A study on microplastics occurrence and dietary intake from staple foods and tap water in western Iran

Abstract

This study investigates the occurrence, characteristics, and estimation of consumer exposure to microplastics (MPs) in a collection of staple foods and tap water in the city of Kermanshah (in western Iran). For this purpose, samples of rice, bread, milk, yogurt, meat, edible oil, and tap water were collected. MPs in each sample were counted using a stereo microscope and classified by shape, size, color, and type (with limitation). The highest average contamination was observed in unrinsed rice (198.00 particles/kg) and the lowest in tap water (4.10 particles/L). The most abundant MPs shape was fiber (76.09%), and blue-colored MPs were also more frequent (69.27%). 31.34% of the particles were in the size range of 500 to 1000 μm, and 30.86% had a size < 500 μm. The identified polymers included (probably) polypropylene (PP), polyethylene (PE: High/Low-Density), and polystyrene (PS). The Cumulative Estimated Annual Intake (CEAI) was 3.8 × 10⁵ particles/kg of bw for children and 8.7 × 10⁴ particles/kg of bw for adults. The Cumulative Estimated Daily Intake (CEDI) was 1047 particles/kg of bw for children and 240 particles/ kg of bw for adults. The results of this research indicate the widespread presence of MPs in the staple food basket of Kermanshah residents, which may raise health concerns (especially for more vulnerable age groups), thus monitoring measures and actions to prevent further contamination are essential. This study has several methodological limitations, including visual MP identification (error risk), no recovery experiments, and polymer analysis only on larger particles. Future work should address these issues.

Introduction

The increasing proliferation of plastics in global ecosystems has become a serious concern for the environment and public health. This problem is rooted in the soaring rise in plastic production since the 1950s^1,2. Microplastics (MPs) are defined as plastic particles or fragments < 5 mm, which are divided into two main categories: primary MPs, which are manufactured on purpose for uses such as cosmetics or industrial applications, and secondary MPs, which result from the breakdown and fragmentation of larger plastic pieces^3,4. To understand the scale of this crisis, it is important to note that approximately 7,800 million tons of plastic were produced between 1950 and 2015, with 79% of it ending up in landfills or the environment¹. This pervasive presence has led to these particles being found virtually everywhere, in water, earth, and air environments^2,5.

MPs are no longer just an environmental pollutant; they have become a concerning factor in food safety and have attracted considerable attention^6,7. Today, the entry of these tiny particles into the human body through the diet is considered inevitable^3,8. Consequently, MPs have now been identified in human feces^9,10, the placenta¹¹, and even blood¹². Furthermore, experimental evidence suggests that exposure to MPs is associated with adverse biological reactions, including cellular apoptosis, cytotoxicity, oxidative stress, inflammation, and genotoxicity¹³. It has been observed that particles up to 10 μm can enter the bloodstream through a process called endocytosis¹⁴, and larger particles up to 150 μm are also capable of crossing the intestinal barrier via persorption¹⁵, which raises concerns about the potential accumulation of MPs in various organs. These studies serve as a serious warning about the potential risks posed by chronic exposure to these particles⁶. Additionally, MPs can function as carriers of other hazardous substances present in the environment¹. Persistent Organic Pollutants (POPs), toxic metals, pesticides, and microorganisms can adhere to their surfaces and be transferred to living systems after consumption, thereby exacerbating their toxic effects^1,16,17.

Across the world, MPs have been found within many different food products and water sources^4,5,18. In aquatic environments, seafood is considered a major source of contamination¹⁹. For instance, a study in Kerala, India, reported an average of 2.7 ± 12 MP particles/kg in the edible tissue of shellfish. This level was also found to be 7.7 ± 20 particles/kg in Uroteuthis (Photololigo) duvaucelii and 3.2 ± 10 particles/kg in Portunus pelagicus⁶. Freshwater sources and drinking water have also been severely affected; MPs have been detected in tap water²⁰, bottled water²¹, and groundwater²² at varying concentrations.

