Young infants' exposure to organophosphate esters: Breast milk as a potential source of exposure

Identification and Prioritization of Organic Pollutants in Human Milk from the Yangtze River Delta, China

Pollutants in human milk are critical for evaluating maternal internal exposure and infant external exposure. However, most studies have focused on a limited range of pollutants. Here, 15 pooled samples (prepared from 467 individual samples) of human milk from three areas of the Yangtze River Delta (YRD) in China were analyzed by gas chromatography quadrupole time-of-flight mass spectrometry. In total, 171 compounds of nine types were preliminarily identified. Among these, 16 compounds, including 2,5-di-tert-butylhydroquinone and 2-tert-butyl-1,4-benzoquinone, were detected in human milk for the first time. Partial least-squares discriminant analysis identified ten area-specific pollutants, including 2-naphthylamine, 9-fluorenone, 2-isopropylthianthrone, and benzo[a]pyrene, among pooled human milk samples from Shanghai (n = 3), Jiangsu Province (n = 6), and Zhejiang Province (n = 6). Risk index (RI) values were calculated and indicated that legacy polycyclic aromatic hydrocarbons (PAHs) contributed only 20% of the total RIs for the identified PAHs and derivatives, indicating that more attention should be paid to PAHs with various functional groups. Nine priority pollutants in human milk from the YRD were identified. The most important were 4-tert-amylphenol, caffeine, and 2,6-di-tert-butyl-p-benzoquinone, which are associated with apoptosis, oxidative stress, and other health hazards. The results improve our ability to assess the health risks posed by pollutants in human milk.

Prenatal exposure to a mixture of organophosphate flame retardants and infant neurodevelopment: A prospective cohort study in Shandong, China

BACKGROUND

Previous studies have suggested that prenatal exposure to organophosphate flame retardants (OPFRs) may have adverse effect on early neurodevelopment, but limited data are available in China, and the overall effects of OPFRs mixture are still unclear.

OBJECTIVE

This study aimed to investigate the association between prenatal exposure to OPFR metabolites mixture and the neurodevelopment of 1-year-old infants.

METHODS

A total of 270 mother-infant pairs were recruited from the Laizhou Wan (Bay) Birth Cohort in China. Ten OPFR metabolites were measured in maternal urine. Neurodevelopment of 1-year-old infants was assessed using the Gesell Developmental Schedules (GDS) and presented by the developmental quotient (DQ) score. Multivariate linear regression and weighted quantile sum (WQS) regression models were conducted to estimate the association of prenatal exposure to seven individual OPFR metabolites and their mixture with infant neurodevelopment.

RESULTS

The positive rates of seven OPFR metabolites in the urine of pregnant women were greater than 70% with the median concentration ranged within 0.13-3.53 μg/g creatinine. The multivariate linear regression model showed significant negative associations between bis (1-chloro-2-propyl) phosphate (BCIPP), din-butyl phosphate (DnBP), and total OPFR metabolites exposure and neurodevelopment in all infants. Results from the WQS model consistently revealed that the OPFR metabolites mixture was inversely associated with infant neurodevelopment. Each quartile increased in the seven OPFR metabolites mixture was associated with a 1.59 decrease (95% CI: 2.96, -0.21) in gross motor DQ scores, a 1.41 decrease (95% CI: 2.38, -0.43) in adaptive DQ scores, and a 1.08 decrease (95% CI: 2.15, -0.02) in social DQ scores, among which BCIPP, bis (1, 3-dichloro-2-propyl) phosphate (BDCIPP) and DnBP were the main contributors.

CONCLUSION

Prenatal exposure to a mixture of OPFRs was negatively associated with early infant neurodevelopment, particularly in gross motor, adaptive, and social domains.

Opportunities in Assessing and Regulating Organohalogen Flame Retardants (OFRs) as a Class in Consumer Products

BACKGROUND

In 2015, the U.S. Consumer Product Safety Commission (CPSC) received and then, in 2017, granted a petition under the Federal Hazardous Substances Act to declare certain groups of consumer products as banned hazardous substances if they contain nonpolymeric, additive organohalogen flame retardants (OFRs). The petitioners asked the CPSC to regulate OFRs as a single chemical class with similar health effects. The CPSC later sponsored a National Academy of Sciences, Engineering, and Medicine (NASEM) report in 2019, which ultimately identified 161 OFRs and grouped them into 14 subclasses based on chemical structural similarity. In 2021, a follow-up discussion was held among a group of scientists from both inside and outside of the CPSC for current research on OFRs and to promote collaboration that could increase public awareness of CPSC work and support the class-based approach for the CPSC's required risk assessment of OFRs.

