Abstract
BACKGROUND:
Organophosphate esters (OPEs) have replaced flame retardant polybrominated diphenyl ethers as flame retardants in consumer products, but few longitudinal studies have characterized childhood OPE exposure.
OBJECTIVE:
We aimed to examine the exposure pattern of urinary OPE metabolites in children.
METHODS:
We quantified three urinary OPE metabolites five times in children (1, 2, 3, 5, 8 years) from 312 mother-child pairs in the Health Outcomes and Measures of the Environment (HOME) Study, a prospective pregnancy and birth cohort in Cincinnati, Ohio, USA. We examined the associations of average maternal OPE metabolite concentrations with OPE metabolite concentrations in childhood, characterized childhood OPE trajectories with latent class growth analysis (LCGA), and examined factors related to trajectory membership.
RESULTS:
Bis(2-chloroethyl) phosphate (BCEP) had the lowest median concentrations over time (0.66–0.97 mg/L) while the median concentrations of bis(1,3-dichloro-2-propyl) phosphate (BDCIPP) increased with age (1.44–3.80 mg/L). The median concentrations of diphenyl phosphate (DPHP) fluctuated between 1.96 and 2.69 mg/L. Intraclass correlation coefficients for urinary metabolites measured at five time points indicated high variability within individuals (0.13–0.24). Average maternal urinary BCEP and BDCIPP were associated with concentrations in early childhood. Maternal education, the birth year of the child, and having a carpet in the main activity room were associated with BCEP and BDCIPP trajectory while none of the factors were associated with DPHP trajectory.
SIGNIFICANCE:
The trajectory analysis showed different patterns of urinary OPE metabolite concentrations, suggesting the need to collect multiple samples to adequately reflect OPE exposure.
IMPACT STATEMENT:
In this well-established cohort, we evaluated the patterns of urinary OPE metabolites in children ages 1–8 years. The number of repeated measures over childhood has not been achieved in prior studies. Our results suggested the high variability of urinary OPE metabolites within individuals. Maternal metabolite concentrations during pregnancy were related to child concentrations at ages 1–3 years. BCEP, BDCIPP, and DPHP demonstrated different trajectories in children, which suggests that multiple samples may be required to capture OPE exposure patterns in childhood.
Keywords: Organophosphate ester, Urinary metabolites, Cohort study, Childhood
INTRODUCTION
After the phase-out of polybrominated diphenyl ethers (PBDEs) in the early 2000s in the United States, organophosphate esters (OPEs) were increasingly added to industrial and commercial products as flame retardants; non-halogenated OPEs are also used as plasticizers [1]. OPEs are a heterogeneous class of phosphoric acid esters differentiated by the moieties replacing the hydrogens in the phosphate group [2]. Due to widespread use, OPEs are frequently detected in water, air, indoor dust, and food, suggesting ubiquitous and chronic exposure in humans [3–6].
In toxicological studies, scientists found that prolonged, high OPE exposures were carcinogenic, neurotoxic, hepatotoxic, and endocrine disruptive, but less is known about the risks associated with human exposure [1, 7–9]. Humans are exposed to OPEs via inhalation, dust ingestion, dermal uptake, and dietary intake. The contribution of different exposure pathways varies by developmental stage and chemical properties [7]. For example, inhalation is the main exposure route for low molecular weight and relatively more volatile OPEs (such as tris(2-chloroethyl) phosphate [TCEP] and tris (1-chloro-2-propyl) phosphate [TCIPP]); for toddlers, dermal uptake of OPEs from indoor dust is the major exposure pathway for chlorinated OPEs while dietary intake was the principal pathway for alkyl OPEs [7, 10]. Children have higher exposures than adults because they have a lower breathing zone and more hand-to-mouth behaviors [7, 11].
