A systematic review and quality assessment of estimated daily intake of microplastics through food

Search strategy

This study was conducted according to Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) guidelines [36]. The literature search was conducted from January 2 to 20, 2024, and five academic databases (PubMed, Scopus, Web of Science, Embase, and Cochrane Library) were examined. There was no limitation on publication date, and all studies published until December 31, 2023, were included in the analysis. Search terms such as (‘Microplastic’ OR ‘Microplastic contamination’ OR ‘Nano-plastic’ OR ‘Nano-plastic contamination’ OR ‘Plastic particle’ ‘Micro particle’) AND (‘mussel’ OR ‘bivalv*’ OR ‘shellfish’ OR ‘clam’ OR ‘shrimp’ OR ‘sardine’ OR ‘sprat’ OR ‘fishes’ OR ‘crustacea’ OR ‘seafood’ OR ‘salt’ OR ‘honey’ OR ‘sugar’ OR ‘rice’ OR ‘tap water’ OR ‘water’ OR ‘drinking water’ OR ‘milk’ OR ‘beverage’ OR ‘beer’ OR ‘food’ OR ‘fruit’ OR ‘vegetable’ OR ‘Meat’) AND (‘annual intake’ OR ‘daily intake’ OR ‘estimated daily intake’ OR ‘intake’ OR ‘ingestion’ OR ‘dietary intake’) were used. Additional literature was included through manual searches. Details on the search strategy can be found in the supplementary materials (see Table S1). The retrieved literature was collected and organized using EndNote 21 and Excel programs.

Inclusion and exclusion criteria

We used the ‘Participants, Exposure, Comparison, Outcome, and Study Design (PECOS)’ format to decide the eligibility criteria of the studies for inclusion in this review. The contents of PECOS are as follows:

1. Participants (P): The general adult population.

2. Exposure (E): Exposure to MP or nano-plastic particles via food ingestion.

3. Comparison (C): Not included in this study.

4. Outcome (O): Estimated daily or annual intake of microplastics.

5. Study design (S): Cohort studies, case-control studies, and cross-sectional study designs.

The exclusion criteria for this study are as follows: (1) Studies involving examination of animal subjects; (2) Studies examining non-food items, such as exposure via food containers; (3) Unavailability of the original text; (4) Articles not published in English; (5) Master’s thesis, doctoral dissertation, and academic conference abstracts were excluded to ensure research quality; and (6) Review articles were excluded from this study.

Selection process of studies

Eligible studies were extracted by two researchers (SJH, JHK). After the two researchers independently searched and reviewed the literature according to the inclusion and exclusion criteria, literature for analysis was selected through consensus. The data selection process is shown in Figure 1. First, a total of 4,650 studies were retrieved through databases and manual search, and the titles and abstracts of 3,139 studies were reviewed, excluding 1,511 duplicates. According to the selection and exclusion criteria, 2,970 articles that did not meet the selection criteria were excluded, and the full text of 169 articles was reviewed. After reviewing the original text, we excluded articles that did not report estimated daily or annual intake (n=40), those that did not report the characteristics of MPs (n=8), those that did not examine exposure via food ingestion (n=8), those for which full text was not accessible (n=4), and those that did not incorporate experimental studies (n=33). Therefore, 76 articles were finally included as studies for systematic review.

Figure 1:

Flow diagram of study selection process based on preferred reporting items for systematic reviews by PRISMA. MP, microplastic.

Data extraction and analysis

For the 76 selected studies, data from individual studies were extracted, as shown in Table 1. Food items, species, number of samples, identification of MPs (analysis, shape, polymer type, color, abundance, particle size), outcome measurements (estimated daily or annual intake), and study information (author, publication year, country) were extracted from individual studies. In addition, based on the research quality assessment results, a radar chart for each food group was constructed. The distribution of the estimated daily average intake of MPs for each food group was presented as a bar graph. Data pertaining to seafood were presented by subdividing into three categories: bivalves, fish, and other seafood. Similarly, information on drinking water was categorized by tap water and bottled water.

