Microfibers from synthetic textiles as a major source of microplastics in the environment: A review

Abstract

Microplastic fibers, also known as microfibers, are the most abundant microplastic forms found in the environment. Microfibers are released in massive numbers from textile garments during home laundering via sewage effluents and/or sludge. This review presents and discusses the importance of synthetic textile-based microfibers as a source of microplastics. Studies focused on their release during laundering were reviewed, and factors affecting microfiber release from textiles and the putative role of wastewater treatment plants (WWTPs) as a pathway of their release in the environment were examined and discussed. Moreover, potential adverse effects of microfibers on marine and aquatic biota and human health were briefly reviewed. Studies show that thousands of microfibers are released from textile garments during laundering. Different factors, such as fabric type and detergent, impact the release of microfibers. However, a relatively smaller number of available studies and often conflicting findings among studies make it harder to establish definitive trends related to important factors contributing to the release of microfibers. Even though current WWTPs are highly effective in capturing microfibers, due to the presence of a massive number of microfibers in the influent, up to billions of fibers per day are released through effluent into the environment. There is a need to establish standardized protocols and procedures that can allow meaningful comparisons among studies to be performed.

Keywords

Microplastics microfibers wastewater treatment plant pollution marine biota

The burst of the third industrial revolution has made plastics an integral part of our everyday lives. Due to versatility, lower density, ease of molding, biological and chemical inertness, and lower cost of production, a world without plastics has now become unthinkable.¹,² At present, plastics are ubiquitous, and the use of plastics generally leads to a massive amount of plastic waste disposed of annually worldwide. Data from the United States Environmental Protection Agency estimates that 29.6 million tons of plastic waste per year is being dumped into the environment in the United States.³ Plastic waste is not limited to inland areas; rather it has now been reported in freshwater and deep ocean environments.^4–6 Reports of plastic waste in marine systems emerged as early as the 1960s.⁷,⁸ An overwhelming proportion (70–80%) of plastic-based marine litter is predicted to originate from inland sources and is transported by rivers to oceans.⁹,¹⁰ Plastics are notorious for their persistence because they are not completely biodegradable. When disposed of in landfills and/or released in the natural environment, they accumulate and eventually degrade under the effect of mechanical weathering and photo-oxidation over a very long period of time. The accumulation of plastics in the environment is a growing concern because permanent elimination of plastic waste relies solely on thermal treatments, such as combustion or pyrolysis. This method leads to the release of harmful and/or toxic compounds into the atmosphere such as carbon monoxide and carbon dioxide. Therefore, plastic contamination most likely occurs almost permanently once the plastics are discarded in the environment.¹

A particular type of plastics have caught the attention of researchers, scientists, and environmentalists in recent years after their problematic occurrence in oceans, in massive quantity, was reported for the first time in 2004. In their consequential study, Thompson et al.¹¹ used the term “microplastics” to denote microscopic plastic particles such as fragments, beads, and fibers. Microplastic objects have now been increasingly considered as a serious pollutant because of their persistence and non-biodegradability, accumulation in the environment, buoyancy and consequent dispersal over large distances, and even entry into the food chain.¹² Although the vast majority of the studies on microplastic pollution are mainly focused on marine environments,¹³ more studies are visualizing its impact on freshwater⁴,^14–16 and terrestrial ecosystems as well.¹⁷,¹⁸ Studies have revealed that the increased amount of microplastics in the environment is correlated with population density, land use, and level of sewage treatment.¹⁶,^19–21

Microplastics have usually been broadly defined as polymer particles or fragments of less than 5 mm in size.^22–25 Microplastic fibers (simply microfibers)²⁶ are produced by fragmentation of large plastic particles in the environment²⁷ or produced via fragmentation even before entering into the environment such as during washing of fabrics.¹⁵ Among different forms of microplastics found in the environment, studies have found that the fibrous and filamentous form is predominant in both marine^28–30 and freshwater ecosystems.¹⁶,³¹ Massive numbers of microfibers, both synthetic and natural based, are released from common textile garments during domestic and industrial laundering processes.²²,²³,^32–34 Therefore, microfibers released from synthetic textiles could be one of the major contributors to the microplastic pollution in the environment, especially in urban areas. Domestic sewers and wastewater treatment plants (WWTPs) are considered as the main pathways of textile microfiber release in the environment.²²,³⁰

Because of their relatively smaller size (100–800 µm),²² significant numbers of these microfibers escape the traps in the WWTP and other industrial facilities, and, in a dire scenario, up to 40% of microfibers can enter rivers, lakes, and oceans downstream.³⁵ Moreover, compared with their natural counterparts, synthetic fibers appear to be removed in the WWTP to a lesser extent.³⁶ Because the use of synthetic textiles continues to increase and the world production of synthetic fibers (e.g. polyester) has surpassed the demand of natural fibers,³⁷ the problem of microfibers released in the environment may be exacerbated in the future.³⁸ Synthetic fibers are produced in very large quantities with figures of more than 45 million tons per year for polyester alone, the most important global textile fiber in terms of production and consumption.³⁹ Legislative measures have already been enacted in an effort to eliminate primary microplastic pollution such as microplastic scrubbers (microplastic beads) from consumer goods (i.e. cosmetics and body washes).⁴⁰ Therefore, it is reasonable to envision that microfibers shed from synthetic textiles will be one of the main sources of microplastics to consider in domestic drainages in the future. Furthermore, with increasing per capita income in the developing countries, especially in the two most populous countries (China and India), per capita consumption of textile is also increasing. The world has witnessed a steady increase in global yearly consumption of textile fibers over the last two decades, with an increase of 79.3% between 1992 and 2010. The growth is almost exclusively attributed to close to 300% increase (from 16 to 42 million metric tons) in the consumption of synthetic fibers over the same period of time. The purchase of synthetic textiles by consumers in developing countries is larger compared with developed countries (68% versus 48%).⁴¹ Because over all per capita consumption of textiles is predicted to increase in the future, the increase in the release of textile microfibers in the environment is inevitable.

Given the importance of microfibers originating from textile products as an important contributor to the global microplastic pollution problem, this review focuses on studies related to shedding of textile fibers from garments especially during laundering. This review starts with a general overview of microplastics with respect to size, material, type (origin), and forms. Important studies on microfiber release during laundering are reviewed and discussed. The role of WWTPs in the release of microfibers into the environment is also examined. Finally, conclusions and outlook are presented.

Microplastics

Size definition

Even though microplastics have been broadly defined as polymer particles or fragments having a size of less than 5 mm,^22–25 there is a clear ambiguity about the size limits of microplastics in the literature. Some studies have used an upper size limit of microplastics of 10 mm, whereas some researchers suggested that using the term macro⁴² or mesoplastic would be more appropriate for plastic particles greater than 1 mm, which are easily visible to the naked eye.²⁴,⁴²,⁴³ Eriksen et al.⁴⁴ estimated the total number of plastic particles and their weight afloat in the world’s ocean for the first time. The authors classified microplastics into two categories: small microplastics (0.33–1 mm) and large microplastics (1.0–4.75 mm).⁴⁴ Generally speaking, the lower limit of the microplastics is not well defined. This might be due to limitations in sampling and detection of very small objects, which relates more to the method of sampling (e.g. mesh size of plankton nets used in sampling from water) and/or method of detection (e.g. spectroscopy).²³,⁴⁵ In a recent study, Velasco et al.⁴⁶ reported microplastics in a size range between 125 µm and 5 mm. The lowest size mesh used during sample processing had pores of 125 µm, which is identical to the lower limit of the microplastics the authors reported. This indicates that the lower limit of microplastics can vary among studies based on the sample collection and processing parameters. Indeed, a wide range of lower limits between 5 µm and 500 µm has been reported based on the pore size of the filters used to retain microplastics before detection and analysis.³⁸ Even with the use of Fourier transform infrared spectroscopy (FTIR), a spectroscopic technique considered powerful compared with other methods of identification of microplastics,⁴⁵,⁴⁷ particles of less than 2 µm are unlikely to be identified.⁴²,⁴⁸ However, a few papers specifically defined the size range of microplastics. For example, Cole⁴⁹ specifically defined microplastics as objects with size between 0.1 µm and 5 mm. Yet, the smallest microplastic particles identified so far in the environment are reported to be around 20 µm.²³ According to another study, the smallest microplastic ever to be detected in marine sediment samples had a diameter of 1 µm and a length of 20 µm.⁵⁰ Although almost all of the studies use only one dimension to define microplastics and as described above in the majority of the cases less than 5 mm, it is highly likely that objects such as fibers might be larger than 5 mm longitudinally but be substantially smaller than 5 mm in the transverse dimension. Therefore, the definition of microplastics as plastic particles having less than 5 mm in their smallest dimension²³ appears to be optimum and inclusive.

