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]
 

Another route of accumulation of microplastics is external exposure when microplastics contact the outer surfaces of the organism and are translocated from the outside into the organism. The extent of external exposure depends on the concentration and size distribution of the microplastic particles and upon the specific nature of the organism.

Microplastic Contamination of Food Products
Recent studies on microplastics in seafood have confirmed that commercially important fish species such as Atlantic cod and Atlantic horse mackerel are often contaminated with microplastics.[6] According to the Food and Agriculture Organization of the United Nations,[7] of the 25 fish species of commercial significance, 11 were found to contain microplastics.

Several researchers have conducted extensive surveys on the extent of contamination of marine and freshwater species with microplastics. Foremost among them was the study conducted by van Cauwenberghe and Janssen,[8] who calculated that in European countries with high shellfish consumption, consumers ingest up to 11,000 microplastic particles per year, whereas in countries with low shellfish consumption, consumers ingest an average of 1,800 microplastic particles per year. The European Commission’s Rapid Alert System for Food and Feed’s portal and the European Food Safety Authority’s website report the presence of these contaminants in a wide variety of human food items.[9,10]
Studies have also been conducted on the concentration of microplastics in other food products, such as beer, honey, salt, drinking water, and mineral water.[6]

Microplastics and Food Safety
The study of microplastic contamination of food products and its impact on human food safety is an emerging field, and there are many gray areas. Risk associated with ingestion of microplastics into the human body is a function of hazard and exposure. Evaluating the risks from microplastics requires knowledge of the hazard (the potential to cause adverse effects), exposure levels (the quantities detected in human food), and their effects (the identification of dose-response relationships and threshold levels).

Microplastics may act as vehicles or carriers for environmental contaminants and other chemicals that are added during their manufacturing process. Chemicals such as styrene, toxic metals, phthalates, bisphenol A, polychlorinated biphenyls, and polycyclic aromatic hydrocarbons may be absorbed on the surface of microplastics and may act as “substrates.” These pollutants and additives can be transferred from ingested microplastics to animal tissues and cause impairment of key body functions.

Dumping of plastic waste in the ocean is a global practice. Monitoring studies have established the presence of pathogenic bacteria such as Vibrio spp., Escherichia coli, and Bacillus cereus on plastic debris. The scientific community has expressed concerns about new contamination routes for the introduction of pathogens into the food supply via microplastics. It is still unknown whether pathogenic organisms on plastic debris survive until the end of the food chain.  

The risk assessment of microplastics in food safety is still in its infancy, and additional information regarding their occurrence, risk assessment, and mode of action is needed to include them as potential hazards in a food safety plan.

How Do Microplastics Get into Food Systems?
Plastic debris, whether on land or in the sea, has an adverse impact on all forms of life. Plastics enter the food chain when they degrade, and the rate of degradation depends on a variety of factors, such as the chemicals added during the manufacturing of plastic and the physical environment that surrounds them (presence of salt, water temperature, light intensity, etc.). Information about how various types of plastics break down into microplastics in the environment and get into the food chain is scanty. However, it has been established that microplastics enter the food chain when animals eat or ingest contaminated food materials. The food web is extremely complex. Zooplankton, the microscopic sea organisms at the bottom of the food chain, is eaten by all kinds of fish. Fish ingest small pieces of plastic due to their continuous uptake of water. Microplastics get into the next level of the food chain when other animals eat fish contaminated with microplastics. Eventually, microplastics move all the way up to the top of the food chain. This has been well documented (Figure 3[11]).

Effect of Microplastics on Human Health
It is evident that the potential accumulation of microplastics in the food chain could have adverse effects on human health like other chemical contaminants relevant to food safety. Studies have confirmed unusually high levels of microplastics in seafood. Therefore, there is no doubt human beings are exposed to higher levels of microplastics. Several studies have confirmed adverse effects on animals as follows:

•    Reproduction in marine animals is affected by exposure to polystyrene microplastics[12]

•    Endocrine disruption in adult freshwater fish from ingestion of PE[13]

•    Altered gene expression was observed in male fish exposed to plastic

However, there is still a knowledge gap regarding the specific threats, toxicities, and adverse health effects in humans posed by the ingestion of microplastic-contaminated food. 

Conclusion
Microplastic contamination in the food chain has kindled a lot of interest recently among consumers and the scientific community alike. The information available currently on the potential adverse effects of microplastics on human health is scanty and sporadic. Additional research is needed to evaluate the extent of microplastics in the food chain. The scientific community should come up with qualitative and quantitative data including the type, size, and components of microplastics in the food chain.

Currently, there is no regulatory requirement globally to increase human food safety against plastic contamination, due to a lack of qualitative and quantitative information on the levels of microplastics in various foods, their adverse effects on human health, and a lack of effective and comprehensive mitigation strategies to control microplastic contamination. Baby steps are being taken to mitigate the microplastic contamination of the ecosystem. President Barack Obama signed the Microbeads-Free Waters Act in 2015 that banned microbeads from rinse-off cosmetics. In order to reduce microplastic contamination of the ecosystem, global companies such as Johnson & Johnson and Unilever made a commitment that their products would be plastic-free within the next few years and that they would use natural substitutes instead. Other cosmetic companies have come forward to phase out microplastics too.

The most prudent approach is to reduce the problem at its source, namely, reduce the use of plastics in our daily life. This will collectively need the involvement of the society we live in, companies that produce plastics, and regulatory bodies. We should act without further delay to protect our food chain.   

Dr. Ramakrishnan Nara is a technical adviser/consultant for the food, pharma, and dietary supplements industries.

References
1. www.worldwatch.org.
2. www.unep.org.
3. marinedebris.noaa.gov/file/2192/download?token=5dvqb-YY.
4. www.gesamp.org/publications/reports-and-studies-no-90.
5. www.ocean.org.
6. Barboza, LGA, et al. 2018. “Marine Microplastic Debris: An Emerging Issue for Food Security, Food Safety, and Health.” Mar Pollut Bull 133:336–348.
7. FAO. “The State of World Fisheries and Aquaculture 2016,” in Contributing to Food
Security and Nutrition for All (Rome, 2016).
8. Van Cauwenberghe, L and CR Janssen. 2014. “Microplastics in Bivalves Cultured for Human
Consumption.” Environ Pollut 193:65–70.
9. webgate.ec.europa.eu/rasff-window/portal.
10. www.efsa.europa.eu/.
11. www.annualreviews.org/doi/abs/10.1146/annurev-environ-102016-060700.
12. www.pnas.org/content/113/9/2430.
13. Rochman, CM, et al. 2014. “Early Warning Signs of Endocrine Disruption in Adult Fish from the Ingestion of Polyethylene with and without Sorbed Chemical Pollutants from the Marine Environment.” Sci Total Environ 493:656–661.