Macroissues with Microplastics: A Review on Distribution, Environmental Impacts, Pollutant Interactions, Toxicity, Analytical Methodology and Mitigation Strategies

Abstract

Although plastic has many desirable properties and numerous social benefits, it is a serious ecological problem due to massive application and difficult decomposing. Various environmental and anthropogenic impacts indicate that plastic breaks down into small particles that are ubiquitous in the environment. Microplastics (MPs) are detected in oceans and seas, freshwater, wastewater, glaciers, soils, air, sediments, precipitation, plants, animals, humans, food and drinking water worldwide. Traces of MPs have been found even in remote and sparsely populated areas, indicating far-reaching movement through environmental compartments. Inadequate waste management and wastewater treatment is considered the major source of MP pollution. MPs are persistent contaminants that can adversely affect the ecological balance of the environment and may damage the health of living organisms, including humans. This review emphasizes the current global problems of MP pollution. It covers different areas of MPs, which include basic characteristics, interactions with other pollutants, occurrence and impacts in the environment, toxic effects on living organisms, sampling, sample pre-treatment and analytical methodology for the identification and quantification of MPs in different matrices as well as potential reduction and remediation strategies and the possibilities for effective control of MPs in the environment. Various interesting and useful previously published knowledge collected in this review can serve as a valuable foundation for further MP research.

1. Introduction

Plastic is a very important and indispensable synthetic, organic, long-chain polymer material that has many desirable properties that improve life quality [1,2]. A total of 353 million tons of plastic waste were produced worldwide in the year 2019, which is twice as much as in 2000. Only 9% of this amount was recycled, while 50% was disposed of in landfills, 19% was burned and 22% was thrown into nature at uncontrolled sites [3]. The durability makes plastics highly resistant to degradation, which leads to its omnipresence in the environment [4]. Various environmental factors promote plastic degradation into smaller particles (Figure 1) that are distributed and accumulated in the environment [5].

Ryan (1990) [6] first used the term microplastics (MPs) for plastic particles smaller than 20 mm found on South African beaches. The first article that described and defined MPs was published by Thompson et al. (2004) [7], which brought the issue of MP pollution into focus. No consensus has been reached yet on the definition that accurately encompasses all criteria that could potentially describe what a MP is [8]. The most common division of plastic fragments derives from particle size, defining plastic particles with a diameter ranging from 1 µm to 5 mm as MPs and those with a diameter less than 1 um as nanoplastics (NPs) [9].

MP pollution is a serious and growing global environmental problem [2]. MPs are increasingly widespread and circulate through all environmental compartments, including distant and sparsely populated areas, adversely affecting the ecological balance [10]. They enter living organisms, including humans, directly or through trophic transfer. The potential toxicity of MPs is increased by their interaction with environmental pollutants and additives added in the production process due to their ability to transfer toxic chemicals and pathogenic microbes in the environment and organisms.

Since the awareness of the existence of MPs in the environment has been present for no longer than 20 years, much is still unknown regarding MP characteristics and their impacts on the environment and living organisms. The objective of this review is to emphasize current global issues of MP pollution on various examples of previously published research, considering their increasing presence in the environment, circulation through environmental compartments and entry into living organisms. Much attention is paid to interactions of MPs with other pollutants, which contribute to their toxic effect on biota and human health. The MP identification and quantification methodology is described through various sampling techniques, the pre-treatment of samples and analytical methods applicable for different matrices. Potential possibilities for the reduction, remediation and control of MPs in the environment are also reviewed.

The MP topic is highly relevant among scientists involved in environmental research, ecosystem pollution, climate change and related fields; therefore, new findings are frequently emerging and being published. The useful basic knowledge together with various interesting insights covering different areas of MPs collected in this review represent a valuable foundation for further MP research as well as to direct and motivate researchers towards new research ideas.

2. Data Acquisition

Relevant publications for this review were gathered through online search in the scientific bibliographic databases Scopus, Web of Science and Google Schoolar using the key word “microplastic” combined with “distribution”, “environment”, “soil”, “freshwater”, “marine”, “air”, “interactions”, “pollution”, “toxicity”, “plant”, “terrestrial animals”, “aquatic animals”, “human”, “health”, “methodology”, “sampling”, “extractin”, “identification”, “quantification”, “mitigation” and “remediation”. For practical reasons only available studies published in English or Croatian language were taken into account for screening. Publications considered relevant for this review were those that provided significant and useful insights into each particular area of the topic, which we considered interesting and valuable in future research into current global issues related to MPs. A total of 113 references were considered, spanning from the initial use of the term MP in 1990 through 2024.

3. Occurrence, Distribution and Environmental Impacts of MPs

The intensive use of plastic associated with poor waste management has led to a huge accumulation and circulation of plastic waste in the environment [11]. MPs are nowadays ubiquitous in the environment. They are reported in oceans and seas, freshwater, wastewater, glaciers, soils, air, sediments, precipitation, plants, animals, humans, various food for human consumption and drinking water worldwide [12]. Circulation pathways of MP in the environment are presented in Figure 2. The ultimate sink for plastic waste is the marine ecosystem, which receives the vast majority of plastic waste and its residues [13]. MPs that reach the environment persist, accumulate and over time reach concentrations that affect ecosystem biodiversity [14].

Table 1. Abundance of microplastics in different environmental compartments [15,16,17,18,19,20,21].

Environmental Compartment	Estimated Abundance (2024)	Units	Sources	Notes
Marine Water	0.1–10 particles/m3 (surface water)	Particles per cubic meter [m3]	[15]	Higher concentrations near coastlines and urban areas; varies with depth and region.
Rivers and Lakes	10–1000 particles/m3	Particles per cubic meter [m3]	[16]	Varies significantly depending on proximity to urban discharge and industrial areas.
Drinking Water	110,000–370,000 particles/L (nanoplastics < 1 µm)	Particles per liter [L]	[17]	Most are <1 µm in size; recent findings (2024) using advanced spectroscopic techniques.
Soil	63,000–430,000 tons/year input	Tons per year	[18,19]	From agricultural sources, sludge application, plastic mulching.
Atmosphere (Urban Air)	0.01–5 particles/m3	Particles per cubic meter (m3)	[20,21]	Highly variable by location and weather; data still emerging.
Atmospheric Deposition	Up to 700 particles/m2/day	Particles per m2 per day	[21]

shows an estimate of the abundance of microplastics in water, soil and atmospheric environments in Europe in 2024. These values are ranges from different 2024 studies or previously established estimates still in use due to limited standardized monitoring. Units and measurement methods vary because of a lack of standardized global monitoring.

3.1. MPs in Aquatic Environment

A total of 60–80% of the marine litter are plastic materials [22]. Population size and the quality of waste management systems largely determine which countries contribute the greatest mass of uncaptured waste available to become plastic marine debris [23]. MP fragments suspended in the water column or deposited in sediments have various negative impacts on the marine ecosystems. Eighty percent of the plastic found in marine waste originates from the mainland [24,25]. MPs that enter rivers directly, by wastewater effluent or through leachate water from landfill, are eventually transported to the sea [26,27,28]. MPs used in cosmetics can enter waterways through domestic or industrial drainage systems. A large proportion of MPs passes through filtration systems of wastewater treatment plants. Seasonal and climatic conditions can exacerbate the transfer of terrestrial debris to the sea and to riverine ecosystems, especially extreme outbursts like floods and hurricanes [27,28,29,30]. Several studies have confirmed that the abundance of MPs in marine environments in Asia is higher during the rainy season than in dry season [31].