Origin

Microplastics found in the environment are classified into two groups based on their origin: primary microplastics and secondary microplastics. Primary microplastics are particles intentionally manufactured for some specific purposes (e.g. microbeads used in peeling lotions and body scrubber, cosmetics, etc.).²³,²⁴,⁵¹ On the contrary, secondary microplastics are particles produced due to fragmentation of large plastic particles as a result of mechanical weathering or abrasion, hydrolysis, photodegradation, and biodegradation. The secondary microplastic particles are often found as fragmented pieces or fibers.⁵² Sources of secondary microplastics in the environment include large plastic waste items, plastic mulches, microfibers from textiles, car tires, and so on.¹²,²⁶,⁵³ Similar to the size limit of microplastics, there is also noticeable confusion in the literature regarding primary and secondary microplastics. Different definitions have been adapted in the literature.⁵⁴ Secondary microplastics originate from fragmentation of large plastic particles once these large plastic particles are released in the environment. Microplastics originating from abrasion of large plastic particles during use or maintenance such as erosion of tires while driving or abrasion of textiles during washing have been considered as primary microplastics in a few studies.⁵⁴,⁵⁵ Table 1 summarizes typical examples of primary and secondary microplastic particles and the likely cause of their generation.

Table 1.

Examples of primary and secondary microplastics and cause of their occurrence in the environment

Microplastic particle	Type	Likely cause of occurrence in the environment	Reference
Microbeads (cosmetic products/cleaning products/personal care product)	Primary	Discharging effluents from WWTPs	¹²,²²,²⁴,⁵¹
Industrial abrasive/sandblasting media	Primary	Runoff from WWTPs	¹²,²⁵
Preproduction pellet	Primary	Accidental slippage during shipping, discharged wastewater	⁵⁶
Car tire dust	Secondary	Regular wear and tear	⁴⁰
Fragments of agricultural plastic	Secondary		⁴⁰
Microfibers from textile	Secondary	Runoff from WWTPs	²²,⁴⁰,⁵⁷
Littered plastic fragments	Secondary	Overtime degradation	⁴⁰
Fishing gear	Secondary	Careless handling	⁵⁶
Fragmented packing material	Secondary	Discarding in the land	⁵⁶
Plastic mulch fragments	Secondary	UV light degradation	¹²
Drink bottle fragments	Secondary	Discarding as municipal solid waste	⁵³
PVC pipe fragments	Secondary	Construction and demolition waste	⁵³

PVC: polyvinyl chloride; UV: ultraviolet; WWTP: wastewater treatment plant.

Form and shapes

Microplastics found in the environment are present in various shapes such as tablet-like, spherical, oblong, cylindrical, and disk shape. The shape of microplastics varies highly depending on the type of the degradation of the plastic, including biological, photodegradation, and weathering, and the type of force, such as the force of the current, wind, and sand, applied to the plastic during its handling or after-use.²⁵,⁵⁸ Any degradation changes the surface morphology, including micropores, creases, and fractures, such as conchoidal and linear cracks, on the surface of the plastic, which results in the formation of microplastic of different shapes.⁵⁹,⁶⁰ Ultraviolet radiation causes oxidative degradation of plastics.²⁵,⁶¹ Polyethylene plastic tends to break down more upon the stress from chemical and mechanical weathering.⁵⁸ Microplastics can be divided into five different categories according to their shapes³¹,⁴⁵,⁶²,⁶³ (Figure 1).

Fragments, defined as broken plastic particles with jagged edges.⁶⁰ The shape of microplastic fragments depends on the process of fragmentation and exposure time in the environment.⁴⁵ Fragments may have sharp edges or smooth edges, indicating recent or earlier fragmentation of a larger piece of plastic, respectively. Larger chunks are more elongated in shape, while smaller fragments become circular progressively. Highly eroded plastic fragments may also have surface scratches and abrasion marks, generated by physical breakdown⁶⁴ and particle−particle collision.⁵⁸

Films, defined as thin plane flimsy plastic such as a thin piece of material from wrappers or plastic bags that can show transparency.⁶⁰ They are mostly from polystyrene, high-density polyethylene, low-density polyethylene, and polypropylene.⁴⁵,⁵⁹

Foams/sponges, defined as lightweight plastic such as foam cups and packaging. However, sponges differ from foams by their porosity.⁶⁰ A higher percentage of foams collection was reported by Brown et al. from estuarine shorelines.⁴²

Fibers/lines, defined as thin or fibrous plastic with a length in the range between 100 µm and 5 mm and a width of at least 1.5 orders of magnitude shorter such as fibers from textiles and lines from fishing nets. They are another form of fragmented plastic. The majority of microplastics collected from tributaries,³¹ sandy beaches,⁵⁷ and sea coasts⁶² are reported microfibers/lines. Fragmented larger lines may create a lot of fibers/lines.⁵⁹

Pellets/beads, defined as spherical plastic, which is derived from preproduction plastics and personal care items. Pellets, mainly used as raw material for manufacturing plastics,⁶⁵ can be found in flat, ovoid, and disk shape.⁴⁵ Eroded pellets undergo some sort of weathering.⁶⁶ In the environment, pellets can be found in intact form and also in embrittled and degraded forms.⁶⁷ The size of pellets ranges from 1 mm to 6 mm in diameter.⁴⁵ They work as organic medium for absorbing persistent organic pollutants (POPs).⁶⁵ The prevalence of pellets is very high nearby industrial sites.⁶⁸ A considerable proportion of microbeads escape from cosmetic and personal care products (facial scrubs, shower gel, liquid soap, and toothpastes) and enter into the aquatic environment through a filtration system.³¹,⁶⁹ The sorption properties of rough microbeads for POPs is higher than the smooth microbeads.⁶⁹

Figure 1.

Different categories of microplastics: (a) fragment, (b) film, (c) foam, (d) fiber, (e) line, and (f) pellet.⁷⁰ Source: reproduced with permission from Elsevier, 2014, reference number 70.