After absorption, OPEs can be quickly transformed into hydrophilic metabolites and excreted via urine, so the diester metabolites in urine are suitable biomarkers for exposure assessment [12, 13]. For example, exposure to TCEP, tris (1,3-dichloro-2-propyl) phosphate (TDCIPP), and triphenyl phosphate (TPHP) can be measured as urinary concentrations of bis (2-chloroethyl) phosphate (BCEP), bis (1,3-dichloro-2-propyl) phosphate (BDCIPP), and diphenyl phosphate (DPHP), respectively [14–16].
Many epidemiological studies have examined the profiles of urinary OPE metabolites in pregnant women [17, 18], children [16, 19–27], and mother-child pairs [15, 21, 28, 29], but few have examined the longitudinal relationship between maternal OPE exposure during pregnancy and exposure in their children. Also, the within- and between-individual variability of OPE metabolites among children has not been fully characterized. Thus, we aimed to identify patterns of children’s exposures to OPEs using repeated measurements of urinary OPE metabolites at ages 1, 2, 3, 5, and 8 years. We also examined whether maternal OPE exposure during pregnancy and sociodemographic factors are associated with childhood exposures.
METHODS
Participants
The Health Outcomes and Measures of the Environment (HOME) Study is a prospective pregnancy and birth cohort in the greater Cincinnati, OH metropolitan area. Between March 2003 and January 2006, pregnant women were recruited if they met the following criteria: living in the study region, <19 weeks pregnant, ≥18 years old, residing in a home built in or before 1978, planning to continue prenatal care and deliver at the collaborating clinics and hospitals, planning to live in the greater Cincinnati area for the next year, and fluent in English. The exclusion criteria included: living in a mobile or trailer home, being HIV-positive, taking medications for seizures or thyroid disorders, being on radiation treatment or chemotherapy for cancer, or being diagnosed with diabetes, bipolar disorder, or schizophrenia. The cohort has been previously described [30]. Data analyzed in the present paper were collected during pregnancy, at delivery, and at ages 1, 2, 3, 5, and 8-year follow-up visits.
Of the 389 mothers who gave birth to singleton infants, the current analysis included 312 children without congenital malformations, who provided at least one urine sample between ages 1 and 8 years that was analyzed for OPE metabolites, and whose mothers also had at least one measurement of urinary OPE metabolites during pregnancy or at delivery. The study protocol was approved by the Institutional Review Board (IRB) at the Cincinnati Children’s Hospital Medical Center (CCHMC). The Centers for Disease Control and Prevention (CDC) deferred to the CCHMC IRB as the IRB of record. All mothers signed written informed consent for themselves and their children.
Quantification of urinary OPE metabolites
Urinary OPE metabolites have been used as an index of exposure to OPE in humans [31]. Maternal urine samples were collected in polypropylene specimen cups at an average of 16 weeks and 26 weeks of gestational age, and delivery. Children’s OPE exposure was assessed in urine samples collected at ages 1, 2, 3, 5, and 8 years. We collected urine samples using Kendall abdominal pads placed inside the diaper for nontoilet-trained children, a training potty lined with Kendall abdominal pads for children undergoing toilet training, or polypropylene specimen cups for toilet-trained children [32]. All the samples were refrigerated for up to 24 h, stored at −20 °C, and then shipped overnight on dry ice to the CLIA-certified analytical chemistry laboratory at CDC’s National Center for Environmental Health for analysis.
A previously published approach was applied to quantify three diester metabolites (BCEP, BDCIPP, DPHP) in urine [33, 34]. Detailed information on analytical and quality control methods used for the analysis of the samples can be found elsewhere [17]. Briefly, after enzymatic hydrolysis, urine samples (200 μL) were concentrated by automated off-line solid-phase extraction (SPE; 60 mg Strata XAW polymeric SPE packing with 1.5 mL liquid space, Phenomenex, Torrance, CA) and the analytes were eluted with 3 × 400 μL of 2% (v/v) NH4OH in methanol. The targeted metabolites were separated and detected via isotope dilution high-performance liquid chromatography-tandem mass spectrometry. The limit of detection (LOD) for each metabolite was 0.1 μg/L. The accuracy of this approach ranged from 98 to 108%, and intra- and inter-day imprecision was <10% [17]. To account for inter-individual differences in urine dilution, we standardized metabolite concentrations by specific gravity, which was measured using an Atago model PAL-10S handheld refractometer. We used the following formula for standardization [35]:
where OPE metabolite SGstd is the specific gravity standardized urinary OPE metabolite concentration, OPEi is the measured analyte concentration, SGi is the actual specific gravity of the sample, and SGm is the median specific gravity of the cohort at each time point.