Tourism, recreational activities, commercial fishing, sea vessels and maritime industries (e.g., aquaculture, offshore exploitation, etc.) represent a direct source of MP in the seas and oceans [29,32]. Marine waste arises mainly from crumbling nets, ropes and abandoned vessels [25]. Discarded or lost fishing gear, including plastic monofilament line and nylon netting, is usually neutrally buoyant and therefore can float at various depths within the ocean [22,33]. A study conducted by Zeri et al. (2018) [27] in the Adriatic Sea showed larger amounts of MPs in the coastal area (≤4 km) than further offshore. The main reason is the proximity of land sources of MPs. The presence of some rare polymers and waxes used in food and dentistry indicated wastewater treatment plants as potential sources of MPs [27]. By monitoring the amount of MPs in sand and sediment samples from four selected beaches in Croatia during and after the tourist season, a higher amount of MPs was determined during the tourist season. Furthermore, the amount of MPs detected at the beach near the river delta was significantly higher than at the other beaches [34]. One of the highest recorded concentration of MPs in sea water so far is reported in the Gulf of Trieste (northern Adriatic) [35]. The main reason for the accumulation of MP waste in the Adriatic Sea is its narrow elongated sub-basin with a high ratio of land to sea surrounded by seven countries and many tourist destinations with strong anthropogenic influence [27,34,35].

3.2. MPs in Soil

Although it was generally believed that the majority of MPs is distributed in aquatic environments, some recent studies indicate that larger amounts of MPs are present in terrestrial ecosystems. It is estimated that the annual amount of MPs deposited in the soil is two to three times higher than the amount of MPs found in seas and oceans. The main sources of MPs in the soil are improper waste management and agriculture. MPs in agriculture originate from artificial fertilizers, mulching, plastic packaging, nets, protective covers, greenhouses, seed treatment with polymer coatings, using wastewater and dried sludge to irrigate or supplement the soil, etc. [36].

The accumulation of MP particles in the soil disturbs the soil structure and modifies its physicochemical properties by increasing porosity, changing acidity and alkalinity (pH), changing the aggregate structure, influencing diffusion of moisture and water holding capacity and affecting greenhouse gas emissions [36,37]. According to Zhao et al. (2021) [38], MP fragments increase soil pH, which may be caused by increased soil permeability and porosity. MP type and soil type condition pH changes [39]. The vegetation can potentially influence the effects of MPs on the pH of the soil by moderating the effects of MPs on the soil pH compared to unplanted ground [38,39]. Furthermore, plastic materials affect the enzymatic activity of the soil, which is an indicator of soil fertility and plays a role in nutrient cycling. The mobility of nutrients is disturbed by MPs, and the availability of nutrients is reduced [40]. The microbiota found on the surface of MP particles can change the basic ecological functions of the soil and biochemical processes and adversely affect the microbiological diversity of the soil [14,36]. The influence of MPs on symbiotic fungi in the soil, soil loosening organisms and plant pollinators can impair the functionality of the entire ecosystem [41]. Microorganisms in the soil or in symbiosis with plant roots have the ability to decompose MPs and thus enable soil bioremediation [36].

MPs in the soil make sowing and plant growth difficult. Large quantities of MPs can fill and block the pores and reduce the infiltration capacity of the soil, which will disrupt the cycle of nutrients in the soil, i.e., the transport of fertilizers and water between the soil and plants, as well as change the microbial structure. A reduction in soil–plant nutrient transfer can cause plant malnutrition, reducing crop growth and yields [42]. The frequent application of plastic mulches in agriculture can result in significant environmental pollution due to the progressive addition of plastic residues to agricultural soils [36].

3.3. MPs in Air and Precipitation

The main sources of MP in the air are urban dust, synthetic textile fibers, tire erosion and the abrasion of plastic materials. Due to their small size and light weight, MP particles can easily be transported to distant and sparsely populated areas over hundreds of kilometers by air masses. German scientists recently reported significant amounts of MPs in snow samples from the Arctic and the Alps, which proves the long-range transmission of MPs by air and precipitation. About 14,400 MP particles were found per liter of melted arctic snow, which is a strangely high amount for such a remote region with an extremely low population [43]. MP fibers were also found in a remote lake located within the Masurian Landscape Park in Poland, which suggests that MPs are carried by the wind and rain and enter freshwater isolated from sewage outlets [44]. A study on airborne MPs conducted in France reports five times higher concentration of MPs in city dust after rain and other precipitation [45]. Wetherbee et al. (2019) [46] collected rainwater samples at eight locations across the mountains of Front Range in Colorado, USA, to track the trajectory of airborne nitrogen that is being deposited in the national park. A microscopic examination of filtered rain discovered MP particles in 90% of the samples, mostly blue fibers that are likely originating from synthetic clothing.

4. Interactions of MPs and Other Pollutants

MP particles come into contact with various organic and inorganic pollutants during the production process (chemical additives), during release into the environment (mostly from landfills and wastewater) and in the environment itself (water, soil, air, sediment and biota) [36,47].

Several thousand synthetic chemicals with potentially toxic effects and a tendency to migrate into the environment are used to produce plastics [48,49]. Additives are added to plastic materials to improve their functional properties (PACs): plasticizers, flame retardants, heat stabilizers, lubricants, pigments, antistatic agents, antioxidants, dyes, pigments, biocides, fillers, etc. [50,51,52]. The most prominent chemicals added to plastic products are formaldehyde (carcinogen), bisphenol A (endocrine disruptor), phthalates [48,53] and heavy metals (cadmium, zinc and lead) [48,54]. About 4% of MP mass is made up of additives [49]. Polyvinyl chloride (PVC) plastic contains the largest amount of additives and is the main source of heavy metal contamination from plastic waste in the oceans [54]. In addition, it is often considered the most dangerous plastic due to the high content of chlorides and additives and the formation of dioxins during the production process and burning [55].

Additives are not chemically bound to the plastic material, but they diffuse into its’ interior down a thermodynamic concentration gradient and later leachate into the environment, where they potentially become biologically available [41,56]. The lifetime of plastic materials such as textiles, furniture, household appliances and electronic devices is several decades, whereby PACs are continuously released into the environment by direct evaporation or abrasion of the surface layer. The emission of PACs depends on their physicochemical properties, characteristics of plastic products and environmental conditions (temperature and UV light) [56]. The European Chemicals Agency (ECHA) proposed the restriction of additives added to plastics in 2019; therefore, some countries limited the amount of additives in plastic products that come into contact with food but still not in other plastic materials (e.g., building materials and rubber) [57].

MPs have the potential to bind, accumulate and release organic and inorganic chemicals. This ability can contribute to the reduction of the concentration of organic pollutants in the environment [36], while at the same time, it represents a serious problem for the environment and human health. Acting as a vector that transports the pollutants in the environment, MPs contribute to the imbalance between environmental compartments and represent an entry route for pollutants into living organisms [36,48,58]. Interactions of MPs with various pollutants that are present in the environment from various anthropogenic sources were investigated, such as heavy metals (iron, aluminum, manganese, lead, copper, silver, zinc and cadmium), pharmaceuticals and hydrophobic organic pollutants (HOCs) or persistent organic pollutants resistant to degradation by biochemical and photolytic processes (POPs): polycyclic aromatic hydrocarbons (PAH), polychlorinated biphenyls (PCB), dioxins, polybrominated diphenyl ethers (PBDE), organochlorine pesticides (DDT and HCH), herbicides and their metabolites, etc. [36,41,48,49,58]. According to some studies, MPs react most easily with pharmaceuticals, heavy metals and pesticides [58].

