Liquid Crystal Monomers in Human, Dog and Cat Feces from the United States

Abstract

Little is known about exposure of humans and companion animals to liquid crystal monomers (LCMs), which are extensively used in digital displays. We determined the concentrations of 52 LCMs in feces of humans, pet dogs and cats from New York State, USA, using gas chromatography-high resolution mass spectrometry (GC-HRMS). Twenty-four, eight, and six LCMs that were predominantly fluorinated were detected in human, dog, and cat feces, respectively. ΣLCMs concentrations in the feces of humans (mean: 8.01 ng/g dry weight [dw]) were significantly higher (p <0.05) than those of dogs (mean: 1.82 ng/g dw) and cats (mean: 1.24 ng/g dw) and with concentrations measured as high as 39.8 ng/g dw. Rel-4'-((1r,1'r,4R,4'R)-4'-ethyl-[1,1'-bi(cyclohexan)]-4-yl)-3,4-difluoro-1,1'-biphenyl (RELEEBCH or 2bcHdFB) was found at the highest detection frequency (DF) among LCMs analyzed in human (DF: 89%), dog (DF: 28%), and cat (DF: 50%) feces, although this compound accounted only <4% of ΣLCM concentrations. The mean cumulative daily intakes of ΣLCMs, calculated through a reverse dosimetry approach, were 71.7, 87.5, and 10.7 ng/kg body weight (bw)/day for humans, dogs, and cats, respectively. This study provides evidence of exposure of both humans and pets to LCMs, highlighting the importance of assessing sources of exposure and associated health risks.

Graphical Abstract

1. INTRODUCTION

Liquid crystal monomers (LCMs) are extensively used in the production of liquid crystal displays (LCDs) of various electronic devices. Primarily based on biphenyl or bicyclohexane structures, these compounds have unique optical properties essential for display technologies [1]. Global production of organic light emitting diode (OLED) and LCD in 2020 was 349 million m² and is forecasted to reach 450 million m² in 2025 [2]. More than 1170 LCM analogues have been reported to exist and hundreds of them are persistent and bioaccumulative [3]. LCMs are released into the environment during various stages of an electronic product’s lifecycle, including production, usage, and disposal [4]. Recent studies have documented the presence of LCMs in various environmental matrices such as landfill leachate, sewage sludge, sediment, air, and soil [5-9], with particularly high concentrations in soil/dust from electronic waste recycling facilities [10-12].

LCMs exhibit both acute and chronic toxicities [13,14]. Zebrafish exposed to LCMs showed accumulation of these chemicals in the intestine, brain, and gills, with greater accumulation of cyanobiphenyl-based compounds than fluorine and alkyl-based compounds [15]. Although LCMs were reported exhibit low toxicity, some of them can cause sensitization, irritation, and corrosion to eyes and skin [16], disrupt metabolism and induce oxidative stress in human renal epithelial cells, affect immune responses, act as antagonists of peroxisome proliferator-activated receptor γ [17], and modulate gene expression in chicken embryonic hepatocytes [18,19]. Studies have reported biotransformation of LCMs in vitro and in vivo [20,21]. Some biotransformation products were more toxic than the parent LCMs [22].

Studies have reported the occurrence of LCMs in human skin wipes, serum, and breast milk [10,23,24]. Skin wipes from hands and foreheads of workers at an electronic waste recycling facility contained >21 LCM analogues, with ΣLCM concentrations in the ranges of 10–1,200 μg/m² and 13–2,000 μg/m², respectively [10]. Serum ΣLCM concentrations among electronic waste dismantling workers and a reference group ranged from 7.8 to 280 ng/mL and 3.2 to 29 ng/mL, respectively [23]. Human breast milk from China was also reported to contain ΣLCMs at a median concentration of 133 ng/g lipid weight [24].

Very little is known about the occurrence of LCMs in human general population. In our earlier study we found measurable concentrations of LCMs in the feces of dogs compared to those found in urine [25]. Pet dogs and cats are sentinels of human exposure to indoor chemicals [26], as they share a common living environment with humans [27,28]. In this study, we determined concentrations of 52 LCMs in the feces of humans, dogs and cats, using gas chromatography-high resolution mass spectrometry (GC-HRMS), to provide baseline information on exposures. Gender, age, body weight, and sampling location related variations in fecal LCM concentrations were examined. We also calculated cumulative daily intakes (CDIs) of LCMs, based on the concentrations measured in feces, through a reverse dosimetry approach.