Statistical analysis
Before specific-gravity standardization, we replaced the OPE metabolite concentrations below the LOD with the LOD/2 when the percentage of concentrations <LOD was below 10% [36]. When the percentage of concentrations <LOD was 10–30%, we replaced the non-detectable concentrations with single-imputed values generated by a fill-in approach using the left-truncated distribution [37]. We log10 transformed specific gravity standardized concentrations to approximate a normal distribution and reduce the influence of outliers.
We first calculated univariate statistics of urinary OPE metabolite concentrations at each of the five time points (1, 2, 3, 5, and 8 years). Then, we visualized age-specific distributions of BCEP, BDCIPP, and DPHP using boxplots. We also calculated intraclass correlation coefficients (ICCs) for urinary metabolite concentrations across the five-time points using both crude and specific gravity standardized values from linear mixed effect models. We followed ICC cut-offs described by Rosner to assess reproducibility: ≤ 0.4 (poor), 0.4–0.75 (fair to good), and ≥0.75 (excellent) [38]. Besides, pairwise Spearman correlation coefficients (rs) were calculated between urinary metabolites at each time point.
To examine the relationship between maternal urinary OPE metabolites and corresponding postnatal urinary metabolites across five-time points, we used linear mixed effect models with the main effect of the average maternal OPE metabolite concentrations during pregnancy and their interaction terms with the children’s concentrations at the postnatal time points. The interaction terms for maternal OPE metabolite concentrations and the time points of postnatal urinary metabolite measurement were considered significant if p < 0.1. Similarly, we also examined the associations of the average OPE metabolite concentrations in early childhood (1–3 years) with later childhood (5 and 8 years). We selected covariates a priori based on published literature: season of urine collection (winter, spring, summer, fall), child sex (female, male), birth year of the child (2003, 2004, 2005, 2006), maternal age at delivery (<25 years old, 25–34 years old, ≥35 years old), maternal race/ethnicity (non-Hispanic white, non-Hispanic black and others), marital status (married or living with a partner, not married or living alone), parity (0, 1, ≥2), maternal education (high school or less, some college/2-year degree, bachelor’s, graduate or professional), household income (<$40,000, $40,000-$79,999, ≥$80,000), and maternal serum cotinine level at 16 weeks of pregnancy [17, 39]; we further adjusted for visible cleanliness in the home (clean, some evidence of housecleaning, or no evidence of housecleaning) and floor type (hard floor, or carpet) at the 1-year visit when examining the associations between the OPE metabolite concentrations in early and later childhood.
With latent class growth analysis (LCGA), we empirically estimated trajectories of postnatal OPE metabolite concentrations from ages 1 to 8 years [40, 41]. The LCGA modeling framework allowed us to include all children with OPE metabolite concentrations measured at least once. For each OPE metabolite, we modeled specific gravity standardized OPE metabolite concentrations as a continuous function of child age at the time of sample collection and estimated the posterior probabilities of trajectory membership for each child. We tested models with varying numbers of trajectories (1–3) and trajectories of different orders (linear [1], quadratic [2], and cubic [3]). The final models were chosen based on the Bayesian Information Criterion (BIC), logged Bayes factor, and trajectory membership posterior probabilities. The threshold of the mean posterior probability for satisfactory trajectory assignment is 0.70 [40]. Then, we used logistic regression to identify characteristics related to a child’s trajectory membership, in which we treated the trajectory membership as the outcome. In bivariate models, we examined the following variables: child sex, birth year of the child, maternal age at delivery, maternal race/ethnicity, marital status, parity, maternal education, household income, residential cleanliness at the 1-year visit, and the presence of a carpet in the main activity room at the 1-year visit. Variables with p-value from bivariate associations <0.10 for any of the OPE trajectories were included in OPE-specific multivariable logistic models. All the analyses were performed using SAS 9.4 (SAS Institute Inc., Cary, North Carolina), including Proc Traj for trajectory analysis [42].