Interactions of MPs with organic and inorganic pollutants depend on its physicochemical and structural characteristics, such as the type of material, chemical composition, surface structure, size, shape, density, concentration, aging, surface-to-volume ratio of particles and hydrophobic properties of MPs and pollutants [47,48,58,59]. Different types of polymers have different surface structures and thus a different affinity for pollutants. The hydrophobic or lipophilic properties of pollutant molecules and the specific surface area or the surface-to-volume ratio of MP particles turned out to be the main contributors to the capacity of MPs to concentrate pollutants [47,48]. Smaller particles have a higher adsorption capacity due to a larger specific surface area. MPs are exposed to mechanical, thermal, chemical and photo-oxidative stress and biodegradation in the environment, which cause surface structure changes and molecular weight decrease, thereby increasing the adsorption capacity. Thus, MPs that are present longer in the environment have a greater adsorption capacity for pollutants [47,58,60]. A greater diversity and concentration of pollutants was observed in black-colored MPs, which indicates complex physical and chemical interactions [47,60]. The color is often conditioned by the age or degree of decay of the MP. As a result of the degradation process, the surface of the MP changes and the polarity increases, which increases the porosity and charge so particles become more reactive [48,61]. In general, higher concentrations of pollutants were found in discolored and yellowed MPs [48].

Interactions of chemicals and MPs also depend on environmental conditions, such as weather conditions, humidity, solar radiation, pH, salinity, water temperature, dissolved organic substances and microbiological activity [41,47,61,62]. Higher salinity favors changes in the size and surface area of particles and an increase in adsorption capacity; therefore, the transport of pollutants via MPs is more pronounced in the sea environment [61]. Lower pH and higher temperature contribute to a faster release of bound pollutants [47]. The presence of organic matter in the surrounding water significantly reduces the binding of organic pollutants to MPs because of direct competition, i.e., part of the pollutants bind to the organic matter itself [61]. Biofilm formed by environmental microorganisms on the surface of MP particles, can improve the adsorption of pollutants, especially metals, by increasing the surface area of the particles and changing the polarity [47,48]. Most studies on pollutant binding to biofilms have been conducted on metals, so further research on organic chemicals is needed [48].

Figure 3 highlights factors that determine interactions of MPs with pollutants in the environment. Due to the lack of standard procedures for the preparation and analysis of MP samples, the results of studies examining the affinity of different types of MPs for different groups of pollutants are often incomparable [58]. There is an urgent need for a coordinated strategy to develop mechanical models and analytical tools as well as to standardize sampling methods and laboratory analyses, which would strengthen the overall understanding of the environmental fate, transport and exposure of MPs and provide reliable data on MPs in air, water, soil, sediment and biota [56]. Also, more detailed research is needed to reveal the source of toxic chemicals found in MPs to determine whether they originate from additives added in production procedure, from the environment to which MPs are exposed or as a consequence of the joint action of both mentioned [48].

5. Toxicity of MPs

The toxic effect of MPs on living organisms results from the characteristics of the MP particles (type of plastic material, shape and size, chemical structure of the surface), additives that are added in the production process (PACs) and later released into the environment and biota and the vector capability of MPs to the transfer of pollutants in the environment into living organisms [14,36].

One of the primary environmental risks related to MPs is their availability to living organisms in the environment [48]. Animals often exchange MPs for food and swallow them or ingest other organisms that are already polluted with MPs. If the organism is exposed to MPs for a long time, poisoning may occur due to the presence of the toxic chemicals [5]. Digestive fluids and low pH conditions potentiate the release of the pollutant ingested together with MPs, which can increase their assimilation in the organism [47,48]. The hydrophobic properties of POPs favor their accumulation in the fatty tissue of living organisms [47]. MPs are detected in marine and freshwater animals (birds, fish, turtles, crustaceans, bivalves, echinoderms, anemones, arthropods, polychaetes, mammals), soil invertebrates (colembolas, cnidarians and isopods), plants, algae and microorganisms (bacteria, fungi and protozoa) [14,25,41]. A large number of contaminated living organisms at different trophic levels indicates the transfer of MPs through food chains [41] and consequently into the human body. Figure 4 shows some examples of food chains in aquatic and terrestrial ecosystems.

The mechanisms of MP impact on living organisms are not completely clarified [41,63], but numerous studies point to harmful effects at the level of populations (the number of species and biomass), organisms (survival, reproduction, growth, feeding, embryonic development, motility and photosynthetic activity) [63], organs and tissues (the damage of digestive and respiratory organs, inflammatory processes, metabolic disorders, hepatotoxicity, neurotoxicity and immunotoxicity) [47,54] and cells and molecules (reduced lysosomal stability of digestive glands, oxidative damage, damage DNA, reduced antioxidant capacity, changes in gene expression, enzymatic activity and ion exchange) [63]. The studies on the effects of MPs on living organisms and their modes of action report the four most important mechanisms of harmful effects that stand out: reduced food intake (i.e., reduced nutritional value of food), internal physical injuries due to sharp edges, external physical injuries and oxidative stress [63]. In addition, MPs can have harmful effects by transferring pathogenic microbes to living organisms, which are often colonized on the surface biofilm (e.g., Vibrio spp.) [36,61].

MP pollution is spread worldwide, but the potential consequences and risks for human health and the environment are greater in specific areas, where the abundance of MPs is greater due to specific industries, wastewater discharges [64], wastewater treatment plant effluents [65] and inefficient waste management (inappropriate landfills, burning, dumping in soil and water, etc.) [66]. Various enzymatic tests, histological analyses and fluorescence monitoring are used to examine the effect of POPs introduced by MPs on aquatic organisms [47]. However, according to current guidelines, toxicological testing of chemicals is conducted using high concentrations of individual substances, and the results are then extrapolated to low doses. It is not possible to precisely determine the risk of exposure to low doses with such procedures nor to obtain data on the risks of simultaneous exposure to a combination of different pollutants [49]. Laboratory studies have proven that the simultaneous exposure of organisms to MPs and other chemicals can induce and/or change their toxic effects [48,49]. Many authors have reported the combined and synergistic lethal effects of MPs and organic pollutants [36]. Since the interaction of MPs and pollutants is complex, to clarify the mechanisms of action and determine the effect on biota, further research is needed on various organisms, different types, sizes and shapes of MPs and in diverse environmental conditions [52]. On the other hand, some scientists report that the amount of pollutants introduced into living organisms through MPs is not significant compared to direct introduction from the environment (water or air) [41,63].

It is difficult to quantify the vector effect of MPs with regard to other routes of exposure to pollutants in the environment [47,63], especially in polluted areas where concentrations of POPs in the environment are high. Studies on the vector effect of MPs and the toxic effects on biota are mostly carried out in controlled laboratory conditions since in situ studies have difficultly distinguishing the effect of pollutants introduced via MPs or directly from the environment. Huge differences between the results of field and laboratory studies indicate the necessity of measuring potential effects on living organisms in real environmental conditions [47].

5.1. Impact of MPs on Aquatic Organisms

Many studies provide examples of MP presence in the aquatic environment and its negative effects on living organisms, especially on aquatic species that are considered valuable sources of nutrients for human consumption. MPs have been determined in 800 aquatic species at all trophic levels [48,49]. The shape and density of MP particles determines their distribution in the aquatic environment (surface, water column or sediment) and thus determines the trophic level of organisms exposed to it [47].

Aquatic animals are exposed to MPs due to entanglement in plastic waste objects [48,49], the consumption of contaminated prey [49] or the direct intake of MP particles that they swallow unknowingly due to their small size or mistaking them for food due to their similar shape and color (e.g., white, transparent and blue) [41,45,49]. The first potential problem after the MP intake is the mechanical blockage of the digestive system, which causes injuries or death [41,47,48,67], followed by the high toxicity of additives and organic pollutants adsorbed into MP particles. Mesopelagic fish that swallow MPs cannot return to deeper water due to their buoyancy, and therefore, they die [41]. Sediment animals can passively infiltrate MPs. Organisms inhabiting marine sediments are more exposed to the negative effects of MPs because they bind hundred times higher concentrations of pollutants than the sediment itself [5].

