From Sea to Plate: The Plastic Pollution Problem in the Food Chain ^†

Abstract

The rising concern over plastic pollution is not only related to pollution in marine and terrestrial habitats but also effects humans. This study analyzes the trophic transfer of microplastics throughout the food chain, with an emphasis on the effects on human health. It provides a review of 12 articles analyzing the microplastic intake by humans via ingestion of fish and environmental exposure. In particular, the reviewed studies focused on microplastic ingestion by fish and animals intended for human consumption, the distribution of microplastics in human tissues, and human blood. The results of this analysis can extend our understanding of microplastic transfer in the human body, with implications for future research.

1. Introduction

Plastic pollution in both the ocean and on land is not a novel environmental problem for the scientific community. However, the great depth of danger that comes from the transfer of plastic—specifically microplastics—in the food chain is poorly understood. Microplastics are classified as plastics measuring less than 5 mm and are mostly made from the degradation of larger plastic items.

In 2023, global plastic production reached approximately 413.8 million metric tons [1], and approximately half of that was single-use plastic [2]. This represents a continued increase from previous years, with production rising by more than 1.5% between 2022 and 2023 [3,4]. This upward trend is expected to continue, with projections indicating that annual global plastic production could exceed 1.3 billion metric tons by 2060 [4].

Most of this plastic is non-biodegradable, not recycled, and ends up all around the world—including the land and ocean [4,5]. Reports show that 1.1 to 8.8 million metric tons of plastic waste per year enter the ocean from land [6].

Additionally, plastic is found in an abundant number of everyday items, thus making plastic exposure to humans inevitable. In fact, clothes, medicine, dust, cosmetics, and food all contain microplastics that enter the human body through ingestion, dermal contact, or inhalation. This can lead to long-term effects, including oxidative stress, inflammation, and ultimately cancer [7].

Within the food chain, plastic particles can accumulate through either biomagnification or bioaccumulation. Biomagnification is the increase in plastic particles seen throughout the food chain—starting from small organisms such as zooplankton, which absorb plastic particles and eventually end up ingested by humans. For example, when zooplankton containing microplastics are eaten by fish, which are then consumed by larger fish and eventually by humans, the amount of plastic accumulates at each step of the food chain. As a result, humans end up with the highest concentration of plastic due to this progressive buildup across trophic levels [8]. Plastic ingestion begins with fish in the ocean and can make its way to the dinner plates.

This paper consists of a review of articles assessing the effects of microplastic intake on human health via consumption of fish and environmental exposure.

2. Materials and Methods

A literature search on the Scopus and Google Scholar databases was conducted in June 2024 with the aim of providing insightful information on the effect of microplastic intake on human health. Keywords used for the search included “microplastic*” AND “fish *” AND “human*” AND “toxic” OR “ingest*”. A simplified version of the PRISMA 2020 protocol [9] was used for the literature review. A total of 100 articles met the authors’ filtering criteria of the effect of microplastic intake on human health. Following, abstract screening and full-text review were performed, and only 12 studies were included in the analysis based on their relevance for answering the research question.

Based on the full-text review, included studies were classified into three thematic areas: (i) microplastics found in animals for human consumption, (ii) microplastics found in human tissues, and (iii) microplastics found in human blood.

Table 1. Overview of the included articles.

Thematic Area	Data Collected	Contributions	References
Microplastics found in animals for human consumption	Gastrointestinal tract of fish	Impacts of plastic ingestion	[10]
	Gastrointestinal tract of fish	Comparison of ingestion rate by species	[11,12]
	Gastrointestinal tract of invertebrates and larger animals	Trophic transfer across species	[13]
	Fish brain	Behavioral changes due to the presence of microplastics in the brain	[14]
	Sea salt samples	Impacts of ingesting microplastic-rich sea salt	[15]
Microplastics found in human tissues	(review)	Microplastic exposure pathways	[7,16,17]
	(review)	Microplastic transfer in tissues	[18]
	(review)	Impacts of microplastics on the fetus	[19]
Microplastics found in human blood	Human blood	Microplastics transfer in blood	[20]

shows an overview of the articles included.

3. Results and Analysis

3.1. Microplastics Found in Animals for Human Consumption

Ref. [8] examined the contents of 32 fish along the coast of Salvador in Northeastern Brazil. The fish were captured in two fishing ports in Salvador, Brazil, between April and May 2011, by fishermen who used hook and line techniques. The researchers performed biometric measurements and taxonomic identifications. The fish were cut open to examine the stomach contents using a binocular dissecting microscope. These fish were separated into what was found in their stomach content–natural foods or plastic. The plastic debris was further separated, quantified, and classified according to shape and color. For each fish, the place of capture, presence of plastic, number of plastic items, and their frequencies were recorded. The largest pellet size recovered was between 2 and 5 mm and was found in five fish of the Scomberomorus cavalla taxa. Following that, the next size was 1–3 mm, found in two Rhizoprionodon lalandii taxa. Based on this analysis, the authors concluded that microplastic ingestion was most likely due to fish and other marine life mistaking plastic particles for food.

