Environmental Microplastic and Nanoplastic: Exposure Routes and Effects on Coagulation and the Cardiovascular System

Abstract

Plastic pollution has been a growing concern in recent decades due to the proliferation and ease of manufacturing of single use plastic products and inadequate waste and recycling management. Microplastic, and even smaller nanoplastic, particles are persistent pollutants in aquatic and terrestrial systems and are the subject of active and urgent research. This review will explore the current research on how exposure to plastic particles occurs and the risks associated from different exposure routes: ingestion, inhalation, and dermal exposure. The effects of microplastics on the cardiovascular system are of particular importance due to its sensitivity and ability to transport particles to other organ systems. The effects of microplastics and nanoplastics on the heart, platelet aggregation, and thrombus formation will all be explored with focus on how the particle characteristics modulate their effect. Plastic particle interactions are highly dependent on both their size and their surface chemistry and interesting research is being done with the interaction of particle characteristics and effect on thrombosis and the cardiovascular system. There is significant uncertainty surrounding some of the findings in this field as research in this area is still maturing. There are undoubtedly more physiological consequences than we are currently aware of resulting from environmental plastic exposure and more studies need to be conducted to reveal the full extent of pathologies caused by the various routes of microplastic exposure, with particular emphasis on longitudinal exposure effects. Further research will allow us to recognize the full extent of physiological impact and begin developing viable solutions to reduce plastic pollution and potentially design interventions to mitigate in-vivo plastic effects following significant or prolonged exposure.

Graphical Abstract:

Introduction

The global increase in environmental plastic has been a topic of increasing discussion and concern over the past decades. Ease of manufacturing, durability, inexpensiveness, and resistance to corrosion are desirable characteristics for consumer products but the resulting environmental longevity is causing a massive proliferation of plastic pollution. When these plastics enter the waste stream they can break down into small microplastic and nanoplastics, and become aquatic, terrestrial, and airborne pollutants. There are many different plastic polymers that make up plastic pollution: polystyrene (PS), polyethylene (PE), and polypropylene can all be found as contributing environmental pollutants. The most abundant microplastic in any given environment largely varies from location-to-location (Andrady, 2011; Browne et al., 2011; Guo et al., 2019). Polystyrene has grown in popularity due to its flexibility, low density, minimal cost of manufacturing, and it is used in the production of packing foam, toys, toothbrushes, and CDs to list a few, making it the subject of frequent studies (Kik et al., 2020). Despite being a recyclable material and being environmentally toxic, plastics in the United States have a recycling rate of just 8.8% as of 2016 (Gu & Ozbakkaloglu, 2016).

This review primarily focuses on data generated using polystyrene (PS) because it is both easily available for use experimentally and is a common environmental pollutant. Plastics present in the environment range in size from macroplastics to nanoplastics with this review primarily focusing on particles in the range of 20 μm to 5 nm. These microplastics can be grouped into two categories: primary and secondary microplastics. ‘Primary microplastics’ are plastics that are initially produced at sizes below 5mm and are commonly found in biotech applications or personal care products. ‘Secondary microplastics’ are plastics that are produced when larger plastic material is broken down to smaller fragments via mechanical, UV damage, or other destructive processes. (Andrady, 2017; Filella, 2015) (Gewert et al., 2015). Secondary microplastics that are under a micron have different properties and behavior in the environment compared to larger particles. These ultrasmall plastic particles are often classified as ‘nanoplastics’, although some literature still uses the term microplastic for particles this size. UV degradation of plastics has been shown to effect the polarity, zeta potential (a physical property involved in aggregation), and crystallinity which can result in increased fragmentation and affect sorption (Liu et al., 2020.). The shapes of secondary microplastics are variable, with fibers, spheres and irregular fragments all being possible (Figure 1.). The high variability possible in environmental particles in terms of size, shape, associated chemical pollutants, and even biofilms that can form on the surface of the plastic particle, make choosing a common standard for the average environmental microplastic difficult.

Figure 1:

Scanning electron microscope (SEM) image showing different microplastic morphologies: a. mixed sample of microplastics; b. fibers; c. spheres; d. fragments (Pivokonsky et al., 2018). Figure reproduced with permission.

The presence of microplastics has been documented in the ocean and a wide range of consumables including drinking water, aquiculture, honey, and table salt (Desforges et al., 2014; Norén, 2007) (Oßmann et al., 2018; Yang et al., 2015) (Liebezeit & Liebezeit, 2013). Microplastics have additionally been documented in air pollution as well as a variety of cosmetic and cleaning products regularly applied to the skin (Lassen et al., 2015). The full scope of microplastic’s interaction with, and possible effects on, human health has not been determined. However, we are beginning to understand some ways in which these particles could disrupt homeostasis and impact cardiovascular health as well as promote thrombus generation.

The aim of this review article is to discuss the routes of plastic particle exposure and examine its size and chemistry dependent effect on thrombosis and the cardiovascular system. This review explores the current state of the field as it relates to circulating microplastic and nanoplastic. Detailed focus will be given to potential mechanisms leading to an increased risk of thrombus formation. While plastic exposure is not currently considered a risk factor of venous thromboembolism (VTE), this review will discuss evidence of plastic particles prothrombotic effects. Plastic particle initiated thrombus generation is contingent upon both surface modification and particle size. Various studies have been carried out both in-vitro and in-vivo using marine organisms and mammalian models. Arterial occlusion studies have also demonstrated the ability for plastic particles to induce thrombosis. On a smaller scale, various physiochemical modifications have been observed to take place in platelets following exposure to plastic particles and these changes are dependent on the surface modifications. Additional effects such as bradycardia and hemolysis will be discussed in detail to present an inclusive review of the cardiovascular effects of microplastic exposure. Finally, this review will aim to identify gaps in the literature that require more study before a true conclusion on the danger of microplastic pollution to human health can be reached.