RESULTS
We measured urinary OPE metabolite concentrations in serial samples from gestation to age 8 years among 312 mother-child pairs. Over 60% were non-Hispanic white persons, with maternal age at delivery younger than 35 years, and with household income <$80,000 (Table 1). The analytical population (N = 312) was comparable to the original cohort (N = 389 singletons) for these characteristics (Supplementary Material Table S1).
Table 1.
Characteristics of maternal-child pairs with prenatal and postnatal OPE measurement in the HOME Study (2003–2006) (N = 312).
| N (%) | |
|---|---|
| All participants | 312 |
| Race/ethnicity | |
| Non-Hispanic White | 213 (68.3) |
| Non-Hispanic Black and others | 99 (31.7) |
| Marital status | |
| Married/living with partner | 261 (83.6) |
| Not married, living alone | 51 (16.4) |
| Child Sex | |
| Male | 144 (46.2) |
| Female | 168 (53.8) |
| Maternal Age at delivery, years | |
| <25 | 60 (19.2) |
| 25–34 | 202 (64.7) |
| ≥35 | 50 (16.1) |
| Maternal Education | |
| High school or less | 62 (19.9) |
| Some college/2 yr degree | 76 (24.4) |
| Bachelor’s | 103 (33.0) |
| Graduate or professional | 71 (22.7) |
| Household Income during pregnancy | |
| <$40,000 | 105 (33.6) |
| $40,000–$79,999 | 115 (36.9) |
| ≥$80,000 | 92 (29.5) |
| Parity | |
| 0 | 138 (44.2) |
| 1 | 100 (32.1) |
| 2+ | 74 (23.7) |
| Birth year of the child | |
| 2003 | 28 (9.0) |
| 2004 | 127 (40.7) |
| 2005 | 111 (35.6) |
| 2006 | 46 (14.7) |
Among the three metabolites, urinary BCEP concentrations and detection frequency were consistently lower than the other analytes across five-time points; DPHP had the highest concentrations before age 3 years, and BDCIPP had the highest concentrations at age 3, 5, and 8 years (Fig. 1; Supplementary Material Table S2). When compared between ages, median urinary BCEP concentrations tended to decrease while urinary BDCIPP concentrations tended to increase with age; DPHP concentrations were highest at age 2 years (2.69 μg/L) and then decreased with age (Supplementary Material Table S2). Both BDCIPP and DPHP concentrations were lower in early childhood, especially for BDCIPP; in contrast, BCEP concentrations at ages 5 and 8 were lower than those at ages 1–3 years (Fig. 1; Supplementary Material Table S3).
Fig. 1. Distributions of specific gravity standardized urinary OPE metabolite concentrations between ages 1 and 8 years (μg/L).
BCEP bis(2-chloroethyl) phosphate, BDCIPP bis(1,3-dichloro-2-propyl) phosphate, DPHP diphenyl phosphate.
The ICCs for urinary OPE metabolites showed poor reproducibility (Table 2). Specific gravity standardized ICCs were slightly lower than those calculated with non-standardized concentrations except for DPHP. At a given age, BDCIPP was moderately correlated with BCEP concentrations except for concentrations at age 8 years; BCEP and BDCIPP were weakly to moderately correlated with DPHP (Table 3).
Table 2.