Beyond this, MPs have contaminated a wide range of everyday consumer products⁵. Commercial salts from various countries have shown MP contamination²³. Studies have reported the presence of MPs in raw milk, processed milk, and infant formula^24–26. For example, MP levels in milk samples in Mexico ranged from 3 ± 2 to 11 ± 3.54 particles/L²⁶. For meat products, observed levels were between 0.03 ± 0.04 and 1.19 ± 0.72 particles/g in chicken and between 0.014 ± 0.024 and 2.6 ± 2.8 particles/g in fish²⁷. Other foodstuffs such as honey²⁸, soft drinks²⁹, beverages³⁰, sausages³¹, packaged tea³², and herbal distillates³³ have also contained MPs. The sources of this contamination can include the water used, plastic packaging materials, and food processing equipment^5,33.

Despite the growing volume of global research on MP pollution⁵, we still don’t have enough data on the extent of human exposure to these particles, particularly in certain geographical areas; especially through the investigation of a comprehensive basket of staple foods and water sources. Although various studies have been conducted in Iran focusing on bottled water³⁴, salt¹⁷, vinegar³⁵, and various bottled beverages²⁹, a comprehensive analysis of the occurrence, characteristics, and dietary exposure levels to MPs across a wider range of major foodstuffs and tap water in a specific urban environment like Kermanshah has not yet been performed.

Therefore, the primary objectives of this study were threefold: first, to determine the level of microplastic contamination in a selection of highly consumed foodstuffs and tap water in Kermanshah; second, to identify the characteristics (shape, color, size and polymer type) of the identified MPs; and third, to estimate the potential annual/daily intake of MPs from these sources for the residents of Kermanshah. The current research is expected to improve the worldwide knowledge of MP pollution in the food and serve as guidance for national food safety policymaking.

Materials & methods

Approach and sample collection

The study is a cross-sectional study designed and implemented to determine the frequency and characteristics of MPs in essential food items and tap water consumed in Kermanshah, Iran (between early and mid-2024). The selection of food items was made after consultation with nutrition specialists and based on the key components of the local people’s diet, which included tap water and several important food groups: rice, bread (Lavash, a thin traditional flatbread, and Sangak, a traditional Iranian whole-wheat flatbread), dairy products (milk and yogurt), edible liquid oil, meat (chicken and red meat), sugar, and salt.

For rice sampling, ten popular and high-consumption brands (including Iranian, Indian, and Pakistani varieties) were selected based on online retail website data, and a 200-g sample of each was collected in glass/metal containers. Bread samples, including Lavash and Sangak, were obtained from six different bakeries across the city and collected in cotton bags. For dairy, three top-selling brands of milk (in carton packaging) and yogurt (in plastic containers) were chosen. Additionally, four brands of edible liquid oil in PET (polyethylene terephthalate) bottles and 200-g samples of chicken and red meat were purchased from three separate poultry shops and three butcheries, respectively. It should be noted that the concentrations provided in a recent study¹⁷ were used to assess the exposure level for sugar and salt.

To obtain a representative sample of the urban water network, tap water was collected from 10 different geographical locations throughout Kermanshah (including Ferdosi, Shariati, Pardis, Vakil Agha, Kian Shahr, Dowlat Abad, Golestan, Zafar, Anahita, and Sorkh Lijeh neighborhoods - names transliterated from Persian; spelling may vary). At each location, the tap was run for 20–30 s to flush out potential sediments, and then a 1.5-liter sample was taken in a glass container with a metal lid. Finally, after the completion of the process, all samples were transferred to the laboratory, and samples that were not immediately analyzed were kept refrigerated (between 3 and 4 °C) until preparation.

Sample preparation and MPs extraction

To separate MPs from diverse matrices, a combination of protocols was utilized. For rice samples, the density separation method was employed; each 200-g sample was divided into two 100-g subsamples. One subsample was washed three times with tap water to simulate consumer practice, while the other remained unwashed. Saturated sodium chloride (NaCl) solution was then added to both subsamples to separate the lighter MPs from the heavier rice grains by floatation³⁶. For samples with high organic content, such as bread, dairy products, and meat, the oxidative digestion protocol was used. To achieve this, 100-g portions of each material (including pieces of bread, milk, homogenized yogurt, and mixed meat) were combined with 100 mL of 0.05 M Fe (II) (FeSO₄·7H₂O) and 100 mL of 30% hydrogen peroxide (H₂O₂) in separate Erlenmeyer flasks³⁷. H₂O₂ has previously shown suitable efficiency in digesting organic materials³⁸. The mixtures were then stored in a dark environment for at least two days to allow decomposition of the biological matrix. To extract MPs from edible oil, a solvent extraction method was used: 20 mL of oil was transferred to a glass Erlenmeyer flask, and 60 mL of hexane (a 1:3 oil-to-hexane ratio) was added, the resulting mixture was then vortexed. Adding hexane to oil will significantly reduce the viscosity and improve the fluidity of the entire mixture. As a result, the oil dissolved in hexane is able to pass through the filter³⁹. Tap water samples, due to their lack of a complex organic matrix, did not require these pre-treatment steps.