Ref. [11] studied 1203 common species of fish found in the North Sea–Herring, Gray Gurnard, Whiting, Horse Mackerel, Haddock, Atlantic Mackerel, and Cod. Fish were taken from the southern and northern regions of the North Sea. The fish were captured during daylight hours in two periods: July to October 2010 and January to February 2011. At least 80 individuals from each of the seven species were collected to investigate their intestines. In 2010, the fish’s length and weight were recorded as well as the contents of the esophagus, stomach, and intestines. In 2011, the length and weight measurements were taken, and the digestive tract was removed. A stereomicroscope was used to identify and count plastic particles. The color and the shape of the particles were described, as well as the recording of the size (median particle size = 0.8 mm). Following, the authors compared rates of plastic ingestion by species. The largest percentage of plastic was discovered in the Cod fish found in 49–55° N with 14.9%. The second greatest percent was found in the Cod at 49–60° N with 13%. The remaining percent of plastic found in the species was between 0 and 6.2%. The large percentage of plastic found in the Cod would eventually end up in the stomachs of humans who consume fish, along with the high percentage of plastic. The six plastic particles in the study were analyzed with FTIR, and the particles consisted of PE, PP, PET, and SA.

Ref. [12] studied plastic ingestion by fish in the South Pacific Islands. Researchers collected 126 fish from local fishermen and markets in remote areas around South Pacific locations, including French Polynesia, Lord Howe Island, and Henderson Island, between May 2015 and May 2016. The fish used in the study were intended for human consumption and were caught using hook and line methods. The entire gastrointestinal tract of each fish was analyzed by hand in a bowl of salt water to aid in plastic flotation. The researchers recorded the plastic’s color, size, and type. The fish species were assigned to trophic levels and guilds, as well as feeding locations. Fish were categorized as small (<300 mm), medium (300–500 mm), and large (>500 mm). Statistical analysis was performed using R-Studio to test for significant differences in plastic ingestion using a generalized linear model. The study’s results show that microplastics were found in 7.9% of individual fish and 25% of the analyzed species; however, this is likely an underestimate of the true levels of plastic in the South Pacific islands.

Plastics have not only been found in the stomachs and intestines of fish, but also in the guts of invertebrates, turtles, and other larger animals that are often intended for consumption by humans [13]. In 2014, at least 170 marine species were exposed to plastic ingestion, and this number increased to 690 in 2015. This entry of plastic into the food web occurs both directly and indirectly across all trophic levels. Direct entry occurs through filter feeding, through the gills, and through consumption from floating in the water. Indirect entry occurs when a predator consumes through biomagnification.

While many organisms are affected by plastic pollution, an exact number is very difficult to estimate due to the billions of organisms affected. In 2015, ref. [14] attempted to answer this question as they discovered that the number of marine species that swallowed plastic or became entangled in it had doubled since 1997, from 267 to 557. In current times, this number has risen over 2000, while only a limited number of species have been studied. The same study addresses the problem of nano-plastics entering the brain of fish through the trophic transfer and leading to abnormal behavior. The nano-plastics were found in algae that are eaten by water fleas, which are then eaten by fish. Simulations of this food chain with the presence and absence of nano-plastics showed that fish that were not in an environment with nano-plastics did not experience abnormal behavior. The abnormal behavior of the fish exposed to nano-plastics included slower eating habits and hyperactivity. The accumulation of nano-plastics in the fish began to slow them down, leading to their becoming easier prey, which can disrupt the natural food chain.

Finally, ref. [15] mapped the amount of microplastics that were found in sea salt, as the plastic was first thought to be from the production of seawater. The scientists found 550–681 particles kg⁻¹ in salt, 43–365 particles kg⁻¹ in sea salt, and 7–204 particles kg⁻¹ in rock salt. More than half of the microplastics discovered were less than 200 μm. Additionally, the particles consisted mostly of fibers and fragments, which made the scientists realize that this was not from the production of seawater. The plastic found was, in fact, from polluted waters that contained microplastics.

