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. 2025 Sep 9;12(10):1314–1319. doi: 10.1021/acs.estlett.5c00774

Silicone Ankle Bands as a Tool to Assess Infant Exposures to Semivolatile Organic Chemicals in Indoor Environments

Catherine F Wise †,‡, Elizabeth Boxer †, Jillian Hurst §, Rebecca M Hoehn †, Nicholas J Herkert †, Duncan Hay †, Ellen M Cooper †, Heather M Stapleton †,‡, Kate Hoffman †,‡,*
PMCID: PMC12529947  PMID: 41112140

Abstract

Exposure to organophosphate esters (OPEs), used as flame retardants and plasticizers, is highly variable in the general population, and limited data exist on exposures in young children. This study evaluated the use of silicone ankle bands to assess OPE exposure in infants under 18 months of age. Infants (n = 21) wore silicone ankle bands for three consecutive days, and spot urine samples were collected using either pediatric urine collection bags or toddler training toilets. Ankle bands were analyzed for 20 OPEs; seven were detected in >70% of samples. TDCIPP and TPHP were the most abundant compounds on bands (medians = 57.5 and 53.0 ng/g, respectively). All targeted urinary metabolites were detected in most samples, with BDCIPP being the most abundant biomarker (median = 3.7 ng/mL SG-corrected), 2.5 times higher than DPHP. Significant positive correlations were observed between urinary metabolites and parent compounds on the ankle bands (rs = 0.40–0.73, p < 0.05), suggesting that silicone samplers reliably capture exposure trends. These findings support ankle bands as a practical, noninvasive tool for assessing OPE exposures in infants, offering an alternative to urine-based biomonitoring.

Keywords: Exposure, urine, infant, silicone wristband, organophosphate esters, biomonitoring

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Introduction

Young children are highly susceptible to environmental exposures, including organophosphate esters (OPEs), a class of semivolatile organic compounds (SVOCs) widely used in consumer products as flame-retardants and plasticizers. Due to their vapor pressures, OPEs can migrate from treated materials, such as furniture and electronics, into indoor environments, resulting in chronic, low-level exposure through inhalation, dust ingestion, and dermal absorption. Infants may face heightened risks of exposure to environmental chemicals compared to older children and adults because of their greater respiratory rate, higher food and water consumption rates per unit of body weight, and behavioral differences (e.g., time spent on the floor, mouthing behaviors, and restricted environments). They also may be more vulnerable to exposure due to rapid development, unique physiology, and limited ability to process chemicals.

Quantifying infants’ OPE exposure remains a significant challenge due to the limitations of traditional biomonitoring, which relies on spot urine samples. Small sample volumes, rapid OPE metabolism, and logistical difficulties in obtaining samples from infants contribute to these challenges. , Collecting urine from non-toilet-trained infants is particularly complicated, with noninvasive collection methods such as clean-catch, pediatric urine bags, cotton diapers, gel diapers, and absorbent materials (e.g., cotton pads or gauze) offering variable effectiveness. − These methods raise concerns about analyte stability, potential contamination, parental burden, and processing complexity. Despite these challenges, spot urine sampling remains the most common exposure assessment method, underscoring the need for a noninvasive, practical approach to accurately assess exposure in young children.

Silicone wristbands are a promising passive sampling tool for assessing personal exposure to SVOCs, including OPEs. , Wristbands have demonstrated utility in various populations. − Silicone wristbands monitor exposure across multiple environments and time points, providing a less invasive and potentially more integrated measure of chemical exposure than traditional biomonitoring. The concentrations measured using wristband sampling have been shown to generally correlate with biomarkers of exposures for several classes of SVOCs in adults and older children, − but their use in infants remains unexplored and presents unique challenges. For instance, infants may chew on or remove the bands, and differences in activity or movement patterns may influence chemical uptake rates compared with older individuals.

This study aims to validate silicone ankle bands as a noninvasive tool to assess OPE exposure in infants. We evaluate whether measurements from bands reliably reflect OPE levels detected in urine. This work seeks to establish a practical, noncumbersome method for exposure assessment in early life, advancing understanding of exposure pathways and informing interventions to mitigate risks during this critical developmental period.