In the final stage, all processed liquids from each sample (including the NaCl supernatant, digested solutions, the hexane-oil mixture, and raw tap water) were passed through a vacuum filtration process using a fiberglass filter (Whatman GF/C) with a 1.2 μm pore size and 47 mm diameter. This step allowed for the collection and separation of all remaining MP particles from the solutions. Finally, each filter was carefully transferred to a labeled glass Petri dish and prepared for subsequent analysis.

MPs analysis and quality control

Following the filtration processes, the filters were subsequently analyzed for the quantification and characterization of the MP particles. Initially, the filters were examined using a stereomicroscope (Olympus SZX16), where particles were counted, and each MP was categorized based on a set of physical characteristics, including size range, shape, and color. The particle size was determined by comparing each particle to a reference rod of 0.5 mm under a stereomicroscope. To confirm the identity of particles not readily identifiable as plastic, the hot needle test was employed; the particle’s reaction to heat (such as bending, melting, or shrinking) served to verify its polymeric nature³³. For chemical composition analysis, Fourier-Transform Infrared (FTIR) spectroscopy was carried out by an external accredited laboratory using a PerkinElmer Spectrum spectrometer (PerkinElmer, Inc., USA) equipped with an ATR accessory. The instrumental parameters included a spectral range of 4000 –400 cm^-1, a resolution of 4 cm^-1, and 32 scans per sample. PerkinElmer Spectrum software was used for data acquisition and processing. Due to technical limitations, the FTIR analysis was restricted to particles large enough (generally > 500 μm) to be manually extracted from the filter using tweezers. Several representative particles were selected from each sample group (water and food), and their spectra were recorded. The resulting spectra were then compared with standard polymer spectra to identify the polymer type⁴⁰. Furthermore, to allow for a more detailed examination of the MPs, their surface morphology was investigated using Scanning Electron Microscopy (SEM, Bruker, Germany).

Rigorous quality control (QC) measures were implemented throughout all stages to ensure the reliability of the results and minimize contamination. These steps included keeping all lab doors and windows closed, mandatory and continuous use of cotton laboratory coats and gloves by personnel, and the prohibition of plastic equipment in the analysis area. All glassware was rinsed with distilled water before use and covered with aluminum foil when not in immediate use. To assess the potential influence of airborne MPs in the analysis area on the results, an open, wide-mouthed glass container filled with distilled water was strategically placed as a reagent blank. Furthermore, a visual inspection of every new filter was conducted under the stereomicroscope prior to use to ensure they were free of any particle contamination.

Estimation of dietary MPs exposure

The dietary exposure to MPs was estimated for two distinct population groups: adults with an average body weight (BW) of 70 kg and children with an average body weight of 16 kg. For this assessment, the Estimated Annual Intake (EAI) was calculated based on the number of ingested particles per kilogram of body weight per year (particles/kg bw/year). This calculation was performed using Eq. (1):

1

Where Ab.MPs is the abundance of MPs in each food item (particles/kg or particles/L), APCC is the Annual Per Capita Consumption of that item, and BW is the average body weight of the consumers (70–16 kg).

The APCC data, based on local Iranian statistics, are as follows: 912.5 L per year for tap water, and 124, 55, 30.2, 37.5, 12.7, 21, and 3.25 kg per year for bread, dairy, sugar, rice, meat, edible oil, and salt, respectively. Since reliable APCC data for different age groups were unavailable, the same APCC values were applied for both children and adults. This approach ensures that the difference in the resulting exposure levels for children and adults is solely attributable to the difference in BW.

Finally, to determine the total dietary exposure, the intake values from all analyzed sources including rice, meat, tap water, bread, dairy, edible oil, salt, and sugar were summed. This methodology provided an overall picture of the Cumulative Estimated Annual Intake (CEAI), as well as the Cumulative Estimated Daily Intake (CEDI), for both the children and adult groups, reflecting their total exposure through their typical diet.

