In Utero and Childhood Polybrominated Diphenyl Ether (PBDE) Exposures and Neurodevelopment in the CHAMACOS Study

1Polybrominated diphenyl ether (PBDEs) flame retardant chemicals, used in the manufacture of furniture, infant products, and electronics, are ubiquitous in U.S. households. An unintended consequence of California’s Technical Bulletin 117 (TB 117)—a fire safety law promulgated in the 1970s which requires that furniture, baby, and other household products resist open flame—is that PBDE concentrations in California children are now among the highest measured worldwide. Until 2005, the predominant chemical flame retardant used to comply with TB 117 was pentaBDE (comprising congeners BDEs 47, 99, 100, and 153). Although pentaBDE was banned in California and phased out by the manufacturer in 2004, pentaBDEs continue to leach from older household items. Exposure is also perpetuated by decaBDEs, still used in many electronic products, which can break down into lower-brominated congeners. Because PBDEs are semivolatile and not chemically bound to substrates, they migrate into house dust, placing young children, who crawl on the floor and exhibit frequent hand-to-mouth behaviors, at risk of higher exposures.

2PBDEs are endocrine-disrupting compounds with half-lives in humans ranging from 2 to 12 years. Recent research suggests that PBDE exposures are associated with altered thyroid hormone levels in pregnant women and infants, and negatively associated with birth weight. Research also suggests possible neurotoxic effects of in utero and early childhood exposure to PBDEs. reported significant decrements in motor and mental development at ages 1–6 years associated with in utero PBDE exposures in New York children (n = 100). In a study of 62 5- to 6-year-old Dutch children, reported that in utero exposure levels were negatively associated with fine motor coordination and sustained attention, although improved coordination and visual perception and fewer internalizing and externalizing behaviors. Recently, reported that 4-year-old Spanish children with detectable blood concentrations of BDE-47 were significantly more likely to demonstrate attention symptoms [DSM-IV (Diagnostic and Statistical Manual of Mental Disorders, 4th ed.) scores > 80th percentile] than less-exposed peers, but not motor or cognitive deficits. Cord blood BDE-47 concentrations were not associated with any neurobehavioral parameters at 4 years of age. found a positive association between breast milk levels of BDEs 47, 99, and 100 and externalizing behaviors, specifically activity/impulsivity behaviors in 220 30-month-olds.

3In this analysis, we examined the relationship of prenatal maternal and child PBDE concentrations with attention, cognition, and motor development in California children at 5 and 7 years of age.

4The Center for the Health Assessment of Mothers and Children of Salinas (CHAMACOS) is a longitudinal birth cohort study of predominantly Mexican-American families in California’s Salinas Valley. Detailed methods for CHAMACOS are published elsewhere. Eligible pregnant women (≥ 18 years old, < 20 weeks gestation, Spanish- or English-speaking, qualifying for low-income health insurance, and planning to deliver at the public hospital) were recruited between October 1999 and October 2000 from community clinics. The cohort included 601 women, 526 of whom delivered live-born singletons.

5Women were interviewed twice during pregnancy (at ~ 13 and 26 weeks gestation), after delivery, and when children were 6 months old, and 1, 2, 3.5, 5, and 7 years old. Mothers completed the Peabody Picture Vocabulary Test (PPVT) or Test de Vocabulario en Imágenes Peabody (TVIP) of verbal intelligence at the 6-month visit and the Center for Epidemiologic Studies Depression Scale (CES-D) at the 1-year visit. Age-appropriate versions of the HOME (Home Observation for Measurement of the Environment) survey were completed at most postdelivery visits. Birth weight and gestational duration were abstracted from medical records.

6Neurobehavioral assessments were performed by bilingual psychometricians, and children were assessed in their dominant language. A total of 310 children were assessed at 5 years (mean = 60.0 ± 2.6 months) and 323 at 7 years (85.2 ± 2.9 months). The present analysis excludes four children with autism, Down syndrome, cerebral palsy/hydrocephalus, or deafness and 63 children who lacked PBDE measurements.