Materials and Methods

Study Population

Children aged 6–18 months were recruited through the HOPE 1000 pregnancy cohort and through community outreach in central North Carolina, November 2023–July 2024. Participants (n = 21) received kits for band and urine collection. Urine kits contained pediatric urine bags, toddler training toilets and specimen cups. The band kit consisted of a mylar resealable bag with a precleaned aluminum tin that housed the precleaned wristband. Ankle bands were worn for three consecutive days through all daily activities. Demographic and socioeconomic data were gathered through questionnaires. Study protocols involving human subjects were approved by the Duke Health IRB (Pro00112855) and Duke University IRB (2024-0367). Guardians provided informed consent for participation.

Silicone Ankle Bands

Silicone bands were purchased from 24hourwristbands.com. One-inch-thick bands were chosen to reduce choking risk (Supplemental Figure S1). Bands were precleaned using a heat treatment in a 3700 Thermal Vacuum Cleaning System from Entech Instruments (https://www.entechinst.com/) based on a previously published method. They were conditioned at 230 °C for 24 h with constant low-flow nitrogen purge while under vacuum at 1 Torr. A quality control sample was taken from each batch and evaluated for residual siloxanes, which may be present in wristbands after the cleaning step and complicate the analysis of target compounds. After heat conditioning, bands were stored in methanol-cleaned aluminum tins with screw-top lids, inside mylar bags, and distributed to participants. After bands were worn, parents placed them back in the tin and bag. Returned bands were stored in tins at −20 °C until analysis (<1 year).

Extraction methods were modified from previously published methods to maintain method quality while increasing throughput, eliminating the use of carcinogenic solvents, and reducing solvent quantities. For this study, small (∼0.5 g) segments of bands were cut (weights recorded). Pieces were then cut into thirds and placed in a 50 mL centrifuge tube and spiked with a suite of isotopically labeled standards (Table S1). Samples were extracted via three 15 min sonications with 5 mL of a 50:50 (v:v) hexane:acetone mixture. Supernatants were removed between sonications for a final volume of 15 mL. Samples were then concentrated under vacuum with a Savant SPD121P SpeedVac Concentrator (Thermo Scientific) to ∼4 mL. A 12 mL test tube was prepared for each sample for the dispersive SPE cleanup step by adding 0.5 g of 2.5% water deactivated Florisil (Acros Organics Florisil, 100–200 mesh, FisherScientific) and ∼2 g of sodium sulfate to each tube. Concentrated 4 mL sample extracts were then added to the prepared dispersed solid phase extraction (SPE) tubes and brought up to 6 mL of total extract. Samples were then vortexed for 30 s, sonicated for 15 min, and centrifuged for 15 min at 2500 rpm. The supernatant was then transferred to a clean 12 mL test tube. Samples were concentrated to ∼1 mL with a Savant SPD121P SpeedVac Concentrator (Thermo Scientific), solvent-exchanged to high-purity hexane, and concentrated under nitrogen evaporation to 0.5 mL. Samples were then spiked with a second set of isotopically labeled standards to assess the recovery of quantitative internal standards and filtered using 0.45 μm nylon filters (Thermo Scientific Waltham, MA, USA). Samples were analyzed using a Q Exactive GC hybrid quadrupole-Orbitrap GC-MS/MS system (Thermo Scientific, Waltham, MA, USA) using previously described methods.

Urine

Urine collection kits included pediatric urine collection bags or toddler training toilets. Upon collection, urine was transferred to specimen cups, with some samples pooled to obtain a sufficient volume. Participants wrapped cups in sealable plastic bags and stored samples in their home freezers until they were collected by study staff (within 24 h). Samples were transported on ice and stored at −20 °C.

Urine samples were analyzed for OPE metabolites as previously described. Specific gravity (SG) measurements for each centrifuged sample were taken with a digital refractometer (Atago UG-α). To prepare samples for analysis, urine samples were centrifuged at 3000 rpm for 5 min to separate cellular components. Samples (∼5 mL) were spiked with labeled standards (d10-BCIPP at 100 ng, d10-BDCIPP at 10 ng, d10-DPHP at 10 ng, and d16-ip-DPHP at 10 ng). A buffer solution (1.0 M sodium acetate, pH 5.0) and an enzyme solution (1000 units/mL β-glucuronidas and 33 units/mL sulfatase in 0.2 M sodium acetate, pH 5.0) were added to each sample, and samples were vortexed to ensure thorough mixing. Samples were incubated at 37 °C overnight, extracted using mixed-mode anion-exchange solid-phase extraction, and analyzed via liquid chromatography–tandem mass spectrometry (Agilent Model 6460). Recovery of the internal standards was measured using d14-BMPP at 50 ng (Table S1 includes recovery data of the isotopically labeled standards used for quantification of all analytes). Target analytes, including bis­(1-chloro-2-propyl) phosphate (BCIPP), bis­(1-chloro-2-isopropyl) 1-hydroxy-2-isopropyl phosphate (BCIPHIPP), bis­(1,3-dichloro­isopropyl) phosphate (BDCIPP), diphenyl phosphate (DPHP), isopropyl­phenyl phenyl phosphate (ip-PPP), and tert-butylphenyl phenyl phosphate (tbutyl-DPP), were quantified based on previously established analytical methods. Method detection limits (MDLs) were determined using 3 times the standard deviation of blanks, normalized to the sample volume. Quality control measures included analysis of laboratory blanks (n = 3) processed in parallel.