2. MATERIALS AND METHODS

2.1. Chemicals and Reagents

Analytical standards of 52 individual LCMs (purity >95%) were purchased from AmBeed Ltd (Arlington Heights, IL, U.S.). Detailed information including chemical name, abbreviation, formula, and purity of 52 LCMs analyzed in this study is listed in Table S1. Four isotopically labeled polychlorinated biphenyl congeners (PCBs), namely, ¹³C₁₂-PCB-28 (¹³C₁₂–2,4,4′-trichlorobiphenyl), ¹³C₁₂-PCB-118 (¹³C₁₂–2,3′,4,4′,5 pentachlorobiphenyl), ¹³C₁₂-PCB-153 (¹³C₁₂–2,2′,4,4′,5,5′-hexachlorobiphenyl), and ¹³C₁₂-PCB-180 (¹³C₁₂–2,2',3,4,4',5,5'-heptachlorobiphenyl) were used as surrogate standards, and were purchased from Cambridge Isotope Laboratories Inc (Tewksbury, MA, U.S.). ACS/HPLC-grade dichloromethane (DCM) and ACS/HPLC-grade hexane was purchased from Honeywell (Charlotte, NC, U.S.), LC/MS-grade methanol (MeOH) was purchased from Fisher Scientific (Waltham, MA, U.S.), respectively. Information regarding chemical names, abbreviations, and chemical formula of 52 LCMs analyzed in this study is listed in Table S1.

2.2. Sample Collection

Eighteen human feces, 26 cat feces, and 39 dog feces were collected from Albany and New York City, USA, during June 2023–April 2024. Human feces were from anonymous donors. Dog feces were collected from an animal shelter and individual pet owners. One of the cat feces samples was from an individual pet owner, and the remaining samples were from an animal shelter. Further details of sample collection have been provided elsewhere [28,29]. Information regarding gender, age, sampling location and sample extraction is given in Tables S2-S4. Feces of humans, cats and dogs were collected directly into a polypropylene (PP) cup. Samples were transferred to the laboratory immediately and stored at −20°C. Pet feces samples were air-dried at room temperature (~20°C) whereas human feces were freeze-dried. Dried samples were ground using a pestle and mortar, sieved through a 100-mesh stainless-steel sieve and then stored at −20°C until analysis. The moisture content of feces was determined gravimetrically, from the weight of samples measured before and after drying. The institutional review board approval was obtained from New York State Department of Health for the analysis of deidentified human specimens.

2.3. Sample Extraction and Purification

Feces were analyzed by following a method described earlier with some modifications [25]. Feces (200 mg dry weight) samples were spiked with 20 ng each of surrogate standards, ¹³C₁₂–2,4,4′-trichlorobiphenyl (¹³C₁₂-PCB-28), ¹³C₁₂–2,3′,4,4′,5 pentachlorobiphenyl (¹³C₁₂-PCB-118), ¹³C₁₂–2,2′,4,4′,5,5′-hexachlorobiphenyl (¹³C₁₂-PCB-153), and ¹³C₁₂–2,2',3,4,4',5,5'-heptachlorobiphenyl (¹³C₁₂-PCB-180) and equilibrated for 30 min. Then, 8 mL dichloromethane (DCM)/hexane (1/1, v/v) was added, and the mixture was shaken in a reciprocating shaker (Eberbach Corp., Ann Arbor, MI, U.S.) at 250 strokes per min for 20 min, ultrasonicated at 40 kHz (Branson 3510R-DTH, Branson Ultrasonics Corp., Danbury, CT, U.S.) for 20 min, and centrifuged (Eppendorf Centrifuge 5804, Hamburg, Germany) at 3,000 rpm for 20 min. The supernatants were combined after repeating the above extraction steps thrice and concentrated to 1 mL under a gentle stream of nitrogen.

