Microplastics and Nanoplastics as Environmental Contaminants of Emerging Concern: Potential Hazards for Human Health

Abstract

With nearly 40% of the total plastics produced being used for packaging, up to five trillion plastic bags are consumed in the world annually. The inadequate disposal of plastic waste and its persistence has become a serious challenge/risk to the environment, health, and well-being of living creatures, including humans. The natural degradation of plastics is extremely slow; large pieces of plastic may break down into microplastics (MPs) (1 μm–5 mm) or nanoplastics (NPs) (<1000 nm) after protracted physical, chemical, and/or biological degradations. A brief overview of the transport of micro- and nanoplastics in the aquatic, terrestrial, and atmospheric environments is presented. Details are provided on the exposure routes for these waste materials and their entry into humans and other biota through ingestion, inhalation, and dermal contact. The greatest concern is the cumulative impact of the heterogeneous secondary MPs and NPs on planetary and human health. Inhaled MPs and NPs have been shown to affect the upper respiratory tract, lower respiratory tract, and alveoli; prolonged exposure can lead to chronic inflammatory changes and systemic disease. These can also lead to autoimmune diseases and other chronic health conditions, including atherosclerosis and malignancy. Sustainable mitigation strategies to reduce the impact of MPs/NPs include source reduction, material substitution, filtration and purification, transformation of plastic waste into value-added materials, technological innovations, etc. Multidisciplinary collaborations across the fields of medicine, public health, environmental science, economics, and policy are required to help limit the detrimental effects of widespread MPs and NPs in the environment.

1. Introduction

Since the early 1950s, plastics have been used extensively in a wide range of products in daily use. Key advantages, such as high durability, lightweight, high strength, low costs, electric and heat insulation, etc., have resulted in an explosive utilisation of plastics all over the globe. Plastics have displaced several traditional materials, e.g., wood, metal, and glass, from applications currently dominated by almost ubiquitous plastics. Plastics have added much value to our lives as a cheap, versatile, and sterile material for use in various applications, e.g., construction, home appliances, medical instruments, food packaging, etc.

Plastic manufacturing has skyrocketed from 1.7 million tons (MT) in the 1950s to a staggering 400.3 MT in 2022 [1,2]. China accounted for ~32% of the global production in 2022, producing between 6 and 12 MT per month, and North America had ~17% of the share in plastics production [3,4]. Global exports of plastics and/or plastic goods crossed the USD 1 trillion benchmark in 2018, reaching USD 1.2 trillion in 2021 [5,6]. Due to their widespread usage and disposal, plastic waste/garbage has become a serious environmental challenge and a risk to waterways, rivers, oceans, and terrestrial environments [7,8,9,10,11]. More than 90% of the plastic waste primarily consists of low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene (PP), polyvinylchloride (PVC), polystyrene (PS), polyethylene terephthalate (PET), and rubber, among others. Up to five trillion plastic bags are consumed in the world annually, and nearly 40% of the total plastics produced are used for packaging [12].

A variety of techniques have been used to process/reuse/recycle end-of-life plastics, e.g., low-temperature pyrolysis [13,14,15], fuel recovery [16], energy recovery [17], composites [18], carbon resources [19,20], roads and construction [21], etc. However, a major proportion of this waste is still landfilled or incinerated. More than 10% of the plastic waste (e.g., plastic bags, packaging, food containers, etc.) is littered around due to poor collection rates and disposed waste spreading over wide areas and a lack of awareness, discipline, and education among the general populace. It is a global problem of great concern that is not limited to specific countries or regions.

One of the most critical issues here is that the natural degradation of plastics is extremely slow. Out in the environment, plastics can break down into smaller pieces through a variety of weathering processes such as mechanical abrasion [22], photodegradation [23], biodegradation [24], thermal oxidation [25], hydrolysis [26], etc. Chamas et al. used specific surface degradation rates (SSDRs) to estimate the half-lives for various polymers: SSDRs for HDPE in the marine environment ranged from 0 to 11 μm/year, indicating that the degradation of a plastic bottle and a pipe may require ~58 years and 1200 years, respectively [27]. While the SSDRs for HDPE and polylactic acid (PLA) were similar in the marine environment, the PLA was found to degrade ~20 times faster than the HDPE on land.

After physical, chemical, and/or biological degradation, large pieces of plastic may break down into microplastics (MPs) (1 μm–5 mm) or nanoplastics (NPs) (<1000 nm) [28]. Microplastics—plastics less than 5 mm in size—can also be intentionally manufactured for commercial products, e.g., exfoliating beads in facial scrubs [29], industrial pellets [30], etc., but primarily originate from the degradation of larger plastics due to weathering [31]. When the breakdown of plastic continues in the sub-micron range, these are termed nanoplastics. Their behaviour differs significantly from the larger pieces of plastics due to much smaller sizes, very large surface areas, and higher reactivities [32]. Other categories of waste plastics include mesoplastics (5–25 mm) and macroplastics (>25 mm).