OBJECTIVES

Given the extensive data collected to date, there is a need to synthesize what is known about OFR and how class-based regulations have previously managed this information. This commentary discusses both OFR exposure and OFR toxicity and fills some gaps for OFR exposure that were not within the scope of the NASEM report. The objective of this commentary is therefore to provide an overview of the OFR research presented at SOT 2021, explore opportunities and challenges associated with OFR risk assessment, and inform CPSC's work on an OFR class-based approach.

DISCUSSION

A class-based approach for regulating OFRs can be successful. Expanding the use of read-across and the use of New Approach Methodologies (NAMs) in assessing and regulating existing chemicals was considered as a necessary part of the class-based process. Recommendations for OFR class-based risk assessment include the need to balance fire and chemical safety and to protect vulnerable populations, including children and pregnant women. The authors also suggest the CPSC should consider global, federal, and state OFR regulations. The lack of data or lack of concordance in toxicity data could present significant hurdles for some OFR subclasses. The potential for cumulative risks within or between subclasses, OFR mixtures, and metabolites common to more than one OFR all add extra complexity for class-based risk assessment. This commentary discusses scientific and regulatory challenges for a class-based approach suggested by NASEM. This commentary is offered as a resource for anyone performing class-based assessments and to provide potential collaboration opportunities for OFR stakeholders. https://doi.org/10.1289/EHP12725.

Associations between exposure to organophosphate esters and overactive bladder in U.S. adults: a cross-sectional study

Background

The relationship between exposure to organophosphate esters (OPEs) and the risk of developing overactive bladder (OAB) is uncertain. The purpose of this study is to examine the potential link between urinary metabolites of organophosphate esters and OAB.

Method

Data from the National Health and Nutrition Examination Survey (NHANES) database of the 2011-2016 cycles were utilized. Four urinary metabolites of organophosphate esters: diphenyl phosphate (DPHP), bis (1,3-dichloro-2-propyl) phosphate (BDCPP), bis (2-chloroethyl) phosphate (BCEP), and dibutyl phosphate (DBUP) were included in the study. Multivariate logistic regression and restricted cubic spline (RCS) were used to evaluate the relationship between urinary OPEs metabolites and OAB. Interaction analysis was conducted on subgroups to confirm the findings.

Results

A total of 3,443 United States (US) adults aged 20 years or older were included in the study, of whom 597 participants were considered to have OAB. After adjusting for potential confounding factors, we found a positive association between DPHP and the risk of overactive bladder. The risk of overactive bladder increased with increasing DPHP concentrations compared with quartile 1 (quartile 2, OR = 1.19, 95% CI, 0.82-1.73, P = 0.34; quartile 3, OR = 1.67, 95% CI, 1.10-2.53, P = 0.02; Q4, OR = 1.75, 95% CI, 1.26-2.43, P = 0.002). However, after dividing the participants by gender, only the female group retained consistent results. Additionally, restricted cubic spline analysis revealed a nonlinear dose-response correlation between DPHP and OAB in female participants. In the subgroup analysis based on age, race, body mass index (BMI), recreational activity, smoking status, drinking status, hypertension, diabetes, and stroke, the interaction analysis revealed that the findings were uniform.

Conclusion

Our findings indicate that exposure to DPHP could elevate the risk of OAB in US adult females. Further experimental studies are needed to explore the underlying mechanism in the future.