Data analysis

This study used descriptive statistics only. Microsoft Excel (2016) was used to calculate mean values, standard deviations (SD), and exposure estimates. MP concentrations are presented as mean ± SD. Shape, size, and color distributions are reported as percentages of total particles counted. GraphPad Prism (version 10.4) was used solely for data visualization and figure generation.

Results and discussion

Occurrence, type, and possible sources

Results showed that the obtained data were highly unlikely to be affected by MP contamination in the laboratory environment because no MPs were observed in the control sample. It was also confirmed that the new filters were not contaminated with MPs before the work began. A sample of stereo microscope images of the separated MP particles is shown in Fig. 1. Clearly, MPs have various shapes and sizes, including fibrous particles of different sizes (Fig. 1a-c) and a blue fragment-shaped particle (Fig. 1d). As shown in Fig. 2, the results of this study indicated that MPs were present in all examined samples of food and tap water. The mean MP concentration varied significantly between different food items and even between different samples of the same specific item (Fig. 2a-h). The highest contamination level was observed in unwashed rice samples with an average of 198 particles/kg (Fig. 2a and h), and the lowest contamination level was in tap water with an average of 4.1 particles/L (Fig. 2g and h). Following rinsed rice, with an average of 100 particles/kg (Fig. 2a and h), edible oil also had an average of 100 particles/kg (Fig. 2e and h), and yogurt followed with an average of 63.33 particles/kg (Fig. 2c and h). Among the meat types, red meat showed higher contamination with an average of 53.33 particles/kg compared to chicken meat with an average of 16.67 particles/kg (Fig. 2d and h). Sangak bread had a higher contamination level with an average of 16.67 particles/kg than Lavash bread with 13.33 particles/kg (Fig. 2f and h). Regarding dairy products, yogurt with an average of 63.33 particles/kg (Fig. 2c and h) had a higher contamination than milk with an average of 43.33 particles/kg (Fig. 2b and h).

Fig. 1.

Stereomicroscope images of some MPs (a-d) identified from food and water samples.

Fig. 2.

Abundance of MPs in samples. (a-g) show the concentration of MPs (particles/kg or particles/L) for each brand or type of rice, milk, yogurt, meat, edible oil, bread, and tap water from different areas. (h) shows the mean abundance ± standard deviation (SD) for each item category.

A study in Mexico found MPs in 48 out of 57 samples of energy drinks, soft drinks, beer, and cold tea, with the highest concentration reaching 28 particles/L⁴¹. In Ecuador, the results of analyzing milk, soft drinks, honey, and beer showed an average of about 40 MP particles/L (ranging from 10 to 100 particles/L)²⁸. Another study reported that each gram of fish and chicken meat purchased in the Middle East could contain up to 2.6 and 1.19 particles, respectively²⁷. A study on common food and beverage containers under typical storage and washing conditions revealed that all food and beverage containers tested released MPs under both hot (95°C) and cold (ice) conditions. The overall average MP release was 176 particles/L, with the highest amount (261 particles/L) coming from Polystyrene (PS) boxes under hot conditions. In HDPE bags, hot conditions resulted in almost double the MP release compared to cold conditions (192 Vs. 90 particles/L)⁴². The presence of MPs has also been reported in various human-consumed plant products. This contamination can occur through multiple factors, including the degradation of plastic items in agricultural mulches, irrigation, the use of sewage sludge for soil enrichment, and other indirect pathways³.

FT-IR was used to identify the chemical composition of the particles (Fig. 3). Based on this, the identified polymers included Polypropylene (PP) (Fig. 3a), High-Density Polyethylene (HDPE) (Fig. 3b), Low-Density Polyethylene (LDPE) (Fig. 3c), and PS (Fig. 3d). These are predominantly polymers used in food packaging and other industrial applications^42,43. Of all the identified MP, 7 particles were large enough to be manually extracted and analyzed by FTIR. Among these, 6 particles (85.71%) were confirmed as synthetic polymers, while 1 particle (14.28%) produced an ambiguous spectrum and was subsequently excluded from the final count. Due to the limited FTIR analysis on small particles, a precise assessment of false results was not possible through this method. The hot needle test, although useful as a rapid screening tool, may in some cases produce false results (e.g., some heat-stable plastics may not exhibit obvious morphological changes). These factors indicate that the abundance of MPs should be interpreted conservatively when evaluating the results (see the Limitations and Future Recommendations section).