3.2. Microplastics Found in Human Tissues

Humans can be exposed to plastic particles via inhalation, ingestion, and dermal contact. Ingestion is the major route of entry of microplastics in humans, and particles are absorbed by the intestine. According to [10], the estimated intake of microplastics is 39,000–52,000 particles ${person}^{-1}$ ${year}^{-1}$. Once particles reach the gastrointestinal system, the body can respond through inflammation, increased permeability, and experience changes in the gut microbiome, including alterations to metabolism and composition. Humans may also ingest microplastics through the food they eat. This includes fish, honey, salt, bottled water, milk, fruit, and meat. Additionally, the plastics in food packaging also contribute to the ingestion of microplastics. In fact, human ingestion is very likely because of the large amounts of contamination in our food and environment. Inhalation of microplastics occurs when they are released into the air through synthetic textiles and abrasions of car tires and building materials. Humans are estimated to inhale an average of 26–130 airborne microplastics ${day}^{-1}.$ Furthermore, a male whose day consists of light activity inhales around 272 microplastics per day.

Ref. [18] also discovered that “fibers of 250 μm have been detected in human lung biopsies, including in cancer biopsies”, with a maximum human exposure of 6110 microplastic particles per year. With the rise in microplastic exposure to humans every year, actions should be taken to reduce plastic use and find alternatives that are healthier for humans and the environment.

Ref. [16] reported that humans are also exposed to microplastics through personal care products. This includes shampoos, conditioners, body lotion, soap, deodorants, cosmetics, moisturizers, serums, and sunscreen. Many of these products contain microbeads, microfibers, and microcapsules that, when washed off the skin, enter the environment through the drainage system and can potentially be ingested by marine organisms. This can eventually end up in the human’s intestines when consuming seafood that has previously consumed microplastics from personal care products. The study showed that cosmetic products, on average, contain microbeads from 0.5 to 5% of their total composition. Toothpaste alone releases approximately 4000 microbeads per single use.

Ref. [17] discovered that more than 37 billion microbeads were released annually into the environment in this densely populated area. These microplastics come from households, landfills, construction, factories, farmland, ships, and marine platforms. Within the household, washing machines play a huge role in the release of microplastics into the environment. According to this study, washing machines produce more than 1900 fibers for every cycle.

Additionally, ref. [19] concluded that action must be taken to minimize the ingestion of microplastics because they can also reach the placenta in a pregnant mother, to the very susceptible fetus. They found that the mother accumulates microplastics in the fat tissue, which then dissolves through the mother’s blood and into the fetus. In fact, 93% of individuals contain BPA in their urine. This can interfere with hormones and lead to the accumulation of endocrine-disrupting chemicals. Substances in the environment, such as air, soil, water supply, food sources, and personal care products, interfere with the normal function of one’s body’s endocrine system. The endocrine system is essential, and much harm can come from introducing microplastic particles because the endocrine system includes the brain, thymus, pancreas, thyroid, adrenal glands, and other key players in the body. The harm to one of these organs can cause tremendous damage to an individual. To prevent the overwhelming increase in the number of microplastics, individuals should avoid single-use plastics, avoid plastic when microwaving food, purchase mostly organic clothing, buy plastic-free cosmetics, and use public transportation. With enough individuals actively participating in limiting the use of plastic, the health of both the individuals, the environment, and the marine ecosystem can all benefit greatly and even flourish again.

3.3. Microplastics Found in Human Blood

Ref. [20] discovered and quantified the pollution of plastic particles in human blood. They conducted research on human blood because blood makes up 6–7% of an individual’s body weight. Since blood transports plastic particles to the body’s organs and other tissues, the only way for the plastic to exit the body is through elimination by renal filtration, biliary excretion, or deposition into the liver or spleen via fenestrated capillaries. This study focused on the analytical method development and measurement of the blood to quantify five of the high production polymers found in plastic—PPMA, PP, PS, PE, and PET. Blood samples were obtained from 22 anonymous, healthy, non-fasting individuals from the Netherlands. Double-shot pyrolysis, gas chromatography, and mass spectrometry were used to analyze the blood samples. The samples were extracted and measured in duplicate, consecutive series, with many blanks in each series as the control group. Because depolymerization products of PET could be present, the researchers used a reagent to form DMT. The study concluded that analyzing human blood is especially important because the human placenta is permeable to plastic, which can also be transported to other organs through the bloodstream.

4. Conclusions

The increasing number of plastics found in marine life endangers both the life and the food chain, and overall, the public health of humans [15]. In this study, we reviewed a number of articles discussing the effects of microplastic intake on humans via consumption of fish or environmental exposure. Although the long-term effects of microplastics are unknown in the human body, the adverse effects reported in the studies reviewed include oxidative stress, disruption of energy balance and metabolism, and disruption to the immune system.

Limitations of this study are associated with sample selection and the depth of data collection. Further research should be performed on the long-term effects on human health.