Quality assessment

To assess the quality of MP studies, we conducted a quality assessment of studies that reported MP intake via food, beverages, and drinking water according to the standards presented in previous studies [33], 34]. These assessment tools can be used to quantitatively evaluate the reliability of studies on exposure to MPs via ingestion by evaluating sampling, extraction, and identification methods for MPs [33]. First, the quality of studies on MPs in food and beverages was evaluated according to 10 criteria (‘sampling methods,’ ‘sample size,’ ‘sample processing and storage,’ ‘lab preparation,’ ‘clean air conditions,’ ‘negative controls,’ ‘positive controls,’ ‘target component,’ ‘sample treatment,’ and ‘polymer identification’) presented by Hermsen et al. [34], and each category was scored from 0 to 2 points. The possible total accumulated score (TAS) ranged from a minimum of 0 to a maximum of 20. The quality of studies on MPs in drinking water was evaluated according to nine criteria (‘sampling methods,’ ‘sample size,’ ‘sample processing and storage,’ ‘lab preparation,’ ‘clean air conditions,’ ‘negative controls,’ ‘positive controls,’ ‘sample treatment,’ and ‘polymer identification’) presented by Koelmans et al. [33], and each category was scored from 0 to 2 points. The possible TAS range was from 0 to 18. In this study, the quality of studies on food and beverages and drinking water were independently evaluated by two researchers.

Study characteristics

The general characteristics of the selected studies are shown in Table 1. Foods were classified into seven categories (seafood, drinking water, table salt, fruits and vegetables, beverages, condiments, and meat), and most studies examined the consumption of MPs through seafood (n=36). In addition, 19 articles specifically focused on drinking water (bottled water & tap water), 13 on table salt, 3 on fruits/vegetables, 7 on beverages, 4 on condiments, and 1 on meat were included. The number of MP samples varied from 3 to 2,310, and the study investigating ingestion of MP via commercial mollusks (clam, snail, oyster, and mussel) had the largest number of samples. The country where the highest number of studies were conducted was China (n=13), followed by Türkiye (n=8), Iran (n=7), India (n=6), Korea, and the UK (n=4, respectively). All studies were published after 2010, and the largest number of studies were published in 2023 (n=29). This was followed by the number of studies published in 2021 and 2022 (n=12, respectively), 2020 (n=10), and 2018 (n=6). The most widely used analysis method to identify MPs was FTIR. μ-Raman microscopy, stereomicroscope, and laser direct infrared (LDIR) were also frequently used. The properties of the MPs were reported by shape, polymer type, color, and size. The most common particle shapes were fiber, fragment, and film, and the predominant polymer types were polypropylene (PP), polyethylene (PE), and polystyrene (PS). The size of MPs varied from 0.1 μm to 8,201 μm depending on the study sample, and the most common colors were blue, black, and red (Table 1).

Estimation of MP intake

Seafood

The types of seafood were classified into bivalves (clam, oyster mussel, and scallop), fish (including tuna, mackerel, anchovy, and salmon), and other seafood (squid, crab, sea cucumber, seaweed, and prawn). A total of 23 studies reported the ingestion of MPs through bivalves. The concentration of MPs in bivalves ranged from 0.0144 to 69.9 MP/g (or 0.12–99.85 MP/individual), and the concentration of MPs in fish ranged from 0.004 to 2 MP/g (or 0–39.65 MP/individual). The concentration of MPs in other seafood ranged from 0.003 to 5.3 MP/g (or 0.14–1.44 MP/individual) (Table 1). The average values of daily and annual intake of MPs via seafood are shown in Table 1 and Figure 2. Hence, an average of 0.01–1,620 MPs were ingested per day (or 0.00012 mg/kg/day to 91.58 g/kg/day) through bivalves, and an average of 4.2–218,400 MPs were ingested per year. The estimated daily intake (EDI) and estimated annual intake (EAI) values of MPs from fish intake were 0.48–490 MP/day and 0.17–178,220 MP/year, respectively. Additionally, the ranges of EDI and EAI values of MPs consumed via other kinds of seafood were 0.04–5.1 MP/day or 13–1,862 MP/year, respectively.