Intra-class correlation coefficients (ICC) of postnatal urinary OPE metabolitesa.
| OPE metabolites | Crude ICC | Specific gravity standardized ICC |
|---|---|---|
| BCEP | 0.22 | 0.22 |
| BDCIPP | 0.24 | 0.23 |
| DPHP | 0.13 | 0.15 |
BCEP bis(2-chloroethyl) phosphate, BDCIPP bis(1,3-dichloro-2-propyl) phosphate, DPHP diphenyl phosphate.
Urinary concentrations were measured in samples collected at 1, 2, 3, 5, and 8 years.
Table 3.
Spearman correlation coefficients between specific gravity standardized urinary OPE metabolite concentrations assessed between ages 1 and 8 years.
| Age 1 | |||
|---|---|---|---|
| BCEP | BDCIPP | DPHP | |
| BCEP | 1 | 0.50* | 0.21* |
| BDCIPP | 1 | 0.41* | |
| DPHP | 1 | ||
| Age 2 | |||
| BCEP | BDCIPP | DPHP | |
| BCEP | 1 | 0.52* | 0.35* |
| BDCIPP | 1 | 0.29* | |
| DPHP | 1 | ||
| Age 3 | |||
| BCEP | BDCIPP | DPHP | |
| BCEP | 1 | 0.51* | 0.33* |
| BDCIPP | 1 | 0.42* | |
| DPHP | 1 | ||
| Age 5 | |||
| BCEP | BDCIPP | DPHP | |
| BCEP | 1 | 0.57* | 0.44v |
| BDCIPP | 1 | 0.50* | |
| DPHP | 1 | ||
| Age 8 | |||
| BCEP | BDCIPP | DPHP | |
| BCEP | 1 | 0.28* | 0.21* |
| BDCIPP | 1 | 0.31* | |
| DPHP | 1 | ||
Indicates p < 0.05
BCEP bis(2-chloroethyl) phosphate, BDCIPP bis(1,3-dichloro-2-propyl) phosphate, DPHP diphenyl phosphate.
Table 4 shows that each log10 increase in average maternal specific gravity standardized urinary BCEP concentration was associated with a 69% higher concentration at age 2 years (95% CI: 10–160%) and a 61% higher concentration at age 5 years (95% CI: 8–139%). A ten-fold increase in maternal BDCIPP concentration was also positively associated with 57% higher postnatal BDCIPP concentration at age 2 years (95% CI: 10–123%) and 52% higher concentration at age 3 years (95% CI: 3–125%) (Table 4). We did not observe statistically significant associations between maternal and child DPHP concentrations across any of the five-time points. Additionally, average BDCIPP concentrations between ages 1 and 3 years were positively associated with BDCIPP concentrations at age 5 years but with a wide 95% CI (Supplementary Material Table S4).
Table 4.
Adjustedapercent difference and 95% CIs in postnatal specific gravity standardized urinary OPE metabolite concentrations between ages 1 and 8 years per log 10 increase in averagebmaternal specific gravity standardized urinary OPE metabolite concentrations during pregnancy (μg/L).
| Average maternal specific gravity standardized urinary OPE metabolite concentrations | Ratio change percent (95% Cl) of corresponding postnatal OPE metabolite concentrations at age 1 year | Ratio change percent (95% Cl) of corresponding postnatal OPE metabolite concentrations at age 2 years | Ratio change percent (95% Cl) of corresponding postnatal OPE metabolite concentrations at age 3 years | Ratio change percent (95% Cl) of corresponding postnatal OPE metabolite concentrations at age 5 years | Ratio change percent (95% Cl) of corresponding postnatal OPE metabolite concentrations at age 8 years |
|---|---|---|---|---|---|
| BCEP | 40 (−6, 108) | 69 (10, 160)* | 42 (−4, 111) | 61 (8, 139)* | 26 (−23, 105) |
| BDCIPP | 46 (−3, 119) | 57 (10, 123)* | 52 (3, 125)* | 5 (−34, 67) | 25 (−12, 74) |
| DPHP | 22 (−15, 76) | 1 (−27, 41) | 26 (−14, 84) | 14 (−30, 87) | 17 (−25, 83) |
Adjusted for maternal age at delivery, race, household income, education, marital status, child sex, parity, sample collection season, birth year of the child, maternal serum cotinine at 26 weeks of gestation; ratio change is calculated as (10regression coefficient − 1) * 100%.