7Compared with children in the cohort who were not followed, children included in the present analyses were more likely to be female and born full term, with mothers who were older, breastfed longer, and were less likely to smoke or drink during pregnancy (data not shown). They did not differ according to other sociodemographic characteristics or by their maternal prenatal PBDE levels [median = 24.9 ng/g lipid; interquartile range (IQR) 14.0–42.1] for those followed versus 23.8 ng/g lipid (IQR = 14.9–41.3) for those not followed].

8Mothers provided written informed consent at both visits, and children provided verbal assent at 7 years of age. Study activities were approved by the University of California at Berkeley (UC) Committee for the Protection of Human Subjects. A technical assistance agreement was established between the Division of Laboratory Sciences at the National Center for Environmental Health, Centers for Disease Control and Prevention (CDC), and UC Berkeley.

9Attention. At the 5-year visit, mothers completed the Child Behavior Checklist (CBCL)/1.5–5 (CBCL). We analyzed two subscales as continuous raw scores: the Attention Problems scale and the DSM-IV–oriented Attention Deficit/Hyperactivity Disorder (ADHD) Problems scale. We also analyzed a “borderline clinical range” (≥ 93rd percentile in the standardization sample) indicator variable for each scale. In addition, children were assessed on the Conners’ Kiddie Continuous Performance Test (K-CPT), a 7-min computerized vigilance task that assesses reaction time, accuracy, and impulse control. We determined continuous T-scores (standardized to a nonclinical population) for errors of commission, errors of omission, and hit reaction time. We also examined the continuous ADHD Confidence Index score, which indicates the probability that children are correctly classified as having clinical ADHD, and a binary variable indicating a Confidence Index score ≥ 70th percentile.

10At child’s age 7 years, mothers and teachers completed the Conners’ ADHD/DSM-IV Scales (CADS) and the Behavior Assessment System for Children, 2nd edition (BASC). CADS data from four subscales (Conners ADHD index score, and DSM-IV–based Inattentive, Hyperactive/Impulsive, and Total ADHD scores) were analyzed both as continuous, standardized scores (T-scores; mean ± SD = 50 ± 10) and as a binary variable indicating scores in the “Moderately” or “Markedly Atypical” range (T-score ≥ 65). BASC data from Hyperactivity and Attention Problems subscales were analyzed as standardized T-scores and as a binary “at-risk” or “clinically significant” variable (T-score ≥ 60).

11Motor function. At ages 5 and 7 years, children’s gross motor skills were assessed using select subscales of the McCarthy Scales of Children’s Abilities. Their fine motor dexterity was assessed with a pegboard test (Wide Range Assessment of Visual Motor Ability; WRAVMA) (age-standardized mean = 100 ± 15) and with a finger-tapping task [at 5 years: Behavioral Assessment and Research System (BARS); and at 7 years: Reitan Neuropsychology Laboratory (Tucson, AZ)]. We standardized McCarthy gross motor and finger tap scores within our study population (z-scores, mean = 0 ± 1).

12Cognitive functioning. At 5 years of age, children completed tests of receptive verbal intelligence in both English and Spanish using the PPVT and TVIP, respectively. We analyzed children’s continuous standardized scores (mean = 100 ± 15) in their language of best performance. We assessed children’s performance intelligence (PIQ) with the Wechsler Preschool and Primary Scale of Intelligence, 3rd edition (WPPSI-III) (mean = 100 ± 15).

13At age 7 years, children were assessed on four subdomains of the Wechsler Intelligence Scale for Children–Fourth Edition (WISC-IV): Verbal Comprehension, Perceptual Reasoning, Working Memory, and Processing Speed. A Full-Scale IQ was also calculated (mean = 100 ± 15 for the Full-Scale IQ and all components).