Materials

Until recently, as their name suggests, microplastics designation was limited to objects that originate from synthetic polymers.⁵⁷,⁷¹ They include small plastic fragments, fibers, and granules usually of different chemical composition such as polyethylene, polypropylene, polyamide, polystyrene, polyester, polyurethane, and polyethylene terephthalate.²²,⁷² Conventionally, when analyzing collected samples of microplastics, careful removal of any fibers and other objects of natural origin is practiced and precaution is taken to avoid contamination from fibers of natural origin during handling and laboratory analysis of the samples.⁵⁷ However, recent studies have revealed the presence of a considerable number of micro-sized objects of natural origin, mainly fibers, such as natural fiber (cotton)^51,73 and manmade fiber of natural origin (rayon),⁷¹,⁷³,⁷⁴ along with true microplastics (Figure 2). Moreover, more and more researchers are pointing to the fact that microfibers of natural origin (e.g. cotton and rayon) could be problematic in the same manner as true microplastics (i.e. synthetic polymer-based), especially as a conduit for transfer of associated chemicals such as dyes and additives.⁴⁶,⁷⁵ Therefore, new approaches of designation are emerging because of the heterogeneous nature of the micro-sized materials present in the samples. In a recent study on microplastic ingestion by marine fish species, Peters et al.²⁷ considered manufactured products of natural origin also as microplastics whereas other studies preferred using the terms “microlitter,”⁵¹ “anthropogenic microlitter,”⁷⁴ or “small anthropogenic litter” (SAL)⁷⁶ rather than microplastics. Some researchers have also indicated that fibers that are blends of natural and synthetic materials and artificial fibers, which are manufactured by transformation of natural polymers, should also be designated as microplastics.⁷⁷

Figure 2.

Microscopic anthropogenic litter (MAL) including natural fibers extracted from gastrointestinal tracts of terrestrial birds.⁷⁴ Source: reproduced with permission from Elsevier, 2016, reference number 74.

Microfibers

Most abundant type of microplastics

Among different forms of microplastics found in the environment, the fibrous/filamentous form is predominant both in marine^28–30 and freshwater ecosystems.¹⁶,³¹ In a recent study, Ling et al.²⁸ evaluated seafloor concentrations in marine sediment samples from coastal and estuarine sites spanning pollution gradients across southeastern Australia. They reported the occurrence of 84% filaments of the total number of microplastics collected. Baldwin et al.³¹ published similar results for the freshwater system. The authors studied plastic debris in 29 Great Lakes tributaries. They reported that the concentrations of plastic particles ranged from 0.05 to 32 particles/m³ (median: 1.9 particles/m³, mean: 4.2 particles/m³). Ninety-eight percent of the sampled plastic particles were in the microplastic size range; of those, 72% were in the smallest size range (0.355–0.99 mm), and 26% were in the 1.0–4.75 mm size range. The most frequently occurring plastic particle type was fibers/lines, making up on average 71% of each sample. The majority of plastic particles categorized as fibers/lines were fibers rather than lines. Miller et al.⁶³ reported the presence of anthropogenic microfibers throughout the Hudson River, even in remote locations. From their investigation, they demonstrated that there could have been a flow of 200–800 million fibers through the Hudson River every day in June 2016. Yu et al.³⁰ observed the presence of microplastics of different shapes in sand samples collected from multiple coastal sites of the United States. They reported fibrous microplastic as the predominant type. Marine biota can easily take up micron-size fibrous particles. Leslie et al.⁷⁸ detected a very high concentration of microplastics (105 particles/g dry weight) in blue mussel, with microfibers as the dominant type. Another study reported also the predominance of microfibers of different polymer types in the extracts of intestines of three different commercial fish species from the natural estuarine environment (Figure 3).⁷⁹ Because the density of individual microplastic pieces has been concluded as one of the major consequential factors determining their distribution,²¹ the dominance of microfibers among other forms of microplastics found in the environment can be attributed to their low weight and greater potential for long-distance oceanic transportation than for heavier plastic particles.²⁸ Similar dynamics can be expected to come into play in the freshwater and terrestrial environments.

Figure 3.

Microfibers of different polymer types extracted from intestines of fish in natural estuarine environment: (a) polypropylene, (b) rayon, (c) polyester, (d) polyacrylonitrile, and (e) nylon.⁷⁹ Source: reproduced with permission from Elsevier, 2018, reference number 79.

Emerging focus on microfibers of natural origin

Research focus has concentrated on the potential of synthetic fibers (synthetic textiles) as a contributor to microplastic contamination; almost no systematic study has been reported on the adverse effects of microfiber pollution emanating from natural fiber textiles (e.g. cotton) and regenerated natural fiber textiles (e.g. rayon). Cotton and regenerated cellulosic fibers (mainly rayon) are the two important natural-based textile fibers. Nonetheless, cotton is the most important natural textile fiber in terms of production and consumption in the global textile fiber market and is second only to polyester, the predominant synthetic textile fiber. Some studies³⁰,⁷⁹ have shown that cellulosic fibers constitute a large proportion of microfibers found in the environment. Yu et al.³⁰ studied the occurrence and distribution of microplastics in the southeastern coastal sites in the United States in sediment samples. The study showed that manmade cellulosic fiber, rayon, comprised 68% of the total fibers tested (approximately half of the total samples). Because cellulose is considered biocompatible and presumably biodegradable,⁸⁰,⁸¹ cellulose-based microfibers may have been considered innocuous and the need for keeping them under scrutiny for potential risk posed by their release might have been overlooked. However, natural and regenerated natural-based textile fibers are exposed to a number of chemicals such as dyes and finishes during the textile manufacturing process. Nowadays, surface modification of textiles by different chemicals is also frequently performed to impart special functionalities.⁸² Therefore, once released in the environment, natural fiber-based textile microfibers may carry harmful chemicals similar to synthetic microfibers. Yet, those chemicals might be more readily released from cellulose fibers than from synthetic fibers because cellulose fibers are degraded faster than synthetic fibers once in the environment. The chemicals on the synthetic microfibers might be degraded due to various reasons before being actually released in the environment. Owing to these reasons, a few researchers have pointed to the need of placing importance on microfiber pollution issues for natural fibers as well.^83–85

Methods of detection of microfibers

Because microfibers are predominant among different types of microplastics (e.g. beads and fragments), common procedures that are applied for the detection of microplastics in general is discussed here. Common procedures employed for the detection of microplastics have also been able to detect natural-based microfibers such as cotton and rayon.⁷¹ Prior to analyzing collected samples (e.g. sediments, water columns, gastrointestinal tracts of biota, etc.) for occurrence of microplastics/microfibers, it is necessary to extract/isolate potential microplastic particles from the raw samples.⁴²,⁴⁶,⁷⁵,⁸⁶ The density separation method is usually used especially for sediment samples to separate lighter potential microplastic particles by floating them in saturated NaCl solution where heavier sand particles are sedimented at the bottom of the NaCl solution in a container.⁴⁵ Typically, for gastrointestinal samples extracted from biota, non-plastic organic matter is digested using a concentrated solution of hydrogen peroxide (30% H₂O₂) or potassium hydroxide (10% KOH).⁷⁵,⁷⁸,⁷⁹ Digestion treatment might also be applied to samples collected from the environment (e.g. water column) to digest suspended non-plastic organic matters based on the need to improve the visual identification of microfibers in later stages.⁴⁶ Once the raw sample is processed to isolate potential microplastics/microfibers as explained above, it is filtered to capture the potential microplastics/microfibers in the filters. Then, the retained particle in the filters are analyzed for identification of microplastics/microfibers.⁴⁵ Even though a few studies solely depend on visual methods, either by the naked eye⁴⁷,⁸⁷ or with the aid of light microscopes for the detection of microplastics/microfibers,⁷⁰,⁷⁸,⁸⁸ most of the recent studies combine visual detection methods with more sophisticated physical and chemical characterization techniques such as spectroscopic techniques (i.e. FTIR and Raman spectroscopy).⁷¹,⁷²,⁸⁶,^89–91 FTIR and Raman spectroscopy techniques help minimize the problem of false identification and underestimation of microfibers associated with the preliminary detection and identification by light microscopy.⁴⁵,⁴⁷ These spectroscopic techniques produce spectra of samples specific to particular functional groups or bonds, and when the spectra are compared with the reference spectra of the library, the microplastics identity (i.e. nylon, polyethylene, polystyrene, cellulose, etc.) is ascertained based on the match.⁵⁷ Nonetheless, some practical problems might be encountered during matching due to the fact that reference spectra in the library are typically generated from clean and ideal samples different from what is normally found in the environment.⁹¹ The use of spectroscopic techniques can also help identify associated chemicals on the microfibers such as chemical identity of the compound producing color.⁷¹ In addition to the use of FTIR and Raman spectroscopy, some studies have also used scanning electron microscopy (SEM) techniques.⁷¹,⁹³ Remy et al.⁷¹ reported that observing microfibers in the samples under SEM led to confirm the identity of cellulosic microfibers as regenerated cellulose fiber rayon not natural cellulose fiber cotton based on the difference in the morphology between rayon and cotton. Table 2 summarizes the methods employed for the detection and analysis of microfibers along the nature of the samples.

