**1. Introduction**

Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) are a group of halogenated aromatic hydrocarbons that exhibit high toxic potential and that include polychlorinated dibenzo-dioxins (PCDDs) and polychlorinated dibenzo-furans (PCDFs), comprising a total of 210 congeners [1]. They typically accumulate in the air, soil, sediment, fish, human adipose tissue, and milk [2,3]. Dioxin-like polychlorinated biphenyls (DL-PCBs) are persistent organic pollutants that remain in the environment for long periods of time [4]. They are hailed as the most hazardous elements for human health and were enlisted during the Stockholm Convention owing to their high carcinogenicity [5]. DL-PCBs have a lipophile and stable chemical structure, making them resistant to degradation [6]. They are introduced into the aquatic environment, tend to settle in the sediment, enter the food chain via numerous routes, and accumulate in living organisms [7]. The International Agency for Research on Cancer (IARC) has classified PCDD/Fs and DL-PCBs as "Group I human carcinogens " [8]. As inducements, PCDD/Fs and DL-PCBs have become research

**Citation:** Wang, Y.L.; Fei, S.Y.; Wang, T.W.; Liu, X.T.; Gao, X.N.; Wu, H.T.; Hu, K. PCDD/Fs and DL-PCBs in Chinese Mitten Crab (*Eriocheir sinensis*) and Its Farming Environment in Shanghai, China. *Foods* **2022**, *11*, 2556. https:// doi.org/10.3390/foods11172556

Academic Editors: Fatih Öz, Dapeng Peng and Yongzhong Qian

Received: 21 July 2022 Accepted: 20 August 2022 Published: 24 August 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

hotspots. Previous studies revealed that the by-products of industrial manufacturing processes involving chlorine remain the major sources of PCDD/Fs and DL-PCBs, which are strongly associated with natural heat [9]. Subsequently, their footprints have also been found in the North and South Poles and may play a significant role in migration processes, posing a serious threat to the earth's ecological environment [10,11]. In the atmosphere, PCDD/Fs and DL-PCBs may be transported into the ocean and rivers via environmental deposition and may affect the health of aquatic animals. The PCDD/Fs and DL-PCBs are widely distributed in the aquatic environment and accumulate in aquatic organisms, being integrated into the food chain [12]. Hyo-bang Moon et al. [13] recorded the highest levels of PCDD/Fs and DL-PCBs residues in crustaceans followed by fish. Dietary intake is believed to be the main route of human exposure to dioxins, accounting for more than 90% of total human exposure [5]. They may be present in one's diet, as it is possible for them to be passed up through the food chain and to accumulate in humans at the top. At the same time, the presence of these compounds in the human body leads to serious health concerns due to their toxicity, indirectness, stability, and generational hereditary. They can also interfere with the immune, reproductive, and nervous systems and render a carcinogenic impact on human beings [6]. Therefore, food safety issues related to dioxins are attracting more and more attention. The tolerable daily intake (TDI) levels recommended by the World Health Organization (WHO) at a 1990 dioxin conference in the Netherlands were 10 pg TEQ/(kg (weight)·d) [14]. Since then, following the emergence of new epidemiological data and data regarding the significant effects of dioxins on neurological development and the endocrine system, the WHO experts consulted these data and re-evaluated their stance on dioxins, determining a new allowable TDI range of 1–4 pg TEQ/(kg (weight)·d) to ensure the safety of food and to build health barriers [15].

The Chinese mitten crab (*Eriocheir sinensis*) is a common freshwater crab, accounting for 18% of shrimp crab culture. They are widely distributed from Northern China to South Korea, existing in freshwater as juveniles and adults. Upon reaching sexual maturity, this crab migrates to brackish water to mate, lay eggs, and hatch, and it then returns to its freshwater habitat [16]. Its delicious meat, richness in unsaturated fatty acids, unique pleasant aroma, and delicious taste have promoted the Chinese mitten crab as a popular aquatic product. In 2021, global sales of the Chinese mitten crab market reached hundreds of millions of dollars, providing a substantial advantage to the marine industry. However, according to Taiwan's Food and Drug Administration, in October 2018, Taiwan had imported about 196 tons of Chinese mitten crab from the mainland, 40 tons of which contained more dioxin than standard levels, accounting for 20% of the total product (https://www.mohw.gov.tw/cp-16-44846-1.html (accessed on 21 January 2022). The sampling limit was four times higher than the maximum of 27 pq/g TEQ/g ww (more than 6.5 pq/g TEQ/g (wet weight)). In October 2017, the dioxin content of Chinese mitten crab from Hunan province exceeded the standard limit. Among them, 4.114 tons of hairy crabs constituted 4.1 pg/g of dioxin, and 4.455 tons of hairy crabs were found to constitute 12.1 pg/g of dioxin and PCBs, seriously affecting crab sales (https://www.ereying.com/articles/83.html (accessed on 27 January 2022).

The accumulation of contaminants in aquatic organisms is closely related to the food chain and its culture environment [17]. Studies have shown that animal feed remains one of the primary sources of pollution, and contaminated animal feed or feed additives may lead to food contamination [18,19]. In 1999, dioxin contamination was found in chicken feed in Belgium [20]; in the USA, livestock, aquaculture, and poultry feed exceeded dioxin limits from 2002 to 2003, and this revealed that dioxins enter the food chain via feed, potentially contributing to dioxin accumulation [21]. Other studies have shown that soil is the source of dioxin contamination in eggs from free-range hens on Polish farms [22]. As omnivorous animals, mixed feed is the primary source of nutrition for Chinese mitten crabs. Moreover, Chinese mitten crabs also eat aquatic plants and benthic animals. Contaminants in sediment might be the source of dioxins in samples of these crabs [23]. Some substances, such as

water bodies, food residues, and feces, will eventually form sediments, which may lead to dioxin accumulation and contamination in sediments and the development of secondary pollution [24]. Sediment is also a feeding route for crabs, so the chemical constituents in sediment can affect crab health. For Chinese mitten crab, the cultural environment has gained gradual prominence in its growth and development. The wide variety of substances consumed by Chinese mitten crabs indicates that the sources of dioxins and biphenyls may be complex and that the food and culture environment of Chinese mitten crabs may be contributing factors.

Currently, the sources of PCDD/Fs and DL-PCBs found in Chinese mitten crab in Shanghai are not clearly understood. This study investigated the PCDD/F and DL-PCB contents in Chinese mitten crabs and the potential sources of these contaminants. The amount of pollution found in Chinese mitten crab on the market was analyzed to provide a scientific basis for targeted prevention and control measures, public scientific consumption, and essential support to ensure the healthy breeding and sustainable development of livestock, poultry, and aquatic products.