Statistical Analysis

Data analyses were performed using JMP Pro (version 17.2, SAS Institute). Concentrations below MDLs were imputed at 1/2 MDL and normalized to silicone mass or urine volume extracted. Statistical analyses were conducted on chemicals with detection frequencies >70%. Visual assessment of distributions and statistical evaluation indicated that data were non-normally distributed, and as a result, nonparametric statistical methods were used for all analyses. Spearman’s correlations were conducted to evaluate relationships between parent chemicals on bands and urinary metabolites, and Wilcoxon rank-sum exact tests were used to compare chemical concentrations between groups. Certain parent chemicals are metabolized to multiple metabolites; associations were evaluated using individual metabolites and their sum. Urine analyses were conducted using raw concentrations and SG-corrected values. We focused on the SG-corrected values.

Results and Discussion

Infants were 6–18 months old (n = 21; median = 10 months; Table S2 includes a breakdown of demographic characteristics). More than half were male (62%). Eighty-one percent of parents identified their infant as white; one reported Hispanic ethnicity. Most infants (90%) had at least one parent with a college degree or higher, and 71% had a parent with a postgraduate degree. Eighty-one percent of families had an annual income >$70,000.

Ankle Band OPEs

Ankle bands were analyzed for 20 OPEs; seven were detected in >70% of samples (Table ). TDCIPP and TPHP were the most abundant compounds detected on bands (medians = 57.5 and 53.0 ng/g, respectively). Notably, ankle bands captured a broader range of parent compounds than were detected via urine biomarkers in this study, reflecting their ability to assess ambient exposure across a wider chemical spectrum. In contrast, urine biomonitoring reflects the internal dose but is inherently limited to compounds for which validated metabolites and analytical methods exist. In addition, bands capture the parent chemical compounds, while urine typically captures metabolites that, in the case of OPEs, can be derived from multiple parent compounds, complicating source determination.

1. Descriptive Statistics for Urinary OPE Metabolites (ng/mL SG) and OPEs Measured in 21 Ankle Bands (ng/g) Worn by Infants Ages 6–18 Months .

Chemical Abbreviation MDL DF (%) Min Med GM Max
URINARY METABOLITES
bis(2-chloro-isopropyl) phosphate BCIPP 0.007 71 <MDL 0.05 0.06 0.93
bis(1-chloro-2-propyl) 1-hydroxy-2-propyl phosphate BCIPHIPP 0.0005 100 0.012 0.20 0.24 2.65
bis(1,3-dichloro-2-propyl) phosphate BDCIPP 0.062 100 0.651 3.70 3.38 41.9
diphenyl phosphate DPHP 0.125 76 <MDL 1.48 1.95 420
mono-isopropyl phenyl phenyl phosphate ip-PPP 0.001 86 <MDL 0.03 0.02 0.10
mono-tert -butyl phenyl phenyl phosphate tb-PPP 0.006 52 <MDL 0.04 0.04 0.42
 
