Toxicological impacts of microplastic fibers: A review assessing risk to human and aquatic health

Abstract

Microplastics (particles less than 5 mm in size) are emerging contaminants that are widely distributed in the environment. Among the various morphologies, microplastic fibers (MPFs) are one of the most prevalent in environmental matrices and at multiple levels of biological organization. Most existing literature on the toxicological implications of microplastics on organisms utilizes morphologies that are not commonly recovered from the field (i.e. spheres or beads), therefore limiting our understanding of true toxicological concerns. Thus, this scoping review aimed to summarize and critically discuss the available data on the toxicological impact of MPFs, providing recommendations for future assessments based on the current knowledge gaps in the literature. The novelty of our review lies in identifying similarities across studies to better understand how laboratory approaches can inform toxicological outcomes. Our review found that most of the literature meeting our search criteria focused on work specific to aquatic and mammalian systems, with the latter yielding very few studies. Through the analysis, multiple knowledge gaps in MPF research emerged, such as MPF toxicity can be an artifact of the fiber dimensions (i.e. length or aspect ratio), and that subsequent leaching of additives from fibers may contribute to toxicity. Additionally, we found that there are varying responses to MPFs versus natural fibers, and a limited understanding of the organismal response to MPFs in the presence of other co-contaminants.

1. Introduction

The widespread occurrence of synthetic fibers in the environment is attributed to a global increase in synthetic textile fiber production of ~61 % from 2000 to 2020 (Surana et al., 2024). Additionally, in recent years, the production of polyester, a highly utilized synthetic fiber, increased from 57 million tonnes in 2020 to 61 million tonnes in 2021 (6.7 % increase) (Surana et al., 2024). Polyamide, also known as nylon, represents the second most widely used synthetic fiber, with its production volume increasing from 5.4 million tonnes in 2020 to 5.9 million tonnes in 2021, accounting for ~5 % of global fiber production (Surana et al., 2024). Other synthetic fibers, such as polypropylene and acrylics, make up ~5 % of the global fiber market. It has been projected that by 2030, the demand for fibers will increase by up to 2–3 % (Gschwandtner, 2022). The increased demand for fibers, specifically synthetic ones derived from plastic polymers, poses a threat to environmental and human health.

Due to the increased production of plastics, their widespread use in products, and inadequate waste management, microplastics (MPs) have become abundant environmental contaminants (Nelms et al., 2018; Rahman et al., 2021), and exist in a variety of shapes, including fibers, fragments, beads, films, and filaments (Weinstein et al., 2016; Carney Almroth et al., 2018; Rahman et al., 2021). Microplastic fibers (MPFs) that originate from the degradation of macroplastics and release from synthetic textiles represent a major source of MPs in the environment, where they are the predominant form in marine and freshwater habitats (Gago et al., 2018; Kwak et al., 2022). Murphy et al. (2016) reported that wastewater treatment facilities can introduce up to 65 million MPFs into receiving water daily. MPFs can also be transported and deposited in the air, allowing these particles to enter a wide range of environments and interact with different trophic levels of biological organization, with atmospheric concentrations in urban, rural, coastal, and remote areas as high as 5.83 × 10⁴ particles m⁻³, 5.83 × 10⁴ particles m⁻³, 451 particles/m⁻³, and 0.146 particles/m⁻³ (Tatsii et al., 2023; Li et al., 2025), and median indoor air concentrations of 528 particles/m⁻³ (Yakovenko et al., 2025).Over the past 20 years, laboratory studies have primarily utilized beads or spheres to assess MP toxicity to organisms, which have predominantly been performed in aquatic species, mostly marine (Thompson et al., 2024; Gray and Weinstein, 2017; Chisada et al., 2021; Gambardella et al., 2017; Magni et al., 2019). While such studies do provide important information regarding MP-induced toxicity in biota, multiple reports have highlighted that the use of commercial MPs (e.g., beads or spheres) has limited environmental relevance as fibers are the predominant shape of MPs present in the environment (Athey and Erdle, 2022; Deng et al., 2020; Zhu et al., 2021).

Furthermore, biological organisms have been shown to contain a higher abundance of MPFs compared to other shapes (Kutralam-Muniasamy et al., 2020; Gray et al., 2024, Weinstein et al., 2022). These data suggest that MPFs are a high-priority type of MP to better understand exposure and health risk at different levels of biological organization. MPFs can interact with cells and tissues differently than spheres, fragments, or films (Allegri et al., 2016; Wieland et al., 2022), leading to a shape-specific toxicity that needs to be better understood.