Microplastic Sources and Modifications

There are two categories of microplastic pollution: primary and secondary. Primary microplastics are plastics that were originally produced to be <5mm in size and released directly into the environment as waste (Smith et al., 2018). These plastics are derived from paints, cosmetics, medical devices, and packing materials, just to list a few. Secondary microplastics are the result of plastic particles decomposing from larger plastics into smaller particles by either physical or chemical processes after they have been released into the environment (Andrady, 2011; Gopinath et al., 2019; Lambert & Wagner, 2016). Physical degradation is fragmentation due to mechanical forces, or pressures while chemical breakdown primarily involves weakening and fragmentation resulting from photodegradation. Secondary microplastics can also be generated from inadequate industrial material processing safeguards and gaps in waste management systems. A study by Lambert and Wagner in 2016 found that nanoplastics formed from degradation of a disposable PS coffee cup lid resulted in a particle release rate of 1.26 × 10⁸ particles/mL over 56 days (Lambert & Wagner, 2016). While there is plastic pollution in terrestrial systems, microplastics in the ocean are better studied with 80% of microplastics in the ocean estimated to have come from river run-off and 20% are directly from marine sources like improper fishing net and rope disposal (Kumar et al., 2020) (Ritchie, 2021). An estimated 268,940 tons of plastic have accumulated at the ocean surface functioning as a large source for continuously rising secondary MP release into the environment (Cole et al., 2011; Eriksen et al., 2014). The exact size of what is considered a microplastic or a nanoplastic is not strictly defined, but commonly <5mm in diameter is accepted as a microplastic and anything <1μm is considered a nanoplastic (Figure 2B). Microplastics in the millimeter range are a true concern environmentally, but are too coarse to be considered a threat for most biological systems as their large size limits adsorption and translocation. Smaller microplastics in the micrometer range, and nanoplastics, are of greater concern for entering and effecting organisms. Nanoplastics are distinct from microplastics in terms of physical properties as a nanoplastic can be colloidal and is generally difficult to isolate from the environment or tissue samples making its environmental prevalence not well characterized. A wide range of particle sizes are used in microplastic research, but the majority of the studies covered in this review are between 20 μm and 5 nm. (Gao et al., 2021) (Gigault et al., 2021)

Figure 2:

A) Commonly found microplastic surface modifications. B) A diagram of the size range and associated general classifications for plastic pollutants.

Surface modifications are frequently used to change or improve plastic material properties to modulate hydrophilicity, reactivity, and to improve bonding ability for further processing or coating applications (Kik et al., 2020). The surface of polystyrene is commonly modified through a variety of chemical processing to achieve various levels of charged states and crosslinking. This process is carried out both intentionally to improve end-product characteristics for unique applications such as for biotech and consumer product use as well as during secondary MP fragmentation in the environment by oxidation and surface breakdown (Kik et al., 2020). Common modifications of PS are amination and carboxylation which are the addition of amine or carboxyl groups to the surface, respectively (Figure 2A). These modifications to the surface chemistry result in a net positive or negatively charged microparticle surface at neutral pH.

Chemical modifications are not the only modifications that can occur to environmental microplastics. Due to varying surface charge PS microplastics are known to interact with proteins both in the environment or after entering an organism. This interaction produces a PS-protein complex, effectively acting as a coat for the microplastic. The protein-PS interaction primarily takes place due to Van der Waals interactions. This protein coat is commonly referred to as a corona and enables the PS to evade the immune system and persist in circulation (Park, 2020). The corona layer additionally demonstrates an ability to increase uptake and subsequent toxicity as has been shown in the species Daphnia magna (Nasser & Lynch, 2016). SDS-page analysis of MP isolated from circulation revealed that the corona is mostly made up of fibrinogen, globulins, and albumin which may contribute to underlying pathogenesis following MP exposure as well as promote thrombus generation.

The MP surface modifications subsequently determines how the plastic particle interacts with the environment and how it impacts biological function following exposure as we will explore in greater detail below. There are many other surface modifications to plastic products but this review will focus on neutral, amine, and carboxy modified MPs as these are the most well studied.

Routes of Human Microplastic Exposure

Humans are exposed to microplastics on a near daily basis and this is likely to only increase over time. One study estimates that if current waste production and disposal patterns continue that by 2050 there will be 12,000 million metric tons of plastic waste in landfills and the environment and by 2060 plastic waste in the environment is set to triple (Geyer et al., 2017; Lebreton & Andrady, 2019). This is likely an underestimate as current consumption trends for single-use plastics and plastic used in personal protective equipment (PPE), such as face masks, have spiked with the COVID-19 pandemic and are generating concern over long-term effects on waste management (Benson et al., 2021). There are various routes by which humans may be exposed; some of which have more associated risk to human health than others. The three primary routes for human exposure to plastic microparticles are oral, respiratory, and dermal (Figure 3). A 2019 study estimated that the annual microplastic consumption by Americans ranges from 39,000 to 52,000 particles depending on age and sex. This increases to 74,000 – 113,000 particles when inhalation of microplastics is considered. The study modeled average intake by using demographics such as consumption of foods known to be contaminated with microplastics and time spent in indoor spaces or areas with high respiratory risk of microplastic exposure. These values were then compared to the median number of particles that the individual would be exposed to during those activities estimated from across the literature (Cox et al., 2019). Although these numbers are subject to variation, these exposure values are likely underestimates and will vary greatly from one individual to the next.

Figure 3:

Flowchart showing pathways of human microplastic exposure. Secondary microplastics from the degradation of plastic waste can enter into the food chain by being directly consumed from contaminated food and drink or via consumption of aquatic organisms who have consumed microplastics. Microplastic exposure can also occur by breathing aerial particles or topical exposure to the skin.

Oral Consumption of Microplastics

Exposure to microplastics by ingestion can be split into two broad categories of direct and indirect ingestion. Direct ingestion of microplastics occurs by consuming food that has been contaminated with MP because of its processing and packaging, or by drinking water that was directly contaminated by primary or secondary MPs. (Rochman et al., 2015; Smith et al., 2018; Van Cauwenberghe & Janssen, 2014). An example of direct ingestion is through drinking liquids from returnable and single use plastic bottles that can contain as many as 15 MPs in every liter (Schymanski et al., 2018). Additionally, microplastics have been found to contaminate other common consumables such as beer, honey, and sea salt (Smith et al., 2018). Indirect ingestion of MPs involves the consumption of organisms that have ingested MPs, and is also referred to as trophic transfer (Miller et al., 2020).

Ingestion of MPs has been well documented in planktonic organisms, algae, and larvae that comprise the bottom of the food chain as well as fish and other invertebrates in marine environments that prey upon them. These plastics can be transferred by consumption up the food chain and can accumulate in predatorial species. In a study of fish species in the English Channel 36.5% of the fish sampled had microplastic contamination and the consumption of microplastic was independent of feeding location or species (Lusher et al., 2013; Mattsson et al., 2015). Many of these organisms known to contain microplastics are part of a typical human diet such as: invertebrates, crustaceans, and fish. Plastic particles are most commonly found to accumulate in the digestive tract, suggesting increased indirect exposure when the entire organism is eaten whole. (Smith et al., 2018). This is the case when consuming shellfish such as oysters, clams, and mussels. Determining how consumed microplastics move through the marine food web is an active field of study (Chae et al., 2018; Miller et al., 2020).

Once ingested, plastic particles undergo a variety of absorption and translocation processes that are impacted by particle size, particle composition, and the unique biology of the organism’s gastrointestinal (GI) tract ranging from filter feeder through mammalian physiology. GI absorption after ingestion of PS MPs is commonly studied in filter feeding invertebrate species, such as the common mussel Mytilus edulis. When exposed to 10 nm unmodified nanoparticles, the particles accumulated in the digestive tubules and translocate into to the circulatory system where they persisted for over 48 days (Browne et al., 2008). Two fluorescent MP sizes were tested (3 and 9.6 μm) and both were found inside hemocytes. This provided evidence that the particles could be taken up from the water column and, due to the size of the particles, likely required phagocytosis.