Fig. 3.

Fourier transform infrared (FT-IR) spectra of some MPs isolated from the samples. (a, b, c, and d) have similarities (probably) to the Polypropylene (PP), High-Density Polyethylene (HDPE), Low-Density Polyethylene (LDPE), and Polystyrene (PS), respectively.

The main polymers identified in food samples in an Indian study were Nylon, PET, Polyacrylamide copolymer, PS, Polychloroprene, and Polynorbornene³⁷. In Ecuador, the analysis of honey, milk, and beverage samples showed that the identified MPs primarily consisted of Polyethylene (low- and high-density), PP, and Polyacrylamide²⁸. A review study of 40 published studies (2017 to 2024) indicates that products such as meat, seafood, fruits and vegetables, dairy, edible oils, salt, and water are vulnerable to MP contamination, and the most common polymer types found are Polyethylene (PE), PP, and PS⁵.

MP contamination of tap water can originate from various sources in the water supply chain. These sources include the water source itself (such as freshwater systems where MPs are abundant) and subsequent stages like water treatment and distribution processes^3,44. The abrasion of plastic equipment used in water treatment and transfer systems, such as pipes (often Polyvinyl chloride- PVC, or PE), fittings, and tanks (epoxy resin coatings to prevent corrosion), is a major factor contributing to the presence of these particles in drinking water¹⁸. Furthermore, since conventional urban water treatment processes are unable to completely remove these fine particles, one of the key points regarding the presence of MPs in food items relates back to the water source used in their preparation. Any food item with a high-water content or that interacts with water during its manufacturing process is susceptible to MP contamination^28,33.

In addition, air plays a significant role. Fine plastic particles released into the air from urban activities, factories, and traffic may mix with food items during production or packaging stages²⁸. MPs also enter food through the abrasion of factory equipment. Plastic or rubber parts on the production line wear down over time, releasing small particles that find their way into the final product²⁸.

Clearly, packaging itself can be a source of contamination. When food is packaged in plastic containers, plastic particles may detach from the container walls and enter the product^34,45,46. Moreover, everyday food preparation tools can directly introduce MPs into food⁴³. For instance, PE-based plastic kitchen cutting boards can contaminate raw fish and chicken meat through mechanical abrasion²⁷. It has even been reported that using a plastic blender for just 30 s to crush ice can release up to 360,000 MP particles⁴⁷. Additionally, the repeated washing and use of plastic containers like melamine bowls have been shown to lead to a significant release of MPs, with the amount increasing tenfold after 100 washes (from 18 to 394 particles/L)⁴².

Numerous studies have been dedicated to investigating the transfer of MPs in the food chain^48–50. For example, a study examining the transfer of MPs from soil to terrestrial organisms related to human consumption provides evidence of this transfer in traditional Mayan home gardens in southeastern Mexico. Quantitative results showed that MP concentrations increase along the transfer chain; this concentration rises from the soil with an average of 0.87 particles/g, to earthworm casts with 14.8 particles/g, and the highest level in chicken feces with 129.8 particles/g. Furthermore, the chicken gizzard (used for human consumption) contained 10.2 particles, while no MPs were found in the chicken crop or crop products⁵⁰. The widespread use of fish meals in poultry and pig diets can also act as a transfer route, leading to the transmission of MP particles to non-marine food sources⁴⁹.

In summary, MPs enter our daily dietary items not only through direct contact (such as packaging) but also through more complex environmental and process-related pathways^3,43,47,50. Consequently, addressing this issue requires simultaneous attention to production methods, packaging types, and the state of the environment.

Morphology and shape

SEM images of several plastics isolated from the samples are presented in Fig. 4. The images are provided at magnifications of 400× (Fig. 4a-c) and 100× (Fig. 4d), showing that the particles have rough and cracked surfaces, which may indicate the potential for degradation and erosion of larger plastics in the environment and their conversion into secondary MPs. Overall, the SEM results in this study reveal a high-detail variety in the morphology and extent of surface degradation of the MPs, ranging from smooth to heavily eroded surfaces. This information is valuable for identifying and understanding the fate and potential source of these particles. In a study that analyzed MPs in commercial beverages, SEM images of the surface texture of MP fibers generally showed an elongated, cylindrical shape with relatively homogenous features. These fibers often appeared disintegrated, which may suggest they underwent mechanical abrasion. Some images also clearly revealed pores and rough surface features⁴¹. Commercial milk products have also been studied using SEM, which reported small holes and cracks on the surface of the identified MPs²⁶. Furthermore, when studying PS-MPs, SEM examinations have shown the surface of the particles to be porous and composed of folds, a structure hypothesized to facilitate the adsorption of other pollutants⁵¹.