Gestational exposure to organophosphate esters and adiposity measures of children up to 6 years: Effect modification by breastfeeding

Organophosphate esters (OPEs) are synthetic chemicals used in various commercial products. Accumulating evidence has shown that they may act as metabolic disruptors. However, no study has investigated the long-term effects of gestational OPEs exposure on childhood adiposity. Breast milk represents the optimal nutritional form of feeding for infants and may protect against the adverse effects of gestational OPEs exposure on offspring development. Using data from the Shanghai-Minhang birth cohort study, we investigated the associations of gestational OPEs exposure with adiposity measures in children up to 6 years of age, and whether breastfeeding could modify these associations. A total of 733 mother-child pairs with available data on OPE concentrations and child anthropometry were included. Eight OPE metabolites were assessed in maternal urine samples collected at 12-16 weeks of pregnancy. Information on children's weight, height, arm circumference, and waist circumference was collected at birth and 0.5, 1, 4, and 6 years of age. Weight-for-age and body mass index-for-age z scores were calculated. The duration of children's breastfeeding was categorized as ≤4 months or >4 months. The generalized estimate equation and Bayesian Kernel Machine Regression models were used to examine the associations of OPEs exposure with children's adiposity measures. Selected OPEs exposure was associated with higher children's adiposity measures. Particularly, we found stronger associations of bis(1-chloro-2-propyl) phosphate (BCIPP), bis(2-chloroethyl) phosphate (BCEP), bis(1,3-dichloro-2-propyl) phosphate (BDCPP), and di-o-cresyl phosphate and di-p-cresyl phosphate (DCP) with higher adiposity measures in children breastfed for ≤4 months, while little evidence of associations was found among those breastfed for >4 months. Our study suggested that gestational OPEs exposure could alter children's adiposity measures, but the potential effects were attenuated if children were breastfed for >4 months.

Gestational exposure to organophosphate esters and infant anthropometric measures in the first 4 weeks after birth

BACKGROUND

Few studies have examined whether gestational exposure to organophosphate esters (OPEs), widely used chemicals with potential endocrine-disrupting potency and developmental toxicity, is associated with impaired infant growth.

METHODS

We analyzed data from 329 mother-infant pairs in the Health Outcomes and Measures of the Environment (HOME) Study (2003-2006, Cincinnati, Ohio, USA). We quantified concentrations of four OPE metabolites in maternal urine collected at 16 and 26 weeks of gestation, and at delivery. We calculated z-scores using 2006 World Health Organization (WHO) child growth standards for the 4-week anthropometric measures (weight, length, and head circumference), the ponderal index, and weekly growth rates. We used multiple informant models to examine window-specific associations between individual OPE metabolites and anthropometric outcomes. We further modeled OPEs as a mixture for window-specific associations with 4-week anthropometric outcomes using mean field variational Bayesian inference procedure for lagged kernel machine regression (MFVB-LKMR). We stratified the models by infant sex.

RESULTS

Diphenyl phosphate (DPHP) in mothers at 16 weeks, and bis(2-chloroethyl) phosphate (BCEP) and bis(1,3-dichloro-2-propyl) phosphate (BDCIPP) at delivery were positively associated with z-scores of weight, length, and head circumference in all infants at 4 weeks of age. After stratifying by infant sex, positive associations were only observed in males for DPHP at 16 weeks and BCEP at delivery and in females for BDCIPP at delivery. Negative associations not present in all infants were observed in males for di-n-butyl phosphate (DNBP) at 26 weeks of gestation with weight z-score and DPHP at delivery with head circumference z-score. Results were generally similar using MFVB-LKMR models with more conservative 95 % credible intervals. We did not identify consistent associations of gestational OPE metabolite concentrations with the ponderal index and weekly growth rates.

CONCLUSION

In this cohort, exposure to OPEs during gestation was associated with altered infant anthropometry at 4 weeks after birth.

Associations between organophosphate esters concentrations and markers of liver function in US adolescents aged 12-19 years: A mixture analysis

