Purpose: The discussion of human microplastic exposure by contaminated seafood.
Recent Findings: Shellfish pose a potential exposure risk and it is presumed to be dependent on the dose, polymer type, size, surface chemistry, and hydrophobicity of the plastic polymer.
Summary: Human activity has led to global use and pollution of plastic. Aquatic life ingested this plastic and there is evidence emerging that it poses a toxic threat towards aquatic life and the humans that consume it. The paper will highlight gaps in our knowledge regarding microplastics and recommends future research on seafood consumption. The researchers also emphasize the importance of lessening the dependence on plastic or ensuring recycling or a longer lifetime for plastic products.
A conservative estimate suggests that there are roughly 5.25 trillion plastic particles circulating in ocean waters. 80% of these plastic particles are suspected to have entered from land-based sources. They enter as trash, industrial discharge, litter through inland waterways, wastewater outflows, and transport by winds or tides. The physical and chemical degradation of these plastics can form smaller particles known as micro or nano plastic. Micro plastic is anything smaller than 5mm and nano plastic smaller than 1 nanometer in size. Degradation of the plastic polymers is dependent upon their polymer size, age, type, and environmental conditions such as weathering, temperature, irradiation, and pH. The type of plastic and its density also determines what depth it remains at within the ocean. Studies have shown that nanoplastic can be found in every organ. There is a growing body of evidence demonstrating a relationship between micro/nanoplastic exposure, toxicology, and human health.
The researchers did a thorough literature review using several databases such as PubMed, Google Scholar, Natures database, and Science Direct. They focused on publications after 2004 given that the term “microplastic” was introduced. Other organizations such as Food and Agriculture Organization (FAO), The Group of Experts on Scientific Aspects of Marine Environmental Protection (GESAMP) of the United Nations, European Food Safety Authority (EFSA), UNited States Department of Agriculture (USDA), Food and Drug Administration (FDA), and National Oceanic and Atmospheric Administration (NOAA) for relevant information and sources.
Background on Microplastics
Sources and Distribution
Microplastics are a mixture of varying plastic shapes, sizes, and types. There are two types of microplastics. First, primary microplastics were purposely made to be less than 5mm. Such an example would be microbeads found in a variety of hygiene products before they were banned in 2015. The second type of microplastics are called secondary microplastics that are formed from the breakdown of larger plastic types into a size smaller than 5mm. Some examples would be microfibers from clothing and tire dust.
Physical and Chemical Properties
Some lighter plastic types float to the top of seawater and some denser ones are found at varying depths with some expected to sink to the bottom. Plastic is made when two individual monomer units of a compound join together to make a polymer made of multiple monomers. Plastic is given special qualities in part by the additives used. There are several thousand all of which give the plastic certain characteristics. The polymer of plastic itself is non-reactive and thus largely non-toxic. However unreacted monomers and additives are extremely toxic to life. As the plastic degrades, the surface area to volume ratio will increase and thus increase the expected amount of additives that will leach out into the surrounding water.
Microplastics in the ocean also absorb persistent organic pollutants (POP)s. These chemicals bind to the microplastic more readily than water and tend to concentrate themselves onto the plastic surface. The exposure of POP’s to humans may be rather small via the microplastic pathway, but it should not be ignored.
Degradation of Marine Plastics
Plastic can be degraded slowly by microorganisms, heat, oxidation, light, or hydrolysis. The rate of these processes is highly dependent upon the environmental conditions present.
Exposure to Microplastics by Marine Animals
In 2016 the UN report documented more than 800 animal species that were contaminated with plastic via ingestion or entanglement. This is 69% greater than a 1977 review that estimated only 247 contaminated species. Microplastics have been found to be present in marine mammals, fish, invertebrates, and fish-eating birds. The micro/nano plastic is concentrated in the gut, but can move into the other organs and circulatory systems
Human Exposure Pathways
Exposure to microplastic via seafood is one method of exposure. Ingestion of microplastic by aquatic life can occur directly or via trophic transfer. It has even been documented in planktonic organisms and larvae which constitute the bottom of the food chain. Likely exposure could occur through bivalves and small fish. Microplastics have also been documented to be present in a beer, honey, and sea salt. This list is not exhaustive. The plastic in these foodstuffs could be due to uptake by the product itself, simple impurities formed during processing, or packaging contaminants.
Due to gaps in data it is unknown what the human health effects are. However, it is most likely related to the concentration of plastic exposure. As stated earlier the exposure to microplastic also means exposure to associated chemicals. However, there is no research studying the effect of additives. There is mounting evidence that microplastic exposure and its associated chemicals is a threat to marine life. The health implications for humans requires a standardized and reproducible method. No standard exists as of yet.
Toxicity to Humans
Microplastics may cause harm to humans both physically and chemically. However, with current data it is impossible to break down the pathways , but we the authors of this paper break it down for the purpose of discussion.
Potential Physical Effects of Microplastic
The physical effects are understudied, but preliminary research suggests some potentially harmful effects. There is a possibility for enhanced inflammation, size-related toxicity of particles, chemical transfer of adsorbed pollutants, and disruption of gut microbiome. The functional groups size, shape, surface charge, buoyancy, and hydrophobicity predict microplastic uptake into the body. Models of mammals suggest that certain microplastics can be taken into the body by M cells or dendritic cells and enter the lymphatic system where they will eventually accumulate in other organs. Some research suggests that it can cause inflammation, cellular proliferation, cell death, and compromise the immune system. In order to further test the effect of microplastic on humans it is important to monitor shellfish consumption and its effects.
Nanoplastic particles are transported into the blood via M cells into the lymphatic system were they are able to travel throughout the body. Their hydrophobicity allows them to pass through the placenta and blood-brain-barrier. Nanoplastics have a higher surface area to volume ratio which could result in a more reactive compound. Studies show that introduction of nanoplastic to lung, liver, and brain cells in test tubes results in toxicity. It has been shown to produce cardiopulmonary responses, changes in metabolism, damage to the genome, inflammatory responses, oxidative stress, effects on nutrient absorption, gut microflora, and reproduction.
Potential Effects of Chemical Additives
Exposure to chemicals present on microplastics poses a particular problem to juvenile humans and animals even at a lower dose. The interaction of microplastics will vary depending on the organism, however studies show increased toxicity from multiple microplastics and their associated chemicals. The exposure may be low, but if the effect is cumulative then this becomes of greater concern. The researchers suggest further research to estimate toxicity doses in humans from microplastics in seafood and several other studies.
Microplastic is used to deliver medication inside the body, but its effects are generally unknown. It may produce a localized toxicity, but we currently have no method of verifying this. There is significant correlation between the amount of microplastic (BPA) in a persons urine and both cardiovascular disease and type 2 diabetes. However further research is strongly advised by the authors of this paper.
Mitigation of and Adaptation to Risks
The effect of microplastic on human health is uncertain, but should not be ignored. The government, industry, and civil society all have vital roles in reducing the flow of plastic into the environment.
Humans ingest microplastics. However, with the research available it is uncertain how the toxicity, kinetics, exposure, and bioavailability play a part in this issue. It is likely that toxicity is dependent upon the size, shape, polymer size, concentration, and associated chemicals. More research is required in this field and the authors list multiple future suggestions for potential studies to help fill the gaps.