The occurrence, quantity and type of MPs in the gastrointestinal tract of sole fish (Solea solea) in the northern and central Adriatic was investigated in 2014 [68]. About 95% of the sampled fish contained MPs, and more than one type of MP was found in 80% of the fish. MPs are mostly accumulated and distributed in the digestive tract, gills and muscles of aquatic organisms [9]. The transfer of MPs from the digestive system to the bloodstream can cause damage and function impairment of many organs [41,47,61,63]. The test organisms mostly used as biological models for evaluating the impact of MPs in aquatic environments are fish, snails, bivalves, crustaceans and insect larvae [65,69].

5.2. Impacts of MPs on Soil Organisms

MPs can interfere with soil organisms (animals, plants and microorganisms) indirectly by changing the physicochemical and biogeochemical properties and the function of the soil or directly by affecting their condition and the function of the organism [41,53,69]. MPs accumulated in the soil reduce plant development and growth, reduce seed germination and slow down the root development during germination [36,37]. Until a few years ago, MP particles were thought to be too large to pass through the physical barriers of undamaged plant tissue. Research on fluorescently labeled polystyrene microspheres entering rice plants by Liu Y. et al. (2022) [70] provided solid evidence that micro-sized MPs can be absorbed by roots and subsequently translocated to aerial parts of plants, thereby possibly transferring to higher trophic levels through food chains. However, MPs are rarely observed in above-ground plant tissue [70]. Plants that have accumulated MPs from the soil express weaker growth and lower yield [36,37,48] due to reduced nutrient intake and the accumulation of MPs in roots, stems and leaves [69]. Enhanced activity of oxidative stress enzymes was observed in plants exposed to MPs and bound pollutants such as heavy metals and additives [69].

The main model organisms for soil research are earthworms (Oligochaetes). Together with others soil invertebrates, they assure loose, fertile and good-quality soil. They directly ingest MPs and thereby contribute to the production of secondary MPs and participate in the transport of plastic through the soil [14]. When consuming a large amount of MPs, Oligochetes die due to a lack of nutrients and energy caused by damages of the digestive system wall [41]. MPs increase the mortality of earthworms up to 25% [71]. Other commonly studied soil organisms are nematodes, isopods, collemboles, gastropods, mites, birds and mammals. Studies on the impact of MPs on animal organisms in soil indicate physical injuries due to ingestion and disorders of growth, development, reproduction, immune system and intestinal microbiota [14,36,41,72].

5.3. Human Exposure and Health Impacts of MPs

The human organism is exposed to MPs through food, water, pharmaceuticals, cosmetic products and airborne dust. The main routes of MP entry into the human body are ingestion into the digestive system, inhalation into the respiratory system and dermal absorption [73] (Figure 5). The estimation of human exposure to MPs varies between 74,000 and 121,000 particles per year, considering consumption and inhalation [74]. The intake of MPs into human organisms is inevitable [75] because MP contamination has become evident in many categories of food products (Figure 6), including those that are consumed frequently and in large quantities [11], namely drinking water [61] and table salt [49,76,77]. MP presence has also been reported in fresh and processed seafood [49,76], honey [78], beer [79], sugar [80], milk [81], eggs [82], tea [76], soft drinks [83], infant formula [84], foods of plant origin (seaweed, rice [25] and various fruits and vegetables from shopping centers, such as carrots, lettuce, broccoli, potatoes, apples and pears [85]), foodstuffs packed in canes [86] and plastic packaging [76], etc. It is estimated that people who order take-out food 4–7 times weekly may intake 12–203 pieces of MPs through containers made of polymer materials [87]. MP content may increase or decrease during food processing or cooking raw food [76], which depends on many factors such as ingredient characteristics, MP types and the process itself (e.g., no significant change in MP content in eggs was observed after cooking [82]).

There is a high probability that humans are most exposed to MPs due to the consumption of seafood (shellfish, crabs, fish and seaweed) and products from the sea (table salt). Smith et al. (2018) [49] estimate that MP intake through the consumption of shellfish in Europe is up to 11,000 particles per year per person. Considering that MP particles are less often found in the tissue of marine organisms compared to the digestive tract, it is recommended to thoroughly clean or remove the digestive organs before consumption. In addition, it has been confirmed that washing the digestive system of bivalves significantly reduces the proportion of MPs [76].

The presence of MPs was reported in 128 brands of commercially available table salt from 38 different sources from five continents [77]. The highest concentration of MPs was found in fine sea salt from Croatia (800–19,800 particles per kilogram) [76,77]. Based on MP concentrations in commercially available salt brands and the recommended daily intake of salt (5 g), Peixoto et al. (2019) [77] calculated the annual intake of MPs through salt consumption, which is 1460–36,135 particles per year for consumers in Croatia.

The presence of MPs in drinking water was first reported in 2017 within a global survey of tap water from six regions on five continents by Kosuth et al. (2017) [88]. Eighty-three percent of the 159 samples analyzed were found to contain plastic particles. Surface and underground sources of drinking water are polluted by MPs due to inadequate disposal of plastic waste in the environment. It is estimated that individuals who meet the recommended water intake through only bottled sources may be ingesting 90,000 MP annually compared to 4000 MP for those who consume only tap water [74]. The main source of MPs in bottled drinking water is the packaging and the filling process, which was proven by a comparative analysis of water from the same source packed in plastic and glass bottles [61]. MPs released from plastic bottles or bottle caps into content were also reported for vinegar [89] and white wine [90].

MPs have been detected in human feces, removed intestinal fragments, saliva, bronchial mucus, skin, hair, lymph nodes, livers, spleens, placentas and blood [54,61,75,91]. Schwabl et al. (2019) [92] studied MPs in human feces and detected nine types of plastic with the average amount of 20 pieces per 10 g sample, size 50 to 500 µm. A more recent study [93] reported 15 different types of MP in human feces.

More than 90% of ingested MPs from the human body are excreted in feces [48,77]. MP excretion or retention in the body depends on the size, shape, length, type of plastic, additives and bound pollutants [48,61], e.g., longer, fibrous particles are more difficult to be excreted from the lungs [48]. The prevailing opinion earlier was that MPs pass harmlessly through the digestive system of living organisms [54]. Research on mammalian model organisms and cell cultures indicates the possibility of small MP particles passing through the cell membrane and entering the lymphatic and vascular systems and the placenta [54,61]. This feature enables the transfer of MPs to distant organs and accumulation as a result of chronic exposure [49,61]. MP particles were found in human blood samples, which are believed to cause certain dysfunctions in the body that have yet to be identified [54].

Animals and humans inevitably inhale MP particles from the air. A study carried out by Vianello et al. [94] using a thermal breathing mannequin in an indoor space reported that a person inhales up to 272 MP particles per day during light physical activity. During play and movement on the floor, children ingest large amounts of dust directly into their mouths [64]. The protective mobile phone cases can release MPs to human hands during use [95]. Various respiratory disorders have been observed in personnel working in the textile and PVC industry as a result of MPs entering their respiratory system (e.g., increased lung permeability and chronic inflammation) [36]. Small MP particles of only a few hundred micrometers have been found in malignant lung samples [96]. Because MPs are small in size and have a large surface area, once they enter the respiratory system, there is a possibility that of inducing chemotactic factors that stop macrophage migration and enhancing permeability, resulting in chronic inflammation [36].

