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Bioaccumulation is the gradual build‐up of the concentration of a compound in an organism relative to that in its environment, due to ingestion or other modes of intake. Biomagnification is an increased concentration of the compound in the predator relative to that in its prey (Miller et al. 2020). A compound that is ingested, but neither metabolized nor excreted at a rate faster than it is consumed by the organism bioaccumulates in its tissue. This is especially true of compounds with a high partition coefficient, (see Chapters 2 and 9). Where the compound is toxic, as with organic mercury, this is a serious concern. Biomagnification results when a predator organism ingests multiple prey organisms, each with a high bioaccumulation of the compound or pollutant (Drollliard 2008; Mizukawa et al. 2009). This leads to a faster build‐up of the chemical compound in predator tissue compared to that in the prey. For instance, when DDT was used liberally, it ended up in the water and invariably in fish. The insecticide was biomagnified in predatory birds such as Ospreys feeding on the fish, resulting in an abnormal thinning of the shells of their eggs.

A highly bioavailable compound is readily transported from the gut into the systemic circulation. When a contaminated plastic fragment is ingested, the contaminant must leach out in the gut and permeate through the gut wall, for it to be bioavailable to the organisms. Otherwise, it is egested and has a little physiological effect. Bioavailability can therefore also, depend on lipid levels in the diet, the presence of gut surfactants, and the gut pH (Koelmans et al. 2014; Kwon et al. 2017).

The bioavailability and toxicity associated with MPs in fish have been recently reviewed (Wang et al. 2020). But, the kinetics of leaching and the mechanism of bioaccumulation remains undefined (Qu et al. 2020). It is also reasonable to expect the bioavailability of POPs in the MPs to ingesting animals to be low (Koelmans et al. 2016; Ziccardi et al. 2016), and the MPs may instead even “clean” the gut environment by removing any existing hydrophobic pollutants (Lee et al. 2019; Scopetani et al. 2018). Black Sea Bass (Centropristis striata) presented with PVC pellets loaded with 10 wt% of dioctyl phthalate (DOP) plasticizer, ingested them at the same rate as “clean” or virgin PVC pellets, but the egested pellets showed no change in the DOP level (Joseph et al. 2020). Ingestion of MPs of PE spiked with benzophenone by rotifers, copepods, bivalves, echinoderms also did not result in any toxic outcomes (Beiras et al. 2018). The level of POPs delivered to organisms may be low as the fraction of MPs in the diet has to be minuscule. But, pollutants such as endocrine disruptor chemicals or antibiotics, act at very low concentrations, some displaying a non‐linear dose‐response curves, allowing them to elicit adverse physiological responses at unexpectedly low doses. Also, the physiological effects in these studies were monitored only over the short term. The data taken together do not rule out the possibility of MPs transferring POPs to biota via ingestion, at least in some species.

Pathways that potentially contribute to the dietary intake of MPs, and especially NPs, in humans are now receiving the focused research attention they deserve (see Chapter 13). While the presence of MPs/NPs in food (Kosuth et al. 2018) and beverages (Schymanski et al. 2018; Shruti et al. 2020), and especially seafood (Smith et al. 2018), is well established, no adverse effects on human health have yet been linked to them (see Chapter 13). But, the relevant data, when considered together, suggest the accumulation of NPs and small MPs may have adverse long‐term effects (Yong et al. 2020). An interesting and worrisome development are the findings that show NPs enter systemic circulation via the gut (Revel et al. 2018); some report (Hussain et al. (2001) unexpectedly find MPs as large as 100 μm to translocate into lymphatic circulation from the gut in humans. Ragusa et al. (2021) recently reported 5–10 μm MPs in the human placenta; 5 particles were isolated from 4 placentae, with less than 5% of the placental mass being analyzed. At this size range, however, MPs may even compromise the blood‐brain barrier (Barboza et al. 2018), and those <20 μm have been shown to access all internal organs (Campanale et al. 2020). A few in vivo studies (Deng et al. 2017; Jin et al. 2019) on mice, including one on effects on offspring (Luo et al. 2019), show physiological effects of ingesting particles ~5 μm in size. However, an in vitro study on human cell lines (human colon epithelial cell) co‐cultured with BeWo b30 (human placental trophoblast cell) did not show the same (Hesler et al. 2019). The study found that 0.5‐μm PS NPs did not significantly compromise the in vitro placental and intestinal barriers. This is a topic with profound implications that deserves focused research attention.