Figure 2:

Distribution of average estimated daily intakes of microplastics. This bar graph shows the distribution of the estimated daily intake of microplastics for each food category, presented by country (year). For data with large gaps, the median value was omitted and has been indicated with a wavy line on the vertical axis. (a) Bivalves, (b) fish, (c) other seafood, (d) tap water, (e) bottled water, (f) table salt, (g) fruits and vegetables, (h) beverages, (i) condiments. ^*Indicates the average value of the data presented as a range. MP, microplastic.

Quality assessment

The results of the quality assessment of studies that examined MPs in food, beverages, and drinking water are presented in the last column of Table 1. The average TAS among food and beverage studies (n=62) was 11.5, and the average TAS score among drinking water studies (n=19) was 11.16. The results categorized based on the food subgroup are summarized as follows: First, the TAS score in the seafood studies averaged 11.87 points (range: 6–15) for bivalves, 11.67 points (range: 8–15) for fish, and 12 points (range: 9–14) for other seafood. In the study examining drinking water, the TAS score averaged 10.6 points (range: 7–14) for tap water and 11.33 points (range: 9–15) for bottled water. The average score of TAS in the table salt study was 9.54 points (range: 5–16), and the average TAS score in the study examining fruits/vegetables was 13 points (range: 11–14). In addition, the average score (calculated from a range) of TAS for beverages and condiments was 12.71 (range: 10–15) and 12 (range: 10–14), respectively. Meat was reported in one study, and the quality assessment score of that study was 11. A graph visualizing the results of the study quality assessment is presented in Figure 3.

Figure 3:

Results of quality assessment of studies examining microplastics. This radar chart shows the distribution of results across quality assessment criteria for food and drinking water research. ‘Sampling method (blue line)’ refers to the sum of the items ‘sampling methods’, ‘sample size’, and ‘sample processing and storage’, and ‘contamination control (orange line)’ implies ‘lab preparation’. ‘Clean air’ refers to the sum of ‘conditions’, and ‘negative controls’. ‘Sample purification & chemical analysis (gray line)’ refers to the sum of ‘positive controls’, ‘target component’, ‘sample treatment’, and ‘polymer identification’. The yellow line represents the distribution of total scores. (a) Seafood, (b) drinking water, (c) table salt, (d) beverages, (e) other food (fruits/vegetables, condiments, and meat).

Study characteristics

Humans can be exposed to MPs primarily via ingestion [37]. Here, a total of 76 studies were systematically reviewed, and the concentration of MPs across seven food categories was analyzed. Specifically, seafood accounts for the highest number of studies investigating MP intake estimates, and exposure through consumption of meat has been reported in only one study. These results imply a remarkable contamination of the marine environment by MPs [38]. Asian continent was associated with the highest number of studies examining MP intake (n=36). In addition, 16 studies were conducted in the Middle East, 14 in Europe, 5 in Africa, 4 in America, and 1 in Australia. All studies were conducted after 2010. In particular, the largest number of studies was published in 2023 (n=29). This finding can be attributed to the fact that the interest in research on MPs has increased since 2013 [39].

Methods for identifying MPs include visual identification (naked eye), optical microscopy, thermal identification, and spectroscopic identification methods, depending on the size and characteristics of the particles to be analyzed [40], 41]. Among the studies we examined, FTIR was the most commonly used analysis method for MPs.

Analysis of MPs can be performed using one or two or more methods depending on the purpose of the study [40]. Scanning electron microscopy-energy dispersive X-ray analysis is one of the optical microscopy methods that can be used to observe the characteristics of MPs, such as elemental composition, size, shape, and color. In addition, FTIR, Raman spectroscopy, and attenuated total reflection, which are spectroscopic identification analysis methods that can confirm the chemical structure of MPs, can be used [41]. LDIR analysis can be used as an alternative to FTIR and has the advantage of rapid identification and quantification of MPs [40], 42]. However, no standardized methods for collecting and analyzing MPs are available [29], 40], 41], 43].

The most frequently detected polymer types in all food groups were PP, PE, and PS, consistent with the results of previous studies [29], 41], 44]. The predominant shapes of MPs were fiber, fragment, and film. The shape of MPs provides information about the source from which the particles originated. For example, fibers are one of the most frequently ingested types of MPs, primarily from textiles [45], 46]. Fibers are introduced into the marine environment through various pathways, such as from washing clothes, and can affect humans through the consumption of seafood, tap water, or table salt [45]. Indeed, these fibers can pose significant health risks, as they have the potential to induce adverse health effects, even at low doses [29], 46]. Fragments are generated when other plastics or fibers break down [44]. Because the toxicity of MPs varies depending on the characteristics of the particles, their effects on humans also vary [47].