Concerns about the effects of MPs on human health have started to spread as MPs are being detected in various human biological fluids and tissue samples (Barceló et al., 2023). Additionally, the widespread identification of fibrous MPs in air (O’Brien et al., 2023) and food (Mamun et al., 2023) has raised concerns about human inhalation and ingestion of such contaminants, respectively. For example, recent reports have revealed that humans can inhale up to 68,000 MPs per day in the 1 μm–10 μm size range in indoor air (Yakovenko et al., 2025). The well documented adverse health effects associated with inhaled particles <2.5 μm (e.g. PM2.5) suggest that inhalation exposures are a critical route of exposure to consider. Despite the body of literature on MPs and adverse health outcomes, the effects of MPFs on human health remain poorly understood, as laboratory studies (i.e. in vivo and in vitro) have largely focused on exposure to beads (Çobanoğlu et al., 2021; Dong et al., 2020; Goodman et al., 2021; Mattioda et al., 2023; Wu et al., 2020). Furthermore, parameters like size, shape, and chemicals adsorbed to MPs may influence disease outcomes (Prata, 2018; Wieland et al., 2022); however, the impact of these factors on MPF toxicity is not yet well understood. Over the years, non-plastic fibers (i.e., asbestos) have been robustly studied due their negative impact on human health, highlighting a potential role for MPFs in the progression of fiber-associated diseases such as fibrosis, chronic obstructive pulmonary disease (COPD), and cancer (Wieland et al., 2022). Therefore, a better understanding of MPF toxicity on human health is crucial as MPF exposure may lead to detrimental long-term health effects.

The goal of this review is to critically assess existing knowledge on the toxicological and human health risks associated with MPF. The specific objectives include identifying a) the MPF polymer type most used in toxicity studies, b) the various methodologies used to create experimental MPFs, c) role of fiber shape or aspect ratio in toxicity, d) the influence of MPF additives in toxicity studies, e) differences between MPF and natural fibers, and f) MPF toxicity outcomes in studies that compared particle shapes such as spheres, beads, or fragments. The primary outcome of this review is to provide researchers with context about toxicological studies utilizing MPFs and how endpoints measured can be influenced by the design.

2. Methods

Literature Search:

This scoping review was performed to analyze MPF studies where toxicty effects were assessed. Studies were identified in PubMed and Scopus from the past five years (January 1, 2019 - April 3, 2024) utilizing abstract, title, and keywords. The search words included (“microplastic fiber*” OR “fibrous microplastic*” OR “microplastic” OR “microfiber*”) AND (“human health effect*” OR “health” OR “toxicity” OR “toxic effect*” OR “toxicological” OR “toxic” OR “disease”). A total of 1054 studies were imported into the Covidence database, and 802 studies were identified for screening after the removal of duplicates. Screening for inclusion of the remaining 61 studies was based on the reading of abstracts and then full-text papers to determine (a) if the study utilized MPFs (b) if the study assessed any toxicity endpoint., Studies were excluded that only focused on other MP forms (i.e., spheres, beads, fragments), only investigated natural fibers, only evaluated bioaccumulation, occurrence and distribution in the environment as an endpoint. This latter point removed 9 papers that primarily used organisms such as house cricket, algae, terrestrial worms and snails, etc., leaving only papers that focused on human and aquatic systems (i.e., utilized aquatic species, human cell lines, and/or mammals (i.e., mice) as the model organisms)., . Literature/systematic reviews, metaanalyses, epidemiological studies, editorials, book reviews/chapters, perspectives, opinion/commentary pieces, conference abstracts, and grey literature were also excluded. in. Following the inclusion and exclusion criteria, 41 final papers were subjected to review (Fig. 1).

Fig. 1.

Flow diagram detailing the framework utilized for identifying articles to include (n = 41) in the scoping review. The review process is represented by four steps, including identification, screening, eligibility, and final papers included. The number of papers included and excluded at each stage is depicted.

Data analysis:

Numerical data was analyzed using descriptive statistics (including percentages and number of papers). GraphPad Prism Software Version 7.0 (San Diego, CA, USA) was used to create the graphics.