The average maternal specific gravity standardized urinary OPE metabolite concentrations during pregnancy were calculated as (concentrations at16w + concentrations at26w + concentrations at delivery)/3
Indicates p < 0.05
BCEP bis(2-chloroethyl) phosphate, BDCIPP bis(1,3-dichloro-2-propyl) phosphate, DPHP diphenyl phosphate.
We found that two trajectory models were optimal for BCEP, BDCIPP, and DPHP (Supplementary Material Table S5; Fig. 2). For BCEP, one trajectory was characterized by low concentrations at each time point (“persistently low”, N = 190, 57.6%). The other trajectory of BCEP was defined by high concentrations during early childhood but lower concentrations after 3 years of age (“early peak”, N = 122, 42.4%). One trajectory of DPHP also showed persistently low concentrations (N = 261, 76.3%) while the other trajectory remained at high concentrations across five-time points despite a slight decrease in later childhood (“persistently high”, N = 51, 23.7%). Similarly, 184 participants (58.5%) had high concentrations of BDCIPP at all time points (“persistently high”). The remaining trajectory of BDCIPP was characterized by an increasing trend between ages 1 and 8 years (“continuously increasing”, N = 128, 41.5%). The mean posterior probability of trajectory membership ranged from 0.77 to 0.85, suggesting a high likelihood of fitness between a child’s actual exposure pattern and trajectory assignment (Supplementary Material Table S6).
Fig. 2. Trajectories of specific gravity standardized urinary OPE metabolite concentrations (μg/L) between ages 1 and 8 years (n = 312) were estimated using latent class growth analysis.
Bands represent 95% CI. The “persistently low” trajectory serves as the reference group for BCEP and DPHP; the “continuously increasing” group serves as the reference group for BDCIPP. BCEP bis(2-chloroethyl) phosphate, BDCIPP bis(1,3-dichloro-2-propyl) phosphate, DPHP diphenyl phosphate.
We included the following variables in the multivariable logistic regression models examining factors associated with OPE trajectory membership: maternal age at delivery, maternal race, marital status, infant sex, maternal education, household income, parity group, birth year of the child, and having a carpet in the main activity room. We found that children born in later years (2005–2006) were more likely to be assigned to the “persistently high” group (vs. “continuously increasing”) for BDCIPP (OR = 2.02, 95% CI: 1.24, 3.30; Fig. 3). In addition, mothers with higher education and who reported having a carpet in the main activity room had lower odds of assignment to the “early peak” group (vs. “persistently low”) for BCEP or “persistently high” group (vs. “continuously increasing”) for BDCIPP (Fig. 3). None of the factors were associated with DPHP trajectory membership.
Fig. 3. Odds ratios (95% CIs) from logistic regression models of factors associated with urinary OPE metabolite trajectories between ages 1 and 8 years.
The “Persistently low” trajectory serves as the reference group for BCEP and DPHP; the “Continuously increasing” group serves as the reference group for BDCIPP. BCEP bis(2-chloroethyl) phosphate, BDCIPP bis(1,3-dichloro-2-propyl) phosphate, DPHP diphenyl phosphate.
DISCUSSION
This study examined the patterns of three urinary OPE metabolites in children ages 1–8 years from a well-established birth cohort. We previously quantified maternal urinary OPE metabolites at three time points during pregnancy in the same cohort. Maternal DPHP had the highest geometric mean urinary concentrations at all time points, followed by BDCIPP and BCEP [17]. Similar to maternal BCEP, postnatal BCEP tended to be the lowest among the three metabolites at all time points. But DPHP was the highest only at ages 1 and 2 years, which might be due to the relatively higher maternal transfer of the parent compounds [43]. Future studies are needed to estimate the parent compounds’ clearance and half-lives during pregnancy or childhood, which may be helpful to illustrate this point. BDCIPP showed an apparent increasing trend over time, which might be related to the increased use of infant products [23].