The predominant colors of MPs found in the entire food group were blue, black, and red, which are commonly detected in food. Furthermore, the color of the MPs can be determined by the polymer type of the MP. Marine organisms may be influenced by ingesting particles of their preferred color [44]. The largest MPs were detected in seafood (0.002 mm to 8,201 μm), followed by beverages (10–5,050 μm), table salt (20–5,000 μm), drinking water (1–5,000 μm), fruits and vegetables (0.1 μm to 1 mm, analyzed after peeling), condiments (<500 μm), and meat (300–450 μm). Particle size can vary depending on the sampling and sample processing methods used in each study [44].

Estimation of MP intake through food

Plastic waste can pollute the marine environment as it undergoes fragmentation into minute particles through physical or chemical weathering. Marine organisms ingest these fine particles [48]. Additionally, MPs ingested by marine organisms are a serious problem as they can enter the human food chain through the consumption of seafood, leading to potential exposure in humans [10], 46], 49]. This study reported the presence of MPs among marine in bivalves, fish, and other seafood. The concentration of MPs was 0.0144–69.9 MP/g (or 0.12–99.85 MP/individual) in bivalves, 0.004–2 MP/g in fish (or 0–39.65 MP/individual), and 0.003–5.3 MP/g (or 0.14–1.44 MP/individual) for other seafood, with the highest concentration in bivalves. Types of bivalves reported included clams, oysters, mussels, and scallops. Among these, exposure to MPs through mussels was reported in most studies. In particular, in a study examining mussels collected in Italy, the estimated intake of MPs reported was 1,395–1,620 MP/day, the highest among seafood [50]. Among fish, a study examining Mugil cephalus (Flathead gray mullet) caught off the coast of Nigeria showed the highest value (i.e., 490 MP/day). The higher microplastic concentration in M. cephalus than in other species can be explained by its habitat and dietary characteristics. The species is frequently found in marine and freshwater habitats and is omnivorous, consuming a variety of foods, from microalgae to bivalves and small fish, potentially resulting in substantial exposure to microplastics [51]. Among other types of seafood, a study examining sea cucumbers sold in China showed that canned sea cucumbers had the highest value at 5.1 MP/day. Based on these results, humans can be exposed to an average of 0.01–1,620 MPs per day through seafood consumption. Seafood is commonly consumed in daily life, with per capita consumption reported to exceed 20 kg per year worldwide [10]. Hence, because seafood is an important part of the global food supply chain, it can pose an even more serious threat to human health [52]. The method of calculating the EDI of MPs varies among studies. However, it is generally calculated by multiplying the abundance of MPs (MP/g) in the food by the average daily intake (g/day) and then dividing by the average body weight by age. For example, the average body weight of adults in the presented studies was mostly in the range of 60–70 kg; however, these values vary depending on the country where the studies were conducted [42], [53], [54], [55]. The intake of microplastics may also vary depending on the seafood consumption in each country. Europe is a country with a high consumption of seafood. Up to 11,000 MPs are estimated to be ingested annually via the consumption of bivalves. However, even within European countries, the intake differs remarkably from country to country. Hence, the average daily seafood intake per individual was high in Belgium at 72.1 g/day but low in France and Ireland at 11.8 g/day [26]. In addition, an analysis of European food consumption data in a previous study showed that the intake can differ by up to 70 times between seafood consumers and non-consumers [26], 56]. The presence of MPs in seafood can be influenced by several causes. For example, the concentration of seafood may vary depending on whether the organism is sourced from farmed or wild habitats [37]. Because farmed fish sold for human consumption undergo a depuration process, the concentration of MPs may be lower than that in wild fish. However, based on the results of previous studies, microplastic contamination in fish can be affected by various factors, such as the degree of habitat contamination or the influx of microplastics in the aquaculture environment. Therefore, we cannot conclude that farmed fish will have lower microplastic concentrations than those of wild fish and the results should be interpreted with caution [57]. Additionally, the concentration of microplastics can vary depending on the cooking method. For example, if possible, seafood should be boiled or steamed, the use of plastic cooking utensils should be minimized, and the use of disposable items when holding food should be reduced [50], 58], 59]. If seafood consumption is unavoidable, the degree of exposure can be reduced by applying safe cooking methods. Therefore, if consumption of seafood is unavoidable, the degree of exposure can be reduced by changing cooking methods.