Fig. 4.

SEM images of some MPs isolated from the samples (a-d).

The distribution of different MP shapes across various food categories and tap water samples from different regions is presented in Fig. 5. A nearly identical pattern was observed across most food and tap water samples, namely that fibrous MPs were the dominant shape (Fig. 5a-i). Overall, fiber-shaped MPs were the dominant morphology among all samples with a frequency of 76.09%, followed by fragment-shaped MPs with a contribution of 23.91%. The shape of MPs often indicates their origin. For instance, primary MPs are particles intentionally produced, and they usually exhibit a regular shape, often spherical or fibrous, whereas secondary MPs are the result of environmental fragmentation or erosion of larger plastics, and typically have an irregular shape^1,4. Among the various food products investigated in studies to date, fragments and fibers have been widely reported as the most common morphologies identified in foods and beverages^{5,33,37,41,52}. Fragments are formed from the breakdown of larger plastic items or fibers, while fibers are defined as particles where the length significantly exceeds the diameter^1,28. Analysis of milk products in Mexico showed that fibers were the dominant shape (97.5%), while fragments comprised a small fraction (2.5%)²⁶. In takeaway food, fragments were identified as the main shape (70%)⁵².

Fig. 5.

(a-h) Percentage distribution of MP shapes for each specific food and water category, and (i) the overall MP shape distribution.

Size distribution

The size distribution of MPs in the different food categories examined and in tap water from various locations across the Kermanshah city is presented in Fig. 6. The results for almost all items analyzed indicate that smaller particles were more abundant (Fig. 6a-i). Overall (Fig. 6i), the highest frequency of MPs belonged to particles with sizes of 500 to 1000 μm (31.34%) and less than 500 μm (30.86%). These two categories collectively accounted for more than 62% of the total MPs identified in the analyzed items. Conversely, the frequency of particles decreased as the size increased, such that particles larger than 4000 μm constituted only 2.63% of the total. The size of MPs is a fundamental characteristic that determines their environmental fate, ecological impacts, and potential risk upon entering the human body via food and water matrices^16,33.

Fig. 6.

(a-h) Percentage distribution of different MP size classes for each food and water sample, and (i) the overall MP size distribution.

Analysis of edible shellfish tissues showed that MPs were predominantly in the 100 μm to 300 μm range, with 61.5% of them falling between 100 μm and 200 μm⁶. Similarly, studies on commercial fish species repeatedly identified MPs smaller than 1000 μm, which constituted 71% to 84% of the total contamination in the fish gastrointestinal tract¹⁶. For staple consumables like salt, a study in China reported that MPs in sea salt are often smaller than 200 μm⁵³. Analysis of bottled water using Micro-Raman Spectroscopy showed that the majority (80%) of the identified MPs were in the size range of 1 μm to 20 μm⁴⁶. This indicates that the detection limit of analytical methods is key, a factor which is likely one of the main limitations of our study, as the criterion for MP detection was based on stereo-microscope examination. Furthermore, in beverages sampled in Mexico, MP sizes varied from 0.1 mm to 3 mm, with a large portion falling in the 0.1 mm to 1 mm range⁴¹.

It is important to note that size is also a critical determinant for biological interaction and toxicity^16,51. Smaller particles, particularly those below 150 μm, have the capacity to pass through the human intestinal barrier, potentially leading to systemic exposure. Particles with sizes less than 20 μm have the potential to translocate into tissues¹⁶.

Color distribution

Figure 7 presents the frequency distribution of MPs based on their color. According to the results, the color distribution of MPs in the examined items is highly diverse (Fig. 7a-h), but overall, blue was identified as the most prevalent MP color, accounting for 69.27% of the total frequency compared to other colors. Following this, MPs of black (8.78%) and red (7.56%) colors were ranked next. Other colors, such as transparent, white, green, and yellow, comprised a smaller percentage of the total particles (Fig. 7h). The variety of colors suggests potential multiple origins⁴¹.

Fig. 7.