The geometric mean concentrations of DPHP (2.10–2.64 μg/L) and BDCIPP (1.49–3.81 μg/L) between ages 1 and 8 years were comparable with concentrations reported in another study with urine samples collected between 2009 and 2013 among school-aged children in Baltimore City [24], but were lower than in other studies that measured DPHP and BDCIPP in infants (age 2–18 months, North Carolina, 2014–2015) [23] or child urine samples (age 1–5 years, New Jersey, 2013–2014) [44] (Supplementary Materials Table S7). These differences may be related to study design, year of sample collection, geographical location, analytical method, and the variation in child behaviors related to exposure, among other reasons.
Consistent with our results, DPHP and BDCIPP are usually the OPE metabolites most frequently detected in urine samples. In Australian children, DPHP concentrations from pooled urine samples of 0–5 years old were one order of magnitude higher than our findings [27, 45]; concentrations of BDCIPP were comparable (mean of 2.6 μg/L) and the concentrations of BCEP were much lower (detection frequency: 15%, maximum of 0.036 μg/L) [45]. In a Norwegian mother-child cohort with a sampling campaign in 2012, the medians of DPHP and BDCIPP in children (age 6–12 years old) were reported to be 1.0 and 0.23 μg/L, respectively, both lower than the concentrations in our cohort [15]. A more recent study (sampling period: 2018–2019) in China reported that the medians of DPHP and BDCIPP in child urine samples were 6.55 and 0.17 μg/L, respectively [46]. A study in Japan did not identify DPHP or BDCIPP as the most frequently detected metabolites in children [19] (Supplementary Materials Table S7). The results may reflect different exposure patterns of OPEs in children globally.
We observed weak to moderate correlations among postnatal OPE metabolite concentrations at a given time point, similar to the correlations among maternal OPE metabolite concentrations during pregnancy as previously reported; the ICCs of postnatal OPE metabolite concentrations were between 0.13 and 0.22, indicating high variability within individuals [17]. Currently, most studies examining inter- and intra-individual variability of OPE urinary biomarkers focused on pregnant women or adults [4, 17, 18, 47, 48]. Only one study characterized within-week and seasonal intra-individual variability of nine urinary OPE metabolites among school-age children and reported moderate within-week correlations (rs: 0.31–0.63) and weak to moderate seasonal reliability (ICC: 0.18–0.38) [24].
We reported previously that concentrations of parent compounds in dust samples collected around 20 weeks of pregnancy were associated with maternal BCEP, BDCIPP, and DPHP during pregnancy albeit not at all time points in the HOME Study cohort [17]. Although published studies have reported positive correlations among mother and child pairs for BDCIPP and DPHP [28, 44, 46], no studies have examined whether maternal urinary OPE metabolite concentrations during pregnancy are associated with postnatal metabolite concentrations. This study identified associations between maternal OPE metabolite concentrations during pregnancy and the corresponding metabolite concentrations in early childhood. Considering previous studies on the adverse associations between OPE exposure and child health, it is important to reduce OPE exposure both during gestation and after birth.
Early life trajectories of PBDEs have been reported [40]. However, no studies have characterized OPE metabolite trajectories in children. We identified different trajectory patterns of three OPE metabolites, which might be explained by differential exposure sources and toxicokinetics. Factors including birth year of the child, maternal education, and floor surface type in the main activity room were associated with the trajectory assignment of BDCIPP or BCEP. Specifically, children born in 2003–2004 were less likely to be assigned to the “persistently high” group. The observed trajectories support that a single spot urine sample may not accurately reflect OPE exposure during childhood, so multiple samples are needed to assess exposure more accurately and avoid misclassification bias that may bias the epidemiologic associations with health outcomes towards null.
