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Hammel et al. J Environ Expo Assess 2024;3:8 https://dx.doi.org/10.20517/jeea.2023.51 Page 5 of 17

method detection limit (MDL), values were imputed as MDL/2 and normalized in a matrix-specific manner
(i.e., breast milk, serum, and placenta by lipid concentration and dust by dry mass).

We evaluated associations between breast milk and dust for NBFRs and between breast milk and dust,
placenta, fetal blood, and maternal blood for PBDEs using Spearman correlations. Relationships among
NBFRs and PBDEs, separately, were assessed within breast milk samples, also with Spearman correlations.
We examined additional relationships between log -transformed concentrations and various predicting
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factors using regression analyses. These factors included mother’s age, pre- and term-pregnancy body mass
index, and pregnancy weight gain (evaluated as continuous variables) and parity and previous breastfeeding
(evaluated as categories, dichotomized based on median reported value). For evaluating associations with
breast milk PBDE concentrations, maternal serum and house dust concentrations were split into tertile
categories. Beta coefficients from all of these models were exponentiated and used as an estimate of
multiplicative change in concentrations for each unit of increase for all continuous variables or relative to
the reference category for any categorical variables.

Exposure estimate calculations
Estimates of daily intake were calculated based on breast milk and house dust concentrations of NBFRs and
PBDEs with > 65% detection in a matrix. These estimates were calculated for a 3-month-old infant, given
that the house dust concentrations were collected in homes at 3 months post-delivery. Estimates are
presented in units of ng/day for comparison of exposure pathways and were not normalized to body mass.
The US Environmental Protection Agency Exposure Factors Handbook was used for recommended
estimated daily consumption of breast milk and for unintentionally ingested house dust for 3-month-old
infants. For human milk intake, the weighted mean intake value for exclusively breastfed infants was
[49]
758 mL/day, with breast milk density estimated to be 1.03 g/mL . Dust ingestion for children < 6 months
was estimated to be 20 mg/day . The intake equations are shown below:
[50]

Furthermore, intake estimates of PBDEs from air were calculated based on air measurements in the same
homes days prior to birth as previously presented in Vorkamp et al. . A long-term exposure value for
[41]
inhalation of 3.5 m /d was estimated for infants 1-3 months of age . The equation for intake by inhalation
[51]
3
was then:

RESULTS & DISCUSSION
Study population
A total of 51 women were recruited for the overall study. Of these, four women did not participate in the
follow-up, and another seven did not wish to or were not able to deliver a breast milk sample. This resulted
in a total of 40 breast milk samples and 47 dust samples collected 3 months after birth; details about the
overall study population have been described previously [15,41,42] . The mean age of mothers of the subset of
participants providing breast milk samples was 32.7 years (range: 24-43 years) [Supplementary Table 2]. The
average number of previous children was one (range: 0-3 children) and mothers had previously breastfed
for about a year prior to starting the sample collection (mean: 11 months; range: 0-30 months). Further
details on descriptors are given in Supplementary Table 2 .