Table 2.

Techniques used for the detection and analysis of microplastics/microfibers

Type of sample	Method used for detection and analysis	Reference
Gastrointestinal material from sea macro-crustaceans	Optical microscopy, Raman spectroscopy, SEM	⁷¹
Gastrointestinal material from red mullets	Optical microscopy, Raman spectroscopy	⁸⁹
Sea sediment	Optical microscopy, FTIR, SEM	⁹³
Beach sediment	Optical microscopy, FTIR	⁹⁰
Beach sediment	Optical microscopy	⁸⁸
Lake shoreline sediment, water column	Optical microscopy	⁷⁰
Beach sediment	Visual inspection	⁸⁷
Sea surface water, beach sediment	Optical microscopy, FTIR	⁴⁷
Atmospheric fallout (dry and wet deposition)	Optical microscopy, FTIR	⁷⁷
Coastal sediment	Optical microscopy, FTIR	³⁰
Marine sediment, WWTP effluent, gastrointestinal material from marine invertebrate biota	Optical microscopy	⁷⁸
Gastrointestinal material from different commercial fish species	Optical microscopy, FTIR	⁷⁹
Lake water column, shoreline sediment	Optical microscopy, FTIR	⁴⁶
WWTP effluent	Optical microscopy, Raman spectroscopy	⁹⁴
River surface water	Optical microscopy, FTIR, SEM-EDS	⁷²
Harbor water column and sediment	Optical microscopy, FTIR	⁹²
Gastrointestinal material from marine fish	Optical microscopy, FTIR	⁷⁵
Deep sea sediment	Optical microscopy, FTIR	⁸⁶
Estuarine sediment	Optical microscopy, FTIR	⁴²
WWTP effluent	Optical microscopy, FTIR	⁹¹

EDS: energy dispersive spectroscopy; FTIR: Fourier transform infrared spectroscopy; SEM: scanning electron microscopy; WWTP: wastewater treatment plant.

Release of microfibers from textile products

Microfibers released from synthetic textiles could be one of the major contributors to microplastic pollution in the environment, especially in urban areas. Domestic sewers and WWTPs are considered as two main pathways of textile microfiber deposition in the environment.²²,³⁰

In one of the first studies of its kind, Browne et al.⁵⁷ suggested that an important source of microplastics deposition appears to be through sewage contaminated by fibers from washing synthetic fiber cloths. Synthetic textiles have been shown to shed numerous microfibers during conventional washing. During conventional washing of textile garments, thousands of microfibers are released into wash water.⁵⁷ The authors showed that a single polyester garment can shed more than 1900 fibers per wash with fleece being the most notorious one.⁵⁷ A more recent study conducted in Finland suggested an even bleaker picture. Researchers found that the shedding of microfibers from textile garments during machine washing was two to three orders of magnitude higher than the figure reported by Browne et al.³⁴ Polyester, acrylic, and polyamide represent the major fibers found in microplastics released as fibers during the washing of clothes.⁹⁵ Browne et al.,⁵⁷ based on samples collected globally from shoreline sediments, showed the predominance of polyester microfibers (56%) followed by acrylic (23%). Other important microfibers present were polypropylene (7%), polyethylene (6%), and polyamide fibers (3%). The study also showed that effluent discharged from sewage treatment plants contained polyester (67%), acrylic (17%), and polyamide (16%) microfibers. The resemblance in proportions of microfibers present between sewage effluent and sediments from disposal sites indicates that a large amount of microplastic fibers found in marine environments may have originated from sewage as a result of washing of clothes. However, recent studies found no correlation between the concentrations of microfiber particles and wastewater effluent.³¹,⁶³

Several subsequent studies have shown that a massive number of microfibers, both synthetic and natural based, are released from common textile garments during domestic and industrial laundering processes.²²,²³,^32–34 Even more interestingly, a study reported 3.5 times higher release of synthetic microfibers from textiles during tumble drying than during actual laundering (washing).⁹⁶ Napper and Thompson²³ tested three different fabrics that are commonly used to make clothes, namely polyester, polyester-cotton blend, and acrylic. The study estimated that, on average, more than 700,000 microfibers could be released per 6 kg wash load of acrylic fabric. De Falco et al.³³ also reached a similar estimation in their study with synthetic fabrics. According to their estimation, more than 600,000 fibers could be released in a typical 5 kg wash load of polyester fabrics depending on the type of detergent used. Sillanpää and Sainio³⁴ compared the release of microfibers during sequential machine washings with the wastewater from synthetic (polyester) and natural fiber (cotton) textiles. The number of microfibers released from polyester textiles in the first wash varied in the range from 2.1 × 10⁵ to 1.3 × 10⁷ per kilogram depending on the garments, whereas the number of released microfibers from cotton garments was between 3.6 × 10⁶ and 4.6 × 10⁶ per kilogram of garments during the first wash. The number of fibers released, in general, decreased in subsequent washing in both cases. The study also estimated the annual emission of polyester and cotton microfibers from household washing machines to the wastewater to be 154,000 (no. of fibers released: 1.0 × 10¹⁴) and 411,000 kg (no. of fibers released: 4.9 × 10¹⁴), respectively, in Finland with population of 5.5 million.³⁴ The estimation by Sillanpää and Sainio³⁴ of the number of microfibers released per unit mass of textiles as a result of washing is drastically larger than the estimation from other studies.²³,⁵⁷,⁹⁶ Discrepancies in the number of fibers released, as explained by Sillanpää and Sainio,³⁴ could be attributed to the use of filters with smaller pore size (0.7 µm) to collect the samples from the laundry wastewater. The smaller filter used by Sillanpää and Sainio would have retained smaller fibers. The majority of fibers, according to the study, were approximately 10–20 µm in thickness and 100–1000 µm in length and, therefore, part of them may pass through filters with pore sizes larger than 20 µm.³⁴ Yet, smaller size microfibers might have greater environmental implications, especially in the sorption and the transfer of harmful microorganisms and chemicals because the surface area per mass increases inversely with the particle size to the power of two. Carney Almroth et al.⁹⁷ sought to quantify the shedding of microfibers from three different synthetic textile knits (acrylic, nylon, and polyester) using different gauges and techniques during laundering. They used laboratory washing machines, which can mimic those used in households. This study confirmed the findings from Browne et al.⁵⁷ that polyester fleece shows a worst tendency to shed microfibers compared with other types of garments. Polyester fleece fabrics shed the greatest amounts, averaging 7360 fibers/m²/L¹ in one wash, compared with polyester fabrics which shed 87 fibers/m²/L.¹ However, similar to the findings of Sillanpää and Sainio,³⁴ this study points to the fact that the number of fibers released from a polyester fleece per wash according to Browne et al. might be an underestimation (e.g. one fleece garment could release 110,000 microfibers⁹⁷ versus 1900 microfibers⁵⁷).