14Other questions. Mothers were also asked “Has a doctor, nurse, psychologist or teacher ever told you that your child might have 1) attention problems? or 2) learning problems?” Teachers were asked “Do you have any specific concerns about this student (in terms of) 1) emotional problems, 2) behavioral problems, or 3) learning problems?”

15PBDE exposure assessment. Blood samples were collected by venipuncture from mothers during pregnancy (mean = 26.7 ± 2.6 weeks gestation, n = 219) or at delivery (n = 60), and from children at the 7-year visit (n = 272). PBDE serum levels in women with data at both time points were very strongly correlated (Pearson r ≥ 0.98, p < 0.001)]. Samples were immediately processed and stored at –80oC until shipment on dry ice to the CDC (Atlanta, GA). Samples were analyzed at CDC for 10 congeners (BDEs 17, 28, 47, 66, 85, 99, 100, 153, 154, and 183) using gas chromatography isotope dilution high-resolution mass spectrometry. PBDE concentrations are expressed on a serum lipid basis (nanograms per gram lipids). Total serum lipid concentrations were determined based on the measurement of triglycerides and total cholesterol using standard enzymatic methods (Roche Chemicals, Indianapolis, IN). The limits of detection (LODs) for BDE-47 ranged from 0.3 to 2.6 ng/g lipids for maternal samples, and 0.4 to 0.8 ng/g lipids for child samples. For all other congeners, LODs ranged between 0.2 and 0.7 ng/g lipids for maternal and 0.3 and 5.6 ng/g lipids for child samples, respectively. Quality control samples (blanks and spikes) were included in each run.

16We used the sum of BDEs 47, 99, 100, and 153 congeners as our primary exposure measure. Values < LOD were assigned the machine-read value if a signal was detected. If not, all concentration levels < LOD were imputed at random based on a log-normal probability distribution using maximum likelihood estimation.

17We assessed maternal exposure to organophosphate (OP) insecticides as measured by dialkyl phosphate (DAP) metabolites in maternal urine (at 13 and 26 weeks gestation), using an isotope dilution gas chromatography-tandem mass spectrometry method; lead in maternal prenatal and cord blood samples, using graphite furnace atomic absorption spectrophotometry; polychlorinated biphenyls (PCBs) in maternal serum using high-resolution gas chromatography/high-resolution mass spectrometry with isotope dilution quantification; and maternal thyroid stimulating hormone (TSH; using immunochemiluminometric assay) and free thyroxine (T4; using direct equilibrium dialysis followed by radioimmunoassay) (Bayer ADVIA Centaur system; Siemens Healthcare Diagnostics, Deerfield, IL) at 26 weeks gestation.

18Data analysis. PBDE levels were expressed on the log10 scale. To determine the shape of the dose–response function, we ran generalized additive models using cubic splines. If nonlinearity was detected (p < 0.10), additional models were run with categorized PBDE concentrations (quartiles). We re-ran all final models with PBDE concentrations expressed on a serum basis (picograms per gram serum) with total serum lipids as a covariate. We also ran models with the sum of all 10 PBDE congeners; individually for each of the four primary congeners (47, 99, 100, and 153); and excluding outliers (defined as being ≥ 3.5 SD away from the mean for log10 PBDEs or the outcome).

19Variables were identified as potential confounders based on their relationship to neurodevelopment. We examined the following [see Supplemental Material, Table S3 for categories (http://dx.doi.org/10.1289/ehp.1205597)]: maternal age, education, years in the United States, marital status, work outside the home, use of alcohol and tobacco during pregnancy, depression (CES-D), parity, and PPVT or TVIP score; housing density, household poverty, pregnancy exposure to environmental tobacco smoke, number of children in the home, father’s presence in the home, and HOME score at 6 months and 7 years; preschool and out-of-home child care attendance; psychometrician, location, and language of assessment; and child sex, birth weight, preterm delivery status, and handedness (motor outcomes only). Missing values (< 10%) for covariates were imputed by randomly selecting a value from the dataset.