MPs are further categorised as “Primary microplastics” and “Secondary microplastics” [33]. Primary MPs are produced from raw materials during the manufacturing of consumer plastic goods, whereas secondary MPs are generated from the cracking, breakdown, and progressive deterioration of large plastic pieces [34,35]. Primary MPs have been detected in cosmetics, scrubs, facial cleansers, toothpaste, microbeads, exfoliants, abrasives, and even during ‘air blasting’ in some industries [36,37,38]. These are also found in washing powders, synthetic rubber and clothes, tea bags, and drug-production vectors in medicine [29,39,40]. However, most of the environmental MPs are classified as secondary MPs. Originating from the larger plastic litter, these can lead to significantly higher public health risks as compared to the primary MPs [41,42].

Following the COVID-19 pandemic, extensive use of disposable face masks, personal protective equipment (PPE), and plastic packaging caused a surge in plastic waste; it was estimated that ~3.4 billion facemasks were being used every day around the globe [43]. Improper disposal of PPE during the epidemic has increased plastic waste accumulation in the soil by 5–10% [44]. Most of the single-use disposable facemasks were made of nonwoven fabrics from polymers such as polystyrene (PP), polyacrylonitrile (PAN), polypropylene (PP), polycarbonate (PC), polyethylene (PE), polyethylene terephthalate (PET), etc. [45]. The most common plastic polymers used in assembling surgical masks were PS and PP due to their low cost and low melt viscosities [46].

With a brief introduction to the presence of micro- and nanoplastics (MPs/NPs) in the environment, we focus on three key features in this overview. Section 2 focuses on the transport of micro- and nanoplastics in the aquatic, terrestrial, and atmospheric environments. Section 3 presents details on the exposure routes for these waste materials and their entry into humans and other biota. Section 4 provides a detailed impact on health and the associated aspects. Section 5 briefly presents sustainable mitigation approaches to reduce/overcome the environmental impact of MPs/NPs. These will be followed by a concluding summary of the key findings from this study, with perspectives on minimizing the environmental damage associated with MPs and NPs.

2. Transport of Micro-/Nanoplastics in the Environment

2.1. Aquatic Environment

Marine litter is defined as discarded, disposed, or abandoned human-made solid materials in the marine environment. Up to 60–80% of the total marine litter consists of waste plastics; about 80% of this debris is of terrestrial origin, and about 18% is from the fishing industry [47,48,49]. Land-based sources include waste from overflowing riverbanks, dumpsites near the coasts and beaches, and litter from agricultural, recreational, tourism activities, etc. [50]. Illegal dumping abandoned fishing gear, aquaculture-related activities, and shipping are the likely sources of sea-based sources [51]. It is estimated that about 3.1–8.2 MT of plastic waste becomes deposited in the marine environment every year and may increase to 22 MT or so in the next decade [52,53].

When plastic is released into the aquatic environment, it degrades into smaller pieces through disintegration, mechanical abrasion, UV radiation, biological degradation, etc. [54]. Sadri and Thompson [55] analysed the particle sizes of plastics recovered from marine debris in the Tamar Estuary (UK). The dominant particle sizes of these plastic particulates were found to be less than 10 mm; the size of over 70% of the particulates was less than 5 mm, and 17% was less than 1 mm. Most of the plastics detected in freshwater were found to be less than 5 mm [56]. There is also a strong likelihood of different types of microplastics in freshwater and marine systems. The MPs present in rivers are likely to depend on household and/or industrial wastes close to the rivers, waste discharges, and distance from a point source.

As determined using transient markers and oceanographic data, plastic contamination was found to move from the east coast of the USA to the subtropical North Atlantic Gyre in under 60 days [57]; upon entering the ocean, the plastic waste starts migrating to several neighbourhood locations. Synthetic polymers lighter than seawater, such as polyethylene and polypropylene, float on the ocean surface, are buoyant, and can be transported in seawater under the influence of the oceanic flow, sea breeze, and coastal as well as tidal waves, etc. [44]. Polymers such as polyvinylchloride that are heavier than seawater are transported through underlying ocean currents [58]. Other factors impacting plastic transport include changing ocean levels, impacts of the Coriolis, breeze, heat gradients, brininess, lunation gravity, etc. [59].

The baseline loading and physical and morphological characteristics of the environment, hydrodynamic conditions, etc., are some of the factors that influence the abundance of MPs in rivers and streams [60,61]. Rivers play an important role in transporting MPs along with sediments and debris into the sea [62]. While large numbers of MPs have been detected floating on the ocean surface, observed numbers are up to 100 times smaller than the predicted values [63]. Migration, stranding, transformation, and sedimentation could be responsible for these differences. Degradation caused by photodegradation, biodegradation, biological fouling, mechanical forces, sedimentation, and accumulation on the ocean floor also makes significant contributions to plastic levels [64].