This study had some limitations. First, despite being the most frequently detected biomarkers in urine, only using urinary OPE diesters as surrogates may underestimate exposure [49]. Previous in vitro studies showed that hydroxylated metabolites of some OPEs rather than their diesters were the major metabolites [50, 51]. Nevertheless, for the three OPEs examined, the detection frequency of diesters ranged from 77 to 100%, indicating the diesters as sensitive exposure biomarkers. Also, we only collected spot urine samples without considering diurnal variability. Since exposures are episodic in nature for OPEs and other non-persistent chemicals, collecting multiple samples helps better define long-term exposure. Future OPE exposure assessments would benefit from examining both hydroxylated and diester OPE metabolites as well as from the collection of multiple urine samples. We did not assess ingestion of dust, inhalation, dermal contact, and breastfeeding as potential routes of exposure. Additionally, we did not collect information on outdoor activities and diet, both of which may be closely related to OPE exposures. In the trajectory analysis, individuals were assigned to the trajectory to which their posterior membership probability was the largest, however, their posterior membership probabilities to the other trajectories were not zero. This ‘uncertainty’ in membership assignment could lead to bias in the subsequent analysis examining the association between trajectory membership and demographic and other participant characteristics. However, we would expect this bias to be small, as the mean posterior probability of trajectory assignment in our trajectory analysis was consistently higher than the widely accepted threshold for all trajectories.
Despite the above limitations, the serial urine samples collected during childhood with a moderate sample size enabled us to characterize the exposure pattern of OPEs among children. The application of LCGA modeling allowed us to identify underlying distinct trajectories of the OPE metabolite concentrations. We had maternal urinary OPE concentrations to examine the association between prenatal exposure and childhood exposure. We also prospectively collected a rich covariate dataset, including sociodemographic characteristics, information on house cleanliness, and floor type.
To summarize, we measured urinary OPE metabolite concentrations three times during pregnancy and five times in children between ages 1 and 8 years in a well-established pregnancy and birth cohort. The urinary OPE metabolites were weakly to moderately correlated with each other at each time point. We also observed high variability within individuals. Maternal OPE metabolite concentrations were associated with concentrations in early childhood. We further characterized trajectories for the metabolites and identified that the birth year of the child, maternal education, and the presence of a carpet in the main activity room were associated with exposure trajectories.
Supplementary Material
ACKNOWLEDGEMENTS
We gratefully acknowledged all the funding agencies and the contributions from all the study participants and staff. Joseph M. Braun was financially compensated for his services as an expert witness for plaintiffs in litigation related to PFAS-contaminated drinking water.
FUNDING
This work was supported by grants from the National Institute of Environmental Health Sciences and the US Environmental Protection Agency (NIEHS P01 ES11261, R01 ES014575, R01 ES020349, R01 ES027224, R01 ES028277, R21 ES034187, P30 ES006096; EPA P01 R829389).
Footnotes
DISCLAIMER
The findings and conclusions in this report are those of the authors and do not necessarily represent the official position of the Centers for Disease Control and Prevention. Use of trade names is for identification only and does not imply endorsement by the CDC, the Public Health Service, or the US Department of Health and Human Services.
COMPETING INTERESTS
JMB was financially compensated for his services as an expert witness for plaintiffs in litigation related to PFAS-contaminated drinking water.
ETHICAL APPROVAL
The study protocol was approved by the Institutional Review Board (IRB) at Cincinnati Children’s Hospital Medical Center (CCHMC). The Centers for Disease Control and Prevention (CDC) deferred to the CCHMC IRB as the IRB of record.
Supplementary information The online version contains supplementary material available at https://doi.org/10.1038/s41370-023-00605-2.
DATA AVAILABILITY
Data requests will be approved by the HOME Study Data Sharing Committee for research purposes.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
Data Availability Statement
Data requests will be approved by the HOME Study Data Sharing Committee for research purposes.