Factors affecting the release of microfibers from textiles during laundering

It has now been documented that thousands of microfibers are released from textiles during washing and can potentially have serious environmental impacts. Studies have focused on microfibers from synthetic textiles as an important source of microplastics in the environment. Therefore, insights into some of the important factors that affect the release of fibers from textiles obviously contribute towards mitigation of the problem by minimizing the release of fibers from textiles in the first place. Although there are relatively few studies available that focus specifically on the release of microfibers from textiles during washing, studies so far have shown several factors that increase the release of fibers from textile products.

Shedding of fibers from a fabric can be affected by different variables such as fabric type, texture (open or compact), yarn type (staple fiber spun or filament spun), and nature of different fiber types involved.²² Moreover, during laundering, a complex set of interactions might come into play: for example, temperature, chemicals (detergents, conditioners, etc.), and mechanical stress employed.³³ Browne et al.⁵⁷ reported that the fabric type significantly affects the release of microfibers from textiles during laundering. The study was conducted with three different polyester textiles, namely blankets, fleece, and shirts, laundered using front-loading washing machines without detergent and conditioner and showed that fleeces released more than 180% of microfibers compared with other garments.⁵⁷ The propensity of polyester fleece to shed more fibers compared with other types of textiles has also been supported by recent studies.⁹⁷,⁹⁸

A study was conducted using three different synthetic fabrics (acrylic, polyamide, and polyester). The fabrics were repolished to simulate used clothes. Repolishing and use of detergent resulted in increased fiber loss from the fabrics during laundering.⁹⁸ Petersson and Roslund⁹⁹ studied shedding of microfibers from polyester fabrics and showed that the fabric structure was the most important factor determining the release of fibers from the fabrics. Fabrics with staple fiber yarns (microfiber yarn) released more microfibers than filament yarns. Similarly, higher gauge and aging resulted in more shedding and therefore amalgamation of these three factors should be avoided from environmental perspective.⁹⁹ Pirc et al.⁹⁶ conducted a study with polyester fleece blankets laundered using front-loading washing machines. The authors found no effect of detergent and softener on the release of microfibers and concluded that the mechanical stress is the major factor governing the fiber release.⁹⁶ Napper and Thompson²³ studied the effect of different combinations of factors on three different types of fabrics (100% polyester, 100% acrylic, and 65% polyester-35% cotton blend). They used two temperatures (30°C and 40°C), three detergent variables (no detergent, biodetergent and non-biodetergent), and two conditioner variables (conditioner and no conditioner). According to the study, the least amount of fiber released, in general, was observed with polyester-cotton blend. However, the effects of the temperature, detergent, and conditioner were less consistent and somewhat confounding.²³ Pilling, the formation of fiber balls as a result of entanglement of fibers on the fabric surface, is supposed to be one of the reasons for fiber loss during wearing and washing.¹⁰⁰,¹⁰¹ Although polyester is notorious for pilling, pills are less prone to be washed out because of the higher tenacity of the anchor fibers compared with cotton.¹⁰² Therefore, shedding of more fibers from 100% polyester fabric than from polyester-cotton blend as observed by Napper and Thompson²³ is somewhat counterintuitive and further research is needed to confirm the observed trend. However, if confirmed by future studies, the trend that polyester-cotton blend releases lower amounts of microfibers during laundering can potentially provide some mitigation measures against the release of microfibers by smart textile design.

Hartline et al.³² demonstrated that the age of the garments and the type of washing machine have significant effect on the amount of synthetic microfibers released. The authors reported that seven times more microfibers were released when laundered using a top-loading machine compared with a front-loading machine. The age of garments also resulted in a higher release of microfibers. Overall, 25% more fibers were released as a result of aging. In one of the most comprehensive studies available so far, regarding microfiber release from synthetic fabrics as a result of washing, Hernandez et al.²² considered different factors such as fabric structure (knitting variations) and washing conditions (use of detergents, detergent types, temperature variations, wash durations, and sequential washings). The results showed that the use of detergent increased the amount of microfibers released with no effect of detergent type (powder or liquid) and increased detergent dose. No other factors were found to significantly affect the release of microfibers. Unlike the results from other studies,²³,³⁴,⁹¹,⁹² which showed reduction in the amount of microfibers released over subsequent washes, Hernandez et al.²² reported on the steady release of microfibers in each successive wash and speculated that fiber staple length and/or fiber debris entrapped inside the fabric could be responsible for the release of fibers rather than the breakage of fibers from fabrics.²² However, De Falco et al.³³ reported a significant effect of detergent type on the release of microfibers from synthetic textiles. When powder detergent was used, a significantly higher amount of microfibers were released as compared with liquid detergent. The potential reason for the higher amount of microfiber release associated with powder detergent, as explained by the authors, might be due to the presence of inorganic compounds such as zeolite in powder detergent that could cause increased friction on the fabric surface. Another important finding by the authors was the reduction (up to 35%) in microfiber release when a softener was used.³³ This reduced amount of microfiber release was attributed to the softener’s role to reduce the surface friction of the fabrics and also to improve the fabric abrasion resistance.¹⁰⁰ However, previous studies reported no effects of softeners and conditioners during laundering.²²,⁹⁶ The study of Falco et al.³³ also showed that woven polyester fabric released a higher amount of microfiber compared with its knitted counterparts.

Regarding the factors affecting the release of microfibers from textiles during laundering, the relatively smaller number of available studies specifically focusing on textile microfibers coupled with, sometimes, conflicting findings make it difficult to establish a clear trend. For example, the use of detergent resulting in increased amount of microfiber release²²,⁹⁷ versus no effect of detergent;⁹⁶ the use of softeners resulting in remarkable reduction in the amount of microfiber release³³ versus no statistically significant effect of softener.²² Therefore, further studies, focused specifically on microfibers, are needed to provide unambiguous information on the dynamics of fiber release from textiles during laundering, which can ultimately serve as a guide to deal with potentially important sources of microfibers in the environment.

Role of WWTPs as potential source of microfibers

Microfibers released from textiles during machine washing reach WWTPs via domestic sewers. A typical process of municipal wastewater treatment comprises preliminary treatment, primary treatment, and secondary treatment.¹⁰³ An additional tertiary treatment process may be included in WWTPs as required at specific locations to protect human health and/or environmental quality¹⁰⁴ (Figure 4). Municipal WWTPs have been frequently suspected as significant sources of microplastics/microfibers through effluents. However, the precise role of WWTPs in the contribution to microfiber release has not been definitely stated. This might be due to the fact that several studies, conducted to evaluate the role of WWTPs as conduits of microplastics and microfibers to the environment, have shown that the existing wastewater treatment processes are highly effective in removing microplastics and microfibers.⁵¹,⁹¹,¹⁰⁵ A reduction of more than 98–99% of microplastics and microlitters in the influent after the secondary treatment in the conventional WWTPs has been reported.⁵¹,⁹¹,¹⁰⁶ However, because of the sheer volume of effluent discharged from the WWTPs, millions of microplastics are released in the environment from a WWTP on a daily basis.⁷⁶ Therefore, even an advanced WWTP that uses a tertiary treatment process may act as a significant source of microfibers in the environment.⁵¹,¹⁰³

Figure 4.

Different stages of wastewater processing in a typical wastewater treatment facility.¹⁰³ Source: reproduced with permission from Elsevier, 2016, reference number 103.