Previous studies have shown that the distribution of MPs in organisms is species-specific (Deng et al., 2017). For example, 5 μm MPs can relocate to the liver of zebrafish (Lu et al., 2016; Pitt et al., 2018); 10 μm MPs are translocated into the cardiovascular system of mussels following ambient exposure (Browne et al., 2008), and 8–10 μm MPs were primarily retained in the foregut of the crab (Carcinus maenas) following dietary exposure (Watts et al., 2014). It has been proposed that the mussel may initially uptake particles utilizing microvilli on the gill surface. Cilia movement may transfer many particles to the digestive system where they can accumulate in digestive tubules (Ribeiro et al., 2019). Translocation of PS microplastics and nanoplastics from the digestive system in bivalves has been previously documented (Browne et al., 2008). Translocation of microplastics of 2 and 4 μm from the GI system into other tissues has also been suggested for mussels (von Moos et al., 2012). MPs may use a variety of methods to enter cells including pinocytosis, phagocytosis, and using clathrin-coated vesicles (Gopinath et al., 2019). These studies utilized a wide variety of particle sizes, particle compositions, test species, exposure conditions, and quantitative assessment tools to determine MP exposure and accumulation levels making direct comparison of their ultimate findings difficult.

When considering the broad impact of MP exposure risk and its potential effects on human health it is necessary to consider MP absorption differences in mammals as well. Rats were fed unmodified 60 nm polystyrene particles to determine where in the digestive tract that absorption occurred. Results demonstrated that 60% of the absorption in the small intestine occurred through Peyer’s patches; responsible primarily for immune system function in the GI tract (Hillery et al., 1994; Hwang et al., 2020). Despite these patches representing just a small portion of the small intestine (<1%) most of the PS uptake occurred here. Specialized microfold cells in the Peyer’s patches are known to transport plastic particles from the GI lumen toward follicles via phagocytosis, and finally into the circulatory system. Some uptake was also observed in the large intestine, specifically where the lymphoid tissue was most dense (Hillery et al., 1994). Approximately 1–4% of PS that enters the intestine are thought to translocate into the bloodstream (Hwang et al., 2020). Approximately 10% of the administered PS NP dose that the rats received was recovered from the GI tract (Hillery et al., 1994). Absorption is not the only factor of concern for toxicity as retention time for how long a microplastic remains in the body is also important and was not explored in this study.

A 2017 study in mice exposed to 5 and 20 μm PS-MP water sources found that tissue concentrations (liver, kidney, and gut) reached a steady-state value within 14-days of initial exposure. Researchers demonstrated accumulation in these tissues by fluorescent spectrophotometry after administration of microplastic polystyrene particles via oral gavage (Figure 4) (Deng et al., 2017). The maximum concentration of the 5 μm MPs were higher than their 20 μm counterpart in the kidneys (20% more) and gut (2-fold). However, the liver retained a higher concentration of the 20 μm MP. Both sizes of MPs were still detected 7 days after the termination of exposure, demonstrating the biological persistence of the PS MPs within the tissues. The 20 μm particles appeared to have relatively uniform distribution across all tissues while the 5 μm particles had the highest accumulation in the gut demonstrating that biodistribution is a function of particle size. Collectively, this data is evidence that PS-MPs do not exclusively accumulate in the gut; but have mechanisms in place to translocate particles into the circulatory system resulting in systemic distribution (Deng et al., 2017). However, this same result has been disputed, with another in-vivo study using mice showing nearly undetectable uptake of microplastics given orally over a 28-day period (Stock et al., 2019). Some studies even question the relevance of bioaccumulation, since some organisms, such as krill, have shown the ability to clear nearly all MPs within 5 days of exposure (Dawson et al., 2018). It is important to note that it is unlikely that human exposure to MPs is through acute exposure events followed by periods of no exposure. This acute vs long-term continual exposure distinction is important when considering an organism’s overall capacity to clear accumulated microplastics and the associated risks of those exposures.

Figure 4:

Accumulation of different sizes of MPs in mouse tissues following exposure for 28 days. (Deng et al., 2017) Figure and legend reproduced with permission.

While the severity and duration of MP uptake following ingestion is largely disputed, its generally agreed that uptake can and does happen in endothelial and immune cells of the GI tract and it remains unclear exactly how the body is responding to these foreign particles. A 2016 European Food Safety Authority report on the occurrence of microplastics in seafood states that the human body is likely able to dispose of >90% of ingested microplastics through excretion in urine or feces (Schwabl et al., 2019) (Smith et al., 2018). Plastic accumulation has been studied in marine life and rodents, but the biodistribution of plastic in humans has yet to be thoroughly explored. Human biodistribution studies looking at MP translocation and accumulation in tissues have been limited since controlled microplastic exposure in humans raises significant ethical concerns.

Inhalation and Respiration of Microplastics

Airborne microplastics are produced primarily through degradation of larger plastics via UV light or secondarily from clothing and health care products. After degradation, these particles can easily be blown into the atmosphere by air currents due to their small size and low density (Enyoh et al., 2019). Inhalation of these fibers has been reported in occupationally exposed individuals such as those working in factories that use high volumes of plastics or produce textiles, often coinciding with reports of dyspnea caused by an inflammatory response (Prata, 2018). Inhalation of flock, 0.2–0.5mm polymer threads used to produce fur-like or fleece fabrics, has also been associated with respiratory injury (Kern et al., 1998). Synthetic fibers have been detected in biopsies of human lungs (Churg & Brauer, 2000; Pauly et al., 1998). General population exposure to inhaled microplastics is less significant than occupational exposure risks; however, there is little known thus far regarding what the average airborne MP exposure for a person is. Some studies of urban air conducted in metropolitan centers found as little as 29 to as high as 313 particles/m^2/day in the air depending on factors like time of day and weather utilizing a passive collection system. (Amato-Lourenço et al., 2020; Cai et al., 2017; Dris et al., 2015). Research in this field is gaining momentum as more and more environments have their aerial microplastic pollution quantified (Enyoh et al., 2019). An additional complication is that people spend much of their time indoors, which is an easier environment to measure, but varies highly based on furnishings and level of activity (Q. Zhang et al., 2020). It is common for interior air quality to be poorer than that of environmental air quality, but research in this area has largely focused on volatile organic compounds (VOC) and hazardous material emission rather than on plastic particulate exposure (González-Martín et al., 2021; Lucattini et al., 2018). All of these factors make determining the actual risk presented by airborne microplastics difficult to assess.