Plastic contamination is an increasing environmental problem in marine systems that has spread globally to even the most remote habitats. While the highest amounts of plastics near some large lakes were found close to highly populated areas, industries, and tourist areas [65], MPs were also detected in remote lakes, probably due to their long residence times [66]. The ocean floor is another large reservoir/sink of waste plastics; due to the lack of removal, it will be a long-time depository of plastic waste [67]. Large plastic objects are likely to be on top of the ocean floor, and small plastic particulates could be mixed into bulk ocean sediment [68]. Plastic degradation is also expected to be extremely slow due to low levels of oxygen and UV radiation [69]. It is estimated that more than 5 trillion plastic pieces weighing over 250,000 tons may be afloat at sea [70]. Figure 1 shows a representative example of the deposition and circulation of MPs/NPs in the marine environment.

2.2. Terrestrial Environment

The transport of MPs/NPs present in the terrestrial environment occurs through human activities, animals, water flow, and diffusion in a complex and intricate ecosystem with a range of pore structures [71]. While the extensive use of agricultural membranes, sludges, and irrigation methods transport waste plastics to the soil surface, local disturbances during farming, harvesting, and other agronomic practices could make these plastics spread around or move deeper underground [72]. Water flow in the soil during irrigation and/or rainfall from top to bottom can transfer MPs/NPs downwards into soil voids, with a possibility of entering groundwater [73]. The size, shape, and ageing behaviour of MPs/NPs also affect their transport behaviour; it was observed that NPs were easier to transfer downwards as compared to MPs penetrating the soil layers into local groundwater networks and systems [74]. It has also been reported that MPs/NPs may adhere to the body surface of earthworms (a common underground invertebrate animal), causing the spatial transport of plastics [75,76]. A representative example of ground transport of MPs/NPs is shown in Figure 2.

Geyer et al. [64] have reported a global analysis of manufactured plastics. It is estimated that nearly 8300 MT of virgin plastics were produced worldwide in 2019 and are expected to double in size by 2040 [77,78]. Nearly 79% of end-of-life plastic waste is still being dumped in landfills or open natural environments. A number of environmental factors, such as the wind, flooding, runoff, leaching, removal by animals, etc., can act as possible routes for the loss of plastics from landfills [79]. While such losses are minimised in sanitary/engineered landfills with appropriate barriers, open dumps with negligible fencing can spread MPs, NPs, and other fine plastic particulates to the environment [80]. The dominant microplastics found in the vicinity of an abandoned coastal landfill were PS, PET, PA, and PP [81].

Several landfill sites have been identified as major sources of MPs [82], which have been shown to migrate in vertical as well as horizontal directions [83]. MPs enter the landfill soils and groundwater through leaching and aerosol transport and landfill mining in dumpsites [84]. A number of studies were carried out in China, Finland, Iran, Norway, Iceland, and Indonesia on the size distribution of the MPs detected in landfill leachates [85,86,87,88]; their sizes were found to range between 20 µm to 5000 µm, depending on the age/size of the landfill site.

The presence of MPs in soil has been reported for three subgroups of terrestrial environments: agricultural, urban and suburban, and coastal soils [89]. Wang et al. [90] have provided extensive details on the different types of soils, types of plastics detected, their abundance, sizes, etc. Information from other sources has also been included. Some of the key details have been summarised in

Table 1. Microplastics pollution data in a variety of soils.

S. No.	Type of Soil	Polymer	Size	Abundance	Ref.
1.	Industrial soil, Australia	PVC, PE, PS		0.03~6.7 wt.%	[91]
2.	Floodplain soil, Switzerland	PE (88%), PS, PVC, SBR, PP	0.125~5 mm	<593 items/kg	[92]
3.	Vegetable soil, China	PP (50.5%), PE (43.43%), PET (6.1%)	20 μm~5 mm	Shallow soil (0–3 cm): 78.0 ± 12.9 items/kg; deep soil (3–6 cm): 62.5 ± 13.0 items/kg	[93]
4.	Agricultural soil, Chile		0.16~10 mm	Median: 1.1–3.5 items/g dry soil	[94]
5.	Agricultural soil, China	PE and PP	>100 μm	40–100 items/kg	[95]
6.	Coastal beach soil, China	PE, PP, PS, PU	<5 mm	1.3–14,712.5 items/kg dry soil	[96]
7.	Greenhouse soil, China		0.05~10 mm	7100–42,960 items/kg	[95]
8.	Agricultural soil, Germany	Fibres, Films	1–5 mm	0.34–0.6 items/kg	[97]
9.	Agricultural soil, Korea	PE, PP, PET, fragments	<5 mm	10–7630 items/kg	[98]
10.	Agricultural soil, Spain	PP, PVC	0.15–0.5 mm	2030 items/kg	[99]
11.	Urban soil, Iran	PET, Nylon	<0.25 mm	619 items/kg	[100]
12.	Urban soil, India	Fibres, foams	0.02–4.89 mm	933 items/kg	[101]
13.	Urban soil, Turkey	Fibres, pellets	<5 mm	3378 items/kg	[102]
14.	Landfill soil, India	PE, PP, PS	<5 mm	1780 items/kg	[103]
15.	Landfill soil, Portugal	PP, PE, PVC	0.01 to 2.95 mm	>105 items/kg	[104]
16.	Landfill soil, China	PP, PE, PET	<5 mm	570–14,200 items/kg	[80]

.