Murphy et al.⁹¹ conducted a study in Scotland, which claimed that it was the first to describe the fate of microplastics during the wastewater treatment process in a WWTP in detail. On the one hand, the study revealed high effectiveness of the modern WWTP in removing microplastics from the influent.⁹¹ On the other hand, despite being highly effective, the same WWTP can be an important source of release of microplastics to the environment. In their study, a modern and large secondary WWTP (designed to serve a city with population equivalent to 650,000) was sampled for microplastics at different stages of the treatment process to assess the detailed fate of microplastics in the wastewater influent at various points in the WWTP.⁹¹ The influent contained on average 15.70 (±5.23) microplastics per liter, which was reduced to 0.25 (±0.04) microplastics per liter in the final effluent, a decrease of 98.41%. However, despite this large reduction, the WWTP is releasing 65 million microplastics into the receiving water per day.⁹¹ One pertinent finding of the study conducted by Murphy et al. regarding textile microfibers was that the most common polymers found in influents were alkyds (28.7%), polystyrene-acrylic (19.1%), polyester (10.8%), polyurethane (8.9%), and acrylic (8.3%). However, the most common polymers found in the final effluents were polyester (28%) and polyamide (20%), followed by polypropylene (12%), acrylic (12%), alkyd (8%), polyethylene (4%), polystyrene (4%), and polyethylene terephthalate (4%).⁹¹ Polyesters and polyamides are primarily used in the manufacturing of textile products. Although it was not specified what percentages of polyesters and polyamides were in the fiber form, the polyester and the polyamide microplastics most likely originated from polyester and nylon textile garments. The predominance of polyester and nylon in the final effluent despite their lower abundance in the influent compared with other microplastics might suggest that synthetic microfibers manage to escape the WWTP in larger proportions compared with other forms of microplastics.

Mason et al.¹⁰³ conducted a similar study in the United States but on a much broader scale. Altogether 17 different WWTPs of varying sizes, populations served, advanced filtration types, and at multiple locations across the United States were studied.¹⁰³ Averaging all facilities and sampling dates, 0.05 ± 0.024 microparticles were found per liter of effluent. Significant inter- and intra-facility variation regarding the discharge concentration as well as relative proportions of particles was observed. Even though the concentration of microplastic particles, when expressed as the number of particles per liter, appear practically insignificant, the range of daily discharges of microplastic particles was from ∼50,000 up to nearly 15 million particles, averaging over 4 million microparticles per facility per day. On average, fibers (59%) were found to be the most common type of particle within effluents, followed by fragments (33%). Some fibers were suspected to be derived from non-plastic sources.

Talvitie et al.⁵¹ focused on microlitters (synthetic and natural) rather than microplastics. A large advanced WWTP (for a population equivalent to 800,000), which also included not so common treatment processes such as biological treatment (activated sludge) and biologically active filtration, was studied.⁵¹ The study showed high removal efficiency of microlitter from wastewater by the WWTP consistent with the results from previous studies of Murphy et al.,⁹¹ Carr et al.,¹⁰⁵ and Mason et al.¹⁰³ More than 99% removal efficiency was achieved after the secondary treatment process. However, the authors estimated that 2.0 × 10⁸ to 7.9 × 10⁸ microlitter per day and 1.7 × 10⁶ to 1.4 × 10⁸ microplastics per day were discharged into the Baltic Sea with effluents.⁵¹ Unlike findings from other studies, fibers were most efficiently removed and the percentage of the microfiber in the final effluent was surprisingly low (∼14%) compared with their very high abundance (∼70%) in the influent. The findings that 66% of the microfibers were cotton, linen, or wool, with cotton being the most predominant (44%) followed by polyester (33%), are also quite surprising for the WWTP that included advanced wastewater treatment processes such as biological treatment and biologically active filtration. Biological treatment processes and biologically active filters, in usual circumstances, would help remove natural fibers with higher efficiency. A relatively high abundance of cotton microfibers compared with polyester is surprising also because consumption of polyester in the global fiber market is disproportionately higher as compared with cotton (55% versus 27%).¹⁰⁷ It would have been more informative to know the percentages of synthetic fibers present in the influent. However, it might not be always feasible to chemically characterize all microplastics and microlitters in influents due to their large numbers. Spatial and temporal factors are shown to affect the relative abundance of microplastics in the effluent from a WWTP.¹⁰³ These factors might also affect the proportions of textile fibers entering into a WWTP and their subsequent release in the environment. Although polyester is the predominant textile fiber globally, local dynamics might be different.⁵⁴

Michielssen et al.⁷⁶ studied the fate of SAL (microplastics and other anthropogenic litter that has a similar size to microplastics) in the Detroit area in the United States in three WWTPs differing in wastewater treatment processes: secondary treatment (activated sludge), tertiary treatment (granular sand filtration) as a final step, and a pilot membrane bioreactor system that finishes treatment with microfiltration. Similar to other studies, SAL removal efficiency ranged from 95.6% for secondary WWTP to 99.4% for the membrane bioreactor plant.⁷⁶ Fibers were the predominant forms of SAL both in the influent and in the final effluent although both WWTP and the process employed affect their relative proportions. Although the WWTP with tertiary granular sand filtration and the membrane bioreactor exhibited greater overall removal of SAL, fibers represented a larger percentage of SAL in the effluent from these plants (79% and 83%, respectively) than the plant with activated sludge as a final step (44%).⁷⁶ This study by Michielssen et al. also points to the fact that fibers are more likely to be released in the effluent from the WWTP compared with other forms of microplastics (e.g. microbeads) or that SAL as microbeads were totally absent in the final effluent from the WWTP with the tertiary treatment process or with anaerobic membrane bioreactor system.⁷⁶ Based on their results, the authors suggested that as high as 9 billion fibers are released per day from one of the studied WWTPs. Estimates on the release of fibers via WWTPs to the environment based on other studies are similar and are summarized in Table 3.

Table 3.

Comparison of fiber retention rates for wastewater treatment plants across the world and varying scales, ordered by most fibers released.⁷⁶

Treatment plant	% fiber retention	Flow rate (m³/day)	No. of fibers released/day (billions)
Central WWTP of Vodokanal (St. Petersburg, Russia)	65.74	959,000	153.4
Detroit (Michigan, USA)	99.28	2,500,000	8.94
Seine Centre (Paris, France)	88.97	240,000	7.68
Viikinmaki (Finland)	92.33	270,000	3.73
Northfield (Michigan, USA)	97.38	1700	8.9 × 10^–3
Lysekil (Sweden)	99.96	5160	2.1 × 10^–5

Source: reproduced with permission from Royal Society of Chemistry via creative commons CC BY license, reference number 76.

These studies show that even with the near perfect efficiency (>99%) of WWTPs in removing microlitters (microfibers) from wastewaters, millions of microfibers are released per day to the aquatic environment through effluents because of the massive amount of wastewater a WWTP processes.⁷⁶ With technological advances, even when all of the microfibers are retained in WWTPs, which is practically impossible, they can still enter into the environment. Because microfibers are not readily decomposed by aerobic or anaerobic bacteria, microfibers intercepted in the sewage treatment plant will accumulate in sewage sludge.⁹¹ The sludge is later released back into the environment, for example if the sludge is used for agricultural purposes,¹⁰⁴,¹⁰⁸ or dumped into land or at sea.¹⁰⁹ Therefore, there is a very high potential of synthetic textiles as a source of microfibers in the environment.