It not yet entirely clear whether the intake of MPs represents a health risk for humans [75]. Research into the effect of MPs on human health is in its infancy. Preliminary studies point out the potential harmful effects of MPs on various organ systems and functions of the human body depending on the types of MPs, particle size, concentration, exposure time and related additives and pollutants, such as inflammatory processes, respiratory diseases, gastrointestinal diseases, cardiovascular diseases, infectious diseases, gene mutations, oxidative stress, cell death, tissue necrosis, reproductive disorders, intestinal microbiome disorders, embryogenesis disorders, neurotoxic effects and cancer [49,54,61,69,75,77]. These toxic effects of MPs were mainly determined by various biological experiments on human cells in vitro and on experimental animals (invertebrates and small mammals) in vivo, but reliable clinical data are still lacking [54]. Osman et al. (2023) [75] report that by 2023, a total of 10 different cell types were used in cultures to evaluate the effect of MPs on humans. In vitro tests are usually performed on specific cell types of immortal cell lines; therefore, they are not ideal evidence for the correlation of effects on the complex human organism [54]. Given that only a few studies of MP metabolism in the human body have been performed, the results should be interpreted with caution [53]. To understand the effects of MPs on the human body, standardized and reproducible methods of sampling, exposure defining, ecological assessment and health impact assessments are required [49].

6. Methodology for MP Analysis

The methodology of sampling, sample preparation and analysis for the purpose of characterization, identification and quantification of MPs is not yet standardized. Various methods have been used in MP research so far, which are still being developed, improved, optimized and validated. In addition, an international agreement on the results’ expression is needed for different research data to be comparable. Attempts have been made to align different datasets, and some recommended research frameworks and protocols have been developed for sampling, pre-treatment and analysis according to specific features of a particular matrix [97].

6.1. Sampling and Sample Handling

The essential rule when sampling all types of samples for MP analysis is to use non-plastic equipment and devices to avoid possible contamination. If plastic materials are used in any part of the sampling procedure, it is necessary to include a blank sample in the analysis to prevent bias in the results [98].

Sampling for MP analysis must be planned and adjusted depending on the purpose of the analysis and research objectives. When sampling in the environment, it is necessary to consider the spatial arrangement of locations, the area of the sampled territory, the depth of sampling, the appropriate amount of sample, etc., so that the collected samples representatively reflect the pollution of the area being examined. In addition, local environmental conditions must be taken into account, which determine the distribution of MPs in space, i.e., their movement and deposition with regard to the shape, density and size of the particles [99]. When collecting biota samples, the focus is on the key species, their developmental stage, feeding habits, spatial distribution, etc. Kazour et al. (2019) [65] propose the blue mussel (Mytilus edulis) as a suitable bioindicator organism for monitoring MP contamination in the sea, considering its wide geographical distribution, sedentary lifestyle and high-water-filtering capacity.

The sampling of MPs from the aquatic environment can be performed in a dynamic (with towing equipment from a vessel) or stationary manner (from a floating station or shore-fixed equipment) [100]. It is carried out using different types of nets intended for particular sampling depths, e.g., a hand net, manta trawl, plankton net, bongo net or Neuston [53,99] (Figure 7). A flow meter can be mounted on the opening of the net so that the volume of sampled water is quantified [53]. Regardless of the net type, the pore size must correspond to the research needs (typically 50–3000 µm) since a larger mesh allows for smaller particles to pass through, and a smaller mesh is easily clogged. The simultaneous use of several nets of different pore sizes is recommended [98]. For MP particles of size 5–50 µm, continuous flow centrifuges can be used or cascade filtering of a water sample under pressure through a series of filters or different mesh size sieves [99]. For direct water sampling at particular depth, different pumps or deep samplers can be used, e.g., Niskin, van Dorn and Kemmerer [53,99] (Figure 7). Such samples are small, spatially limited and often not representative, so a larger number of repeated samples is recommended to analyze the entire area [53,98]. In general, the load of MPs in an aqueous medium is lower compared to sediment due to water movement and dilution in the water column, so it is often necessary to analyze a larger sample volume to obtain measurable results [98].

Sediment is a static environment that accepts and accumulates settled non-floating particles [98]. For sediment sampling in the coastal area, a benthos net, trawl net, fish net, shovel, hand rake, hand drill or bucket can be used depending on the size of the sediment particles in the sublittoral zone deep rakes (e.g., Ekman or Van Veen), box corers or deep drills [98,99]. (Figure 8). It is necessary to take at least 300–500 g of the sediment sample, which is then placed in a container made of stainless steel and transported and stored at 0–4 °C [101]. In the laboratory, samples are dried at 40 °C for 72 h [53] and thoroughly homogenized before further processing [98].

The usual equipment for soil sampling are metal (e.g., stainless steel) spatulas and aluminium sample containers [36]. The samples are dried and sieved before further processing in the laboratory [53].

A representative sample of the biota consists of at least 50 individuals. Live organisms for MP analysis are collected manually or using traps, grabs, plankton and benthic nets and trawls, electrofishing, etc. Biota samples should be frozen, dehydrated or fixed as soon as possible after collection (e.g., formalin or ethanol) [99].

6.2. Sample Pre-Treatment and MP Extraction for Analysis

During laboratory work and handling with samples, it is necessary to follow the general guidelines for preventing contamination. The recommendation is that all laboratory utensils and equipment are made of glass, metal or ceramic as well as cotton clothing and latex gloves are worn during work [52,65,84,99,102]. It is necessary to use blanks or negative controls to check for plastic contamination during the preparation process [58] and to better understand the origin of potential bias [103]. To avoid contamination with MP particles from the air [104], the sample manipulation should be kept to a minimum [99]; work surfaces should be regularly cleaned with cotton cloths and ethanol [103], acetone or methanol [102]; sample preparation should be performed in a laminar [52,65] or fume hood [84] and washed laboratory equipment should be kept covered with aluminium foil [101].

Prior to identifying and quantifying MP particles using analytical techniques, they must be separated from the rest of the sample without damage [58]. When choosing an appropriate extraction technique, it is necessary to take into account the purpose and goals of the analysis. The pre-treatment procedure depends on the type of sample being analyzed (sea water, freshwater, sediment, soil, biota, air, food, cosmetic products, etc.). Water samples can be directly filtered through a net, sieve [41] or glass or metal fiber filter [99,101]. For clean solid samples, rinsing with distilled water may be sufficient [76]. For more complex samples (e.g., soil, sediment and biota), multiple filtering, sieving, flotation, density separation and chemical or biological digestion of organic matter are used [14,53,76,95,105].

Soil is a mixture of solids (clay and minerals), liquids and significant amounts of dead organic material [14]. Techniques for MP extraction from soil that have given good results are air flotation, density separation, heating at 130 °C [36] and oil extraction [14].

MPs can be separated from the larger sediment using an MP separator based on particle size or an electrostatic separator considering non-electrostatic properties [58,99]. For smaller sediment (<1 mm), density separation (e.g., ZnCL₂ solution in a flotation chamber) and subsequent filtration can be used [99].

The density of MPs is relatively low and mostly varies from 0.9 to 2.3 g cm⁻³ [41]. Density separation pre-treatment is based on adding the sample to a prepared solution of a high density, where the MPs float on the surface, while the mineral parts of the soil or sediment remain at the bottom of the solution. MPs of higher density need a denser solution for efficient separation [103]. Different salts (e.g., NaCl, CaCl₂, ZnCl₂, ZnBr₂, NaI, NaBr, potassium formate, sodium tungstate dihydrate and sodium polytungstate) are used to prepare solutions of known density [14,36,53,65,76,99,101] that have different separation efficiency on individual MP types. Separation can last up to several days (72 h), so various options for shortening this time are being investigated [53,54]. Common laboratory glassware and accessories are most often used for density separation, but some scientists have developed specialized equipment, such as a sediment separator, an overflow column and an elutriation column for sediment samples or a small volume glass separator for ground water samples [103].