The body’s response following inhalation of microplastic air pollution is one of primary interest. Respirable microplastics are defined as those that can reach and deposit in the deep lung, the respiratory zone of the lungs, where the alveoli are located and gas exchange occurs. Although very large microplastics may be inhalable, particles with a size greater than 5 μm show a steep drop-off in lung deposition as they are subjected to mucociliary clearance. Mucociliary clearance protects the lungs from large inhaled particles by suspending the particles in mucus where cilia in the respiratory endothelia push the particles up and out of the airway into the pharynx where it is swallowed and enters the ingested pathway previously described (Bustamante-Marin & Ostrowski, 2017; Gasperi et al., 2018). Intratracheal instillation of PS particles has been shown to increase neutrophil influx, histamine content, and overall pulmonary inflammatory response in a hamster model. This effect was noticeable on broncheoalveolar lavage (BAL) indices and peripheral thrombosis within one hour of instillation which demonstrated the rapid ability of aminated-PS (aPS) to pass into systemic circulation through the lungs (Nemmar et al., 2003). Only the aPS demonstrated an ability to enhance thrombus formation when given intratracheally. However, this prothrombotic effect was only observed at a concentration of nearly 10 times as high as with intravenous administration to induce a similar effect (Nemmar, Hoylaerts, et al., 2002; Nemmar et al., 2003). Smaller particles had significantly greater physiologic impact as compared to larger diameter particles. A study by Donaldson et al on ultrafine particle aerial pollution found that small (< 202 nm) latex particles caused more inflammation then similar doses of larger latex particles in a rat bronchoalveolar lavage model (Donaldson et al., 2000) These findings were corroborated by Brown et al and the inverse relationship between size and inflammation was shown to be linearly related (Brown et al., 2001). Ultrafine technetium-labeled carbon particles (< 100 nm) have been shown to be able to crossover from the pulmonary to the circulatory system in mice and humans and its likely greater absorptivity combined with the much higher surface area of the smaller particles is causing these prothrombotic and proinflammatory effects (Nemmar, Hoet, et al., 2002).

A recent study demonstrated that inhalation of PS NPs caused a variety of alterations in hematologic and biochemical markers following exposure. A 14-day repeated inhalation test at varying concentrations of 100 nm PS NPs was done on Sprague Dawley rats to investigate a variety of post-exposure effects. Aspartate transferase (a liver function test), TGF-alpha, TGF-beta, fibrosis related factor, and white blood cell count were all elevated after exposure in the rats. These markers are indicative of substantial inflammation. Additionally, there was a statistically significant increase in heart size after exposure to the PS NP (Lim et al., 2021). The authors suggest that continuing the exposure would likely cause the molecular changes to begin to bear out into higher level physiological effects. Research into chronic inhalation exposure will only become more relevant as microplastic pollution levels increase.

Dermal Application of Microplastics

Prior to 2019, microplastics had been widely used in hand cleaners (Napper et al., 2015), sunscreens (Duis & Coors, 2016), facial cleaners (Gregory, 1996), toothpaste (Sharma & Chatterjee, 2017), and other skin care products. The FDA recently passed the Microbeads-Free Water Act, which went into effect for products produced after 2017 or 2018 for non-prescription pharmaceuticals. This regulation was enacted restricting the use of MPs in exfoliants, toothpastes, and other rinse-off cosmetics to minimize disposal of MPs into water systems. There is also wide utilization of microplastics in the medical field, including frequent use in plastics used to deliver pharmaceutical agents (Duis & Coors, 2016). Exfoliants for facial cleansers before the Microbeads-Free Water act may have exposed individuals to between 4,594 and 94,500 MPs in a single 5 ml use of the product (Gregory, 1996; Napper et al., 2015). A study conducted by Cosmetics Europe revealed that approximately 93% of the microplastics used in skin cleaning products were polyethylene (Lei et al., 2017). The remaining components primarily included polypropylene and polystyrene. The effects of topical microplastics are well understood in terms of their consequences for downstream contamination of water sources and future repeat plastic particle exposure through ingestion; however, its direct effect on human health is largely unknown. It is currently not well understood if there is a risk of absorption of microplastics or nanoplastics via a dermal exposure route. There have been limited studies of human dermal microplastic exposure with much of the research focus being on driving adsorption of dermally applied nanoparticles for drug delivery applications. Rigid microparticles and nanoparticles have not been observed crossing the subcutaneous barrier under normal conditions; however, they have been shown to accumulate in hair follicles and 40 nm PS NP were observed to be taken up by Langerhans cells. (Baroli, 2010; Lademann et al., 2011; Vogt et al., 2006). Polymeric nanoparticles are commonly used in topical drug delivery for their longevity on the skin in extended-release applications but the particles themselves are not intended to be absorbed into the skin. Compromised skin, due to injury or illness, is known to be more permeable and is a possible route for unintended microparticle absorption stemming from environmental exposure (Zoabi et al., 2021). This is an area in need of continued research efforts to determine if such exposure via this pathway would have significant health consequences.

Cardiovascular Physiological Effects

Considering the exacerbation of plastic pollution in recent decades, it is necessary to be aware of the effects it may have on susceptible organs such as the heart. In a study on zebrafish larvae, PS nanoparticles were found to accumulate in the GI tract, pancreas, gallbladder, heart, and liver, throughout development. However, the highest concentration was found in the pericardial sac (Pitt et al., 2018). In this study the effect was dependent on the concentration of NP, with pericardial localization demonstrated in only the higher concentration groups at 1 ppm and 10 ppm. This same effect was not seen in the lower-dose 0.1 ppm group. A dose dependent decrease in heart rate of 5–10% was observed in all zebrafish larvae groups from 0.1 ppm to 10 ppm concentrations. The presence of cardiovascular dysfunction at 0.1ppm without the pericardial translocation suggests other pathologic mechanisms may be at work. The localization of PS was not found to induce deformities or increase pericardial surface area. Another study on zebrafish larvae in 2017 by Veneman corroborated this information, noting that embryos with 700 nm PS particles injected into the yolk showed uptake and translocation through the circulatory system to accumulate in the pericardium (Veneman et al., 2017).

Another study in 2019 on marine medaka (Oryzias melastigma) had similar findings as Pitt with lower MP exposures. The fish were exposed to environmentally relevant concentrations of 10 μm unmodified polystyrene microplastics. This effect was shown across a generation as parental exposure to 20 μg/L microplastics resulted in an extended incubation period as well as decreased heart rate, decreased body length, and a decreased hatching rate for the offspring (Wang et al., 2019). There are many potential contributing factors to the observed decrease in heart rate. Previous findings in 2005 suggest that PS NPs are localized into cells rather than becoming membrane bound. This scenario creates potential for PS interaction with cardiac sarcomeres and subsequently impacting heart rate (Geiser et al., 2005). Oxidative stress has also been shown to be an effect of PS NP exposure and may additionally play a role in disrupting heart functionality (Q. Chen et al., 2017). A recent study from 2020 in 6-week-old rats exposed to 0.5 μm PS microparticles at concentrations between 0.5 to 50 mg/L over 90 days showed a negative cardiovascular impact from NPs (Z. Li et al., 2020). Serum troponin I and creatine kinase-MB were measured as markers for cellular injury and found to be elevated indicating increased myocardial apoptosis.