ANKLE BANDS
OPEs
tris(2-chloro-1-methylethyl) phosphate TCIPP 4.34 100 5.73 37.9 41.9 426
tris(1,3-dichloro-2-propyl) phosphate TDCIPP 4.60 86 <MDL 57.5 41.1 232
2-ethylhexyl diphenyl phosphate EHDPP 0.53 95 <MDL 10.6 13.0 101
triphenyl phosphate TPHP 0.48 100 6.15 53.0 43.3 268
ITPs
2-isopropylphenyl diphenyl phosphate 2IPPDPP 1.01 67 <MDL 8.99 7.13 82.9
3-isopropylphenyl diphenyl phosphate 3IPPDPP 0.50 52 <MDL 1.08 1.74 33.4
4-isopropylphenyl diphenyl phosphate 4IPPDPP 0.42 76 <MDL 3.42 3.20 32.8
diisopropylphenyl diphenyl phosphate 24DIPPDPP 0.76 52 <MDL 2.34 2.65 22.9
bis(o-isopropylphenyl) phenyl phosphate B2IPPPP 4.89 19 <MDL nd nd 25.2
bis(3-isopropylphenyl) phenyl phosphate B3IPPPP 3.93 0 <MDL nd nd nd
bis(4-isopropylphenyl) phenyl phosphate B4IPPPP 4.83 0 <MDL nd nd nd
bis(2,4-diisopropylphenyl) phenyl phosphate B24IPPPP 0.78 95 <MDL 4.36 4.50 14.8
tris(3-isopropylphenyl) phosphate T3IPPP 4.24 0 <MDL nd nd nd
tris (4-isopropylphenyl) phosphate T4IPPP 4.85 0 <MDL nd nd nd
isodecyl diphenyl phosphate isodecylPP 4.58 38 <MDL nd nd 939
TBPPs
2-tert-butylphenyl diphenyl phosphate 2tBPDPP 0.15 5 <MDL nd nd 0.586
4-tert-butylphenyl diphenyl phosphate 4tBPDPP 0.04 100 0.573 7.56 7.96 337
bis(2-tert-butylphenyl) phenyl phosphate B2tBPPP 0.57 0 <MDL nd nd nd
bis(4-tert-butylphenyl) phenyl phosphate B4tBPPP 5.16 14 <MDL nd nd 172
tris(4-tert-butylphenyl) phosphate T4tBPP 5.18 5 <MDL nd nd 10.6
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a

Detection frequency (DF), method detection limit (MDL), and geometric mean (GM). Urine values are specific gravity corrected. Descriptive statistics were not done (nd) for chemicals with a DF less than 50%.

Several studies have used silicone wristbands over varying durations in older children (3–9 years) to assess OPE exposures. ,,, For comparison, we divided the reported concentrations by wear time (Table S3) because many chemicals exhibit a linear uptake over time. Previous US studies reported a similarly high abundance of TDCIPP and TPHP, variability in which was found at higher concentrations. In contrast, a study in Uruguayan children found higher levels of EHDPP and TCIPP, suggesting possible geographic differences in exposure. However, due to differences in study design, sample size, laboratory methods, and geographic context, direct comparisons between studies should be interpreted with caution.

Urinary Biomarkers

Urine was analyzed for six OPE metabolites; five were detected in >70% of samples (Table ). BDCIPP was most abundant (median = 3.7 ng/mL SG), 2.5 times higher than the next most abundant biomarker (DPHP). These are within the range of values previously been reported for US children under age 2 years. ,, Variability in urinary OPE levels may reflect differences in infant behaviors, product use, and environmental exposures, with higher urinary OPE levels linked to children’s products in the home. Sociodemographic factors, such as lower maternal income, have been associated with higher DPHP levels in toddlers, while seasonal, dietary, and micro­environmental factors also influence exposure. ,,

OPEs on Ankle Bands and Urinary Biomarkers

OPEs detected on bands were positively correlated with corresponding urinary biomarkers (Table , Figure ). Each urinary metabolite had at least one parent chemical detected on ankle bands that was positively correlated (rs = 0.40, p < 0.05 to rs = 0.73, p < 0.001). A single urinary metabolite, BCIPP, was not quite significantly correlated with its parent compound (TCIPP) when using SG-corrected values (rs = 0.39, p = 0.08); however, the correlation was significant when uncorrected values were used (rs = 0.53, p < 0.05). Furthermore, TCIPP also metabolizes to BCIPHIPP, which had a significant correlation using SG-corrected values (rs = 0.59, p < 0.01). The sum of both metabolites was significantly correlated with TCIPP (rs = 0.60, p < 0.01).

2. Spearman’s Correlation Coefficients between Urinary Biomarkers and Ankle Band OPE Levels (n = 21) .

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a

Gray-shaded cells represent known parent–biomarker pairs. *p < 0.05, **p < 0.01, ***p < 0.001.

1.