2.3. Atmospheric Environment

The presence of MPs/NPs in the atmosphere can be divided into three categories, namely, suspended particulates, indoor and outdoor dust, and atmospheric fallout [105]. While the outdoor transport and movement of fine particulates are affected to a great extent by meteorological factors, e.g., winds, wind speeds and directions, rains, humidity, snowfall, etc. [106,107], indoor movements and particulate transfer are controlled by human activities, air-conditioners, humidity, blowers, thermal differences, etc. [108,109]. The airflow caused by window ventilation was found to cause an exchange of indoor and outdoor MPs. Dris et al. [110] found that the indoor concentrations of MPs (1–59 MPs/m³) were much higher than the outdoor ranges (0.3–1.5 MPs/m³); this was attributed to stagnancies of indoor pollutants and their inability to move into the larger outdoors. It has also been noticed that non-fibrous MPs were affected strongly by human activities, whereas suspended MPs were likely to be transported by wind, rain, snow, etc. [111]. Some PET MPs have been observed on snow pits and snow surfaces in the Austrian Alps.

Airborne MPs can enter aquatic environments or deposit into soils through dry or wet deposition, making significant contributions to MP levels [112,113]. Airborne MPs are a serious health hazard, as they can be inhaled or ingested with dust, causing potential adverse effects on human health [11]. As the atmosphere is essential for all living creatures, the movement of air, various activities, and pressure systems cause a continuous exchange of matter and energy between the land and ocean, upper air and ground, between the upper and lower air of the atmosphere, the Northern and Southern hemispheres, etc. Because of perpetual movements, there is a strong likelihood of transport of MPs/NPs between the three environments.

3. Exposure Routes

The ingestion of water and/or food contaminated with MPs/NPs, inhalation of contaminated air, and having physical and/or dermal contact with contaminated water, air, foods, textiles, and cosmetics are some of the key exposure routes of micro-/nanoplastics (see Figure 3) [7,114,115].

3.1. Ingestion

Oral consumption (ingestion) is a major exposure route for micro- and nanoplastics in humans [116]. These have been reported in drinking water [117], bottled water [118], salts from oceans and lakes [119], commercial salts [120], foods from fish and marine cultures [121], mussels, etc. [122]. The most common polymers found in bottled water were identified as PP and PET [123]. Tap water in developing as well as developed countries has been found to be contaminated (in up to 88% of the samples tested) with MPs and NPs [124]. Even teabags, coffee, and tea in cafeterias and food stores in Canada and Germany were found to be contaminated with MPs/NPs [125], while a Mexican study reported the presence of up to 14 particles/L of milk in the size range of 0.1 to 5 mm [126]. The long-term exposure and consumption of contaminated food and drinks pose a serious risk to public health; the consumption of 0.5 mg/day of MPs (5–20 µm) could accumulate in the stomach, kidneys, and liver, resulting in significant damage over time [127]. It has been estimated that up to 39,000 to 52,000 particles of MPs could be ingested with food by one person annually [128]. It has been predicted that dust settling on outdoor meals could cause even more damage than the microplastics already present in food and drinks [129].

3.2. Inhalation

Inhalation of MPs/NPs through breathing is another major exposure route due to their presence in ambient air [130]. This particulate matter becomes directly inhaled due to its small size and can accumulate in the respiratory tract and human lungs [131]; the chemical composition of these particles can cause chronic and acute respiratory problems in the short/long term. MP fibres with sizes larger than 250 μm have been detected in human lungs [132]. While a majority of these particulates are cleared by mucociliary clearance, some may linger in the lungs, causing inflammation and a biological response [133]. Microplastics are released into the air by several sources, such as synthetic textiles, polymers, abrasion of materials (e.g., car tyres), and the resuspension of microplastics in surfaces. Inhalable concentrations of MPs have been estimated to range from 0.3 to 1.5 particles/m³ in outdoor air and from 0.4 to 56.5 particles/m³ indoors. It has been estimated that up to 272 MPs could be inhaled per day depending on space factors, cleaning schedules, furniture, season, activity, etc. [134]. The large surface area of small particulates in the respiratory system can induce the release of chemotactic factors, increasing permeability and chronic inflammation due to dust overload [135]. Emissions of ~136,000 tons/year of MPs have been estimated to be released from the oceans to the atmosphere in the form of sea sprays and could be an important source of MPs near coastal regions [136]. These could be transported over long distances depending on the wind’s speed and direction and the initial concentration. The most typical MPs found were mostly PE, PS, PET, and other fibres in the size range of 10–8000 μm [137,138].

