. 1992. PCDDs and PCDFs in human milk--statistical evaluation of a 6-years survey. Chemosphere 25(7-10):1029-1038.

Furst P, Kruger C, Meemken HA, Groebel W. 1989. PCDD and PCDF levels in human milk--dependence on the period of lactation. Chemosphere 18(1-6):439-444.

Geyer HJ, Schramm KW, Darnerud PO, Aune M, Feicht EA, Fried KW, et al. 2004. Terminal elimination half-lives of the brominated flame retardants TBBPA, HBCD, and lower brominated PBDEs in humans. Organohalogen Compounds 66:3867-3872.

Gonzalez MJ, Ramos L, Hernandez LM. 1995. Levels of coplanar PCBs in human breast milk at different times of lactation. Bull Environ Contam Toxicol 54:349-356.

Hites RA. 2004. Polybrominated diphenyl ethers in the environment and in people: a meta-analysis of concentrations. Environ Sci Technol 38(4):945-956.

Hori S. 1993. Levels of PCDDs, PCDFs, co-PCBs and PCBs in human breast milk at different times of lactation. Organohalogen Compounds 13:65-67.

Jones-Otazo HA, Clarke JP, Diamond ML, Archbold JA, Ferguson G, Harner T et al. 2005. Is house dust the missing exposure pathway for PBDEs? An analysis of the urban fate and human exposure to PBDEs. Environ Sci Technol 39:5121-5130.

LaKind J, Berlin C, Naiman D. 2001. Infant exposure to chemicals in breast milk in the United States: what we need to learn from a breast milk monitoring program. Environ Health Perspect 109:75-88.

Noren K, Meironyte D. 2000. Certain organochlorine and organobromine contaminants in Swedish human milk in perspective of past 20-30 years. Chemosphere 40:1111-1123.

Rabe-Hesketh S, Skrondal A. 2005. Multilevel and Longitudinal Modeling Using Stata. College Station, TX:Stata Press.

Rogan W, Gladen B, McKinney J, Carreras N, Hardy P, Tully M, et al. 1986. Polychlorinated biphenyls (PCBs) and dichlorodiphenyl dichloroethene (DDE) in human milk: effects of maternal factors and previous lactation. Am J Public Health 76(2):172-177.

Schecter A, Papke O, Joseph JE, Tung KC. 2005. Polybrominated diphenyl ethers (PBDEs) in U.S. computers and domestic carpet vacuuming: possible sources of human exposure. J Toxicol Environ Health 68:501-513.

Schecter A, Papke O, Lis A, Ball M, Ryan JJ, Olson JR, et al. 1996. Decrease in milk and blood dioxin levels over two years in a mother nursing twins: estimates of decreased maternal and increased infant dioxin body burden from nursing. Chemosphere 32(3):543-549.

Schecter A, Ryan JJ, Papke O. 1998. Decrease in milk and blood dioxin levels over levels and body burden of dioxins, dibenzofurans, PCBs, DDE and HCB in blood and milk in a mother nursing twins over a thirty-eight month period. Chemosphere 37(9-12):1807-1816.

She J, Holden A, Sharp M, Tanner M, Williams-Derry C, Hooper K. 2007. Polybrominated diphenyl ethers (PBDEs) and polychlorinated biphenyls (PCBs) in breast milk from the Pacific Northwest. Chemospere 67(9):S307-S317.

Sjodin A, Jones RS, Lapeza C, Focant JF, Wang R, Turner WE, et al. 2003a. Retrospective time-trend study of brominated flame retardants and polychlorinated biphenyls in human serum from various regions of the United States, 1985-2002. Organohalogen Compounds 61:1-4.

Sjodin A, LaKind JS, Patterson DG, Needham LL, Wang R, Paul IM, et al. 2005. Current concentrations and changes in concentrations of PBDEs, persistent pesticides, and PCBs in human milk. Organohalogen Compounds 73:1745-1748.

Sjodin A, Patterson DG Jr, Bergman [ANGSTROM][ANGSTROM]. 2003b. A review on human exposure to brominated flame retardants--particularly polybrominated diphenyl ethers. Environ Int 29:829-839.

Skaare JU, Polder A. 1990. Polychlorinated biphenyls and organochlorine pesticides in milk of Norwegian women during lactation. Arch Environ Contam Toxicol 19:640-645.

Snijders TAB, Bosker RJ. 1999. Multilevel Analysis: An Introduction to Basic and Advanced Multilevel Modeling. London:Sage Publications.

Stapleton HM, Dodder NG, Offenberg JH, Schantz MM, Wise SA. 2005. Polybrominated diphenyl ethers in house dust and clothes dryer lint. Environ Sci Technol 39:925-931.

Thuresson K, Hoglund P, Hagmar L, Sjodin A, Bergman A, Jakobsson K. 2006. Apparent half-lives of heptato decabrominated diphenyl ethers in human serum as determined in occupationally exposed workers. Environ Health Perspect 114:176-181.

Wilford BH, Shoeib M, Harner T, Zhu T, Jones KC. 2005. Polybrominated diphenyl ethers in indoor dust in Ottawa, Canada: implications for sources and exposure. Environ Sci Technol 39:7027-7035.

Yakushiji T, Watanabe I, Kuwabara K, Yoshida S, Koyama K, Hara I, et al. 1978. Long-term studies of the excretion of polychlorinated biphenyls (PCBs) through the mother's milk of an occupationally exposed worker. Arch Environ Contam Toxicol 7(4):493-504.

Address correspondence to K. Hooper, Environmental Chemistry Laboratory, 700 Heinz St., Berkeley, CA 94710 USA. Telephone: [phone]. Fax: [phone]. E-mail: [e-mail] We thank the many first-time mothers of California who participated in these studies. This study received support from Pollution Prevention grant NP979442010 from U.S. Environmental Protection Agency Region IX. Views expressed are those of the authors and not necessarily of the sponsoring agencies. The authors declare they have no competing financial interests. Received 13 February 2007; accepted 28 June 2007.

Kim Hooper, (1) Jianwen She, (1) Margaret Sharp, (1) Joan Chow, (1) Nicholas Jewell, (2) Rosanne Gephart, (3) and Arthur Holden (1)

(1) Environmental Chemistry Laboratory, California Department of Toxic Substances Control, Berkeley, California, USA; (2) Division of Biostatistics, School of Public Health, University of California, Berkeley, California, USA; (3) Woman's Health and Birth Center, Santa Rosa, California, USA