The extracts were purified by passage through HyperSep^™ silica cartridges (1,000 mg, 6 mL; Thermo-Fisher Scientific, Waltham, MA, U.S.), which were conditioned by the elution of 12 mL 10% MeOH/DCM (v/v) and 14 mL 5% DCM/hexane (v/v). The sample extracts were then loaded onto silica cartridges and eluted with 8 mL 5% DCM/hexane (v/v). The extracts were evaporated to near dryness, re-dissolved in 1 mL hexane, filtered through a 0.22 μm polytetrafluoroethylene membrane filter to remove particles, and stored at −20°C until instrumental analysis.

2.4. Instrumental Analysis

The target analytes were determined using an Agilent 7890B gas chromatography (GC) connected to a JMS-800D UltraFOCUS magnetic sector high resolution mass spectrometer (HRMS; JEOL, Peabody, MA, U.S.) in an electron impact ionization (70 eV) and selected ion monitoring (SIM) mode. The mass spectrometer was operated at a resolution of R>10,000. An HP-5MS capillary column (30 m × 0.25 mm i.d. × 0.25 μm film thickness, Agilent, Santa Clara, CA, U.S.) was used for the separation of target analytes. The injector and ion source temperatures were maintained at 290°C and 250°C, respectively. Helium was used as the carrier gas at a constant flow of 1.2 mL/min. The column temperature was programmed from 80°C, held for 3 min, increased to 160°C at 20°C/min (held for 1 min) and to 240°C at 10°C/min (held for 3 min), and finally to 300°C at 10°C/min (held for 10 min). The mass scans ranged from m/z 50 to 700. The analytes were confirmed by monitoring quantification ion and retention time (Table S5). GC-HRMS chromatograms of LCMs detected in actual and fortified feces samples are shown in Figure S1.

2.5. Quality Assurance and Quality Control

Target analytes were quantified using an external calibration method. A 10- to 12-point standard calibration curve was prepared at analyte concentrations in the range of 0.01–50 ng/mL, along with 20 ng/mL surrogate standards. Two procedural blanks, two matrix blanks, and two matrix spikes were analyzed with each batch of 20 samples. None of the target analytes was found in procedural blanks. Hexane was injected after every 10 samples to monitor for potential instrumental contamination and carryover of analytes between samples. The limits of quantification (LOQs) of all analytes were defined as 10 times of signal-to-noise ratio (S/N) of chromatograms. LOQs were in the ranges of 0.03–4.53 ng/g dw (Tables S5). The recoveries of LCMs spiked into feces at 10 ng/g were in the range of 68–131% (Table S5).

2.6. Statistical Analysis

OriginPro 2024 (OriginLab Corporation, Northampton, MA, U.S.) and Python 3.7 (Python Software Foundation, Beaverton, OR, U.S.) were used for data analysis. Correlations among the concentrations of analytes determined in feces were investigated using Spearman’s rank correlation. The differences in analyte concentrations between ages, gender, and sampling location were examined using Mann-Whitney U test. A value of p <0.05 was considered statistically significant.

2.7. Exposure Assessment

We estimated cumulative daily intakes (CDIs) of LCMs based on the concentrations measured in feces, using the following equation [30]:

$$CDI=\frac{fecalconcentration\left(\frac{ng}{g}\right)\times fecalexcretionrate\left(\frac{g}{day}\right)}{bodyweight(kg)}$$

The reported average fecal excretion rate for humans, cats and dogs was 128, 19.4 and 254 g wet weight (ww)/day, respectively [30,31]. The body weights of humans, cats, and dogs were obtained at the time of sample collection. For this calculation, dry-weight based concentrations of LCMs were converted into wet-weight basis, based on the measured moisture content of feces (Tables S2-S4). This daily intake calculation assumed that LCMs were fully excreted via feces (due to the lack of toxico-kinetic information) and therefore it should be considered as an estimate. Furthermore, use of mean concentrations (median was not calculated due to the large portion of non-detects for several LCMs) add large uncertainties in intake assessments, but the information would help further refinements in exposure assessments. These values should be considered ballpark estimates.