3.3. Dermal Contact

Dermal contact through the skin is another route for exposure to MPs and NPs. Consumer products containing MPs, such as face creams, lotions, and face wash, can enhance the exposure risk for polyethylene MPs [29]; acrylic components in such products can cause allergic chemical reactions. Although the absorption of MPs/NPs through the skin appears to be quite improbable, some studies have suggested the possibility of NPs (10–100 nm) permeating human skin [139]. Exposure and deposition on the skin from air fallout have also been reported, e.g., up to 800 pieces of MPs/microfibers have been reported in a study [140]. There is also the likelihood of microfibers and MPs/NPs from cosmetics and toothpaste being absorbed by the skin [141]. While dermal contact with microplastics is considered a less significant route of exposure, it is important to investigate the possible detrimental effects of nanoplastics and extensive skin contact with plastic particles, such as dust, microbeads, liquid hand-cleansers, etc. Several personal care and cosmetic products (PCCPs), e.g., toothpaste, shampoos, conditioners, soaps, deodorants, lipsticks, eyeliners, nail polishes, sunscreens, facial creams, etc., contain significant levels of MPs/NPs [142]. Up to 5% of microbeads (~250 µm) and PE-based MPs are added to enhance exfoliation and cleansing activities [143]. The size of MPs in PCCPs has been reported to range between 70–700 µm [144]; however, MPs in this size range cannot enter the human body via dermal contact [145]. While the NPs present in PCCPs could penetrate human skin, no experimental data have been reported in the literature.

4. Impact on Human Health

Planetary health, as defined in 2015 by the Rockefeller Foundation and The Lancet medical journal [146], emphasised the links between human health and the environment. The evidence in the literature suggests that the impact of MPs and NPs on all biological systems will be enormous and complex [147]. However, translating the impact of accumulating MPs and NPs within the terrestrial, aquatic, and atmospheric environments into direct and indirect clinical effects for individuals and community health is quite challenging. Many of the biological molecular and cellular mechanisms involving exposure to heterogeneous MPs and NPs are yet to be defined. The greatest concern is the cumulative impact of the heterogeneous secondary MPs and NPs on planetary health and human health.

Clinical information derived from the current data is presently limited. Most studies have used in vitro studies (mostly cell lines) or in vivo studies (animal models) to investigate the clinical effects of particulate plastics. Additionally, most studies have used homogenous virgin MPs and/or NPs (predominantly polystyrene beads) to assess the clinical responses. On that basis, the only reliable extrapolation we can make is that the anticipated clinical response to heterogeneous MPs and NPs in humans is a pro-inflammatory cascade. Clinical sequelae in response to exposure to heterogenous MPs and NPs will be determined by many factors, including the physical–mechanical–chemical makeup of particulate pollutants, other pollutants in the environment [148], bioaccumulation secondary to the food chain [149], and the individual’s ability to mount a clinical response. The wide range of clinical responses is due to the extensive distribution of NPs by the cardiovascular system to the numerous organ systems, and the amplification of the clinical response is determined by the degree of activation of the immune and endocrine systems.

4.1. The Physical–Mechanical–Chemical Features of MPs and NPs

The heterogenic makeup of secondary plastic particulates following degradation within aquatic, terrestrial, and atmospheric environments include diversity in size [150], shape (irregularly shaped fragments, fibres, beads, foams, and pellets) [151], surface area, charge (neutral, positively charged or negatively charged), hydrophobic states, types, and combinations of MPs and NPs [152].

The age of the plastic debris has been shown to be important in the clinical response, with faster and greater uptake of older degraded plastic MPs and NPs by macrophages [153]. The age, type, and source of the degraded particulate plastic materials can also influence exposure to other environmental pollutants that have endocrine-disrupting properties (such as bisphenols, phthalates, polybrominated biphenyl ether, polychlorinated biphenyl ether, organotin, perfluorinated compounds, dioxins, polycyclic aromatic hydrocarbons, organic contaminants, and heavy metals [144]). These toxic agents can also directly affect an individual’s clinical response [154,155,156].

The clinical presentation in affected individuals is diverse and varied because of the amplification effects via the immune and endocrine systems. The immunogenicity of secondary MPs and NPs can be graded according to the protein corona formed on their surfaces [157]. The clinical effect of these secondary plastic particulates can be significantly exaggerated by the individual’s endocrine system when the weakly adherent endocrine-disrupting chemicals bind to and activate endocrine hormone receptors.

4.2. Organ Systems Affected by MPs and NPs

Variable biological responses between individuals exposed to heterogenous MPs and NPs would be due to the microcellular and cellular pathophysiological mechanisms initiated at different anatomical locations. The diverse physical, mechanical, and chemical properties of the plastic particulates directly affect the local tissue inflammatory responses initiated at the sites of deposition.