3. Study characteristics

3.1. Test species/models

Of the identified studies, 95 % (39/41) evaluated MPF toxicity on aquatic health, while only 5 % (2/41) studied MPF toxicity on human health by utilizing human organoids and/or mammals (i.e., mice) (Fig. 2a) (Table 1). All studies but one were laboratory experiments. Of the aquatic studies, 56 % (22/39) utilized invertebrates as their model organism (Fig. 2b) (Table 2), and 44 % (17/39) utilized vertebrate species (Fig. 2c) (Table 3). Of the aquatic invertebrates, mussels, specifically Mytilus galloprovincialis (23 % (5/22)) and Mytilus edulis (14 % (3/22)), were the most tested species, followed by Daphnia magna and Daphnia carinata (18 % (4/22)). Of the aquatic vertebrates, Danio rerio (35 % (6/17)) was the most commonly used species, followed by Oryzias latipes (24 % (4/17)) and Carassius auratus (12 % (2/17)). Thus, these findings demonstrate that there is a disparity in research between aquatic and human models.

Fig. 2.

Overview of the literature from our review. (A) Number of review papers addressing aquatic or human health topics. (B) Studies reviewed broken down by aquatic invertebrate species. (C) Papers reviewed that utilized aquatic vertebrate species.

Table 1.

Summary of the experimental parameters and results of n = 2 papers included in the scoping review that studied MPF toxicity using human cell lines.

Citation (Country)	Species/Models	Polymer Type	Method used to make fibers/Origin of fibers	Microplastic Fiber Dimensions	Comparison to other microplastic shapes (i.e. beads) or natural fibers?	Dose/Concentration	Duration of Exposure	Effects/Outcomes
Winkler et al. (2022) (Italy)	Human airway organoids	Polyester	Collected fibers from the filtered exhaust air of a tumble dryer after the drying of synthetic fabrics	Mean length of 700 ± 400 μm and mean diameter of 10 ± 5 μm (AR: ~70:1)	No	1, 10, and 50 μg/mL	17 days	Organoid growth was not inhibited, nor was there a significant change in the expression of oxidative stress-related genes.
Song et al. (2024) (Netherlands)	Lung organoid cultures from C57BL/6 mice, human lung organoids, and human primary bronchial epithelial cells cultured at an air-liquid interface	Nylon and polyester	Fibers were purchased from Goodfellow and cut with a cryotome (cited Cole, 2016)	Polyester: median length of 52 μm and median diameter of 15 μm. Nylon: median length of 31 μm and median diameter of 12 μm (AR: ~3:1)	No	2000–5000 fibers/well corresponding to 16–39 μg/mL for nylon and 49–122 μg/mL for polyester	7, 14 and 21 days	Nylon, more than polyester, inhibited developing airway organoids (largely mediated by components leaching from nylon). Transcriptomic analysis revealed Hoxa5 gene may play a role in nylon toxicity.

Abbreviations: Microplastic fiber (MPF); Aspect ratio (AR).

Table 2.

Summary of the experimental parameters and results of n = 22 papers included in the scoping review that studied MPF toxicity on invertebrates.