(a-h) Percentage of different MP colors observed for each food and water category, and (i) the overall MP color distribution.

A study on milk samples showed a variety of colors, including blue (72%), red, pink, and brown²⁶. The predominance of blue MPs is like the results of the present study. Milk analyzed in Turkey reported a spectrum of colors including black, green, brown, gray, blue, and red⁵⁴. An investigation of beverages, including soft drinks, iced tea, energy drinks, and beer, also revealed the presence of MPs of various colors, particularly blue (70.3%), red (15.3%), and black (11.8%)⁴¹. In specific food categories, such as rice and coffee, white MPs were the most common color observed⁵².

The color of MPs is an important characteristic that can serve as an indicator for their origin, polymer type, and even their fate in environmental systems¹. According to the review article by Cverenkárová et al. (2021), yellow and black MPs are often associated with higher levels of adsorbed persistent organic pollutants, and transparent and white MPs are known as particles frequently ingested by marine animals, highlighting the significance of color. Also, according to that study, clean and transparent MPs are often identified as common polymers like PP, while white and opaque types are often PE or LDPE¹. A study that analyzed drinking water from groundwater sources identified dark and transparent fibers; however, these fibers were often associated with the procedural control samples¹⁸.

Dietary intake

The results of the assessment of daily and annual MP exposure for the two age groups, children, and adults, are summarized in Fig. 8. These results reflect the MP concentrations, the per capita consumption of the items, and the body weight of the consumers. The estimated annual intake (EAI) for adults showed that the highest exposure is through the consumption of rice and tap water (both at 2 × 10⁴ particles/kg bw/year). Conversely, the lowest exposure to MPs occurs through the consumption of salt and meat (Fig. 8a). The same pattern was observed for children, with the difference that children generally showed a higher EAI for each of the examined items due to their lower body weight (Fig. 8b). More importantly, the cumulative estimated annual intake (CEAI) for children, at 3.8 × 10⁵ particles/kg bw/year, was significantly higher than for adults, at 8.7 × 10⁴ particles/kg bw/year (Fig. 8c). This difference was also evident in the cumulative estimated daily intake (CEDI), where the results showed that children, on average, receive 1047 particles/kg bw/day, and adults receive 239.31 particles/kg bw/day, indicating the greater vulnerability of children to these contaminants (Fig. 8d). However, as previously stated, the per capita consumption of the examined items was considered equal for children and adults in this study, and BW is the factor creating the difference in the presented results. It is likely that the actual per capita consumption for children will differ from that of adults in most cases, which would therefore affect the CEAI and CEDI results.

Fig. 8.

(a) Estimated Annual Intake (EAI) for adults from each dietary source (particles/kg bw/year), (b) Estimated Annual Intake (EAI) for children, (c) Total Cumulative Estimated Annual Intake (CEAI) for children and adults, and (d) Total Cumulative Estimated Daily Intake (CEDI) for children and adults (particles/kg bw/day).

In addition to ingestion, air inhalation and direct contact are other pathways for MPs to enter the human body⁵⁵. The assessment by Cox et al. (2019) regarding the number of MPs consumed through the daily diet of Americans showed that each person, on average, consumes between 39,000 and 52,000 MP particles annually, which is less than the result of the current study. According to them, this number increases to 74,000 to 121,000 particles when considering MPs inhaled through the air. The same study showed that if only drinking water from plastic bottles is consumed, each person can ingest 90,000 additional particles, compared to 4,000 particles for tap water⁵⁶. In the study by Hee et al. (2022), it was estimated that the annual MPs consumption by an individual in Southeast Asian countries is typically around 195,000 particles via food and beverage containers from fast-food or consumed on-site⁴².

One of the significant non-dietary routes is the inhalation of MPs present in the air and dust^37,57,58. According to one study, the average daily exposure through air inhalation per person in India was estimated at 594 particles, which was higher than the exposure through drinking water (382 particles) and lower than through food (1036 particles). In the same study, the CEDI of MPs from the sources was up to 202 mg per person per day³⁷, which was lower than our study. MPs manufactured for specific purposes (such as microbeads) in health and cosmetic products can also be added to these sources^59,60. For example, the presence of HDPE and PE in toothpaste samples suggests that this source may be underestimated⁶¹. These findings indicate that the sources of human exposure to MPs are far more numerous than the items examined in this study, which focused solely on oral ingestion of particles through the consumption of major dietary items. Nevertheless, the presented results further illuminate the contribution of the diet as a major source of MPs pollution exposure for humans.