Inhaled MPs and NPs have been shown to affect the upper respiratory tract, lower respiratory tract, and alveoli [158]. Du et al. (2023) demonstrated that lung macrophages J774A.1 demonstrated significantly increased uptake of aged polystyrene NPs when compared to new NPs [152]. Potential lung problems include chronic inflammation, asthma, pulmonary fibrosis, respiratory infections, and possibly an increased risk of lung cancer. Similarly, ingested MPs and NPs affect different segments of the gastrointestinal tract (GIT) [159,160] as well as the gut microbiome [161,162]. Clinical problems with the digestive system include the development of inflammatory bowel disease, irritable bowel disease, autoimmune diseases, nutrient-absorptive conditions, and malignancy. Less information is available regarding the impact of plastic particulate materials on the integumentary system, although Aristizabal et al. (2023) demonstrated that MPs and NPs can affect skin integrity, provoke local inflammatory responses, and disturb the homeostasis of the skin’s physiological functions [163].

The mechanism of clearance of the plastic material influences the early intracellular response, which in turn determines the chronic systemic effects. As the MPs traverse through an individual’s GIT and respiratory systems, direct trauma causes local inflammatory changes to the affected tissues. The internalisation of smaller MPs and NPs by phagocytosis, endocytosis, or micropinocytosis into local cells with a subsequent presentation to the individual’s immune system further amplifies the acute inflammatory response. Prolonged exposure to these MP/NP stimuli results in chronic inflammatory changes and systemic disease. Amplification after the widespread dispersion of MPs and NPs via the circulatory and lymphatic systems means that the clinical presentation is more likely to be varied rather than “classical”.

Small MPs and NPs easily enter the circulatory system. This has been demonstrated in the cardiovascular system of fish from aquatic environments via their gills [164]. The MPs and NPs directly affected cardiac function (causing cardiotoxicity, pericardial oedema, and impaired heart rate) and the vascular system (thrombosis, vascular structural damage, hormone changes, reduced immunity, and altered blood biochemistry) of various fish species. In mammals, MPs and NPs have been shown to internalise into cardiomyocytes, where they subsequently cause myocardial damage, fibrosis, altered electrophysiology, and impaired cardiac function [164]. Interaction of MPs and NPs with plasma protein within the vasculature initiates changes in the components of blood (red blood cells and leukocytes) as well as the cellular components of the walls of arteries, capillaries, and veins to initiate widespread pathological changes within the circulatory system.

NPs have been identified in human subjects. Leslie et al. (2022) noted the presence of NPs in blood samples collected from 17 of 22 healthy human volunteers (~77%) from Amsterdam in the Netherlands [165]. Although these volunteers were asymptomatic, it is reasonable to postulate that the accumulation of particulate material within blood vessels will result in pro-inflammatory atherogenic reactions, which, in turn, will lead to arterial occlusive disease. A prospective, multicentre, observational study conducted by Marfella et al. (2024) identified that 150 out of 257 (asymptomatic) patients (58%) who underwent carotid endarterectomies had substantial quantities of heterogeneous MPs and NPs (polyethylene and polyvinyl chloride being the most prevalent) present within the endarterectomised plaque [166]. Their analysis showed that the patients with MPs and NPs within their plaque were more likely to smoke and also have diabetes, cardiovascular disease, hyperlipidaemia, and higher creatinine values. The endarterectomised plaque containing MPs and NPs had substantially higher numbers of pro-inflammatory cells (CD68+ innate immune cells and CD3+ T cells), as well as a substantially greater expression of inflammatory cytokines (Interleukin-1b, Interleukin-6, Interleukin-18, and TNF-a). Follow-up of these patients by Marfella’s group showed a significantly higher association of cardiovascular-related morbidity and mortality over the following 36 months when compared to patients where no MPs or NPs were identified within the endarterectomised plaque. This suggests a more significant inflammatory process within the cardiovascular system, which is likely driven by the heterogeneous MPs and NPs.

The multiple end-organs affected by the widespread distribution and bioaccumulation of MPs and NPs by the circulatory system include components of the neurological, musculoskeletal, genitourinary, and endocrine systems. Pro-inflammatory mechanisms triggered by MPs and NPs (and associated endocrine-disrupting chemicals) at these secondary locations result in multiple downstream biochemical effects, including chronic inflammation and carcinogenic sequelae [160]. MPs and NPs have been implicated in several animal models of liver disease (resulting in endocrine as well as exocrine dysfunction) [157,167,168,169,170]. MPs and NPs have also been identified in liver biopsy tissue of patients with liver cirrhosis [171]. Other endocrine systems affected by MPs and NPs include the thyroid, pancreas, and reproductive organs. Impaired production and regulation of thyroid hormones adversely affect an individual’s metabolism, growth, and development. Furthermore, the impact of MPs and NPs on insulin regulation and fat metabolism can increase the incidence of metabolic disorders such as obesity, diabetes, and metabolic syndrome [172]. Actions of MPs and NPs on the reproductive organs can alter sex hormone levels, leading to fertility issues and developmental problems in offspring [173]. Additionally, foetal or early-life exposure to MPs and NPs can result in developmental and behavioural sequelae, affecting growth, brain development, and overall health.