Another important aspect worth considering may be the fact that the release of microfibers to the environment in developing countries might be more serious. Textile microfibers (synthetic) released from clothes during laundering represent a major key source of microplastics in the oceans in India, South Asia, and China.⁵⁴ A putative reason for this might be that WWTPs in these regions may not be as effective as reported for developed countries of Europe and North America in removing microfibers from influents. Furthermore, the portion of the population that has access to a WWTP system in this region is low especially in South Asia. Only less than one-third of the population has access to a wastewater treatment system globally. The amount of synthetic textile fibers originating from the laundering process that contribute to primary microplastics in the ocean is largest from India-South Asia and China (15.9%, 10.3% respectively) compared to North America-Europe and Central Asia (2.6 % and 4% respectively).⁵⁴ However, once in the sea, microplastics are transported around the globe by ocean currents; consequently microplastics have been found in almost every habitat around the world⁴³ and even in the most virgin environments.¹¹⁰

Potential hazards posed by microfibers

Adverse effect on aquatic environment

The wide dispersion of microplastics in the aquatic environment, from the surface water to sediment, has made an extensive range of aquatic biota occupying different habitats, including marine (sea, ocean)^111–113 biota and freshwater (river, lake, estuary) biota,¹¹⁴,¹¹⁵ susceptible to microplastic pollution. The consequences of microplastic exposure bring detrimental effects to aquatic organisms, both flora and fauna.^116–119 There are many studies to investigate the ecotoxicological impacts of microplastics on aquatic biota. Most of the studies regarding microplastic impact on aquatic biota are focused on marine biota.⁶,^111–113,¹¹⁹ Although freshwater ecosystems suffer a similar level of microplastic contamination, there are limited studies to evaluate the effect of microplastic pollution on freshwater fauna.¹¹⁴ The evidence of microplastic contamination has been found in a wide variety of phytoplankton and fauna.¹¹⁸,^120–122 There is a potential risk of transferring contamination into the aquatic food chain from microplastic ingestion by biotas.¹¹⁹ Mostly, the uptake of microplastics is accidental, because aquatic organisms cannot differentiate microplastic particles from natural preys.¹¹⁴

Microfibers have potential to have a toxic effect in waterways and subsequently in the food chain on a tremendous scale. As discussed above, owing to their relatively smaller size (100–800 µm), a significant number of microfibers escape the traps in the usual WWTPs and other industrial facilities and ultimately enter rivers, lakes, and oceans downstream. Once in the aquatic environment, a relatively smaller size of microfibers could be readily consumed by fish and other aquatic faunas. Indeed, recent studies have shown the presence of various microfibers (synthetic- and cellulose-based artificial fibers (rayon)) in the digestive tracts of fish and crustaceans.²⁷,⁷¹,^123–125 Compared to synthetic textile microfibers, artificial cellulose microfibers may not be an environmental issue in their pristine forms. Nonetheless, associated dyes or additives could potentially cause harmful effects to the aquatic fauna once ingested. However, synthetic microfibers are particularly problematic because fibers and associated chemicals have the potential to bioaccumulate, concentrating toxins in the bodies of larger animals higher up in the food chain.

Threat to marine biota

In a particularly relevant study regarding microfiber ingestion, Peters et al.²⁷ investigated the frequency of microplastic ingestion by six marine fish species collected from the Texas Gulf Coast. They analyzed a total of 1381 fish and reported that the frequency of microplastic ingestion was very high, specifically the ingestion of microfibers (86.4%). Microplastics are transferred to food chains by a range of organisms.¹¹¹,¹¹⁸,¹¹⁹ Setälä et al.¹¹⁹ showed the potential transfer of 10-µm polystyrene microplastics from one trophic level to a higher level. Microplastics can clog the gills or the glands of the digestive system of marine biota.¹²² Romeo et al.¹¹⁸ investigated the presence of plastic particles in the stomach of large pelagic fish. They found the presence of abundant mesoplastics (5–25 mm), macroplastics (>25 mm), and microplastics (<5 mm) in the stomachs of swordfish, bluefin tuna, and albacore, respectively. Van Franeker et al.¹¹¹ also showed the abundance of plastics in the stomachs of fulmars collected from different regions of the North Sea. In a five-year period of the analysis, on average 35 pieces of plastic particles were found in the stomachs of 95% of fulmars.¹¹¹

Smaller plastic items, such as microfibers, ingested by marine species cause physical impairments such as blockage in the gut.^126–128 The physiological process of marine worms can be disrupted even by ingestion of small quantities of microplastics.¹²⁹ The ingestion of microplastics by marine biota is associated with other physical hazards, including disruption of the endocrine system,¹³⁰ reproductive impairment,¹³¹ oxidative stress,¹¹³ and disturbance in the body functions including respiration, segregation of digestive enzymes, absorption of nutrients, and storage of energy reserves.¹²² Besseling et al.¹¹² studied the effect of bioaccumulation of POPs in polystyrene microplastics on the fitness of the marine lugworm. They reported that the bioaccumulation of POPs negatively affected the fitness of the lugworm by reducing its efficiency of energy assimilation. Brennecke et al.¹²² showed the presence of microplastics in different organs of tropical fiddler crab Uca rapax. This demonstrated that plastic particles can potentially harm marine invertebrates.

Threat to freshwater biota

Microplastics have morphological and physiological impacts on freshwater phytoplankton, such as inhibition of growth, decreased photosynthetic activity, damaged cell membrane, and distorted thylakoids.¹²⁰,¹³² Mao et al.¹²⁰ and Besseling et al.¹³² elucidated the negative impacts of microplastic exposure to freshwater algae, Chlorella pyrenoidosa and Scenedesmus obliquus, respectively, found in worldwide freshwater, to polystyrene microplastics.

The presence of significant amounts of microplastic particles in freshwater biota has been reported in different studies.¹²¹,¹³³,¹³⁴ A study by Hurley et al.,¹²¹ calculated the number of plastic particles ingested by a commonly found invertebrate, Tubifex worm, in freshwater systems. They detected 129 ± 65.4 microplastic particles/g in the tissue of Tubifex worm, 87% of which was reported to be microfibers, presenting the accumulation of a higher concentration of microplastics in tissue than other aquatic organisms. The retention time of the ingested microparticles in the gut was more prolonged than other non-plastic matrices of the sediment, posing a potential risk of transferring plastic contaminants to the aquatic food chain. The first evidence of freshwater fish contamination was reported by Sanchez et al.¹³⁴ They confirmed the ingestion of microplastic particles by a continental fish, Gobio gobio, found only in freshwater. From their finding, 12% of the collected fish had microplastic contamination. Faure et al. found microplastics in the digestive tract of almost all (eight out of nine) of the dissected birds collected from the shores of Lake Geneva.¹³³ They reported 4.3 ± 2.6 plastic particles per bird.

The toxic effects of polystyrene microplastic accumulation in zebrafish (Figure 5) have been studied by Lu et al.¹¹³ Their findings suggested that ingestion and accumulation of microplastics induced inflammation, lipid accumulation, and oxidative stress, and also caused disruption of metabolic profiles in the liver of fish. Lei et al.¹¹⁶ investigated the effect of different microplastics (polyethylene, polyamide, polyvinyl chloride, polystyrene, and polypropylene) on Danio rerio, a vertebrate fish, and Caenorhabditis elegans, a benthic nematode.¹¹⁶ They demonstrated the correlation of particle size on the lethality induced by microplastics. However, the toxic effects were independent of the composition of microplastics. The access of particles into different organs and the degree of accumulated particles were directly associated with a similar degree of primary damage in the intestine of both species. They observed cracked villi and split enterocytes in the vertebrate fish. They noticed the distended abdomen of D. rerio, owing to the extensive accumulation of microplastic particles, ultimately causing the death of D. rerio. They reported a change in the levels of intestinal calcium in C. elegans. Moreover, the increased expression of gene gst-4, indicating oxidative stress in C. elegans, depended on the ingested microplastic particle size. The highest lethality that occurred in C. elegans was reported due to the accumulation of 1-µm size particles in the intestine. Rehse et al.¹¹⁷ investigated the physical impact of size-dependent ingestion of microplastics on zooplankton, Daphnia magna.¹¹⁷ They reported ingestion of 1-µm polyethylene particles by zooplankton, leading to its immobilization with increased dose and time. However, they observed no ingestion for 100-µm particles, although 100-µm particles could attach to the carapax of D. magna with no immobilization.