Citation (Country)	Species/Models	Polymer Type(s)	Method used to make fibers/Origin of fibers	Microplastic Fiber Dimensions	Comparison to other microplastic shapes (i.e. beads) or natural fibers?	Dose/Concentration	Duration of Exposure	Effects/Outcomes
Choi et al. (2021) (South Korea)	Mediterranean mussel (Mytilus galloprovincialis)	Polyethylene terephthalate	Thread purchased from commercial store and cut into fibers using a cryotome (cited Cole, 2016)	Short fibers (mean length of 44.8 ± 19.9 μm and diameter of 13 μm) and long fibers (mean length of 118.3 ± 66.6 μm and diameter of 13 μm) (AR: ~2–9:1)	No	0.5 μg/L and 100 mg/L	4 days	Increased AChE, and apoptotic and necrotic hemocytes in exposed mussels
Choi et al. (2022) (South Korea)	Mediterranean mussel (Mytilus galloprovincialis)	Polyethylene terephthalate	Thread purchased from commercial store and cut into fibers using a cryotome (cited Cole, 2016)	Mean length of 106.5 ± 74.8 μm and diameter of 13 μm (AR: ~8:1)	No	0.0005, 0.1, 1, 10 and 100 mg/L	32 days	Decreased estradiol and testosterone, and increased activities of antioxidant-related and neurotoxicity-related enzymes in the digestive gland and gill tissues of mussels. Increased apoptosis and DNA damage in a dose-dependent manner.
Pittura et al. (2022) (Italy)	Mediterranean mussel (Mytilus galloprovincialis)	Polyester	Prepared from coloring-and surface treatments-free standard fabrics using a Retsch mill	Mean length of 618 ± 367 μm and diameter of 13 ± 1 μm (AR: ~48:1)	Polyamide fibers: (566 ± 500 μm in length and 11 ± 1 μm in diameter). Cotton fibers: (412 ± 342 μm in length and 16 ± 4 μm in diameter)	50 fibers/L	14 days	Synthetic MPFs impacted immune and antioxidant responses more than natural fibers. MPF exposed mussels were observed to be more susceptible to heat stress postexposure.
Auguste et al. (2023) (Italy)	Mediterranean mussel (Mytilus galloprovincialis)	Polyester	Cryo-milling of polyester fleece blanket	Mean length of 228.6 ± 185.5 μm and mean diameter of 28.3 ± 6.7 μm (AR: ~8:1)	No	In vitro: 0.5 mL hemolymph incubated with MF suspensions 50 and 100 μg/mL in artificial seawater In vivo: 10 μg/L (150 fibers/L/mussel); 100 μg/L (1500 fibers/L/mussel) in artificial seawater	In vitro: 30 min to 4 h In vivo: 24–96 h	In vitro observations included cell rounding, clumping, and instability, with NO production and no significant ROS production. In vivo showed significant effects of NO and ROS at the lowest concentration.
Mai et al. (2023) (South Korea)	Mediterranean mussel (Mytilus galloprovincialis)	Polyethylene terephthalate	Not discussed	200–400 μm range (AR: unknown)	Polyethylene microspheres (27–32 μm)	1, 10 and 100 mg/L	21 days	Exposure of MPFs exaggerated the level of morphological abnormalities, DNA fragmentation, and transcriptional activities modification in dose dependent manner.
Christoforou et al. (2020) (United Kingdom)	Blue mussel (Mytilus edulis)	Nylon	Cut using a cryotome (cited Cole, 2016)	Mean length of 35.20 ± 12.9 μm and diameter of 10 μm (AR: ~3:1)	No	24,000 fibers/L	52 days	Impacted phytoplankton removal capacity by mussels over time; higher MPFs in digestive tract associated with higher algae consumption.
Cole et al. (2020) (United Kingdom)	Blue mussel (Mytilus edulis)	Nylon	Fiber was purchased from Goodfellow and cut using a cryotome (cited Cole, 2016)	Length of 30 μm and diameter of 10 μm (AR:~3:1)	Polystyrene nanoparticles: (50 nm diameter). Polystyrene beads: (20 μm diameter)	500 ng/mL	24 h or 7 days	No significant effect to SOD levels, genotoxicity or DNA damage; aspect ratio did not influence toxicity.
Collins et al. (2023) (Untied States)	Blue mussel (Mytilus edulis)	Nylon	Nylon thread (Coats & Clark® Extra Strong Nylon Upholstery Thread) was purchased and cut using a razor blade	Length of 500 μm and diameter of 30 μm (AR: ~16:1)	Spartina spp. Particles (length of 250–500 μm)	50 and 100 particles/L	21 days	No effect of MPFs on gut microbiome or digestive tissues. Gut microbial communities of mussels exposed to nylon MPF did not differ from those exposed to Spartina spp. Particles.
Kim et al. (2021) (South Korea)	Water flea (Daphnia magna)	Polyethylene terephthalate and polypropylene	Fibers were obtained from the Korea Institute of Industrial Technology (KITECH; Ansan, Korea) and cut using micro-scissors	Polyethylene terephthalate: Mean length of 120.49 ± 82.46 μm and diameter of 3.73 denier. Polypropylene: Mean length of 134.83 ± 88.70 μm and diameter of 1.5 denier	Lyocell fibers (mean length of 141.04 ± 104.39 μm and diameter of 1.2 denier)	1000 and 2,000 mg/L	48 h	Synthetic fibers had a greater impact on immobilization than natural fibers. Natural fibers had greater impact on mortality, growth inhibition, and gut damage post-exposure.
Tourinho et al. (2022) (Czech Republic)	Water flea (Daphnia magna)	Polyethylene terephthalate	Cut using centrifugal mill	Mean length of 366 ± 275 μm and diameter of 14 ± 3 μm (AR: ~26:1)	No	100 mg/L	48 h; co-exposure to silver nanoparticles and silver nitrate	Increased energy consumption in daphnia exposed to silver nitrate alone or co-exposed to MPF and silver nitrate. MPFs alone did not affect survival or mortality of daphnia and did not increase acute toxicity of silver nitrate.