MPs and NPs have been associated with neurological impairment. NPs can directly breach the blood–brain barrier, resulting in the bioaccumulation of NPs in the brain [174], while MPs can induce neuroinflammation (and therefore affect neurological function) by disrupting the gut microbiome [175]. Yang et al. (2023) identified that gut macrophage activation by NPs and MPs in NP-fed mice results in an alteration of brain immunity, resulting in microglial and Th17 activation [176], which, in turn, correlated with a decline in cognitive and short-term memory.

4.3. Mechanisms of Toxicity by MPs and NPs

At a molecular level, a wide range of pro-inflammatory trigger pathways are proposed to elicit a wide variety of clinical responses. Understanding the key microcellular and cellular pathways helps us to understand the potential mechanisms involved in the clinical presentation. Almost every cell type in the body is affected when it encounters MPs or NPs. Although there is no discrimination between different cell types by the plastic particulate materials, some cell types may be more sensitive than others. Generally, cellular interaction with MPs and NPs results in a graded pro-inflammatory cascade, where specific cytokines and chemokines result in the activation of different types of cellular machinery pathways, which subsequently leads to downstream actions. These actions can include alteration of homeostasis and result in progression into the atherogenesis and/or carcinogenesis pathways.

The cellular mechanisms related to MPs and NPs are still being mapped. The mechanisms identified at this stage include the Toll-Like-Receptor (TLR) [152,177,178], NK-kB [152,179,180,181] and Fe²⁺ influx [159] pathways. The subsequent generation of reactive oxygen species (ROS), oxidative stress, and mitochondrial dysfunction result in progression along several possible outcomes, such as apoptosis, carcinogenesis, or atherogenesis, depending on the cell type. More research is needed in this area to help establish future clinical solutions.

5. Sustainable Mitigation Strategies for MPs/NPs Pollution

In this section, a brief overview is presented of the sustainable solutions for mitigating MP/NP pollution in terrestrial, environmental and marine ecosystems. While some of these techniques have achieved a level of maturity, others are still developing and evolving. Various strategies used for reducing the abundance of MPs/NPs in the terrestrial environment are broadly classified into upstream and downstream approaches. Upstream approaches are focused on limiting, intervening, and preventing the release of plastics into the environment during every stage of the plastic life cycle. Replacing conventional plastics with bioplastics from renewable sources is one such approach [182]. With much shorter degradation times (in months or years), bioplastics offer a valuable alternative to single-use plastic packaging, e.g., food packaging, and significantly reduce the consumption of single-use plastics and the associated generation of waste [183]. However, care needs to be taken during the disposal of bioplastics through landfilling or incineration due to environmental and health concerns [184]. To improve plastics waste management through law enforcement and better legislation, penalties for indiscriminate disposal, and improvements in infrastructure for plastics waste management and recycling facilities are among other mitigating approaches [185,186].

While serious efforts are being made towards limiting the generation of plastic waste, downstream approaches for removing plastic waste from the environment also play a crucial role. Capturing devices and meshes can remove or limit the spread of waste plastics; however, their use may be economically and logistically challenging due to the widespread distribution of MPs/NPs and the likelihood of harm/damage to local ecosystems with artificial constraints. Other alternatives for the removal of MPs include biological and chemical approaches [187]. Most of these studies are still in the laboratory stage, as the application of these techniques in open environments under uncontrolled conditions still requires more research [188,189]. Further research in this field would make significant contributions to sustainable soil management practices and the ecological integrity of soils together with long-term productivity.

Several regulations have been enacted by the European Union, including the ‘Water Framework Directive’ for achieving good status on bodies of surface water and groundwater by 2027; on emission limits as well as streamlining legislation; the ‘Common Fisheries Policy’ to reduce chemical and nutrient pollution in aquatic systems [190]; the Integrated Coastal Zone Management and the Marine Strategy Framework Directive for controlling MP pollution, etc. [191]. Similar regulations have been enacted by different countries around the globe. The release of microfibres during the washing cycle of clothes is another major source of water contamination with MPs. It has been estimated that up to 279,972 tonnes of microfibers could be released from laundries worldwide, from which about 123,000 tonnes could annually flow through untreated effluents to rivers, streams, lakes, or directly to the ocean [192,193]. Devices located within the washing drum and at the effluent outlet were found to retain between 25 and 90% of the released microfibres [194]. In another approach, agglomeration induced by Sol–gels has been used to remove MPs from the aquatic systems [195].

