According to the U.S. National Oceanic and Atmospheric Administration (NOAA), microplastics are small plastic pieces less than 5 mm long that can be harmful to marine and freshwater organisms. A variety of sources have been cited for microplastic pollution, such as dumping of plastic waste into the oceans that degrades slowly and the use of microbeads as exfoliants in beauty products. Because of their tiny size, these pollutants escape water filtration systems and end up either in the oceans or in other water bodies, and cause serious environmental and food safety concerns. Extensive and indiscriminate use of food packages and drink bottles, synthetic textiles, car tires, paints, personal care products (e.g., facial cleaners, toothpaste), and electronic equipment is also one of the main contributors to microplastic contamination of the environment and food chain.
According to Worldwatch,[1] the consumption of plastics worldwide has been increasing at an alarming rate. North America consumes approximately 100 kg/person of plastic each year, mostly in the form of packaging, as opposed to 20 kg/person in Asia. The global production of plastics was 335 million metric tons in 2016. According to the United Nations Environmental Program (UNEP),[2] more than 8 million tons of plastic leak into the ocean each year—equal to dumping a garbage truck-load of plastic every minute. Also, plastics for recycling are shipped to less-developed countries for reprocessing; they become an important source of air and water pollution. UNEP recently launched a global campaign to eliminate major sources of marine litter by 2022: microplastics in cosmetics and single-use plastic. Launched in 2017 at The Economist World Ocean Summit in Bali, the “CleanSeas” campaign is urging governments to pass plastic reduction policies, targeting industry to minimize plastic packaging and redesign products, and calling on consumers to change their throwaway habits—before irreversible damage is done to our seas.
What Makes Plastics Problematic?
Chemically, plastic is a polymer, a molecule that consists of repeating identical units (homopolymer) or different subunits in various possible sequences (copolymer). Plastics are categorized as thermoplastics (plastics that soften on heating and therefore can be molded into different shapes) and thermosets (plastics that cannot be molded on heating). Both types of plastics are relevant for causing pollution of marine and freshwater organisms. Further, to improve the properties of plastic materials, numerous chemicals, such as fillers, plasticizers, colorants, stabilizers, and processing aids, are used. These chemicals are also relevant for polluting the food supply chain.
Microplastics include particles of varying size, shape, and chemical composition. The working group on the occurrence, effects, and fate of microplastic marine debris, hosted by NOAA in 2008,[3] suggested an upper size limit of 5 mm for microplastics, based on the available scientific evidence that it would include a wide range of small particles that could readily be ingested by marine organisms, and such particles that might be expected to present different kinds of threats.
Sources of Microplastic Contamination of the Food Supply Chain
Although hundreds of thousands of plastic materials are in use globally, only six are extensively used—polyethylene (PE, high and low density), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS, including expanded PS or EPS), polyurethane (PUR), and polyethylene terephthalate (PET).
The production chain for the most common artificial and natural polymers is illustrated in Figure 1[4].
When we assess the impact of microplastics on the food supply, we must make a distinction between primary and secondary microplastics. Primary microplastics are those materials that were originally manufactured to be that size. On the other hand, secondary microplastics are degradation products of plastic materials from larger items. This distinction will help us evaluate the sources of contamination, work out mitigation strategies, and reduce their input into the food supply chain. Primary sources include plastic powders in molding, microbeads in cosmetic formulations, and plastic nanoparticles in a variety of industrial processes. In addition, virgin resin pellets are widely used during plastics manufacture. Secondary microplastics originate from the fragmentation and weathering of larger plastic items. This can happen during the life cycle of plastic products such as textiles, tires, etc.
This article is not concerned with naturally occurring biopolymers because they are biodegradable and therefore do not pose any threat to marine organisms and the food supply chain. Natural polymers are readily biodegraded into CO2 and H2O in the oceans.
Mapping of Microplastic Contamination
Scientists have used theoretical and numerical modeling to map the extent of microplastic contamination of the oceans. These approaches involve deriving an estimate based on known factors such as sources, transport by ocean currents, sinkability, etc. Figure 2[5] provides an idea of the extent of ocean contamination by microplastics.
Mode of Accumulation of Microplastics in Marine and Freshwater Organisms
Microplastic contamination of marine and freshwater organisms occurs worldwide. Microplastics are highly persistent in the environment and may pose a serious threat to marine and freshwater organisms, as well as to humans because humans are at the end of the food chain. Ingestion of water contaminated with microplastics is the main exposure route for several marine and freshwater species. Recent research has concluded that microplastic ingestion has been observed in fishes, bivalves, and crustaceans. In addition to contaminated water, aquaculture systems where fish or other farmed species are fed with feeding materials produced from fish and other animals (e.g., fish meal) may also be contaminated with microplastics present in these products.[4]