Lee et al. (2023) (South Korea)	Water flea (Daphnia magna)	Polyester	Cut using a cryotome (cited Cole, 2016)	Length of 6 μm (AR: unknown)	No	10 and 100 particles/L	24 h; co-exposure involving freshwater acidification	Co-exposure to fibers and acidification induced synergistic adverse effects at physiological and molecular levels. Changes in microbiome suggested changes to fitness.
Jiang et al. (2023) (China)	Water flea (Daphnia carinata)	Polyethylene terephthalate	Fibers were purchased from Huixiang Fiber Co., Ltd. (Changsha, Hunan) and cut using a cryotome	Mean length of 58.89 ± 7.62 μm and mean diameter of 20 ±4 μm (AR: ~3:1)	No	50 and 500 fibers/mL	7 days	Induced mitochondrial damage and apoptosis, elevated ROS levels, and altered activity of antioxidant enzymes.
Klasios et al. (2024) (Canada)	Zooplankton (mixed mesocosm)	Polyethylene terephthalate	Cut red sewing thread using a scalpel and surgical blade	Length of 1–1.5 mm and diameter of 0.015 mm (AR: ~67–100:1)	No	10 and 50 fibers/L	12 weeks	No impact on zooplankton community composition, abundance, or diversity.
Esterhuizen et al., 2022 (Germany)	Yellow mini clam (Corbicula javanicus)	Polyester	Black fleece jackets were washed and particles filtered and collected	No definitive size or aspect ratio reported	HDPE fragments (5 mm - 1 μm)	81,000 fibers/L	24 h	Exposure to polyester fibers resulted in increased enzyme activities, while exposure to HDPE resulted in decreased enzyme activities. Yellow polyethylene fragments significantly impacted CAT and GST activity compared to red and blue.
Setyorini et al. (2021) (Germany)	Harlequin fly (Chironomus riparius)	Polyethylene terephthalate	Fibers were purchased from Goodfellow and cut using a cryotome (cited Cole, 2016)	Length of 50 μm and diameter of 14 μm (AR: ~4:1)	No	500 particles/kg, 5000 particles/kg, and 50,000 particles/kg dry weight sediment	28 days	No significant effect on the time until emergence of larvae, or on the weight and head capsule lengths in the organisms exposed to MPFs. No significant change in heat shock protein 70 levels between control and exposure groups in the same life stage.
Kim et al. (2021) (Korea)	Brine shrimp (Artemia franciscana)	Polypropylene and polyethylene terephthalate	Fibers were supplied by the Korea Institute of Industrial Technology and cut using microscissors	Polypropylene: Mean length of 182.76 μm and diameter of 22.40 μm (AR: ~8:1). Polyethylene terephthalate: Mean length of 234.43 μm and diameter of 19.32 μm (AR: ~12:1).	Lyocell fibers (mean length of 259.29 μm and diameter of 14.31 μm)	75, 125, 250, 500, and 1000 mg/L	48 h	Gut damage occurred in all exposure groups of synthetic and natural MPFs; however, the most severe damage occurred in PET exposed organisms.
Mohsen et al. (2021) (China)	Sea cucumber (Apostichopus japonicus)	Polyester	Threads were obtained from a local store and cut using scissors	Mean length of 0.87 ± 0.4 mm (AR: unknown)	No	2 fibers/mL	0.5, 2, 4, 8, and 16 h	Top genes dysregulated by MPF exposure related to metabolic processes and signal transduction pathways. Histology of respiratory tree revealed injury and loss of cell components following MPF exposure.
Di Natale et al. (2022) (Italy)	Sea urchin (Paracentrotus lividus) embryos	Polyamide	Fibers were obtained from 100 % woven fabric and milled with liquid nitrogen using a Retsch mill	Mean length of 618 ± 367 μm and mean diameter of 13 ± 1 μm (AR: ~48:1)	Polystyrene particles (mean size of 1.01 μm)	0.5 mg/mL	22 days	Co-contaminants and MPFs altered transcriptional profiles in developmental related genes in embryos.
Détrée et al., 2023 (France)	Oyster (Crassostrea gigas)	Polyester, acrylic, and nylon	Long filaments cut using a cryotome (cited Cole, 2016)	The average length and diameter of all microfibers produced were 100 ± 55 μm and 16 ± 5 μm, respectively (AR: ~6:1)	Natural fibers (wool, cotton, and organic cotton)	0, 10, 100 fibers/L or leachate in aquaria	3 days	No dose-dependent effects on biomarker activity or expression were observed for natural fibers, synthetic fiber exposures, or leachate. Digestive and inflammatory responses were higher from natural MPF exposure compared to synthetic.
Harikrishnan et al. (2024) (India)	Polychaete worm (Hydroides elegans)	Heterogenous mixture: polypropylene, polyurethane, polyester, polyacrylonitrile	Commercial surgical facemasks were purchased, soaked in fresh filtered seawater, and shaken continuously using a mechanical shaker for 24 h to 120 days	<20 μm (AR: unknown)	No	50 fibers/mL	20 min	MPFs produced adverse effects during embryogenesis, slowed down mitotic cell division and significantly postponed the time of larval hatching. Fertilization rate decreased and delayed early embryonic development in eggs.
Kim et al., (2024) (South Korea)	Disk abalone (Haliotis discus hannai)	Polyester	Obtained polyester textiles from Busanjin Market (Busan Metropolitan City, Korea) and cut using a cryotome (cited Cole, 2016)	0–500 μm (AR: unknown)	No	0, 10, and 100 fibers/L	72 h; co-exposure to BPA	Continuous accumulation of BPA and MPF and persistent oxidative stress observed in abalone body. Combination exposure elicited a higher impact on cellular toxicity.
Iwalaye and Maldonado et al., 2024 (Canada)	Marine amphipod (Cyphocaris challengeri)	Polyethylene terephthalate	Blue fleece fragmented with scissors and blended in Milli-Q water using a kitchen blender	Mean length of 357.21 μm and mean diameter of 14.71 μm (AR: ~24:1)	No	10, 100, 1000, 10,000 and 50,000 fibers/L (i.e., 0.013, 0.135, 1.341, 13.406 and 67.02 mg/L)	24, 48 and 72h	Feeding was impacted, but not survival.