3. RESULTS AND DISCUSSION

3.1. Concentrations and Profiles of LCMs in Human and Pet Feces

Among 52 LCMs analyzed, 24, 8, and 6 of them, respectively, were found in human, cat and dog feces. Representative chromatograms of 24 LCMs in real feces samples and fortified samples, are shown in Figure S1. Significantly higher concentrations of ΣLCMs were found in human feces, with a mean concentration of 8.01 ng/g dw, than those of dogs (mean: 1.82 ng/g dw) and cats (1.24 ng/g dw) (p <0.05) (Table 1 and Figure S2). In comparison to other environmental chemicals measured in feces in our laboratory, the mean concentrations of ΣLCMs in dog and cat feces were lower than the concentrations of per and polyfluoroalkyl substances (PFAS) [30], bisphenol A (BPA) [26], nicotine [32], polycarbonate-based microplastics (PC) [26], aromatic amines (AAs) [32], terephthalic acid (TPA) [26], quaternary ammonium compounds (QACs) [28], and polyethylene terephthalate-based microplastics (PET) [26]. In human feces, the mean concentrations of ΣLCMs were lower than the concentrations of PC, TPA, QACs and PET (Figure 1) [29,33]. It is likely that LCMs accumulate in body tissues and/or are bio-transformed. Further studies should focus on biotransformation and bioaccumulation of these compounds. Among several LCM derivatives found in feces, fluorinated LCMs (F-LCMs) were relatively more abundant, accounting for 88–100% of the total concentrations (Figure 2B). Only two non-fluorinated LCMs (NF-LCMs), namely 4-cyano-4"-pentyl-p-terphenyl (CN-PPTP) and 4-cyano-4'-pentyloxybiphenyl (CN-OPB) were found in the feces of humans, accounting for 8% and 4% of the total LCM concentrations, respectively. CN-PPTP (mean: 505 ng/cm²) was previously detected in smartphones [4]. NF-LCMs were not found in the feces of dogs and cats. Among F-LCMs, (trans,trans)-4-(2,3-difluoro-4-methylphenyl)-4'-ethyl-1,1'-bi(cyclohexane) (DFMPEBC) (mean: 2.38 ng/g dw), 4-ethoxy-2,3-difluoro-4'-(4-propylcyclohexyl)biphenyl (EDFPB) (mean: 2.95 ng/g dw), and 4-fluoro-4'-(trans-4-propylcyclohexyl)-1,1'-biphenyl (FPrCB) (mean: 2.00 ng/g dw) were the most abundant in the feces of humans, dogs, and cats, respectively, accounting for 5.85%, 20.8%, and 21.6% of the total LCM concentrations (Figure 2A). However, their detection frequencies (DFs) were only 3–6% (Table 1). Among them, EDFPB (mean: 44,800 ng/cm²) was previously detected in smartphones [34].

Table 1.

Concentrations (ng/g dry weight [dw]) (detection frequency [DF], mean, and range) of 52^a liquid crystal monomers (LCMs) in feces from humans (n = 18), dogs (n = 39) and cats (n = 26).

	humans			dogs			cats
LCM	range	mean ± SD	DF	range	mean ± SD	DF	range	mean ± SD	DF
RELEEBCH	0.015–1.14	0.587±0.309	89%	0.168–2.56	0.505±0.661	28%	0.013–0.48	0.094±0.138	50%
ETDFPPB	NDb	1.662	6%	ND	ND	ND	ND	1.64	4%
ETFMBC	0.05–4.98	1.11±1.22	56%	1.54–1.58	1.56±0.013	15%	ND	1.95	4%
ETFPBC	1.56–1.87	1.71±0.156	11%	ND	1.47	3%	ND	ND	ND
DFECB	1.66–1.94	1.80±0.136	11%	ND	ND	ND	ND	ND	ND
ETeFT	1.39–3.73	1.73±0.761	44%	ND	1.43	3%	ND	ND	ND
TeFPrCB	1.85–2.20	1.93±0.133	28%	ND	ND	ND	ND	ND	ND
FPPrBC	ND	1.93	6%	ND	ND	ND	ND	ND	ND
DFPPrBC	ND	2.07	6%	ND	ND	ND	ND	ND	ND
DFMPEBC	ND	2.38	6%	ND	ND	ND	ND	ND	ND
TFPrCCB	1.96–2.33	2.07±0.152	22%	ND	1.99	3%	ND	ND	ND
TFPxM-DFFPBP	1.12–1.43	1.21±0.128	22 %	ND	ND	ND	ND	ND	ND
DFPrB	1.71–2.06	1.89±0.175	11%	ND	ND	ND	ND	ND	ND
FPrCB	1.85–2.21	1.93±0.127	33%	ND	ND	ND	ND	2.00	4%
TeFPrT	1.51–1.80	1.66±0.149	11%	ND	1.5	3%	ND	1.99	4%
CN-OPB	1.76–1.85	1.81±0.048	11%	ND	ND	ND	ND	ND	ND
TFTPCTFBP	1.39–1.78	1.58±0.195	11%	ND	ND	ND	ND	ND	ND
DFMPPBCH	ND	1.64	6%	ND	ND	ND	ND	ND	ND
EFPrTP	1.15–1.47	1.26±0.149	17%	1.24–8.67	2.79	13%	1.15–2.69	1.57±0.515	27%
EDFPB	1.63–2.04	1.77±0.189	17%	ND	2.95	3%	ND	ND	ND
FPePrT	1.24–1.56	1.36±0.145	17%	ND	ND	ND	ND	ND	ND
CN-PPTP	1.33–5.78	3.15±1.89	22%	ND	ND	ND	ND	ND	ND
TFPrBCB	ND	1.15	6%	ND	ND	ND	ND	ND	ND
PDTFMTFT	ND	0.405	6%	ND	ND	ND	ND	ND	ND
Sumc	ND–39.8	8.01±9.79		ND–8.67	1.82±1.72	ND	ND–7.61	1.24±1.83
CDId	ND–374	71.7±90.6		ND–566	87.5±125		ND–48.7	10.7±14.2