An additional step in the MP extraction procedure is the digestion of samples to remove organic matter. Chemical procedures and reagents that can be used for digestion are shown in

Table 2. Organic digestion possibilities during sample pre-treatment procedure [41,57,76,99].

Chemical Reaction	Reagents	Reference
Acidic digestion	HNO3	[27,99]
	HCl	[76,99]
	HClO4	[76,99]
	H2SO4	[41]
Alkaline digestion	NaOH	[57,76,99]
	KOH	[57,76,99]
Oxidizing digestion	H2O2	[41,57,76,99]
	Fenton reagent	[41,57,76,99]
	Piranha solution	[41,57]
	KClO4	[76]
Enzymatic degradation	Proteinase Trypsin Collagenase	[76]
	Cellulase Lipase Chitinase	[99]

. Digestion is more effective at a higher temperature (50–70 °C) for a longer time (12–96 h), but it is necessary to consider that certain plastic types melt and lose at higher temperatures [76]. It is also necessary to take into account that acidic solutions can decompose plastic, alkaline solutions can lead to surface degradation of plastic and peroxide can change the appearance of MP particles and affect the properties of certain polymers. In addition, the digestion process can stimulate the leaching or conversion of additives embedded in MP composition and thus change the chemical properties of the particles [57]. The efficiency of a particular chemical in removing organic matter from a sample depends on the type of sample, the type of plastic material and the concentration of the solution used. For example, 30% H₂O₂ has an efficiency of 80–86% in sludge and 96–108% in soil, but it is not suitable for all polymer types [53]. Enzymatic degradation proved to be the most efficient (>97%), with minimal MP damage, but the method is expensive, degradation can last from 24 h to 30 days [53,99] and it is not applicable for the analysis of biodegradable polymers [57].

When choosing chemical reagents for treating samples in the process of MP extraction, it is mandatory to consider the stability of MPs because the procedure itself can induce the fragmentation of MPs and affect the increase in the amount of particles in the sample [57]. Each of the above mentioned methods has certain disadvantages, and none of them has maximum MP extraction efficiency [14,36,53,57,76,99]. There is an urgent need to establish and standardize an extraction method that will be fast, efficient and simultaneously applicable for different types of polymers with minimal impact on the samples [104].

6.3. Analytical Methods for Identification, Quantification and Characterization of MPs

Due to the diversity and complexity of properties, MPs are one of the most challenging analytes in the environment, and advanced analytical methods are required for their detection, identification, quantification and characterization [52]. The methods applicable in MP research can be roughly divided into non-destructive and destructive types. Non-destructive methods do not damage the sample during analysis (e.g., microscopy and spectroscopy), while destructive methods result in the complete loss of MP particles due to heating and the impossibility of further analyses (e.g., chromatography) [53,58,64]. Choosing the appropriate method or combination of methods depends on the purpose and goals of the research. For an accurate, reliable and precise analysis of MPs, it is desirable to combine different analytical methods [105].

The most commonly used microscopic, spectroscopic and chromatographic methods for MP analysis of are described in this section. The possibilities of various other single or combined, earlier established or novel analytical techniques are being explored in MP analysis. A great variety of methods used in MP research was reviewed by Ivleva (2021) [52] and Huang et al. (2023) [105].

6.3.1. Microscopic Methods

Light microscopy can be applied to analyze the physical properties of MPs, such as the color, shape, size, surface structure and degree of corrosion and weathering [14,64,105]. For morphological characterization and counting of MPs, one can use a stereoscopic (dissection) microscope for particles larger than 50 µm [41,64] or a classic optical microscope with 10–50× magnification [54]. These devices can be connected to a computer and combined with image analysis programs [54], e.g., Image J [41]. Visual detection is suitable only for clean matrices [76] and if the MP particles are not transparent [54]. The disadvantage of the method is the possibility of misidentification of MP particles due to non-recognition or confusion with other materials (e.g., coal ash, paint fragments, natural fibers: cellulose, keratin and viscose) [14,54,58,64].

Electron microscopes can compensate for the shortcomings of optical microscopes because they reach higher magnification and resolution, thus providing a clearer structural image of the MP surface [64]. For analyzing the surface morphology of MPs, a scanning electron microscope (SEM) is mainly used. An SEM can analyze particles up to 1 nm in size [105] and provides data on size, shape, surface structure, wear, degradation and microbiological colonization [52]. The preparation of samples for SEM microscopy requires additional pre-treatment of coating MP particles with certain chemicals [52,57], so it is not suitable for analyzing the surface structure and color [14].

A polarized light microscope (PLM) can be used to analyze some types of MPs whose crystal structure allows for light to pass through, but the method is not suitable for opaque and thicker MPs [64]. The use of hydrophobic fluorescent dyes for MP staining [105], such as the Nile Red reagent (NR staining method) are shown to be effective in the analysis of sand samples [53]. The stained samples are then analyzed with a fluorescence microscope or a confocal laser scanning microscope. Staining with fluorescent dyes can give false positive results because the bioorganic material contained in the sample is simultaneously stained, and the environmental samples can have fluorescent properties by themselves [105].

The type of polymer cannot be determined by microscopic techniques because they do not provide information on the composition of MP particles [104]. To obtain reliable results, microscopy can only be used as a preliminary technique in combination with other methods. For a simultaneous analysis of the surface structure and elemental chemical composition of MP particles, a combination of an SEM with energy dispersive X-ray spectroscopy (SEM-EDX) can be used [64,105]. A prerequisite for this analysis is the conductivity of the samples. MPs are not conductive, so they require a complex pre-treatment (e.g., gold plating); this technique is currently used only for the analysis of certain types of MPs and is not applicable for environmental samples [105]. Combinations of microscopy with different techniques is recommended, and the most commonly used combinations are the ones with spectroscopic techniques (µ-FTIR and µ-Raman) [41].

6.3.2. Spectroscopic Methods

MPs are typically a heterogeneous mixture of plastic particles of varied and complex compositions, and the analysis of their chemical composition is based on the determination of functional groups, molecular weight, structure and the degree of polymerization [105]. For the representative analysis of MPs, vibration spectroscopic methods (FTIR and Raman) are most often used. These are based on the interaction of radiation and molecular vibrations and can identify the type of polymer, quantify their number and determine the shape, size and distribution of the particles [53]. The recorded spectra of the samples are compared to the reference spectrum from the literature database [54,105]. Commercial spectral libraries usually contain the spectra of original intact synthetic polymers, which can differ significantly in practical application due to the presence of additives and MP changes in the environment. Therefore, some open-access spectral libraries are established that are supplemented with practical examples of MP spectra found in nature (e.g., SLoPP, SLoPP-E, RDWP and Open Spec) [52].

Fourier transform infrared spectroscopy (FT-IR) enables the accurate identification of polymer type based on the measurement of its IR spectrum [54,105]. The analysis provides information on the amount of MP particles and their structure (chemical bonds and functional groups) and enables the assessment of degradation or weathering of the material. The sample must be previously cleaned of other impurities and completely dry because moisture interferes with the analysis, which is the main limitation of the method [52]. The preliminary preparation procedure and measurement device settings should be adapted to the sample type being analyzed since the particle size, color and shape of MPs affect the analysis [53,105]. Each particle in the sample is analyzed separately, which is time consuming. Dark and opaque materials and irregular particles smaller than 20 µm are difficult to analyze [14,105]. The combination with an optical microscope (µ-FTIR) improves the efficiency of identification [105] and enables the analysis of particles up to 5 µm in size [53]. µ-FTIR analysis can be performed on individual manually isolated particles or their optical images as well as simultaneously on the entire filtered area [52].