Types of drinking water include tap water and bottled water, which are supplied to humans after being purified from surface water (rivers, lakes) or groundwater [60]. MPs such as PET, PE, and PP are commonly detected in bottled water and tap water, and in the case of bottled water, MPs can be released as they are worn when opening or closing the bottle cap [60], 61]. The level of exposure to tap water may vary depending on whether MPs are removed during the drinking water treatment process or whether the household pipes through which tap water is supplied are made of plastic [62]. The results of this study showed that the EDI of MPs ranged from 0–660 MP/day in tap water and 0.0091–1,531,524 MP/day in bottled water. Intake of MPs through bottled water was the highest across all food groups. This increase was particularly notable in a previous study that analyzed Italian bottled water samples [63]. Another study also found that exposure to MPs was higher when drinking bottled water compared to tap water [64]. Hence, when meeting the recommended water intake, individuals drinking tap water can consume 4,000 MPs per year, whereas individuals drinking bottled water can consume up to 90,000 MPs. Consumption of drinking water cannot be reduced or avoided. Hence, it is one of the main sources of MP exposure for humans and poses an issue that cannot be overlooked when considering long-term exposure [65]. The level of exposure to MPs through drinking water can vary significantly depending on the source of drinking water. The concentration range of MPs in bottled water investigated in this study was 2–54,200,000 MP/L, which was remarkably higher than that in tap water (0.035–1.753 MP/L). The difference can be attributed to the fact that the packaging materials used to prepare bottled water contribute to the release of MPs [66]. When drinking water from the same brand and source was placed into glass and plastic bottles, the water in the plastic bottle (1,410 MP/L) had a higher concentration of MPs than the water in the glass bottle (204 MP/L) [67]. Another study showed that in the case of reusable plastic bottles, the cap and neck of the bottle may wear out depending on the number of uses, releasing more MPs [68].

Salt is classified into various types, such as sea salt, rock salt, lake salt, and well salt, depending on the source and is obtained via an evaporation process involving the sea, lake, or well, respectively [69], [70], [71]. Salt collected from these sources is packaged in various containers and consumed by humans for various purposes, such as direct consumption or food processing [69], 70]. The presence of MPs in salt may result from contamination during salt collection, transportation, drying, and packaging [72]. This study showed that the concentration of MPs in table salt ranged from 2–1,475 MP/kg for sea salt and 150 MP/kg for bamboo salt. Additionally, the concentration of MPs in lake salt was 2–364 MP/kg, and the concentrations in rock salt and well salt were 9–1,356 MP/kg and 7–185 MP/kg, respectively. Hence, the concentration of MPs in sea salt was the highest compared to other salts, consistent with the results of previous studies [24], 72]. Hence, MP pollution in coastal areas is more serious, and the degree of MP pollution in surface water where sea salt is collected is more severe than in groundwater where rock salt and well salt are collected [71], [72], [73]. The presence of table salt is an indicator of the degree of MP pollution in the marine environment [35]. In addition, depending on the type of salt, the concentration of MPs may vary due to additional purification processes. For example, because bamboo salt is produced by baking at high temperatures, residual MPs can be removed. Hence, the level of MP contamination may be lower than that of other salts [29]. We found that the EDI of MPs associated with table salt intake was 0–117 MP/day, which could result in the ingestion of up to 42,600 MPs per year. A previous study has reported that adults consuming approximately 10 g of salt every day can consume up to 19,000 MPs per year [73]. As of 2018, global salt consumption was approximately 300 million tons, of which approximately 11.6 % was consumed by humans [70], 73]. Salt is a source of sodium, an essential nutrient required by our bodies. The WHO recommends consuming less than 5 g of salt per day. Since salt intake is inevitable for humans, efforts are needed to reduce exposure to MPs during the collection, manufacturing, and packaging stages [69], 74].