1

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Spearman’s correlation plot of TDCIPP and DPHP parent compounds on ankle bands with their corresponding urinary metabolites. Correlations of (a) sum of all potential parent compounds that metabolize to DPHP detected in >50% of ankle bands and DPHP and (b) TDCIPP and BDCIPP. Data are log transformed. Urine values are specific gravity (SG) corrected; Spearman correlations (rs) and p-values for each association are provided.

Previous studies assessing relationships between urinary biomarkers and wristbands worn by adults and older children have reported similar findings. ,,,− TDCIPP and BDCIPP were significantly and positively correlated among preschoolers (rs = 0.52, p < 0.05), as were 4tBPDPP and tb-PPP (rs = 035, p < 0.05). Adult populations showed mixed results, with some significant correlations for TCIPP with BCIPHIPP (rs = 0.52, p < 0.0001) and TPHP with DPHP (rs = 0.39, p < 0.05). , Previous children’s studies suggest that wristband OPE concentrations have greater correlations with urinary biomarkers compared to housedust. , In studies of mother–child pairs, children typically had stronger correlations between wristbands and urinary biomarkers compared to parents. , Higher correlations observed in children may be due to less variability in the microenvironments, more consistent behaviors, and less variable diets.

The magnitude and significance of correlations differ across studies due to factors such as age, geographic location, socioeconomic status, and variability in urine collection methods. Some metabolites, such as BDCIPP, show consistent results across studies, while others, like DPHP, are less consistent, likely due to their derivation from multiple parent chemicals.

Demographics and Chemical Exposures

Associations between demographic and socioeconomic factors and OPEs have been reported. Despite the small sample size, we observed several statistically significant relationships between demographic factors and OPEs (Table S4). Infants above the median age of 10 months had higher levels of TPHP on ankle bands compared to younger infants (p < 0.05) (Figure S2). Similarly, DPHP was 2.9 times higher in urine from older infants. This difference was not statistically significant (p = 0.06), likely reflecting the impact of a very high DPHP concentration (420 ng/mL SG) from a six-month old infant that is 30 times higher than the second highest infant value (14.1 ng/mL SG). Excluding this individual strengthened the trend between age groups (median difference = 3.2 times; p < 0.05). This is consistent with previous reports that have shown that OPE urinary biomarkers increase with age in children, specifically BDCIPP and DPHP. ,,, Considering that this individual did not have elevated levels of TPHP on their ankle band compared to other infants in the study, we suggest that the higher concentration of urinary DPHP is likely due to dietary exposure.

This study demonstrates the potential of silicone ankle bands as practical, noninvasive tools for assessing infant OPE exposures. Positive correlations between band OPE levels and corresponding urinary biomarkers suggest that silicone samplers reliably capture exposure trends in infants. This approach addresses significant challenges in traditional biomonitoring methods, particularly the difficulty of collecting sufficient and representative samples from infants. Silicone bands capture parent OPE chemicals, providing a more direct and regulatory-relevant measure of exposure than urinary metabolites, which may reflect multiple compounds. Additionally, bands can be mailed, allowing for remote collection. Study limitations include a small sample size and a relatively homogeneous study population, which may limit generalizability. In addition, we relied on convenient spot urine samples, which were collected using two different methods (urine collection bags and training toilets), introducing potential variability in the sample composition. Despite these limitations, our findings align with prior research, underscoring the utility of silicone samplers in capturing OPE exposures. Future studies should include more diverse populations, explore geographic and socioeconomic variability, and investigate how exposures may influence health outcomes. By providing a scalable, less invasive method for exposure assessment, this work advances our understanding of early-life chemical exposures and informs potential health interventions.

Supplementary Material

ez5c00774_si_001.pdf (193.3KB, pdf)

Acknowledgments

Funding for this research was provided by the National Institute of Allergy and Infection (Grant R01 AI167850) and the National Institute of Environmental Health Sciences (U2C-ES030857 to HMS and P42-ES010356 to HS and KH). We are thankful to all the study participants. The authors wish to thank Samantha Samon for administrative support.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.estlett.5c00774.

  • Figure S1, Silicone ankle band photo; Figure S2, TPHP and DPHP concentrations by child age; Table S1, Recovery of isotopically labeled standards used for quantification; Table S2, Participant demographic characteristics; Table S3, OPEs (ng/g/day) in children’s wristbands reported in literature; Table S4, Wilcoxon Exact Test results for associations between urine and ankle band concentrations (PDF)

The authors declare no competing financial interest.

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