In some MP studies, a similar technique of NIR spectroscopy was used for measurements at wavelengths close to the IR spectrum [41,103]. This could be used as a screening method before more detailed spectroscopic analyses [52]. Studies also report the application of FTIR with attenuated total reflection (FTIR-ATR) for the description of MP composition [52,54,101,106] and FTIR with FPA detector (focal plane array-based reflectance) and its improved automated version FPA detector-based µ-FTIR imaging [52,105], for which a free software tool for the systematic identification of MPs in the environment (siMPle) was developed [52].

Raman spectroscopy determines the presence and type of MPs in samples based on the scattered light reflected by the sample molecules when they are in an excited state [53]. Different structures of different molecules and atoms have different frequencies of scattered light due to vibrations induced by changes in the polarization of chemical bonds, which results in the Raman spectrum [105]. The resolution of the Raman spectrum enables the analysis of particles up to 1 µm in size [14,105]. A small amount of sample is sufficient for the analysis. The samples do not have to be dried and dehydrated, but they must be completely cleaned of organic matter [53]. The main method limitation is the interference of fluorescent components naturally present in the environmental sample (e.g., clay minerals, dust, hummus, microbiological and biological impurities [52]) or formed during the pre-treatment process (e.g., iron residues after digestion with Fenton’s reagent [57]). Spectra obtained from additives contained in MPs and bound contaminants can overlap with the polymer spectrum, which interferes with the identification of MPs. It was also established that a monochromatic laser light source in a Raman spectrometer can induce light and the thermal degradation of MPs [105]. Many scientists are working on improving the method for the analysis of environmental samples. Classical Raman setups were used only in the earliest research of MPs. A combination with a confocal optical microscope (µ-Raman) began to be used very quickly. Eighty-six percent of MP studies with Raman spectroscopy used the µ-Raman technique [107]. Unconventional methods that enable a more advanced application of Raman spectroscopy are also used, e.g., Raman Tweezers (Rts) (optical tweezers combined with a Raman spectrometer [52,54]), standoff Raman real time detection [107], nonlinear Raman (laser technique), coherent anti-Stokes Raman scattering (CARS) and laser microscopic technique simulated Raman scattering (SRS) with improved sensitivity and without fluorescence interference [52].

A mutual limitation of spectroscopic techniques is the analysis of the surface part of MP particles exclusively, which makes the interference of additives such as pigments possible [52,53]. Contemporary techniques that could compensate for the disadvantages of FTIR and Raman spectroscopy, such as speed and resolution, are Laser Direct Infrared (LDIR) chemical imaging [52,53] and hyperspectral imaging technology [41,52,54,107]. At the same time, more and more scientific effort is devoted to the development of computer programs for automated MP analysis (e.g., GEPARD and TUM-Particle Typer) [52].

6.3.3. Chromatographic Methods

Chromatographic techniques that are most commonly used in MP analysis are thermos-extraction desorption gas chromatography with mass spectrometry (TED-GC-MS) and pyrolytic gas chromatography with mass spectrometry (Pyr-GC-MS). Environmental samples are first subjected to thermal decomposition, which involves heating to a temperature of 500–650 °C in an inert gas atmosphere (TED-GC-MS) or without the presence of oxygen (Pyr-GC-MS) that makes the polymers gradually change from solid to liquid or a vaporous state. Decomposition products are formed, which are first separated on a chromatographic column and then analyzed by mass spectrometry at the molecular level [52,54,105]. Identification is carried out based on characteristic ions and their specific ratios, and the quantity is determined from the number of ions released during pyrolysis or thermal desorption of MPs. Some polymers have similar degradation products, so the results can be misinterpreted [53,105]. Particles smaller than 10 µm can be qualitatively and quantitatively analyzed [53]. The chemical composition of the entire MP particle is analyzed, which enables identification of individual types of polymers and organic chemicals they interact with (additives and pollutants) [52,53,54]. Due to high sensitivity, a small amount of sample is sufficient for the analysis (5–200 µg). Various physical or chemical effects can change the MP quantity during sampling, pre-treatment and analysis, which affects the quantitative concentration in the sample; therefore, using an MS detector to determine the mass concentration of MPs is more reliable and less susceptible to environmental influences [105].

TED-GC-MS technique is also appropriate for analyzing significantly larger samples (up to 100 mg), which enables a lower limit of determination, and it is more suitable for the analysis of complex environmental samples and food [41,52]. No special sample pre-treatment is required, and solid samples can be directly analyzed [105]. However, it has been reported that previous MP extraction or pre-concentration by removing organic and inorganic matter from the sample can improve the performance of the analysis, i.e., increase the reliability of identification and the sensitivity of quantification and, at the same time, reduce background noise and interference [52]. Adding internal standards (e.g., deuterated compounds such as styrene, polystyrene or chlorobenzene) to samples during analysis further improves the quality of the results. It is also recommended to analyze calibration standards and procedural blanks together with the samples for correction of the measured values [52].

The Pyr-GC-MS method was used for the analysis of model and natural samples of seawater, freshwater, sediment, biota, sewage sludge, air, soil, commercial sea salt and drinking water [52,105]. The authors constructed a portable device applicable for rapid field analyses of MPs (Pyr-MS) based on MP pyrolysis degradation and mass spectrometry quantification within 5 min.

Other chromatographic techniques used for MP research are high-temperature gel permeation chromatography (HT-GPC), size-exclusion chromatography with liquid extraction (HDC-SEC) [105] and high-performance liquid chromatography (HPLC) for polymers insoluble in common solvents after the depolymerization process [52].

7. Mitigation Strategies for MPs in the Environment

MP pollution represents a threat to global social, environmental and economic sustainability [55]; therefore, effective measures and strategies for its reduction and control are necessary. It is urgent to standardize analytical methods for reliable identification and quantification of MPs in order to implement effective regulation and environmental pollution control [54]. There is no global strategy that would include practical and measurable interventions in the reduction of plastic pollution [66]. The initiative and engagement of individual countries in the application of certain measures depends on expenses, infrastructural arrangements, economic conditions, public readiness for changes, etc. [75]. Action plans and guides are mainly focused on plastic production and usage limitations and disposal regulation, i.e., plastic waste management [61,64]. Some governments have restricted single-use plastics, such as beverage bottles and plastic bags, straws and cutlery [54,61]. Intentionally added non-essential primary MPs, such as beads in cosmetic products, are banned in many countries (e.g., New Zealand, Canada, France, UK, USA, Sweden, Taiwan, Thailand, Ireland, India, Italy, South Korea, etc.) [1,8,54,61,75], and also, the production of unnecessary and problematic plastic materials that are subject to abrasion and fragmentation is reduced. MP pollution is invisible to the eye and often ignored by individuals and society [55], so it is necessary to raise awareness and educate the public by pointing out the possibilities of individual contribution to solving the problem [108]. Means of individual contribution are replacing plastic with alternative materials, multiple uses of plastic objects, separate collection and disposal of plastic waste [66], wearing clothes made of natural materials, installing filters on washing machines, using natural cosmetics [75], etc. (Figure 9).