The presence of MPs in fruits and vegetables, including lettuce, tomatoes, onions, potatoes, cucumbers, carrots, broccoli, pears, and apples, was reported in three studies. The concentration of MPs among fruits and vegetables ranged from 2.9 to 195,500 MP/g, with the highest concentration in apples. Consuming apples can lead to exposure to 462,000 MPs per day, which is much higher than that noted with other food groups. Although only a few studies have reported the intake and concentration of MPs through fruits and vegetables, previous studies have found MPs with sizes less than 2 μm in eggplants, potatoes, grapes, and bananas purchased from local markets in India [75]. A study conducted in China found MPs smaller than 500 μm in agricultural fields growing vegetables, with an average abundance of approximately 1,400 MP/kg [76]. Another study confirmed that styrene, ethylbenzene, and xylene, which are used in the manufacture of plastics, were detected in vegetables such as parsley, tomatoes, garlic, and spinach, suggesting MP contamination among vegetables [77]. Hence, airborne dust, herbicides, and fertilizers can accumulate in the soil and penetrate through seeds, roots, stems, and leaves, resulting in MP contamination [75], 78]. Exposure to agricultural environments can potentially affect human health as agricultural products are ultimately consumed by humans via the diet [78], 79]. However, measures such as rinsing fruits and vegetables with water or washing them with edible detergent before consuming them can reduce exposure to MPs [80].

The presence of MPs in beverages was reported in soft drinks, drip bag coffee, milk, infant milk powder, processed drinks, and alcoholic beverages (beer). The total concentration of MPs in beverages ranged from 1.75–7,607 MP/L or 5 ± 3 MP/100 g, and the estimated daily intake ranged from 0.002 to 18,140 MP/day. Additionally, barley used to make beer or tea leaves used to produce liquid tea can be contaminated with MPs during the cultivation process [81]. In particular, pouring hot water into a tea bag or drip coffee bag can exacerbate the release of microplastics, with estimates ranging from at least 10,000 to up to 10 billion microplastics being released [82], 83]. Exposure to MPs can also occur during the packaging or processing of products, such as pouring drinks into beverages into plastic bottles [81]. Two studies reported the presence of MPs in milk, and the concentrations of MPs were similar at 6.5 ± 2.3 MP/L and 6 ± 5 MP/L, respectively. The number of MPs that can be exposed via milk consumption was up to 858 MPs per year. Contamination of MPs in milk can be mediated by the milking machines used to collect the milk. Additionally, the concentration of MPs may vary depending on whether packaging materials are used [84]. The concentration of MPs in powdered milk consumed by infants can vary depending on the type of container. Furthermore, boxed milk powder (7 ± 3 MP/100 g) has demonstrated a higher concentration of MPs than canned milk powder (4 ± 3 MP/100 g), which can be attributed to the fact that the inner packaging material of the box-shaped container was made of plastics. Here, we estimated that an average of 305–580 MPs could be consumed per year through powdered milk. This finding can be primarily attributed to exposure through plastic baby bottles rather than MPs in the powdered milk itself [85]. Therefore, exposure to MPs through milk and powdered milk can be a threat, especially to children who consume a lot of dairy products. Furthermore, MPs may also be present in other dairy products such as ice cream, cheese, or yogurt [84]. MPs in alcoholic beverages were reported in two studies, and the concentrations of MPs were 9 MP/L (median) and 4.05 MP/L (mean), respectively. The intake of MPs through beer was estimated based on per capita alcohol consumption, revealing the prevalence of MPs in this beverage [29], 86].

The presence of MPs in condiments was reported for a total of four items (honey, vinegar, soy sauce, and fish sauce). The range of concentration of MPs in condiments was 0.18–0.60 MP/g and 30.0–51.35 MP/L, with the highest concentration reported in bottled vinegar. The presence of MPs in condiments can be determined by the amount of salt added to the product during the manufacturing process or the product packing process [87]. Additionally, MP contamination in honey can occur when bees ingest MPs in the air or flowers while collecting honey [45], 88]. The results of this study showed that the EDI of MPs consumed through condiments ranged from 0.0002–0.004 MP/day, and a maximum of 0.74 MPs could be ingested per year. However, due to the limited number of studies, our findings may be an underestimate of actual data.

