Evaluating oral and inhalation bioaccessibility of indoor dust-borne short- and median-chain chlorinated paraffins using in vitro Tenax-assisted physiologically based method
Introduction
Chlorinated paraffins (CPs), with general formula CnH2n+2-zClz, are among the most commonly used anthropogenic chemicals in the world (Gluge et al., 2016; Hahladakis et al., 2018; Xu et al., 2019). In 2012, the annual global CP production exceeded 1.1 million tons, most of which happened in China (Gluge et al., 2016). Due to their widespread production and usage, CPs have been frequently reported in numerous environmental compartments worldwide (Du et al., 2018; Sun et al., 2020; Wang et al., 2013). CPs are conventionally categorized as short-chain (SCCPs, C10-13), median chain (MCCPs, C14-17), and long-chain CPs (LCCPs, C 18) based on carbon chain length. They represent a suite of lipophilic compounds with a wide range of physical-chemical properties, e.g. log KOW values 4.10–11.34 (Gluge et al., 2013), depending on the carbon chain length and the chlorine content (Hilger et al., 2011). Among CPs, SCCPs have received most environmental concern due to their persistence, bioaccumulative potentials, long-range transport, and mammalian toxicity (Dong et al., 2019; Geng et al., 2015; Zeng et al., 2017). After the regulation on SCCPs by Stockholm Convention in 2017, MCCPs, as alternatives to SCCPs, are expected to have increasing production and usage (Zeng et al., 2015). Recent studies have revealed that MCCPs might share similar characteristics (e.g. persistence and bioaccumulation) as SCCPs (Castro et al., 2018; Gluge et al., 2018).
Ingestion of indoor dust is suggested as one of the major routes for human exposure to flame retardants and plasticizers due to their high occurrences in the residential environment (De Boer et al., 2016; Gao et al., 2018; Wong et al., 2017). The exposure dose can be even greater for toddlers because of the increased hand-mouth behavior and proximity to floors (Cui et al., 2016). Owing to the satisfactory chemical and thermal stability, CP products have been used as flame retardants and plasticizers for several decades. In China, the largest manufacturer of CP products (Gluge et al., 2016), the dust ingestion could account for 59 % and 51 % of the total external exposure to SCCPs and MCCPs, respectively, for toddlers (Gao et al., 2018). In addition to ingestion, inhalation was recently proposed as a potentially significant exposure pathway of CPs for humans (Cao et al., 2019; Friden et al., 2011; Huang et al., 2017; Zhan et al., 2017). During human breathing, the particle-borne hydrophobic organic contaminants (HOCs) are likely to penetrate deeply into the respiratory tract and encounter interstitial fluids (Liu et al., 2017; Wei et al., 2018). Most researchers have used the conservative assumption that all (i.e., 100 %) of HOCs present in soils and dust is available for human exposure, whereas the bioavailability of particle-borne chemicals may be significantly reduced after environmental aging (Humel et al., 2020; Portet-Koltalo et al., 2020). For example, Li et al. (2016) found that the oral bioavailability of dichlorodiphenyltrichloroethane (DDT) in historically contaminated soils was low at 17.9–65.4 %. This likely suggested that health effects depend not only on the amount of HOC the dusts carry, but also the HOC bioaccessibility (Raffy et al., 2018). Traditional assessment approach based on the bulk concentration might overestimate CP exposure risks.
In vitro methods are simple and inexpensive physiologically-based approaches to evaluate bioaccessibility of HOCs, and has been considered as satisfactory alternatives to predict bioavailability (Collins et al., 2015). The majority of in vitro models developed in previous studies statically mimicked human exposure to contaminants by sequential extraction with the simulated digestive fluids for a physiological residence period (Cui et al., 2016; Oomen et al., 2002). Actual gastrointestinal tract and lungs contains lipid membranes which tend to absorb and transfer HOCs. To mimic such process, several materials including C18 membrane, silicon rod, and Tenax were introduced as a sorption sink into the simulated system (James et al., 2011; Juhasz et al., 2014; Li et al., 2015; Zhang et al., 2015). in vitro models have successfully determined bioaccessibility of HOCs, such as DDTs, polyromantic hydrocarbons (PAHs), and polybrominated diphenyl ethers (PBDEs) in ingestion and lung matrices (Cui et al., 2016; Li et al., 2016; Zeng et al., 2019). Although the bioaccessibility of SCCPs through food intake have been reported by Cui et al. (2019), that of particle-borne CP residues for human exposure remains scarcely studied.
In the present study, (colon-extended) physiologically based extraction test (PBET) was applied to investigate ingestion and inhalation bioaccessibility of CPs with Tenax as adsorption sink. Two SCCP mixtures (51.5 %Cl and 63 %Cl) and two MCCP mixtures (42 %Cl and 57 %Cl) were tested. Our research goals were to (a) determine the ingestion bioaccessibility of SCCPs and MCCPs in dust samples and its dependence on their molecular sizes and physicochemical properties; (b) examine the effects of food nutrients on ingestion bioaccessibility; and (c) assess the inhalation bioaccessibility and associated influences of dust properties.
Section snippets
Sample collection and preparation
Original indoor dusts (approximately 50 g) were collected from residential homes in City of Riverside, California, the U.S. All indoor dusts were air-dried, homogenized, and passed through a 150-μm sieve. Our preliminary experiments showed that the SCCP and MCCP levels in these indoor dusts were 4.3 ± 0.9 and 16.7 ± 3.0 μg/g dw, respectively. The dust was divided into four groups (10.0 g dw each), and then spiked respectively with SCCPs 51.5 %Cl, SCCPs 63.0 %Cl, MCCPs 42.0 %Cl, and MCCPs 57.0
Tenax sorption of CPs in digestive fluids
Once CPs enter human digestive system through dust ingestion, gastrointestinal membranes may continuously absorb these dust-borne contaminants by removing their freely dissolved fraction from digestive fluids (Gouliarmou et al., 2013). Therefore, gastrointestinal absorption may significantly affect the extent to which CPs were desorbed from ingested dust. Fang and Stapleton (2014) reported that the addition of Tenax as a sink made the result of an in vitro digestion bioaccessibility test more
Conclusions
The present study clearly showed that Tenax could act as an efficient infinite sink in CE-PBET system to evaluate dust-borne CP bioaccessibility. The ingestion and inhalation bioaccessibility of CPs in dust assessed by Tenax-assisted system was remarkably higher than that by traditional approach (without Tenax). The ingestion bioaccessibility of SCCPs 51.5 %Cl, SCCPs 63 %Cl, MCCPs 42 %Cl, and MCCPs 57 %Cl in dust may be considered in ranges of 21.1–44.0 %, 11.7–45.8 %, 21.9–36.6 %, and 7.9–32.9
CRediT authorship contribution statement
Xinyu Du: Software, Data curation, Writing - original draft. Yihui Zhou: Formal analysis. Jun Li: Supervision, Conceptualization, Methodology, Writing - original draft. Yan Wu: Writing - review & editing. Ziye Zheng: Visualization. Ge Yin: Methodology, Writing - review & editing. Yanling Qiu: Investigation. Jianfu Zhao: Data curation. Guoli Yuan: Funding acquisition, Resources.
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgements
Funding: This work was supported by the National Natural Science Foundation of China [grant numbers 41872100]; and the Fundamental Research Funds for the Central Universities [grant numbers 2652018160 and 2652018158].
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