The problem of MPs/NPs in the environment continues to persist and increase gradually. Local, regional, and global challenges remain along with a lack of appropriate infrastructure. Sustainable plastic waste reduction techniques include source reduction, value addition, regulatory/legislative/policy changes, and beneficiation using material transformations. Many countries, especially poor and developing regions, however, lack appropriate segregation, identification, and quantification techniques to restrict the entry of plastics into the environment. With low values of recovered plastic waste and minimal recovery incentives, there is very little potential for downstream waste mitigation. It is, therefore, important to set up the best waste management practices and treatment facilities [196]. While stringent laws and measures could limit leakage of parent plastic materials in the supply-usage chain [197], the strategies become impractical due to the dispersion and difficulty in collecting MPs and NPs. Concerted efforts should be made to reduce, reuse, and recycle waste to minimise plastic waste [198]. Waste plastics are being recycled and transformed into value-added carbon materials [199,200], fuels, aromatic chemicals [201], construction/cementitious materials [202], polymers, and fibres [203]. Using an array of sustainable approaches, preventive measures, and innovative technologies, the collective pursuit of reducing MP/NP pollution represents vital steps towards safeguarding the environment and health and advancing sustainability with collaborative efforts across the broader public, as well as with researchers, policymakers, industry stakeholders, and governments.

6. Concluding Summary

A brief overview has been presented on the presence of MPs/NPs in the environment, their transport routes, exposure pathways, entry into humans/other species, and their impact on health and associated aspects. The key findings are summarised below, along with a few comments on the current status of the impact of waste plastics on the environment and the health of humans, animals, and other biota.

A tremendous increase has been observed in the use of plastics worldwide, up from 1.7 MT in the 1950s to a staggering 400.3 MT in 2020; the global production of plastic goods has already crossed the one trillion mark. Due to widespread usage and inappropriate disposal, plastic waste/garbage has become a serious source of environmental contamination and associated pollution.

Although polymer degradation/weathering can occur through a number of mechanisms, such as mechanical abrasion, photodegradation, biodegradation, thermal oxidation, hydrolysis, etc., the rates of natural degradation of plastics are extremely slow. Following the degradation over time, large pieces of plastic are likely to break down into microplastics (1 μm–5 mm) or nanoplastics (<1000 nm).

The generation of MPs/NPs occurs over vast areas worldwide and is not localised to small regions/areas; their further transport occurs through three key routes, namely the aquatic environment, terrestrial route, and atmosphere. This results in their widespread distribution, pollution, and environmental impact in regions far away from the generating source.

The key exposure routes of MPs/NPs into humans and other biota include the ingestion of water and/or food contaminated with MPs/NPs, the inhalation of contaminated air, and physical and/or dermal contact with contaminated water, air, foods, textiles, cosmetics, etc. Oral consumption (ingestion) and absorption are considered to be the major exposure routes for these waste polymeric products.

Food chain bioaccumulation and biomagnification along the food chain result in higher concentrations appearing in apex predators, including humans. The cumulative exposure of MPs and NPs increases the potential for health impacts. MPs and NPs trigger immune responses that initiate a combination of acute and chronic inflammatory changes. Chronic exposure could, in turn, lead to autoimmune diseases and other chronic health conditions, including atherosclerosis and malignancy.

A significant portion of modern healthcare waste fails to undergo proper segregation and recycling [204]. The risks presented by MPs and NPs to our planetary health are likely to be greater than currently understood because of the cumulative nature of plastic products. It is essential for multidisciplinary collaboration across the fields of medicine, public health, environmental science, economics, and policy to help mitigate these detrimental effects.

In the healthcare sector, the move to single-use products gained traction following the discovery in 1996 of variant Creutzfeldt–Jakob disease (vCJD), an incurable disease caused by transmissible misfolded proteins (prions) that in genetically susceptible individuals fatally accumulates in and damages the brain [205]. Plastics comprise up to a third of hospitals’ general waste and 40–60% of the clinical waste streams. Modern healthcare has a critical reliance on single-use plastic, which ultimately generates a large-scale waste problem in any hospital and/or healthcare facility.

Limiting/banning single-use plastic products is one such measure that has gained increasing momentum [206,207]. EU rules on single-use plastic products aim to reduce/prevent the impact of various plastic products on human health and the environment, especially the marine environment. Plastic pollution poses the greatest public health hazard faced by humanity. Global efforts are required to limit the plastic waste generated to the extent possible and extent associated with environmental damage.

A number of sustainable mitigation strategies have been reported, including source reduction, material substitution, filtration and purification, transformation of MP/NP waste into value-added materials, technological innovations, etc. Various stumbling blocks, including the small sizes and heterogenous plastic types of MPs/NPs, sorting difficulties, contamination with organic matter, and technological and economic sustainability, will continue to challenge future developments and efforts in this field.