The presence and consumption of MPs in meat were reported in only one published study. Packaged meat (chicken) sold in France contains MPs at an average concentration of 4–18.7 MP/kg, which implies that humans can ingest an average of 0.01–1.4 mg of MPs via meat consumption per day. This outcome was attributed to polystyrene particles present in food tray packaging, whereby chicken could potentially become contaminated as these particles adhere to the surface of the meat. Additionally, the color of particles is also affected by the color of the food tray [89]. Plastic is used to prevent meat spoilage and facilitate its effective storage. Most plastic containers contain polyvinylchloride, polyamide, polyethylene, and PS [89], 90]. Hence, MPs exist in the environment and are converted into consumable food during packaging, contributing to human exposure [90]. Another study confirmed that exposure to MPs can occur through plastic cutting boards that are used when cutting meat in butcher shops [91].

Quality assessment

The absence of standardized protocols for sampling, sample pretreatment, and analysis methods for MPs introduces limitations in comparing results across various studies, which may exhibit inconsistencies. Due to the diversity of food matrices, uncertainty about the concentration of MPs may also arise during the matrix separation process [29]. As the issue of the reliability of research results continues to be raised, previous studies have addressed this issue by proposing evaluation standards for assessing the reliability of research on MPs in drinking water and food [33], 34]. Accordingly, this study also assessed the quality of the study on MPs through each evaluation criteria for food and drinking water (see Table 1 and Figure 3). First, as a result of evaluating the quality of studies on MPs in food according to the standards proposed by Hermsen et al. [34], the total reliability score ranged from 5 to 16 out of 20, and the average of the total score was 11.50. The average scores for each criterion are listed as follows: sampling methods (0.79), sample size (0.66), sample processing and storage (1.24), lab preparation (1.71), clean air conditions (1.19), negative controls (0.94), positive controls (0.13), target component (1.77), sample treatment (1.08), and polymer identification (2.00). Hence, the ‘polymer identification’ was associated with the highest score, indicating that this particular area can be deemed highly reliable and trustworthy. In contrast, most studies had small sample sizes or omitted detailed information on sampling methods and positive/negative controls, resulting in low scores associated with each criterion. Second, upon evaluating the quality of MP research on drinking water according to the standards of Koelmans et al. [33], the total reliability score ranged from 7 to 15 out of a total of 19 points. The average total score was 11.16 points, which was lower than that of the food groups. The average scores for each criterion were as follows: sampling methods (1.37), sample size (1.58), sample processing and storage (1.79), lab preparation (1.63), clean air conditions (1.42), negative controls (0.74), positive controls (0.16), sample treatment (0.53), and polymer identification (1.95). Similar to the quality evaluation results of research on MPs in food, information on ‘positive/negative controls’ and ‘sample (pre)treatment’ were omitted or insufficient, resulting in low scores for each criterion. In both food and drinking water studies, no study scored more than 1 point in each evaluation area. Hence, all studies received a score of 0 in at least one evaluation criterion. Therefore, according to the criteria presented by Hermsen et al. [34], each study may not achieve complete reliability. However, these evaluation results should be interpreted as indicators of the usefulness of the research rather than accepted as an absolute evaluation standard for previous studies [33], 34]. Therefore, these evaluation criteria can be referred to before starting further research on MPs. Additionally, the results of this evaluation suggest that standardization of sampling and analysis methods should be established to improve the quality of MP research in the future.

Strengths and limitations

Upon reviewing the literature, limitations were associated with direct comparisons due to disparities in sampling methods, analysis equipment, and quality practices among previous studies. Additionally, the number of studies for some food groups was limited, rendering the generalization of the results difficult. However, despite these limitations, our study confirmed the presence of MPs in various food groups and characterized the particles. Additionally, by investigating the estimated intake of MPs through food, we were able to determine the extent of the exposure of humans to MPs and possibly pathways to mitigate this exposure. The significance of this study lies in the fact that it is the first study to present a systematic literature review on the estimated intake of MPs.