^a28 LCMs that were not detected in any feces sample, so only 24 LCMs are listed on this table;

^bValues below the limits of quantification (LOQs) are denoted as not detected (ND);

^cThe mean and range of sum of liquid crystal monomers (ΣLCMs) in all samples;

^dThe unit of cumulative daily intake (CDI) is ng/kg bw/day;

Figure 1.

Mean concentrations of sum of liquid crystal monomers (ΣLCMs) measured in feces of dogs, cats and humans from New York State, compared against other chemicals reported previously in feces. PET, polyethylene terephthalate-based microplastics; QACs, quaternary ammonium compounds; TPA, terephthalic acid; AAs, aromatic amines; PC, polycarbonate-based microplastics; PFAS, per and polyfluoroalkyl substances; BPA, bisphenol A; PFOA, perfluorooctanoic acid [26,28-30,32,33].

Figure 2.

Relative distribution of several derivatives of (A) liquid crystal monomers (LCMs) and (B) between F-LCM and NF-LCM measured in human, dog and cat feces from New York State, USA. Average concentrations were used for the calculation of percent contributions.

Rel-4'-((1r, 1'r,4R,4'R)-4'-ethyl-[1,1'-bi(cyclohexan)]-4-yl)-3,4-difluoro-1,1'-biphenyl (RELEEBCH or 2bcHdFB) was the most frequently detected compound in human (DF: 89%), dog (DF: 28%), and cat (DF: 50%) feces (Table 1), but accounted only for <4% of the total LCM concentrations, respectively (Figure 2A). RELEEBCH is a fluorinated biphenyl-bicyclohexane compound. A recent study reported RELEEBCH as a major contaminant in soils collected in the vicinity of LCM manufacturing factories in China [11,35]. RELEEBCH has been detected at low concentrations in LCD panels and indoor dust [25], but several studies reported the occurrence of this compound in foodstuffs [15,24,36]. Overall, human feces contained several NF-LCMs relative to those of pets (Figure 2B) and that those LCMs present in feces were also reported to occur in indoor dust or soil samples [25,37]. Direct exposure of humans to electronic devices may explain higher fecal concentrations of LCMs compared to those of dogs and cats [4,38]. LCMs with lower molecular weights and octanol/water partition coefficients can more easily penetrate human skin and enter the bloodstream [39].

Exposure to LCMs can occur primarily through inhalation, ingestion, and dermal contact. Previous studies have reported widespread occurrence of LCMs across various environmental media, at the following median ΣLCM concentration ranges: air (24.5–204 ng/m³) [7], skin wipes (4,160–62,100 ng/m²) [10], dust (41.6–18,500 ng/g dw) [10,12,18,40-42], water (2.71–16.8 ng/L) [43], and foodstuffs (0.13–9.5 ng/g ww) [14,36] (Table S6 and Figure 3). Volatilization from or dermal contact with digital displays can be a source of human exposure. A few consumer products such as polarized eye glasses may contain LCMs. The most frequently detected LCM in feces, RELEEBCH, was also found in potatoes and seafood [36]. The sources of LCMs found in foodstuffs are not known and further studies are needed on this regard.