Abbreviations: Microplastic fiber (MPF); Aspect ratio (AR); Hours (hrs); High-density polyethylene (HDPE); bisphenol A (BPA); Nitric oxide (NO); Acetylcholinesterase (AChE); Catalase (CAT); Glutathione (GST); superoxide dismutase (SOD).

Table 3.

Summary of the experimental parameters and results of n = 17 papers included in the scoping review that studied MPF toxicity on vertebrates.

Citation (Country)	Species/Models	Polymer Type(s)	Method used to make fibers/Origin of fibers	Microplastic Fiber Dimensions	Comparison to other microplastic shapes (i.e. beads) or natural fibers?	Dose/Concentration	Duration of Exposure	Effects/Outcomes
Qiao et al. (2019) (China)	Zebrafish (Danio rerio)	Polypropylene	Cut using a cryotome (cited Cole, 2016)	Length of 20–100 μm and diameter of 20 μm (AR: ~1–5:1)	Polystyrene beads: 15 μm. Polystyrene fragments: 4–40 μm	10 μg/L (680 fibers/L)	21 days	MP fibers resulted in more severe intestinal toxicity (i.e., mucosal damage, and increased permeability, inflammation and metabolism disruption) than fragments or beads.
Zhao et al. (2021) (China)	Zebrafish (Danio rerio)	Polypropylene	Cut using a cryotome (cited Cole, 2016)	Two different lengths (50 ± 26 μm and 200 ± 90 μm) and diameter of 20 μm (AR:~3:1 and ~10:1)	No	10 and 100 μg/L	2 days (larvae) and 21 days (adults)	Increased oxidative stress, inflammation, and lipid depletion in a length-dependent manner.
Cheng et al. (2021) (China)	Zebrafish (Danio rerio)	Polyethylene terephthalate	Provided by Yineng Plastic Co. (Dongguan, China)	Length of 3–5 mm and diameter of 20 μm (AR: ~150–250:1)	PET granular particles (150 μm in diameter)	20 mg/L; co-exposure to cadmium	24, 48, and 72 hpf	Increased heart rate and blood flow, and inhibited hatching in embryos exposed to both MP particles and fibers. Cd more strongly adsorbed to PET particles over fibers.
Zhang et al. (2023) (China)	Zebrafish (Danio rerio)	Polyacrylonitrile	Fibers were purchased from Huixiang Fiber Co., Ltd (Changsha, Hunan, China) and cut using a cryotome (cited Cole, 2016)	Mean length of 65 μm and diameter of 10–15 μm (AR: ~4–7:1)	No	10 μg/L (600 fibers/L), 100 μg/L (6000 fibers/L), 1 mg/L (60,000 fibers/L), and 10 mg/L (600,000 fibers/L)	7 dpf	Increased intestinal ROS levels, mitochondrial quantity, apoptosis, and disrupted lipid metabolism.
Logan et al. (2023) (United States)	Zebrafish (Danio rerio)	Polyethylene terephthalate	Cut by electrospinning followed by cryosectioning (cited Cole, 2016)	Mean length of 13 ± 7 μm and mean diameter of 207 ± 10 nm (AR: ~63:1)	No	4 μg/mL	24 to 48 hpf	Increased apoptosis during embryogenesis, impaired vascular basement membrane assembly, increased neutrophil counts, and increased expression of pro-inflammatory cytokines.
Missawi et al. (2024) (Belgium)	Zebrafish (Danio rerio)	Polyethylene terephthalate	Fibers purchased from Goodfellow and cut using a cryotome (cited Cole, 2016)	Length of 5–15 μm (AR: unknown)	PET fragments (5–15 μm)	1000 μg/L	48 and 96 hpf concomitant with Aeromonas salmonicida challenge at 2 hpf	MPs alone induced earlier embryo hatching and did not alter heart function; however, a combined exposure of MPs + bacteria significantly slowed down the rate of hatching and increased heart rate. Fragments were more detrimental than fibers on developmental parameters, while bacterial presence damaged body length, eye, and yolk sac surface area.
Hu et al. (2020) (China)	Japanese Medaka (Oryzias latipes)	Polyester and polypropylene	Commercially dyed green polyester thread and transparent polypropylene fibers were purchased from a supermarket (Shanghai Qinhe, China) and cut using micro-scissors	Polyester: Mean length of 350 μm and diameter of 10–20 μm (AR: ~23:1). Polypropylene: Mean length of 380 μm and diameter of 50–60 μm (AR: ~7:1)	No	1000 fibers/L and 10,000 fibers/L	21 days	Increased egg production in females exposed to PP, and increased body weight in males exposed to either MP type. No impact on embryonic mortality, development, or hatching.
DiBona et al. (2021) (United States)	Japanese Medaka (Oryzias latipes)	Polyethylene	Lumat USA provided blue multifilament polyethylene yarn that was cut with a microtome (cited Cole, 2016), or with a paper cutter, ruler, and scalpel	Length of 100 μm (larvae) and 400 μm (juvenile) (AR: unknown)	No	0.5, 1.5, 3, and 6 fibers/fish/day	21 days	Altered expression of genes related to digestion and nutrient uptake. Proteobacteria abundance was decreased in larval samples and increased in juvenile samples.