Liver disease has become a growing health burden, and little is known about the impairment of liver function caused by exposure to organophosphate esters (OPEs) in adolescents aged 12-19 years in the United States. To investigate the relationship between urinary metabolites of OPEs including diphenyl phosphate (DPHP), bis(1,3-dichloro-2-propyl) phosphate (BDCPP), bis(1-chloroethyl) phosphate (BCPP), bis(2-chloroethyl) phosphate (BCEP), and dibutyl phosphate (DBUP) and liver function in US adolescents aged 12-19 years. Liver function tests (LFTs) include aspartate aminotransferase (AST), albumin (ALB), alanine aminotransferase (ALT), alkaline phosphatase (ALP), gamma-glutamyl transferase (GGT), total bilirubin (TBIL), total protein (TP), and AST/ALT. Meanwhile, potential confounding and interaction effects were assessed. The study sample included 592 adolescents aged 12-19 from two consecutive NHANES cycles (2011-2012, 2013-2014). A composite statistical strategy combining traditional linear regression with advanced multi-pollutant models quantile based g-computation (QGC) and eXtreme Gradient Boosting (XGBoost) regression was used to analyze the joint effects of multiple OPEs on liver function indicators, and to describe the interaction between different OPEs in detail. 592 adolescent participants were 15 (14-17) years old, with similar numbers of males and females (304 vs. 288). The analysis results showed that (1) in the linear regression model, individual DPHP, BCEP exposure and ALP changes, BCEP and AST/ALT changes were positively associated. DPHP, BDCPP were negatively associated with TP changes. (2) The combined effects of various OPEs on ALB, ALT, ALP, GGT, TBIL, TP, and AST/ALT were statistically significant. (3) There is no potential interaction between different OPEs. Several OPEs and their combinations are closely related to the 8 LFT indicators. In addition, data suggest that exposure to OPEs in adolescents may be associated with liver damage. Due to limited evidence in the literature and potential limitations of the current study, our findings require more studies to confirm.

An overview of organophosphate esters and their metabolites in humans: Analytical methods, occurrence, and biomonitoring

With the strict regulation of brominated flame retardants, organophosphate esters (OPEs) have been extensively used as replacements. Increasing concerns on OPEs have aroused due to their extensive distribution in the environment and humans, as well as their potential toxicities. Recent studies have demonstrated that some organophosphate di-esters are even more toxic than their respective tri-esters. This review summarized the current state of knowledge on the analytical methodologies (including sample collection and preparation, instrumental analysis, and the feasibility of each potential human matrix), as well as the occurrences of OPEs and/or their metabolites (m-OPEs) in various human matrices. Organophosphate esters are readily metabolized in human thus only limited studies reported their occurrences in blood and breast milk, whereas abundant studies are available regarding the occurrences of m-OPEs rather than OPEs in urine. Since none of the matrix is suitable all the time, appropriate matrix should be selected depending on the aims of biomonitoring studies, e.g., high throughput screening or body burden estimation. Biomonitoring with non-invasive matrices such as hair and/or nail is useful to screen specific populations that might be under high exposure risks while urine is more suitable to provide valuable information on body burden. In terms of urinary monitoring, specific biomarkers have been identified for some OPE compounds, including tri(2-butoxyethyl) phosphate, tri(1,3-dichloro-2-propyl) phosphate, tri(2-chloroethyl) phosphate and tri(1-chloro-2-propyl) phosphate. Further studies are required to identify suitable urinary biomarkers for other OPE compounds, especially the emerging ones.

Infants' diminished response to DTaP vaccine is associated with exposure to organophosphate esters

Organophosphate esters (OPEs) are commonly applied as flame retardants and plasticizers. Toxicological studies suggest exposure effects on immune endpoints, raising concerns as infants' OPE exposures are elevated compared to older children and adults due to hand-to-mouth behavior and breastfeeding. Here, we sought to evaluate the immune responsiveness of infants to a neoantigen (e.g., a newly encountered antigen) in the presence of OPE exposures. As a proxy for immune responsiveness, children were given three doses of the Diphtheria, Tetanus, and Pertussis (DTaP) vaccine as recommended, and diphtheria and tetanus antibodies were evaluated in serum samples collected when children were 12 months old (n = 84). Titers were compared, based on maximum sample overlap, to measurements of OPE metabolites in spot urine samples collected before vaccination (age 2 months, n = 73) and at the time of antibody assessment (12 months of age, n = 46). Metabolites of two chlorinated OPEs were significantly associated with diminished antibodies for diphtheria and tetanus. A metabolite of tris (1,3-dichloroisopropyl)phosphate (TDCIPP) measured at 2 months was associated with decreased diphtheria antibodies (-0.07 IU/mL per log₁₀ increase in metabolite). One metabolite of tris(2-chloroisopropyl)phosphate (TCIPP) measured at 12 months was associated with decreased tetanus antibodies (-0.57 IU/mL per log₁₀ increase in metabolite). These results provide some preliminary insights for OPE exposure impacts on vaccine responses in early life and may have important implications for immune health through childhood and adulthood.