Figure 3.

Reported data on exposure sources of liquid crystal monomers (LCMs) through inhalation, dermal contact, and ingestion. (A) Range of median concentrations of sum of LCMs (ΣLCMs) and number of measured LCMs in reported samples. The raw data are presented in Table S6; (B) three main exposure routes and the range of reported estimated daily intake (EDI); the raw data are presented in Table S6; (C) mean ΣLCMs concentrations in feces of cats, dogs, and humans and number of detected LCMs found in this study.

In serum samples of e-waste dismantling workers, 29 out of 60 targeted LCMs were detected with a median concentration of 35.2 ng/mL (range: 7.78 to 276 ng/mL) [23]. 3,4-Difluoro-4'-(trans-4-propylcyclohexyl)-1,1'-biphenyl (DFPrB), 2',3,4,5-tetrafluoro-4"-propyl-1,1':4',1"-terphenyl (TeFPrT), and 3,4-difluoro-4'-(trans-4-ethylcyclohexyl)biphenyl (DFECB), which were found human feces, were also found frequently in serum. Similarly, (trans,trans)-4-ethyl-4'-(4-(trifluoromethoxy)phenyl)-1,1'-bi(cyclohexane) (ETFMBC), 4"-ethyl-2',3,4,5-tetrafluoro-1,1':4',1"-terphenyl (ETeFT), FPrCB, and rel-2,3',4',5'-tetrafluoro-4-((1s,4r)-4-propylcyclohexyl)-1,1'-biphenyl (TeFPrCB), frequently detected in human feces, were also found in serum [23]. TeFPrT and ETeFT were previously reported to be present in smartphone screens [34].

3.2. Determinants of LCM Exposure

Lower concentrations of ΣLCMs (not statistically significant) were found in dog feces collected from individual pet owners (mean: 1.33 ng/g dw) than those from animal shelter (mean: 2.47 ng/g dw) (Table S7). However, the types/diversity of LCMs found in dog feces from pet owners exceeded those from shelters. Specifically, 4"-ethyl-2',3,4,5-tetrafluoro-1,1':4',1"-terphenyl (ETeFT), 3,4,5-trifluoro-4'-(trans-4-propylcyclohexyl)-1,1'-biphenyl (TFPrCCB), TeFPrT, and EDFPB were found only in pet owners’ dog feces (Table S8). ETeFT (mean: 14,500 ng/cm²), TeFPrT (mean: 14,000 ng/cm²), and EDFPB (mean: 44,800 ng/cm²) were found at significant concentrations in smartphone screens [34]. ETFMBC and trans(trans)-4-ethyl-4'-(3,4,5-trifluorophenyl)-1,1'-bi(cyclohexane) (ETFPBC) were found only in dog feces from shelters (Table S8). Dogs in households may be exposed to a greater variety of LCMs than those from shelters. Household electronics, such as smartphones, desktop monitors, laptops, and televisions, are the major sources of LCMs in the indoor environment [4,38].

The fecal concentrations of ΣLCMs were higher in females than human males, but in pets, feces from males had higher concentrations than females (Table S7). However, these differences were not statistically significant. A weak negative correlation between human age and ΣLCM concentrations was found (r = −0.587, p <0.05). No significant correlation between age of pets and ΣLCM concentrations was found. Similarly, no significant correlation between body weight and ΣLCM concentrations was found (Table S9). Age and body weight were not major determinants of ΣLCM concentrations measured in feces across species (cats, dogs, and humans).