Cardiovascular disorders from exposure to environmental pollutants in the form of airborne particulates is known to occur, and as previously covered in this review airborne pollution is a vector for plastic particle exposure(Münzel et al., 2018). A detailed analysis of MP and NP exposure using the framework of Adverse Outcome Pathway (AOP) found that 45% of the studies that covered NP found an increase in the activation of oxidative stress pathways.(Hu & Palić, 2020). Oxidative stress is a key mechanism for the effects of MP and NP, with much of the current plastic particle research involving examining markers for oxidative stress following exposure. Oxidative stress is often measured indirectly through activation of associated enzymes and signaling cascades or increases in transcription of associated oxidative stress genes. The cardiovascular effects due to oxidative stress that occur from air pollution are due in part to an increase in reactive oxygen species (ROS), which interact with nitric oxide signaling that occurs in endothelial cells by converting it to the cytotoxic peroxynitrite. This reduces the bioavailability of nitric oxide in the endothelium, and high concentrations of peroxynitrite is cytotoxic resulting in damage to DNA, proteins, and lipids in the cells (Gori & Münzel, 2011). Co-exposure of mice to 50 nm and 500 nm nanoplastic particles had a greater effect on reactive oxygen species (ROS) then exposures of the equivalent dose of either alone (Liang et al., 2021). As environmental exposure to microplastics are commonly heterogenous mixtures of sizes, more study into possible combinatorial effects of microplastics in both size and other characteristics could better explore possible exposure risks.

Hemolysis

PS particles in the <10 μm range at high concentrations have been shown to exhibit hemolytic properties. While the exact mechanism is not well understood one possible explanation is associated with increased surface area of the particles as particle size decreases resulting in increased adherence to erythrocytes through Van der Waal interactions (Barshtein et al., 2011; Hwang et al., 2020). There is a well-documented correlation between hemolysis and in-vivo toxicity (Chen et al., 2004; Sayes et al., 2007). PS compatibility was tested in human blood utilizing a hemolysis assay where erythrocytes were incubated with microparticles and the supernatant was read on a spectrograph at 540 nm to quantify free hemoglobin from lysed cells. In the study by Hwang et al, PS particles <5 μm had a hemolytic effect of >4% relative to the control group as measured by post exposure excretion of cytokines and chemokines. Particles smaller than erythrocytes (6–8 μm on average) were more hemolytic and cytotoxic at every concentration, demonstrating a positive correlation with total PS surface area. Therefore, hemolysis was shown to have a negative correlation with plastic particle size (Hwang et al., 2020). Blood plasma and albumin, at physiological concentrations, has demonstrated an ability to modulate the hemolytic effect, suggesting that effects in-vivo may yield different results (Barshtein et al., 2011). These experiments were conducted with unmodified plastic particles, which represents a primary microplastic well, but is not representative of secondary microplastics or environmental nanoplastics.

McGuinnes et al measured hemolysis at similar concentrations of microparticles using modified and unmodified polystyrene latex nanoparticles (PLNP) and found that unmodified PLNP and carboxylated-PLNP did not cause significant hemolysis, but aminated-PLNP caused significant hemolysis. However, this effect was abolished in the presence of 10% plasma. The proposed mechanism is that erythrocytes, with their negative surface charge, interact more with the positively charged aminated microparticles but the presence of proteins in plasma can also associate to the particle surface effectively blocking these charge-charge based interactions making them improbable in-vivo. McGuinnes also noted the zeta potential changing for aminated particles (from +12.1 to −13.4 Mv) with increasing plasma percentage. This remained below the threshold for effecting hemolysis demonstrating that the complexity of the ways in which the cells and particles may interact in solution are not well understood (McGuinnes et al., 2011). Measurement of hemolysis is typically done by measuring free-heme in the blood plasma by spectroscopy, which requires the removal of the blood from the organism and cannot be done in in-vivo or in real-time. The combined modulating effect of albumin observed by Barshtein et al and the difficulty of measuring low-level hemolytic effects suggests that the in-vivo hemolytic effects of microplastics to be minimal.

Thrombosis

Effects on Platelet Aggregation

Thrombosis can occur in both veins (venous thrombosis) and arteries (arterial thrombosis) and tend to occur through different mechanisms. Arterial thrombosis can lead to severe medical issues such as ischemic heart disease (incidence rate of 1,518.7 per 100,000 worldwide) and ischemic stroke (incidence rate of 114.3 per 100,000 worldwide) (Wendelboe & Raskob, 2016). Venous thromboembolism (VTE) is also prevalent with the CDC estimating that between 1 to 2 in 1,000 people will experience a venous thrombosis event each year in the United States and can lead to serious disease such as pulmonary embolism (Beckman et al., 2010). Arterial blood clots are formed under much higher shear than venous clots and are composed of more fibrin and platelets, while venous blood clots consist of mostly red blood cells and fibrin (Chernysh et al., 2020).

A 2002 study done by Nemmar explored the effects of 60 nm PS NP on thrombus formation in a hamster model. The study compared unmodified, carboxylated, and aminated plastic particles. The amine functionalized particles were found to enhance thrombus formation at 50 and 500 μg/kg, but not at 5 μg/kg (Figure 5). Similarly, a 500 μg/kg intratracheal instillation was also found to increase thrombus formation. The carboxylated form was found to have an inhibitory effect on thrombus formation at 500 and 100 μg/kg that was not observed at the lower 50 μg/kg exposure level. There was no effect observed with an intratracheal instillation of carboxylated particles. However, in an in-vitro test using platelet rich plasma preincubated with unmodified, aminated or carboxylated PS, the carboxylated PS demonstrated an ability to weakly enhance platelet aggregation at 25 μg/mL but only when induced with adenosine diphosphate (ADP) (Nemmar, Hoylaerts, et al., 2002). The aminated PS had a stronger effect on aggregation, causing an increase at 50 and 100 μg/mL. Uniquely, the aminated PS was also able to induce platelet aggregation without induction by ADP. The unmodified NP forms had no effect on thrombus formation or platelet aggregation (Nemmar, Hoylaerts, et al., 2002). Further study into the inhibitory effect of the cPS was reported by Griffin et al in 2018, with a study using whole blood and microfluidics to look at cPS inhibition at arterial shear rates. They found that cPS at 60 nm and 510 nm could inhibit thrombosis at arterial shear rates when used in a 1.5–2.0 ratio with the Von Willebrand factor (an important protein in clot formation) in the blood (Griffin et al., 2018).

Figure 5:

Effect of unmodified (A), carboxylate-modified (B), or amine-modified (C) particles on thrombus formation. Cumulative thrombus generation, expressed as total light intensity over 40 minutes, after photochemical injury (2 minutes) to the femoral vein in control hamsters and in hamsters pretreated with an intravenous bolus of UFPs, administered 10 minutes before photochemical injury. Data are mean ± SEM for the number of hamsters indicated in parentheses. Control animals are the same in all three panels. *p < 0.05, **p < 0.01 by Dunnett’s multiple comparison test. AU: arbitrary units. Note: y-axis is different in (C).(Nemmar, Hoylaerts, et al., 2002) Figure and legend reproduced with permission.