Environmental pollution with MPs results from the linear use of plastic materials and the accumulation of plastic waste [108]. Landfills are not the last reservoir for plastics as it was considered earlier, but they are potential sources of MPs in the environment, which is a serious problem considering that MPs are not yet a legally regulated pollutant [65]. One alternative to disposing plastics in landfills is burning them for energy production. However, this merely moves pollution from the ground level into the atmosphere in the form of carbon dioxide and other gases [109]. Effective plastic waste management should be based on the circular concept of the reuse and recycling of plastics [62,64,75]. Waste management systems generally do not have sufficient capacities for safe disposal or recycling of plastics on a global scale; therefore, it is necessary to increase the capacities of collection, sorting and mechanical and chemical recycling [66]. Synthetic textiles are an important source of MPs and should not be left out [110]. New materials often have worse thermal and mechanical properties, the recycling of plastic materials is quite expensive and it depends on human involvement [108]. According to some preliminary studies, MPs can be recycled together with other materials and reused as a new source of energy [111].

Large amounts of MPs reach wastewater treatment plants together with other pollutants [98]. Urban wastewater contains microbeads from cosmetic products and textile fibers that are separated when washing clothes [53,111]. Experiments sampling wastewater from domestic washing machines demonstrated that a single garment can produce >1900 fibers per wash [112]. The efficiency of wastewater treatment plants in removing MPs by secondary and tertiary treatment can reach up to 99.4% [65]. Still, not all MPs from industrial, municipal and agricultural sources can be removed by wastewater treatment facilities, so they represent a significant source of MPs in environmental water [55,58,65], and the soil as well, as a result of sewage sludge deposition on arable land.

The best way to avoid the environmental risks of MPs is to remove them from the environment and then recycle or completely degrade them [75]. Technologies for removing MPs from the environment are intensively studied [69]. The efficiency of MP removal from water can be improved by sedimentation, flocculation, advanced filtration (e.g., disk filter, bio filter or sand filter), membrane bioreactor technology, adsorption (synthetic or natural sponge, biochar, graphene, metal complexes, organic chemicals, untreated coffee grounds or magnetic adsorption), coagulation or innovative procedures of electrocoagulation, electrochemical oxidation and magnetic separation [53,75,113,114]. Flocculation, sedimentation and coagulation technology is applicable for drinking water treatment [113]. There are possibilities of biological removal of MPs from the aquatic environment (Figure 10), such as adsorption on green algae and aggregation, flocculation using jellyfish mucus, decomposition by microorganisms (fungi and bacteria) and ingestion (zooplankton, corals and bivalves) [98,113,114]. Despite the great potential of marine organisms to remove MPs from the environment by ingestion or filtration, it is currently not considered as an elimination strategy [114] because in this way, the MPs are only moved in the environment and introduced into food chains instead of efficient removal [113]. The combination of different removal techniques can achieve better efficiency [114]. However, all suggested MP removal techniques have both positive and negative effects and were mainly investigated in a laboratory rather than under natural environmental conditions [75].

Abiotic (photocatalytic) degradation of MPs occurs spontaneously in the environment under the influence of UV radiation, temperature, air, water and mechanical force. Such mechanical decomposition reduces the size of the particles while their relative weight remains the same. The total amount of MPs is reduced by chemical decomposition. Biotic decomposition is desirable and safe for the environment because it enables complete mineralization of MPs if the appropriate environmental conditions are enabled. In addition, useful intermediate products can be formed in the process [113]. Some polymer types are molecules with a large molar mass and stable covalent bonds, so decomposition takes decades or even centuries [115]. No signs of PVC degradation were found in the soil after 35 years in natural environmental conditions [14]. It is known that many enzymes have the ability to depolymerize MPs, but the mechanism of complete mineralization is still unclear and unexplored [69]. Recently, various complex compounds are being synthesized that would catalyze the degradation of MPs [105]. Most of the abiotic and biological degradation technologies of MPs have been studied under laboratory conditions; therefore, further research and technical improvements are needed before their field application [113].

Some scientists see a possible solution of the MP pollution in biodegradable plastics and bioplastics as alternatives to conventional plastic materials [114,116]. Biodegradable plastics are produced from petroleum resources but can be degraded by microorganisms [8,69], e.g., Staphylococcus, Bacillus, Psudomonas and Rhodococcus [64,113]. On the other hand, the term bioplastics refers to polymeric materials derived from plants, animals and microorganisms [54], i.e., renewable natural resources that are environmentally benign (e.g., thermoplastic starch) [109]. Bioplastics comprise about 1% of the annual amount of 390.7 million tons of plastic produced in 2021 [36,117]. MPs with a half-life shorter than it are defined by the REACH persistence criteria in a particular environmental compartment and are considered degradable [114]. Aliphatic polymers as well as aliphatic–aromatic polymers with a small number of aromatic units are classified as biodegradable [115]. The degradation of MPs occur in two phases that take place in the environment or in living organisms. The first phase is the fragmentation of polymers into monomers (e.g., by hydrolysis, oxidation with or biotic reactions), which changes their original properties, modifies their surface and increases their hydrophilicity [69,108,115,118]. The second phase is bioassimilation by microorganisms, i.e., mineralization to carbon, water, methane and biomass, depending on the oxygen availability. Degradation is dependent on environmental conditions (light, pH, temperature, moisture, microorganisms and the amount and activity of enzymes); therefore, the degree of decomposition of similar MP particles differs in water, soil and organisms. In addition, the mechanism and time of biodegradation are affected by additives present in MP particles [115,118].

Polymers that are prone to biodegradation but the process consumes a lot of time, are not suitable solution to the MP pollution problem [115]. A 1 mm-sized MP particle needs 320 years to reduce to a size of 100 nm during photo-oxidation and biodegradation in laboratory conditions. The same process would take much longer time in the environment due to limited exposure to oxygen, light and bacterial activity [119]. The complete decomposition of biodegradable plastics is possible only under special composting conditions [36] in controlled industrial composting facilities [115]; therefore, it is a misconception that bioplastics and biodegradable plastics do not cause harm if they reach the environment [116]. Some studies have shown that biodegradable plastics have similar toxic effects on living organisms as MPs and equal or even greater ability to bind hydrophobic organic chemicals from the environment [36,61].

8. Conclusions

The interest in MPs is increasing considerably among scientists, proactive associations and the legislative and regulatory bodies globally. Given that many MP characteristics as well as their impacts on the environment and living organisms have been partially explored and explained so far, many scientists are engaged in researching various aspects of the MP issues.

It is evident from many studies that techniques used in previous research on MPs manifest certain limitations, which can serve as guidelines for future studies on MP analysis methodology, with the aim of developing more precise, accurate, efficient, practical, faster and cheaper methods.

For the results of different studies to be comparable, it is necessary to standardize procedures for sampling, pre-treatment, characterization, identification and quantification of MP, i.e., to establish suitable and reliable international analytical standards. This is a prerequisite for confirmation of the validity and justifiability of potential risks that MPs represent for the environment and biota.

More scientific evidence arising from concrete research results are needed to legally declare MPs an environmental pollutant. The European Union made the huge step forward regarding the legislation to regulate synthetic polymer microparticles (MPs) through Commission Regulation (EU) 2023/2002 under Annex XVII of REACH. The provisions of this regulation will be implemented in phases from 2023 onwards and once the limit concentrations for MPs in the environment and organisms are prescribed, the mandatory monitoring of MPs in the environment will be enabled with the aim of MP pollution control and determination of implemented reduction and remediation measures effectiveness.

In conclusion, over the past two decades, substantial progress has been made in understanding and addressing the global issue of MPs. However, there remains significant potential for improving existing technologies, particularly in analytical methodologies (e.g., AI-driven analysis) and mitigation strategies (e.g., cutting-edge remediation technologies), to more effectively address MP challenges.