3.3. Exposure Doses

Little is known about the toxico-kinetics of LCMs in humans or laboratory animals. Available studies indicate preferential accumulation of LCMs in lipid rich tissues [20], while some studies also suggest extensive metabolism of these chemicals [21]. Absorption, distribution, metabolism and excretion of LCMs can vary depending on the structure, molecular weight, molar volume, level of fluorination and oxygenation [21,37]. Nevertheless, given the paucity of information on exposure doses of LCMs in humans and pets, the measured concentrations in feces were extrapolated to estimate exposure doses. This calculation assumes that LCMs are fully excreted in feces and therefore should be considered as a crude estimate. The mean estimated CDIs for ΣLCMs were 71.7 ng/kg body weight (bw)/day for humans, 87.5 ng/kg bw/day for dogs, and 10.7 ng/kg bw/day for cats (Table 1). Although the LCM concentrations in dog feces were lower than those of humans, the fecal excretion rate in dogs were two-fold higher than those of humans. Thus, CDIs of LCMs were higher in dogs than those of humans. The mean CDI of individual LCMs is shown in Table S10. Among LCM analogues, CN-PPTP, TFPrCCB, and 4"-ethyl-2'-fluoro-4-propyl-1,1':4',1"-terphenyl (EFPrTP) exhibited the highest mean CDIs at 31.5, 277, and 16.8 ng/kg bw/day for humans, dogs, and cats, respectively. The CDI values of EFPrTP were far below the reported tolerable daily intakes and no observed adverse effect levels (Table S10) [36].

The reported exposure doses of LCMs through inhalation and ingestion were compared with the CDI calculated in our study. The estimated daily intakes (EDIs) of ΣLCMs through dermal contact of indoor dust were in the range of 0.15–16.5 ng/kg bw/day and those through ingestion of food and dust were in the range of 0.016–173 ng/kg bw/day (Table S6 and Figure 3) [10,36,42]. The EDIs were lower than those of CDIs calculated in our study (up to 374 ng/kg bw/day). These results suggest the existence of other sources of LCM exposure. Several LCMs with high log K_ow may accumulate in lipid rich tissues and therefore our exposure dose is an underestimate of actual exposures [20,21,44]. Furthermore, >1170 LCM analogues have been reported to be found in LCDs [3] and comprehensive studies are needed to determine the occurrence of all LCMs in human specimens for the assessment of exposure doses and health risks. It should be noted that the CDI calculated in this study is a crude estimate and presents large uncertainties due to the use of mean concentrations in the calculation. However, in the absence of any data, our estimates can be used as an information to refine further assessments.

4. CONCLUSIONS

We report the occurrence of and exposure to 52 LCMs in humans, dogs and cats. RELEEBCH was the most frequently detected fluorinated LCM analogue in feces of humans and pets, but its contribution to ΣLCM concentrations was only <4%. The exposure doses of LCMs, calculated based on the concentrations measured in feces, were higher than those reported previously through diet and dust ingestion, suggesting the need for further studies to identify sources of exposure. Our study establishes baseline information on exposure to LCMs and suggest the need for further studies on a broader range of LCMs and their metabolites.

Environmental Implications

Here we document human and pet exposure to LCMs, which are widely used in LCDs. Among >1170 LCM analogues reported to exist in LCDs, studies have reported up to 93 of them in air, skin wipes, dust, water, and food [7,10,12,14,18,36,40-43]. In human feces, only 24 LCMs were detected suggesting that these chemicals are metabolized or sequestered in body tissues. Further studies are needed to elucidate the sources, absorption, disposition, metabolism, excretion and toxicity of LCMs. Similar to legacy persistent organic pollutants, LCMs possess high log K_ow values in the range of 4 to 13 and they accumulate in lipid rich tissues [15]. Extensive metabolism of some LCMs has also been documented [21]. Although analysis of feces offers an approach to assess exposures, this may not reflect overall body burden of chemicals. Future studies should focus on the determination of these chemicals in human and animal tissues.

Environmental implications.

Liquid crystal monomers (LCMs) are widely used in digital displays including smart phone, computer and television screens. Although studies reported the occurrence of LCMs in indoor dust and in electronic products, analysis of these contaminants in biological samples was fraught with potential contamination issues. We used high resolution magnetic sector mass spectrometry to detect and quantify LCMs in feces of humans, dogs and cats and report widespread occurrence exposure for the first time. Our study provides valuable insights into the exposure to LCMs by humans which warrant further studies to assess risks.

Highlights.

• Liquid crystal monomers were found in the feces of humans, dogs and cats.

• LCMs concentrations human feces were higher than those of dogs and cats.

• Profiles of LCMs varied widely among human, dog and cat feces.

• Fluorinated LCMs were the predominant compounds found in feces.

• Estimated intakes of LCMs among humans and pets were few tens of ng/kgbw/d.