In 2010, another study was conducted by Bihari et al focusing on manufactured nanoparticles and microcirculation included aminated and carboxylated nanoplastics.. Aminated and carboxylated PS NPs (60 nm) were injected into small mesenteric arteries of mice and the artery occlusion times were measured. Compared to the saline control group, the aminated group had a decreased occlusion time, indicating increased thrombus formation, while the carboxylated group had an increased occlusion time. The data from this experiment supports Nemmar’s 2002 study. Additionally, the study found that the aminated nanoparticles increased platelet aggregation as was evidenced by increased expression of P-Selectin on the platelet surface. No changes in platelet activation or aggregation were seen with the carboxylated PS particles (Bihari et al., 2010).

Similar studies were carried out by McGuinnes et al in 2011 on healthy human blood from volunteers using PS particles 50 nm in diameter. Both the aminated and carboxylated nanoparticles increased platelet aggregation to a similar degree. The unmodified form had no effect on aggregation. This study notes that the small discrepancy in results from Nemmar’s study may be a result of using a more sensitive flow cytometry approach rather than the platelet aggregometry approach (McGuinnes et al., 2011). Both the aminated and carboxylated PS MPs caused increased platelet-platelet aggregation when compared to neutral PS or saline controls. Carboxylated PS MPs initiated platelet activation by modulation of platelet membrane receptors (McGuinnes et al., 2011). CD26P, otherwise known as P-selectin, is important in the recruitment of leukocytes to the sites of inflammation. CD26P is present in resting platelets and is moved to the cell membrane via exocytosis following exposure to thrombin and subsequent platelet activation. This receptor is commonly used as a marker for platelet degranulation (Daugirdas & Bernardo, 2012). When added to solution, cPS caused upregulation of CD26P and PAC-1 to a degree similar to results seen when platelets are exposed to the potent platelet activator TRAP (thrombin receptor-activating protein) (McGuinnes et al., 2011).

Aminated nanoplastics appear to work uniquely from the carboxylated nanoplastics in that they do not cause an increase in platelet aggregation through upregulation of platelet adhesion receptors. One possible mode of action is through perturbation of the platelet membrane through interactions with anionic phospholipids. This interaction was examined using several generations of carboxy and amine PAMAM dendrimers as uniform model microparticles. While PAMAM dendrimers are not the same as the PS nanoparticles discussed in other studies, being non-rigid, the common surface chemistry can still provide mechanistic insights (Dobrovolskaia et al., 2012; McGuinnes et al., 2011). Plasma samples were treated with dendrimers of various sizes (generations 3, 4, 5, and 6 which range in size 3.1 nm to 7.5 nm) and flow cytometry was used to detect platelet aggregation. The treated plasma was also tested for CD62P as a marker of platelet activation (Dobrovolskaia et al., 2012). Additionally, morphological characteristics in the platelets indicating the presence of storage granules was also monitored. The CD62P probe was observed binding in a non-specific way indicative of membrane perturbation, and intermediate signaling molecules (such as thrombospondin and PDGF) were released rapidly after addition of aminated microparticles. This suggests that the increased aggregation caused by aminated PS exposure is associated with mechanical damage rather than an interruption in signal mediation as the signaling molecules were not being activated, but rather released due to a loss in membrane integrity. This point was further shown by adding inhibitors for multiple common platelet aggregation signaling pathways which did not affect the dendrimer-induced aggregation (Dobrovolskaia et al., 2012).

Another experiment on platelet aggregation with nanoplastic particles by Smyth et al in 2015 looked at the mechanism of NP platelet activation using both aggregation assays and imaging. Platelets were exposed to 50 nm and 100 nm plastic particles that were unmodified, aminated, or carboxylated. Transmission electron microscopy was used to image platelets internalizing amine modified NP, with 100nm showing some minor internalization and carboxylated and unmodified NP not showing any internalization. The carboxylated and unmodified NP were shown associating with degranulated/activated platelets, while the aminated NPs were not associated with activated platelets (Figure 6). They also observed adhering of discoid/inactivated platelets together by chains of 50 nm aminated NPs. This is notably the only experiment to report aggregation being increased with unmodified NP as well as the previously reported animated and carboxylated NP. The effect of the unmodified and carboxylated NP was eliminated by adding GPIIb/IIIa antagonist eptifibatide, while the aminated NP effect was unaffected. Smyth et al also looked at lactate dehydrogenase (LDH) release as a marker for cell toxicity, and found that over the aggregation assay the levels of LDH levels did not increase, making it unlikely that the aminated particles were causing membrane damage that was causing the aggregation. However, on longer exposures in the same aggregation assay (2hrs), they found there was a significant increase in LDH release from platelets treated with the aminated NP. The aminated NP are still causing membrane perturbation, but it is not necessarily causing the aggregation (Smyth et al., 2015). Much more study is needed before any conclusion on mode of action for the effects of plastic particles can be stated with any certainty.

Figure 6:

Electron micrographs of isolated human platelets following exposure to nanoparticles. Stained sections displaying varying magnifications of platelets exposed to 50 μg ml⁻¹ of PLNPs. (A) shows a control platelet in resting conformation. 50 nm cPLNPs (B) and uPLNPs (C) are seen in close proximity and in contact with (arrows) the extracellular surface of activated, degranulated platelets. Non-activated, discoid platelets with 50 nm aPLNPs in contact with the platelet surface (arrows, D) and enclosed within intracellular vacuoles (arrowheads, D). Platelets are also shown tethered to each other by chains of aPLNPs (E). 100 nm aPLNPs and cPLNPs enclosed within intracellular vacuoles (arrowheads, F and G) and in association with the extracellular surface of the platelet membrane (arrows, G and H) (Smyth et al., 2015). Figure and legend reproduced with permission.

Effects on Thrombin Regulation

A study carried out by Oslakovic in 2012 using PS nanoplastics of varying size and modifications explored the effects of PS on thrombin regulation in human plasma. The aminated NPs at 57 and 330 nm diameter were both able to decrease thrombin formation in plasma with the 57 nm group exhibiting a stronger effect when compared to similar particle mass concentrations (Figure 7). This change may be due to binding of aminated PS NPs to human coagulation factors. When PS NP were incubated with plasma, factors VII and IX, which are essential for the intrinsic/contact pathway of blood coagulation upstream of thrombin, were found to bind selectively to the aminated PS NP. This occurs in a surface area dependent manner and when the thrombin generation assay results are analyzed by comparing similar surface areas (0.25 mg/mL of 330 nm amine MPs with 0.06 mg/mL 57 nm amine NPs) the effects on the thrombin generation assay are lost. This indicates that the binding of clotting factors to the amine modified nanoplastics is likely the driver for their effect on thrombin generation.