20We built separate models for attention, cognition, and motor outcomes, and used the same model for all outcomes within a category. In addition to child’s sex and months of age (continuous), final models included all covariates that changed the coefficient for the main exposure and any outcome within the group by > 10%. The covariates maintained in the models are listed in the footnote of the respective tables.

21For sensitivity analyses, we adjusted for birth weight, gestational age at birth, maternal thyroid hormone (TSH and free T4), DAPs, lead, and PCBs in separate models. We evaluated effect modification by child sex. In addition, we included maternal and child PBDE levels in the same models, although doing so reduced the sample size (n = 214).

22Main effects were considered statistically significant with p < 0.05 based on two-tailed tests, and interactions were considered significant if p < 0.10. All analyses were conducted with STATA version 10.1 (StataCorp, College Station, TX).

23For both mothers and children, BDE congeners 47, 99, 100, and 153 had detection frequencies > 97% and dominated the total measure of concentration, with BDE-47 in the highest concentration [for maternal and child measures, see Supplemental Material, Tables S1 and S2, respectively (http://dx.doi.org/10.1289/ehp.1205597)]. Children’s PBDE levels were more than three times higher than the mothers’ for the sum of four congeners, and detection frequencies for most other congeners were also substantially higher in children. The correlation between maternal and child ΣPBDE levels was 0.27 (p < 0.001); the correlation for individual congeners ranged from 0.21 for BDE-99 to 0.30 for BDE-153. Supplemental Material, Table S3, presents the distribution of demographic characteristics for children in the study sample and the geometric means (GM) of maternal and child ΣPBDE concentrations by covariates. Supplemental Material, Table S4, summarizes neurobehavioral scores for the study population.

24Correlations between reports by teachers and parents concerning attention at 7 years of age, and between measures of attention, cognition, and motor skills, were moderate. For example, correlations between maternal and teacher report on the CADS ranged from r = 0.2–0.3 (p < 0.01). Similar measures of attention on the BASC and CADS within a reporter (mother/teacher) were more strongly correlated—for example, r = 0.5 to 0.8, p < 0.001 for maternal report and r = 0.7 to 0.8, p < 0.001 for teacher report. Maternal and teacher CADS scores were negatively correlated with WISC Full-Scale IQ scores (r = –0.2 to –0.3, p < 0.001). Motor skills outcomes tended to be positively correlated with IQ scores (r = 0.1 to 0.4, several p < 0.001) and negatively correlated with attention outcomes (r = –0.05 to –0.2, several p < 0.01) (data not shown).

25Attention. At child age 5 years, maternal prenatal ΣPBDE concentrations (for the 4 main congeners) were marginally associated (p < 0.10) with maternally reported CBCL scores above the 93rd percentile for attention problems [adjusted odds ratio (aOR) for a 10-fold increase in ΣPBDE = 4.6; 95%CI: 0.9, 24.5] [see Supplemental Material, Table S5 (http://dx.doi.org/10.1289/ehp.1205597)], and strongly associated with both errors of omission scores and ADHD Confidence Index scores on the K-CPT (Table 1). Quartile categorization suggested that both errors of omission and the ADHD Confidence Index were primarily elevated in children with mothers in the highest quartile of ΣPBDE exposure (> 42 ng/g) (Figure 1).