Study of the Chemical Ionization of Organophosphate Esters in Air Using Selected Ion Flow Tube-Mass Spectrometry for Direct Analysis

Organophosphate esters are an emerging environmental concern since they spread persistently across all environmental compartments (air, soil, water, etc.). Measurements of semivolatile organic compounds are important but not without challenges due to their physicochemical properties. Selected ion flow tube-mass spectrometry (SIFT-MS) can be relevant for their analysis in air because it is a direct analytical method without separation that requires little preparation and no external calibration. SIFT-MS is based on the chemical reactivity of analytes with reactant ions. For volatile and semivolatile organic compound analysis in the gas phase, knowledge of ion-molecule reactions and kinetic parameters is essential for the utilization of this technology. In the present work, we focused on organophosphate esters, semivolatile compounds that are now ubiquitous in the environment. The ion-molecule reactions of eight precursor ions that are available in SIFT-MS (H₃O⁺, NO⁺, O₂^•+, OH^-, O^•-, O₂^•-, NO₂^-, and NO₃^-) with six organophosphate esters were investigated. The modeling of ion-molecule reaction pathways by calculations supported and complemented the experimental work. Organophosphate esters reacted with six of the eight precursor ions with characteristic reaction mechanisms, such as protonation with hydronium precursor ions and association with NO⁺ ions, while nucleophilic substitution occurred with OH^-, O^•-, and O₂^•-. No reaction was observed with NO₂^- and NO₃^- ions. This work shows that the direct analysis of semivolatile organic compounds is feasible using SIFT-MS with both positive and negative ionization modes.

Atmospheric deposition, seasonal variation, and long-range transport of organophosphate esters on Yongxing Island, South China Sea

The South China Sea (SCS), surrounded by developing countries/regions with a huge consumption of flame retardants, is generally contaminated by organophosphate esters (OPEs). However, studies on the occurrence, deposition and long-range atmospheric transport (LRAT) process over the SCS of OPEs compounds are still limited. In this work, 10 OPEs were measured in 100 atmospheric samples collected from Yongxing Island (YXI) in the SCS. The total OPEs concentrations ranged from 1508 to 1968 pg/m³ with 28.6-1416.9 pg/m³ in gas and 95.2-1066.2 pg/m³ in particle partition. The three chlorinated OPEs are present at higher concentrations than the other seven non-chlorinated OPEs. Most OPEs had clear seasonal variations that followed the order: spring>summer≈winter>autumn except for tri-isobutyl phosphate (TIBP) and tris-(2-ethylhexyl) phosphate (TEHP). The particle-bound fraction of the total OPEs had little seasonal variations with a mean value of 0.35. Comparing J-P model and Koa model, it was found that the gas/particle partition in the study area was in non-equilibrium condition. LRAT, controlled by seasonal wind direction, was the predominated factor that led to the seasonal variations of OPEs on YXI. The average daily deposition flux of total OPEs was 13.0 ng/m² with an annual total deposition of 15.06 g.

Organophosphate ester metabolites in human breast milk determined by online solid phase extraction coupled to high pressure liquid chromatography tandem mass spectrometry

The analysis of metabolites of organophosphate esters (OPEs) in human breast milk is essential to evaluate OPE and OPE metabolite exposure of newborns. In the current study, an analytical method which only needs a small amount of breast milk (100 μl) was developed and validated for six diester metabolites and three hydroxylated metabolites applying salt-induced liquid-liquid extraction (SI-LLE) and dispersive solid phase extraction (d-SPE) for sample preparation and online solid phase extraction coupled to high pressure chromatography tandem mass spectrometry (online-SPE-HPLC-MS/MS) for quantitative measurement. The final method consisted of an extraction with formic acid (FA)/acetonitrile (1:200, v/v) and a cleanup with C18 d-SPE. The final extracts were trapped on a C18 cartridge with application of a wash step of 2 ml 0.1% FA milli-Q/methanol (98:2, v/v). Method detection limits (MDLs) ranging from 21.7 ng/l for BBOEHEP to 500 ng/l for BCIPP and average recoveries ranging from 58% for 5-OH-EHDPHP to 120% for BCIPP were achieved. Thirty-three breast milk samples from the LINC (Linking EDCs in maternal Nutrition to Child health) cohort collected in three distinct areas in The Netherlands were analyzed using the validated method. BCEP, BCIPP, BCIPHPP, BDCIPP, and 5-OH-EHDPHP were not detected in any of the samples, while BBOEP was the most frequently detected metabolite with a concentration range of <MDL to l.47 ng/ml, followed by DPhP and BBOEHEP, detected in ranges of <MDL to 0.09 and <MDL to 0.027 ng/ml. The results indicated that OPEs entering the human body are only to a limited extent excreted via breast milk.

