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.
1.4 Sustainability of Plastics
The notion of environmental sustainability is a complex one (well outside the scope of this chapter) and according to its original definition, refers to a mode of development that “meets the needs of the present generation without compromising the ability of future generations to meet their own needs.” It is a laudable, qualitative statement, but the strategy to achieve this objective is not clear, especially where development involves depleting a fixed reserve of a natural resource such as rare earth elements or oil. Not only is the number of future generations not specified, but they are also assumed to have the same set of needs as the present generation does. A tempting approach is to “decouple” growth in GDP from environmental impacts (Luo et al. 2019), allowing sustainable development to proceed unhindered. This, however, is not realistic (Ward et al. 2016), especially in the future plastics industry.
But what can be realistically implemented to improve the sustainability of plastics in the near or medium term? The goal should be to ensure that the rapidly depleting resource base for resin production lasts long enough for technological advances to perhaps make those resources obsolete by discovering substitutes. Finding ways to minimize the environmental impact of plastics, given the future increase in production levels, is also critical to ensure sustainability. Specifically, three strategies towards sustainability deserve close attention.
1 Energy economy: Plastic resin manufacture and processing into consumer products needs to be more energy‐efficient and, wherever possible, rely on renewable energy instead of conventional fossil‐fuel derived energy. Innovations to capture process waste energy such as low‐grade heat for reuse needs to be enabled.
2 Feedstock economy: Using the minimum amount of plastic materials to deliver the necessary functionality for the performance of the product, needs to be implemented. Conserving fossil‐fuel feedstock by material recycling (as well as chemical or energy recycling) of post‐use plastics and where feasible, by substituting bio‐based plastics in place of fossil‐fuel‐derived resins should be incentivized.
3 Minimizing toxicity: Minimizing the release of toxic chemical by‐products from resin production, processing, use, and waste disposal of plastics especially into the ocean environment, is a priority. This requires urgently substituting some of the toxic legacy additives with known adverse ecological impacts, with non‐toxic alternatives. Also, capturing CO2 and process gaseous emissions for conversion into useful products to reduce the environmental footprint of the material should be encouraged.
In 2020 with the COVID‐19 pandemic slowing down economic activity, the energy demand also decreased, with that for oil and coal dropping by 7–8% each, accompanied by a consequent decrease in the global carbon emissions. Despite this respite, the world energy demand is still expected to grow by nearly 50% by 2050 to reach 240 Quads (quadrillion BTU; EIA 2020), led primarily by industrial growth in Asia. While more renewable energy will be available in the future, their percent contribution to the overall energy supply will still be only about 16%, as opposed to the present 11%. Therefore, conserving energy, along with using more renewable energy and improving efficiency in plastic processing (for instance, with all‐electric molding technology) will be important in making plastics more sustainable in the future.
Moves to encourage economy in using energy and material, conveniently align well