Limitations and future recommendations

Several potential limitations of this study should be clarified, both for consideration when interpreting the results and for guiding future research directions.

• First, in this study, the identification and counting of MPs were performed using a stereomicroscope, which, due to its reliance on human vision, carries a risk of error and the potential to overlook some very small particles.

• Second, density separation using saturated NaCl likely underestimates higher-density polymers, as these polymers may not float and thus remain undetected.

• Third, no recovery experiments (positive controls) were conducted to assess the extraction efficiency of the digestion, density separation, or solvent extraction procedures. Although most protocols were based on previous studies, the absence of recovery testing introduces uncertainty regarding the reported MP concentrations in the samples.

• Fourth, the analysis to determine polymer types was performed only on larger particles that could be manually separated; therefore, the identified polymers cannot definitively represent all MPs present in the samples. Regarding the reliability of visual identification, although approximately 86% of the subset of particles subjected to FTIR were confirmed as plastics, this validation rate applies only to larger, manually extractable particles. As noted, the vast majority of counted particles were not spectroscopically verified due to their small size, and the hot needle test, although applied, may yield false results. Consequently, the reported exposure estimates may be subject to both overestimation (if non-plastic particles were incorrectly counted as MPs) and underestimation (if true MPs were missed or not extracted). These uncertainties should be considered when interpreting the results, and the reported abundances should be viewed as conservative estimates, with future studies aiming to address these concerns.

• Fifth, these methodological uncertainties, together with the use of identical consumption data for children and adults and the absence of non-dietary exposure routes, imply that the CEAI and CEDI values should be regarded as semi-quantitative approximations rather than precise exposure metrics.

Future studies should address the reported gaps through recovery testing, the use of higher-density solutions, and broader spectroscopic verification. The use of more advanced technologies is recommended to enable the identification of smaller MPs. It is also necessary to examine a wider variety of food products and to trace the main sources of contamination entering the supply chain. Conducting comprehensive assessments that simultaneously consider ingestion, inhalation, and even dermal contact pathways could reveal actual exposure. Using precise consumption data for each specific age group to estimate exposure levels more accurately could also bring results closer to reality. A focus on toxicological studies is necessary to understand the long-term health effects of these particles. Finally, to mitigate the risks posed by this emerging contaminant, implementing stricter regulatory policies in the field of food safety appears essential.

Conclusion

This study provided the comprehensive data on the contamination of microplastics (MPs) across a wide range of staple foods and tap water in a specific urban environment in Iran (Kermanshah). We showed that the number of MPs entering our bodies through the consumption of typical dietary items is not insignificant and cannot be ignored. MPs were present in samples ranging from rice and bread to dairy products, meat and drinking water. This concern appears more serious given the potential of MPs to carry and absorb other pollutants. It was estimated that consumers annually ingest 3.8 × 10⁵ particles/kg bw for children and 8.7 × 10⁴ particles/kg bw for adults due to the consumption of common foodstuffs. Furthermore, 1047 particles/kg bw for children and 240 particles/kg bw for adults are ingested daily by these consumers. The higher values of these indices for children compared to adults represent a serious warning for public health. Additionally, this research showed that small-sized particles (mostly < 1000 μm) and blue were the characteristics of most identified MPs. The identification of common polymers such as polypropylene (PP) and polyethylene (PE) may also indicate the prominent role of plastic packaging and human activities as sources of this pollution, although the contamination pathways are highly varied and complex.

Author contributions

M. Pirsaheb: Supervision‎, Conceptualization, Methodology, Data analysis, Writing—first draft, ‎Review and editing. M. Parvizi: Conceptualization, Experiment‎, Methodology, Review and editing, ‎Data analysis. H. Hossini: Conceptualization, Experiment‎, Data analysis, Review and editing. T. ‎Massahi: Data analysis, Software‎, Writing—first draft, Review and editing‎.‎.

Funding

Kermanshah University of Medical Sciences (Grant No. 4020431).

Data availability

All relevant data are included in this document.

Declarations

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.

Ethics approval

Ethically approved (Ethics code: IR.KUMS.REC.1402.098).

Consent to participate

Not applicable.

Using generative AI and AI-assisted technologies in the writing process

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