Figure 7:

Plasma containing amine-modified NPs has decreased thrombin generation. Plasma was incubated in the presence or absence of NPs and tested for thrombin generation using the thrombin generation assay. The first derivative, fluorescence units/min, is shown (means of n = 3) for plasma with 57 nm and 330 nm amine-modified NPs (A & B, respectively) at different concentrations as indicated in the figure. The 0.5 mg/mL line is hidden under the x-axis in (A). Plasma incubated with 23 nm, 200 nm particles, either the sulfonated (just denoted their size) or the carboxyl-modified particles at 0.5 mg/mL (C) (Oslakovic et al., 2012). Figure and legend reproduced with permission.

Carboxylated and sulfonated (another less common modification with a negative charge) NPs increased the conversion of prothrombin to thrombin via the intrinsic pathway; but only with a size of 220 nm. The smaller size of 24 nm had no effect. The larger 220 nm particles were more successful due to the larger negatively charged surface activating Factor XIIa via a known contact dependent pathway. The carboxylated MPs at 50 μg/mL were as efficient as 1 μg/mL kaolin at activating the intrinsic pathway in plasma. This result was further confirmed by adding corn trypsin inhibitor (CTI), an inhibitor of the intrinsic pathway, which negated the effect of carboxylated MPs on coagulation (Oslakovic et al., 2012).

Coagulopathy Effects

The unmodified nanoplastics near unanimously had no effect on platelet aggregation or thrombus formation, suggesting that charge is essential to the coagulopathy effects of plastic particles. Increased platelet activation and aggregation is seen after exposure to both aminated and carboxylated forms of modified PS NP. Aminated PSs have stronger effects on platelet aggregation than carboxylated PS; however, aPS were shown to have a negative effect on thrombin conversion while cPS were shown to increase thrombin conversion. While both aPS and cPS induce clotting they do so by distinct mechanisms. Studies on arterial occlusion in in-vivo models support that cPS has an inhibiting influence on thrombus generation and aPS increases coagulability. The complexity of these experiments with many parameters available for modification results in a wide range of experimental results including those that found no significant effect at all of PS NP on coagulability (Table 1). Coagulation mediated by thrombin and coagulation that is mediated by platelets have significant overlap, but differ particularly where it interfaces with inflammatory effects (Fröhlich, 2016). Due to the large degree of variation in particle sizes, particle composition, density of surface charge, and unique exposure conditions comparing results across studies is difficult. More studies are needed to elucidate the combinatorial effects of particle size and surface modifications on both thrombin and platelet mediated clotting.

Table 1:

PS Modification and Size with Effects on Platelet Aggregation and Thrombus Formation

Modification	Size (nm)	Platelet Aggregation	Thrombus Generation	Reference
Carboxylated	<10	No Effect		(Dobrovolskaia et al., 2012)
	50	Increased	No Effect	(McGuinnes et al., 2011)
	60		Decreased	(Bihari et al., 2010)
	60		No Effect	(Nemmar et al., 2003)
	60	Increased	Decreased	(Nemmar, Hoylaerts, et al., 2002)
	50	Increased		(Smyth et al., 2015)
	100	Increased
	9		Increased: Intrinsic Pathway	(Oslakovic et al., 2012)
	200		Increased: Intrinsic Pathway
	20	Increased		(Mayer et al., 2009)
	200	Increased
	14		No Effect	(Sanfins et al., 2014)
	220		Increased
Aminated	<10	Increased (G4–G6 only)		(Dobrovolskaia et al., 2012)
	50	Increased	Increased	(McGuinnes et al., 2011)
	60		Increased	(Bihari et al., 2010)
	60	Increased	Increased	(Nemmar, Hoylaerts, et al., 2002)
	50	Increased	Increased (with collagen)	(Smyth et al., 2015)
	100	Increased
	9		Decreased: FXII, IX	(Oslakovic et al., 2012)
	200		Decreased: FXII, IX
	20	Increased		(Mayer et al., 2009)
	200	Increased
	60		Increased	(Nemmar et al., 2003)
	400		No Effect
Neutral	<10	No Effect		(Dobrovolskaia et al., 2012)
	50	No Effect	No Effect	(McGuinnes et al., 2011)
	60		No Effect	(Nemmar et al., 2003)
	60	No Effect	No Effect	(Nemmar, Hoylaerts, et al., 2002)
	60	No Effect		(Santos-Martinez et al., 2012)
	20	No Effect		(Semberova et al., 2009)
	200	No Effect
	50	Increased	No Effect (with collagen)	(Smyth et al., 2015)
	100	Increased

Non-Cardiovascular Effects

While this review focused on the impact of NP and MP on the cardiovascular system, there is widespread evidence of exposure toxicity on other systems as well. Plastic particle toxicity is a growing field of interest and there are a wide variety of variables and possible responses for organisms that are being actively studied. Some species show little impact from microplastic exposure such as in the aquatic isopod Idotea emarginata with their resiliency being the result of some interesting anatomic features that include fine mesh membranes that modulate absorption in its short mid-gut restricting transfer of microplastics to its circulatory system. (Hämer et al., 2014). Observation of changes in body mass/composition (Welden & Cowie, 2016), detoxification impairment (Green et al., 2019), and damage of intestinal microvilli (Chae et al., 2018) highlight a few more of the abundant reported physiological effects (Kogel et al., 2020).

Effects of microplastics on fecundity and reproductive health is particularly relevant in toxicology and is also of interest in the study of microplastics and their negative health impact. Alterations involving embryonal development or reproductive success have been studied in many systems including copepods (Jeong et al., 2017; Lee et al., 2013), fish (Qiang & Cheng, 2021) and mice (Jin et al., 2021). Generational effects of exposure to microplastics and nanoplastics in the parent generation and how those effect the subsequent generation is a concern as eggs are more permeable to plastic particles then a fully grown organism. Small fish species have been shown to take up NPs into their egg forms and carry them through development to their larval stage. (Kashiwada, 2006). Following oral exposure to MPs male Balb/c mice showed a decrease in sperm count and a decrease in related molecular markers of reproductive health (Xie et al., 2020). Plastic particles were also seen to accumulate in the ovaries of Wistar rats, and rat ovaries treated with NP had elevated oxidative stress markers and increased apoptosis (An et al., 2021). The epigenetic effects of microplastic exposure are also an area of active interest with recent work by Zhang et al showing epigenetic silencing in a Drosophila melanogaster model when exposed to polystyrene contaminated with cadmium compounds, which are used as dyes and stabilizers in plastics (Y. Zhang et al., 2020). Unmodified nanoplastics showed an effect that persisted through 5 generations in Caenorhabditis elegans after a single exposure to 100 nm particles for 72 hrs in the maternal organism. The subsequent generations had less offspring, increased germline apoptosis, and showed hypomethylation of the promoter ced-3, a major gene in the programmed cell death pathway for C. elegans (Yu et al., 2021).