26At child age 7 years, maternal ΣPBDE exposure was associated with maternally reported ADHD Index scores on the CADS (β = 2.9; 95% CI: 0.7, 5.2), DSM-IV Total scores (β = 2.6; 95% CI: 0.2, 5.0), and DSM-IV Inattention scale scores (β = 2.2; 95% CI: 0.0, 4.5) (Table 1). Although there was evidence of nonlinearity for the DSM measures, quartile categorization showed no clear trends (Figure 1). Maternal exposure was also related to somewhat higher odds of a mother having been told that her child had attention problems (aOR = 2.3; 95% CI: 0.9, 5.8), and to teacher reports of child behavior problems (aOR = 2.5; 95% CI: 1.1, 6.0) [see Supplemental Material, Table S5 (http://dx.doi.org/10.1289/ehp.1205597)]. However, there were no associations between maternal ΣPBDE and teacher ratings on the CADS or BASC, or maternal ratings on the BASC, for continuous or dichotomous outcomes.

27By contrast, child PBDE concentrations were associated with reports of attention problems from teachers, but not from mothers. Specifically, child ΣPBDEs were associated with more adverse teacher reports on CADS ADHD Index, CADS DSM-IV Total, CADS DSM-IV Inattentive, BASC Hyperactivity, and BASC Attention Problems scales [Table 1; see also Supplemental Material, Table S5 (http://dx.doi.org/10.1289/ehp.1205597)]. Associations were particularly pronounced for some of the dichotomous outcomes: Every 10-fold increase in child ΣPBDE level was associated with 4.5 and 5.5 times higher odds of the child being rated by the teacher as being in the “moderately or markedly atypical” range on CADS DSM-IV Hyperactive/Impulsive subscale (95% CI: 1.2, 16.6) and DSM-IV Total subscale (95% CI: 1.5, 20.3), respectively (see Supplemental Material, Table S5).

28Motor function. We observed little evidence of association between either maternal or child ΣPBDE serum concentrations and gross motor performance on McCarthy scales (Table 2). However, maternal ΣPBDEs were related to poorer performance on the WRAVMA pegboard at both 5 and 7 years, particularly for the nondominant hand. For the 5-year-olds, this relationship was observed primarily for the nondominant hand among boys (boys: β = –12.1; 95% CI: –19.4, –4.7; girls: β = 0.8; 95% CI: –6.8, 8.5; pinteraction = 0.09), whereas at age 7, it was seen mainly in the dominant hand in girls (boys: β = –2.7; 95% CI: –10.8, 5.4; girls β = –8.1; 95% CI: –16.3, 0.1; pinteraction = 0.08). Associations between maternal ΣPBDEs and pegboard performance at 7 years showed evidence of nonlinearity, with nonsignificantly poorer performance in children of mothers in the highest quartile of exposure (Figure 1). At 5 but not 7 years of age, maternal ΣPBDEs were also inversely associated with dominant-hand finger taps (Table 2).

29Child ΣPBDEs were marginally related to nondominant hand pegboard performance at age 7 years, but not with other motor outcomes.

30Cognitive functioning. We observed no associations between maternal ΣPBDE concentrations and child PPVT/TVIP or WPPSI Performance IQ scores at age 5 years (Table 3). However, at age 7 years, maternal ΣPBDEs were associated with significant decrements in WISC Verbal Comprehension IQ, contributing to a somewhat lowered Full-Scale IQ. Quartile analysis indicated that the association was primarily driven by a Verbal Comprehension IQ decrement in the highest quartile (β = –6.0; 95% CI: –11.3, –0.7; see Figure 1).

31Children’s ΣPBDE concentrations were also related to Full-Scale IQ at age 7 years (β = –5.6; 95% CI: –10.8, –0.3), particularly with the Perceptual Reasoning IQ, Processing Speed IQ, and Verbal Comprehension IQ subscales (Table 3).

32Sensitivity analyses. The above relationships were not confounded by maternal lead, PCB, or OP pesticide exposures, or substantially altered when controlled (in separate models) for birth weight, gestational age, or prenatal thyroid hormones. Overall, associations with individual PBDE congeners or the sum of all 10 congeners [see Supplemental Maternal, Table S6 (http://dx.doi.org/10.1289/ehp.1205597)] were generally consistent with results for the sum of the four major congeners. Depending on the outcome, there were between 0 and 4 outliers with respect to either ΣPBDE concentrations or outcomes; excluding them did not substantively affect the results (data not shown). Except where noted, we did not find evidence of effect modification by child sex.