Beyond Cholinesterase Inhibition: Developmental Neurotoxicity of Organophosphate Ester Flame Retardants and Plasticizers

BACKGROUND

To date, the toxicity of organophosphate esters has primarily been studied regarding their use as pesticides and their effects on the neurotransmitter acetylcholinesterase (AChE). Currently, flame retardants and plasticizers are the two largest market segments for organophosphate esters and they are found in a wide variety of products, including electronics, building materials, vehicles, furniture, car seats, plastics, and textiles. As a result, organophosphate esters and their metabolites are routinely found in human urine, blood, placental tissue, and breast milk across the globe. It has been asserted that their neurological effects are minimal given that they do not act on AChE in precisely the same way as organophosphate ester pesticides.

OBJECTIVES

This commentary describes research on the non-AChE neurodevelopmental toxicity of organophosphate esters used as flame retardants and plasticizers (OPEs). Studies in humans, mammalian, nonmammalian, and in vitro models are presented, and relevant neurodevelopmental pathways, including adverse outcome pathways, are described. By highlighting this scientific evidence, we hope to elevate the level of concern for widespread human exposure to these OPEs and to provide recommendations for how to better protect public health.

DISCUSSION

Collectively, the findings presented demonstrate that OPEs can alter neurodevelopmental processes by interfering with noncholinergic pathways at environmentally relevant doses. Application of a pathways framework indicates several specific mechanisms of action, including perturbation of glutamate and gamma-aminobutyric acid and disruption of the endocrine system. The effects may have implications for the development of cognitive and social skills in children. Our conclusion is that concern is warranted for the developmental neurotoxicity of OPE exposure. We thus describe important considerations for reducing harm and to provide recommendations for government and industry decision makers. https://doi.org/10.1289/EHP9285.

Sex-specific Disruption of the Prairie Vole Hypothalamus by Developmental Exposure to a Flame Retardant Mixture

Prevalence of neurodevelopmental disorders (NDDs) with social deficits is conspicuously rising, particularly in boys. Flame retardants (FRs) have long been associated with increased risk, and prior work by us and others in multiple species has shown that developmental exposure to the common FR mixture Firemaster 550 (FM 550) sex-specifically alters socioemotional behaviors including anxiety and pair bond formation. In rats, FRs have also been shown to impair aspects of osmoregulation. Because vasopressin (AVP) plays a role in both socioemotional behavior and osmotic balance we hypothesized that AVP and its related nonapeptide oxytocin (OT) would be vulnerable to developmental FM 550 exposure. We used the prairie vole (Microtus ochrogaste) to test this because it is spontaneously prosocial. Using siblings of prairie voles used in a prior study that assessed behavioral deficits resulting from developmental FM 550 exposure across 3 doses, here we tested the hypothesis that FM 550 sex-specifically alters AVP and OT neuronal populations in critical nuclei, such as the paraventricular nucleus (PVN), that coordinate those behaviors, as well as related dopaminergic (determined by tyrosine hydroxylase (TH) immunolabeling) populations. Exposed females had fewer AVP neurons in the anterior PVN and more A13 TH neurons in the zona incerta than controls. By contrast, in FM 550 males, A13 TH neuron numbers in the zona incerta were decreased but only in 1 dose group. These results expand on previous work showing evidence of endocrine disruption of OT/AVP pathways, including to subpopulations of PVN AVP neurons that coordinate osmoregulatory functions in the periphery.