Figure 5.

Representation of toxic effect of microplastic accumulation in zebrafish.¹¹³ Source: reproduced with permission from American Chemical Society, 2016, reference number 113.

Adverse effects on human health

Humans can be exposed to microplastics primarily through inhalation, ingestion, and dermal contact.¹³⁵,¹³⁶ Although direct passing of microplastics through human skin is obstructed, particles possibly can penetrate along two routes: hair follicles and sweat glands.¹³⁷ However, the route of skin contact is mostly associated with microplastic monomers and additives, such as bisphenol A (BPA) and phthalates, leached out from everyday-used products.¹³⁶

The consumption of sea products, such as mussels, oysters, and sea salt, is a significant route of microplastic exposure in humans and it has become a potential risk for food safety.^138–140 From the finding of preliminary annual dietary exposure, Van Cauwenberghe et al. reported a consumption of 11,000 microplastics yearly by European shellfish consumers.138 Microplastic contamination has been reported in commercial salts.¹³⁹,¹⁴⁰ Similar to plastic debris, microfiber pollution especially from synthetic fibers increases human exposure to both plastic particles and release of associated chemical additives from the particles. The uptake of plastics by humans can cause adverse health effects via three different possible means: particle toxicity, chemical toxicity, and pathogen and parasite vectors.^141–143 Even though the knowledge of the interaction of plastic particles with tissues and cells in humans is limited so far, studies have shown that exposure to plastic particles can cause lung and gut injuries in humans, and very fine particles can cross cell membranes, the blood–brain barrier, and the human placenta inducing oxidative stress, cell damage, inflammation, and impairment of energy allocation functions.¹²²,^144–146 Garrett et al.¹⁴⁷ proposed a pathway of the uptake of nano-sized drugs and its distribution in the human body. The ingestion of microplastics via water and food and subsequent circulation and bioaccumulation within the human body can be understood from the pathway shown in Figure 6.¹⁴⁵ Plastic particles can be taken up into the blood from gut through gut-associated lymphatic tissues, especially by microfold cells. From there, they can be translocated into the liver and gallbladder, followed by recirculation through bile to the intestines.¹⁴⁷ Moreover, microfibers may pose shape-dependent hazardous effects when ingested by organisms, which might be different from other forms of microplastics.¹⁴⁸

Figure 6.

Pathway of nanoparticle circulation within the human body.¹⁴⁵ Source: reproduced with permission from Springer Cham via creative commons CC BY license, reference number 145.

Inhaled microplastic particles interact with immune cells, causing inflammation due to the release of oxidative stress, protease, and cytokinin.¹⁴⁹ Chronic inflammation may be crucially involved with the adverse health effect by causing oxidative damage to DNA. It may promote cancer by forming malignant cells.¹⁵⁰

Microfibers and microplastics in the environment can also act as conduits in transferring dyes and other associated chemical additives.¹⁴¹ Many of these chemicals (e.g. BPA, phthalates, and some of brominated flame retardants) impact human health by disrupting endocrine functions.¹⁴⁴,¹⁴⁵ Very fine plastic particles, which generally carry chemical compounds, may cross cell membranes and enhance the chemicals’ bioavailability, similar to nano-sized polymeric drug delivery vehicles.¹⁴⁴ The negative consequences of BPA is very crucial for human health. The BPA chemical, known as an endocrine disruptor,¹⁵¹ is a common plastic additive, used industrially as protective coatings to manufacture food containers made of polycarbonate plastic.¹⁵² Therefore, it is quite common to ingest BPA-contaminated food. Human exposure to BPA is pervasive in the United States. Calafat et al.¹⁵² reported BPA detection in 92.6% of people, with a concentration range between 0.4 µg/L and 149 µg/L.¹⁵² BPA is related to the development of obesity. BPA disturbs the activity of alpha and beta receptors in fat tissues, ultimately changing the level of fat tissue hormones.¹⁵³ The development of heart disease and diabetes is associated with the disruption of the endocrine system due to BPA.¹⁵³ BPA brings an adverse effect on the cardiovascular system. A survey in the United States on 1493 adult people (aged 18–74 years) conducted by the National Health and Nutrition Examination Survey from 2005 to 2006 reported the association of the presence of BPA metabolites in urine with cardiovascular disease.¹⁵⁴ BPA can cause permanent epigenetic changes by modifying histone (DNA methylation, acetylation, chromatin remodeling), inducing a carcinogenetic effect in the cells.¹⁵⁵ The estrogen receptor activity is inhibited at the genomic level and extranuclear level by BPA.¹⁵¹ It results in impairing the functions of the hormone, which is crucial to protect the colon against the growth of cancer. Women exposed to BPA are at high risk of colon cancers. BPA may also enhance the possible risk of inducing breast and prostate cancer.¹⁵⁶

Microfiber particles may also act as a vehicle to carry pathogenic microorganisms and parasites. Harmful pathogens can form stable biofilms on the surface of microfibers when they come into contact in wastewater. When these microfibers enter water, pathogens in the fibers may be released in bathing or drinking water, resulting in human exposure and increased risk of infection. However, due to a lack of intensive studies, it is still impossible to fully comprehend the potential hazards posed to human health by consuming microplastic particles.

Conclusions and future outlook

Microplastic fibers (microfibers) are the most abundant microplastic forms found in the environment. Synthetic textiles are one of the major sources of these microfibers as they are released in massive numbers from common textile garments during laundering processes and are eventually released into the environment via sewage effluents and/or sludge. Once released into the environment, they are readily ingested by both aquatic and terrestrial organisms and enter into food chains. These microfibers have potentially hazardous effects through particle toxicity, chemical toxicity, and as conduits of harmful chemicals and microorganisms. Because per capita consumption of synthetic textiles is increasing, an increased amount of microfiber release can be expected. Due to the limited number of available studies specifically focused on microfiber release from textiles during the laundering process, factors affecting the release of microfibers are yet to be conclusively determined.

There is a lack of standardized methods and practices in sampling and in techniques used to identify and quantify microfibers. In addition, when dealing with textile samples for microfiber release, there is inherent variability among fabric samples. As a result, comparison between two different studies becomes difficult and inconclusive. Therefore, there is a need to establish standardized protocols and procedures for future studies.

Studies have shown the prevalence of microfibers compared with other forms of microplastics (fragments and particles) in both aquatic and terrestrial environments. The prevalence of microfibers is also the case among microplastic forms extracted from the bodies of different organisms. The higher frequency of ingestion of microfibers among other forms of microplastics needs to be ascertained to determine whether this is due to the abundance of microfibers in the environment or to the preferential ingestion of microfibers.

Potential hazards posed by microfibers might be different from other forms of microplastic particles. Studies have shown that nanoparticles behave differently when their shape is altered. However, studies dealing with this aspect of microfibers are non-existent. Future studies dealing with potential hazards of microfibers should take this aspect into consideration rather than generalizing all microplastics. Another important aspect for future research is that natural or natural-based microfibers should also be investigated. Studies have shown the presence of natural or natural-based microfibers in the environment as well as in digestive tracts of organisms. Rather than either overlooking them because of their reputation of being biodegradable and/or biocompatible or generalizing them as similar to synthetic microfibers, it is important that natural and natural-based microfibers are also objectively scrutinized for potential risks by their release in the environment.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The authors received no financial support for the research, authorship and/or publication of this article.

ORCID iD

Noureddine Abidi

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