DiBona et al. (2022) (United States)	Japanese Medaka (Oryzias latipes)	Polyethylene	Lumat USA provided blue multifilament polyethylene yarn that was cut using a paper cutter, ruler, and scalpel	Length of 400 μm (AR: unknown)	No	0.5, 1.5, 3, and 6 fibers/fish/day	21 days	Hatch rates were significantly delayed and reduced, but there was no impact on fecundity or fertility.
Kim et al. (2023) (Korea)	Japanese Medaka (Oryzias latipes)	Polyester	Polyester fabrics were purchased from Busanjin Market (Busan, Korea) and cut using micro-scissors	Length of 500 μm (AR: unknown)	No	500 and 1000 fibers/L	3, 7, 10, 14, and 21 days	Decreased survival and altered levels of GnRH mRNA, CYP19a mRNA, and plasma estradiol in fish exposed to the highest concentration of MPs.
Kim et al. (2023) (Korea)	Goldfish (Carassius auratus)	Polyester	Polyester fabrics were purchased from Busanjin Market (Busan, Korea) and cut using micro-scissors	Length of 500 μm (AR: unknown)	Polystyrene beads (0.2 and 1.0 μm)	10 and 100 fibers/L	6, 12, 24, 72, and 120 h	Increased oxidative stress, apoptosis, and DNA damage in goldfish.
Liang et al. (2023) (China)	Goldfish (Carassius auratus)	Polyacrylonitrile	Red commercial fibers were cut using a cryogenic microtome (cited Cole, 2016)	Mean length of 329.3 ± 164.1 μm and mean diameter of 21.3 ± 5.7 μm (AR: ~2:1)	No	100 and 1000 items/L	2 h and 45 days	Increased coughing rate, heightened secretory capacity of mucus cells, reduced daily intake of food, and decreased predatory behavior in fish exposed to the highest concentration of MPs.
Huang et al. (2022) (China)	Discus fish (Symphysodon aequifasciatus)	Polyamide	Not discussed	Length of 900 μm and diameter of 400 μm (AR: ~2:1)	Polystyrene microspheres (~88 nm)	20 and 200 μg/L	96 h	Fibers reduced growth while spheres weakened swimming performance. Both spheres and fibers inhibited butyrylcholinesterase activity, increased the concentrations of neurotransmitters in the brain, and altered gut microbiota.
Xie et al. (2021) (China)	Asian sea bass (Lates calcarifer)	Polyethylene	Cut sections of a long rope	Length of 2–3 mm and diameter of 200 μm (AR: ~10–15:1)	No	10 g/kg feed	56 days	Induced intestinal microbiome dysbiosis, oxidative stress, and histological damage.
Bunge et al. (2022) (Germany)	Stickleback (Gasterosteus aculeatus)	Polyester	Cut from a pink-red commercial polyester thread	Mean length of 245.6 ± 163.1 μm and mean diameter of 9.7 ± 2.3 μm (AR: ~25:1)	Cotton fibers (mean length of 197.1 ± 148.9 μm and mean diameter of 13.9 ± 3.9 μm)	0.2 and 2 mg/g	9 weeks	No significant change in growth, body condition, gonad development, or total leucocyte count; no significant difference between MP and natural fibers.
MacAulay et al. (2023) (United Kingdom)	Guppy (Poecilia reticulata)	Polyester	A polyester black shirt was cut into pieces with scissors, and then agitated in water to promote fiber shedding	Mean length of 2933.7 μm at start of experiment and 1615.4 μm by end of experiment (AR: unknown)	Bamboo fibers: Mean length of 1867.7 μm at start of experiment and 446.8 μm by end of experiment. Cotton fibers: Mean length of 2273.7 μm at start of experiment and 539.1 μm by end of experiment	~700 fibers/L	Exposed to fibers for 21 days, and then infected with G. turnbulli until takedown on day 52	In uninfected fish, polyester was associated with significantly higher mortality rates compared to natural fibers. Parasites exposed to polyester and cotton fiber leached-out dyes died significantly earlier compared to control parasites, with polyester dyes inducing the highest parasite mortalities.
Seeley et al. (2023) (United States)	Rainbow trout (Oncorhynchus mykiss)	Nylon	Already cut undyed nylon fibers were purchased from Claremont Flock, Inc.	Length of 500 μm and diameter of 10 μm (AR: ~50:1)	Polystyrene particles: median diameter of 26.8 μm. Spartinaspp. Particles: median diameter of 39.2 μm	0.1, 1, and 10 mg/L	31, 35, 42, and 56 days (on day 28, fish were dosed with virus or a mock control)	Increased mortality was not observed in uninfected fish treated with plastic only. Among virus-exposed fish, all MP types increased mortality compared to no particle exposure treatments; however, the largest increase in mortality (~80 %) was observed in the high dose nylon group.

