Accumulation of short-, medium-, and long- chain chlorinated paraffins in tissues of laying hens after dietary exposure

Highlights

• 21 laying hens dietary exposed to SCCP, MCCP and LCCP mixtures for up to 91 days.

• All CPs detected in liver, showing a gastro-intestinal barrier crossing.

• SCCPs and MCCPs reached more easily the internal tissues than LCCPs.

• CPs affinity was differentiated between low and high perfusion rate matrices.

• An uneven mass balance suggested a potential biodegradation of CPs.

Abstract

Reliable human health risk assessment associated with chlorinated paraffins (CPs) exposure is limited by the lack of data on the fate of this complex family of contaminants. To gain knowledge on the accumulation and distribution of CPs in biota after ingestion, laying hens were dietary exposed to technical mixtures of short- (SCCPs), medium- (MCCPs), or long-chain (LCCPs) CPs of various chlorine contents during 91 days, at 200 ng/g of feed, each. Adipose tissue, liver, muscle and serum were collected at the steady-state, along with excreta. All C₁₀-C₃₆ CPs were detected in liver. However, differences were observed in CP distribution: LCCPs high %Cl were retained in the liver; LCCPs low %Cl circulated through the serum and were distributed in the different compartments, but were mostly excreted through the eggs; SCCPs and MCCPs were found in all tissues at similar levels. Finally, a mass balance indicated a potential for biotransformation.

Introduction

Chlorinated paraffins (CPs), a family of polychlorinated n-alkane chains (C_xH_2x+2−yCl_y) with varying chain length and chlorination degrees in the range 30–70% w/w, have been used since the late 1970s in many industrial applications such as lubricants in metal-working fluids, flame-retardants and plasticisers (European Food Safety Authority (EFSA) panel on contaminants in the food chain (CONTAM), 2020).

Consequently, CPs are produced in large volumes (estimated at 1,000,000 tonnes per year, Glüge, Wang, Bogdal, Scheringer, & Hungerbühler, 2016). Unfortunately, part of those CPs are released during production, uses and improper disposal of polymeric products containing these additives (Glüge et al., 2016). Thus, the CPs environmental levels usually surpass most of the levels of other halogenated contaminants such as polychlorobiphenyls, organochlorine pesticides or dioxins (Krätschmer et al., 2019, Niu et al., 2020, Zhou et al., 2018). CPs have been reported in most environmental compartments, such as air (Niu et al., 2020), water (Wang, Jia et al., 2019), soil (Aamir et al., 2019), sediment (Chen et al., 2011), biota (Yuan et al., 2019), food (Harada et al., 2011, Lee et al., 2020) and even human matrices (Wang, Gao, Wang, & Jiang, 2018). The ubiquity of CPs in the environment make them chemicals of concern, notably because they share similar physio-chemical properties with other halogenated contaminants that are considered hazardous for human health.

CPs are sub-categorised into short-chain CPs (SCCPs, C₁₀-C₁₃), medium-chain CPs (MCCPs, C₁₄-C₁₇), and long-chain CPs (LCCPs, C_≥18). SCCPs were classified as possibly carcinogenic to humans in 1990 by the International Agency for Research on Cancer (IARC working Group on the Evaluation of Carcinogenic Risk to Humans, 1990), and were later shown to cause chronic toxicity to marine species and mammals (Wang, Zhu et al., 2019). As a consequence, they have been phased out in Europe and North America (European Union, 2006, Government of Canada, 2009, United States Environmental Protection Agency (US EPA), 2015) since the 2010s and included in the Annex A of the Stockholm convention in 2017 (Conference of the Parties of the Stockholm Convention, 2017). However, there is a lack of toxicity and toxicokinetics data on MCCPs and LCCPs, though recent occurrence studies on terrestrial and marine ecosystems show their potential for bioaccumulation (Yuan et al., 2019). Recently, the EFSA published a scientific opinion on CPs concluding that risk assessment in Europe was hardly feasible, based on the few submitted toxicological and occurrence data (EFSA CONTAM panel, 2020) at the time of their evaluation. In particular, they emphasised the need for further information on the influence of the chain length and the chlorination degree of CPs on their toxicokinetics in humans and experimental animals.

To date, the few published toxicokinetics experiments focused on SCCPs and MCCPs mainly. In rodents, ¹⁴C-labelled SCCPs have been reported to be distributed primarily in fat, liver, bile and egg yolks (Biessmann et al., 1982, Biessmann et al., 1983). The authors also showed that lower chlorinated compounds could undergo degradation to CO₂, whereas the higher chlorinated compounds could not. Later, Fisk, Cymbalisty, Tomy, and Muir (1998) showed accumulation of SCCPs and MCCPs congeners in dietary exposed rainbow trout. In their study, the depuration half-lives varied from 5 to 53 days depending on the octanol–water partition coefficient (K_ow) and carbon chain length, indicating an influence of the homologue structure on the accumulation potential. More recently, Geng et al. (2016) observed SCCPs absorption and depuration in rats after a single-dose exposure. Alike other persistent organic pollutants, SCCPs distributed in tissues sensitive to the chlorine content. These studies demonstrated well the influence of the homologue structure on the bioaccumulation but were limited to C₁₀-C₁₄ chain lengths. In parallel, one study on SCCPs, MCCPs, and LCCPs in exposed aquatic invertebrates via contaminated water and feed revealed the strong potential of LCCPs for accumulation (Castro, Sobek, Yuan, & Breitholtz, 2019). However, contaminants have been shown to feature diverse accumulation behaviours in aquatic or terrestrial species (Sun et al., 2017). It was therefore of particular interest to extend the knowledge on the toxicokinetics of SCCPs, MCCPs and LCCPs at a homologue level on terrestrial vertebrates.

In the present study, the laying hen was selected as model organism, as it is a widely distributed farmed animal around the globe. Two previous experiments on hens confirmed the accumulation of SCCPs in abdominal fat, liver and kidney, although the relative concentration in the tissues did not follow the same trends (Sun et al., 2017, Ueberschär et al., 2007). In a previous work (Mézière et al., 2021), we exposed laying hens to an exposure mixture of CPs with various chain lengths and chlorination degrees, at environmentally relevant levels (5 × 200 ng/g ww, Dong, Li, Su, & Wang, 2019). Substantial amounts of CPs were found in eggs, suggesting that CPs could be absorbed and distributed in laying hens. In this study, we hypothesised that this distribution would be dependent on the homologue formula. We thus collected various tissues and fluids (muscle, liver, fat, serum, and the rest of the carcass) at days 77 and 91, days at which we believed the steady-state was reached. Additionally, excreta (faeces + urine) was collected to attempt a mass balance. The results provide valuable data on bio availability and distribution of CPs in the hens. In particular, to our knowledge this is the first time that LCCPs fate in terrestrial birds has been reported.