33When both maternal and child ΣPBDE levels were entered into the same model (n = 214), associations were attenuated (data not shown) but child ΣPBDE levels were still associated with a borderline increase in teacher-reported scores for inattention on the BASC (β = 2.8; 95% CI: –0.2, 5.7) and maternal ΣPBDE levels were still associated with maternally-reported CADS DSM-IV Total scale scores (β = 2.6; 95% CI: –0.3, 5.5), decreased Verbal Comprehension IQ (β = –5.2; 95% CI: –10.4, 0.1) and Full-Scale IQ (β = –5.2; 95% CI: –10.6, 0.1), and lower nondominant hand pegboard scores (β = –6.5; 95% CI: –13.4, 0.3).

34In the present study, we report associations between mothers’ prenatal serum concentrations of PBDEs and evidence of deficits in attention, fine motor coordination, and cognitive functioning (particularly verbal comprehension) in their children at ages 5 and/or 7 years. Despite only weak correlations between PBDE concentrations in maternal prenatal and child age 7 years blood, we found associations between cognition, motor function and attention with both maternal and child PBDE exposures. The observed results appeared to be independent of associations previously reported in this cohort between maternal PBDEs and maternal thyroid hormone or child birth weight and between maternal organophosphate pesticide exposure and child neurobehavioral development. In addition, associations were not confounded by maternal lead or PCB levels, which were at low background levels.

35This is the largest study to date on the potential neurodevelopmental impacts of PBDE exposures, and largely supports findings from three smaller studies, including those with substantially lower PBDE serum levels. Our results are also similar to those reported between prenatal exposure to PCBs, which are chemically similar to PBDEs, and poorer attention and cognition or mental development in children.

36A notable finding of our study is that, in addition to in utero exposures, childhood PBDE concentrations were also associated with neurodevelopmental deficits. Although we hypothesized a priori that prenatal exposure would be more influential than postnatal exposure, the 7-year-olds’ average PBDE concentrations were much higher than those in their mothers during pregnancy; we attribute this difference in part to the lifetime residence of the children in California compared with mothers, many of whom were recent immigrants to California when their levels were measured.

37In animal studies, PBDE exposure has been associated with increased death of cerebellar granule cells, alterations in neuronal arachidonic acid release, and disruption of calcium homeostasis. Other potential mechanisms include perturbations of the cholinergic neurotransmitter system, interference with cellular signaling, and, because of PBDEs’ structural similarity to T4, effects on maternal thyroid hormone necessary for normal infant brain development. However, maternal thyroid hormone did not appear to explain the associations observed in our study population, as adding it to models did not measurably change the results.

38Important strengths of the current study include its longitudinal design and use of comprehensive neurobehavioral assessments, which incorporate input from multiple informants. Limitations of this study are that we did not observe consistency in associations with PBDEs across informants for measures of attention (although their responses were moderately correlated), and we constructed numerous statistical models (although performance across domains was also moderately correlated), which increased the possibility of a chance finding. We also did not measure some higher-brominated compounds (e.g., BDE-209), which are present in decaBDE. Another study, however, indicates that BDE-209 represents a very small fraction of total serum PBDE concentrations in a different population of California children.

39This study’s finding of significant associations of both maternal prenatal and childhood PBDE exposures with poorer attention, fine motor coordination, and cognition in early school-age children contributes to the growing evidence of adverse associations between PBDE exposure and children’s neurobehavioral development. Although these results are of particular concern for California children, they are also relevant to other locations, many of which contain products manufactured to meet California’s standards. With the phaseout of pentaBDE, other flame retardants have been used to achieve compliance with TB 117. Additional research is needed to determine the potential child health consequences of these new chemical flame retardants.