Studies of the effects of microplastics on metabolism and the microbiome when ingested are ongoing. An experiment in zebrafish in 2020 by Zhao et al showed that microplastic exposure caused a change in the transcription of genes related to glycolipid metabolism in the liver (Zhao et al., 2020). In a 2019 study of microplastic exposure in mice by Jin et al, oral treatment with polystyrene MPs caused disruption of metabolic parameters in serum that involve amino acid and bile acid metabolism. Additionally, the microbiota composition of the mice was strongly altered, with significant changes in the prevalence of 15 different genus including a decrease in the Actinobacteria phylum which are associated with absorption and digestion in humans (Jin et al., 2019).

In another experiment using zebra fish, larva treated with 200 nm PS NP at the study’s higher doses of 1,000 ppb and 10,000 ppb displayed hyperactive swimming behavior. At 100 ppb NPs no significant behavioral differences were noted, but transcriptome analysis found 734 (62% upregulated) differentially expressed genes, including genes related to organismal injury, endocrine disorder, neurological disease, and movement disorder pathways (Pedersen et al., 2020). In a study by Chen et al in 2017 using larval zebra fish, exposure to 50 nm particles at 1 mg/L was shown to cause a decrease in locomotor activity over the course of three light-dark cycles. The same experiment was also run using microplastics (10 μm diameter at 1 mg/mL) and as a co-exposure with high or low doses of a locomotive inhibitor, 17α-ethynylestradiol (EE2). The effect of the EE2 was modulated at lower doses by the microplastics but not nanoplastics. The microplastics and the nanoplastics both caused an increase in hypoactivity when exposed alongside EE2 more than just the inhibitor alone (Qiqing Chen et al., 2017). Swimming behavior in zebra fish is modulated by the circadian rhythm, which has been shown to have some correlation with exposure to aerial pollutants, and dysfunction of the circadian rhythm is associated with negative cardiovascular effects and increased oxidative stress (H. Li et al., 2020). In an experiment on zebra fish testing their circadian regulated swimming activity, a chronic exposure of 5 ppm of 70 nm PS particles dysregulated their circadian rhythm causing the fish to display more “sleep-like” behavioral patterns during both day and night cycles (Sarasamma et al., 2020). Exposing rats to ambient air pollution (particles >2.5 μm) compared to filtered air was shown to cause an upregulation of important circadian rhythm “clock” genes. Mammalian models exposed to environmental air pollution (which consists of many different particles with a diverse range of chemistries and sizes) show changes in the expression of circadian rhythm genes and increases in oxidative stress (Song et al., 2017; Xu et al., 2019).Microplastics also function as possible vectors for spreading chemical and microbial contamination in aquatic environments (Zettler et al., 2013). Transfer of polybrominated diphenyl ethers by microplastics into Allorchestes compressa (a marine amphipod) has been demonstrated (Chua et al., 2014). A recent study by Wang et al showed that nanoparticles could transfer bisphenol A and that exacerbated the oxidative stress effect the nanoparticles caused in Caco-2, a colonic cancer cell line (Wang et al., 2020). The amount that these riders can be absorbed alongside the microparticles and persist is an active field of study and its possible effects on cardiovascular and thrombosis are worth additional research.

Conclusion

Many studies have been carried out looking at microplastic uptake in a variety of species with significant variation in resulting pathologies. Most evidence has shown the general tendency for smaller diameter particles to have more substantial negative impact relative to larger particles. Microplastic exposure studies tend to compare exposure amounts between different particle sizes by relative mass making it difficult to conclude with certainty whether particle size or particle concentration has the larger impact. Standardization of models, particles, and quantification strategies are needed to clarify discrepancies within the literature. Retention rates within different organisms differ significantly with some organisms having unique physiology that provides routes for faster clearance of microplastic challenge depending on the route of exposure. Humans typically lack these unique anatomical features to mitigate or rapidly clear exposures providing for the possibility of accumulation and significant adverse health related consequences. Additionally, pre-existing health conditions such as diabetes, lupus, or coagulation factor disorders may exacerbate the adverse impact of microplastic exposures. There is still much research needed before it can be definitively determined whether or not environmental microplastic exposures pose a significant human health risk.

Microparticles can stimulate platelet aggregation contingent upon multiple factors including diameter, surface charge/modifications, and their concentration in solution. Considering increased utilization of MP/NP in medicine, electronics, and cosmetics; these factors should be considered in manufacturing and engineering to minimize unnecessary negative impact on human health and environmental contamination. Only the unmodified MP/NP unanimously demonstrated little to no effect on coagulability. Although both aPS and cPS have been shown to effect thrombosis, they do so by unique mechanisms. The majority of studies into microplastics on thrombosis have been conducted as acute exposures at a high concentration rather than a more representative chronic exposure model. This highlights the need for better estimations of human microplastic exposure to establish more accurate animal models to test long-term exposure effects. Most chronic exposure experiments available are in invertebrate aquatic models, which are less applicable to human health than mammalian models. Further insight is needed into long-term effects of chronic exposure to microplastics as well as to clarify ambiguous results surrounding thrombus generation in response to modified PS exposure.

Real-life microplastic exposures will be mixed plastics of differing sizes, polymeric make up, surface modifications, routes of exposure, and duration of exposure. Many microplastics ingested are likely to be secondary microplastics which have gone through weathering and possibly have diverse mixed chemical or protein surface coatings. Advanced research integrating some of these complexities will be necessary to achieve a higher understanding of the risk associated with varying environmental microplastic exposures. In-vivo experiments using environmentally sourced plastic particles would answer many questions about microplastic risks, but these experiments would be non-trivial as using environmentally sourced plastics would add significant complexity. Capture and characterization of environmental plastics is an active field of study (Elkhatib & Oyanedel-Craver, 2020; Mintenig et al., 2018; Zhou et al., 2021) and as techniques improve these findings can be used to study organismal uptake and risk. Long-term bioavailability studies of ingested microplastics, including the effects of surface modification on subsequent uptake in addition to effects in circulation before being ultimately cleared, are needed. If the management of plastic waste continues to remain lacking it may also be necessary to consider therapeutic interventions following known significant exposures. Plastic pollution is a persistent and pernicious concern that humans live with now and will continue to live with into the future. With no long-term solution to the problem on the horizon the dangers to human health are well worth exploring and characterizing more completely.

Review Limitations

This review utilized strings searched through PubMed and Web of Science bibliographic databases that included: microplastic, nanoplastic, polystyrene, pollution, platelet activation, coagulation, thrombosis, environment, and exposure. This is not a systematic review, but rather a narrative review to allow for discussion of research trends across a broad range of particle types, sizes, modifications, model organisms, and differing exposure routes, quantities, and durations.

Highlights.

• Absorption and effects of modified microplastics (MP) are not well studied.

• Microplastics in circulation can affect thrombosis through size dependent pathways.

• Chemical modifications influence MP effects on platelet aggregation and hemolysis.

• More studies using modified microplastics in long-term low doses are needed.