Abbreviations: Microplastic fiber (MPF); Microplastic (MP); Aspect ratio (AR); Cadmium (Cd); Reactive oxygen species (ROS); Gastrointestinal tract (GIT); Hours (hrs); Gonadotropin-releasing hormone (GnRH); Cytochrome P450 aromatase (CYP19a); Days post-fertilization (dpf); Hours post-fertilization (hpf); Gyrodactylus turnbulli (G. turnbulli).

3.2. Microplastic fiber (MPF) production

Unlike beads or spheres that can be commercially purchased, MPFs are collected from various environmental sources or created in-house. Of the identified studies, 39 % (16/41) directly cited the methods from Cole et al. (2016) for generating experimental MPFs, which involve the use of a cryogenic microtome to cut fibers at specified lengths precisely. Other studies mentioned the use of scissors (20 % (8/41)), a scalpel/razor (7 % (3/41)), or cryo-milling (10 % (4/41)) to make microfibers. Alternatively, some studies collected fibers from a tumble dryer after the drying of synthetic fabrics (Winkler et al., 2022), blended fleece in a kitchen blender (Iwalaye and Maldonado, 2024), or placed larger plastic pieces on a mechanical shaker for an extended period to generate MPFs (Harikrishnan et al., 2024). In-house generation of MPFs using the latter methods tend to generate heterogeneous mixtures of MPFs in terms of size, whereas a cryogenic microtome is more controlled, yielding the ability to execute precise cutting and therefore more homogeneous fibers. Because fiber morphology (e.g., length) can significantly influence biological responses, it is important to consider how the method used to create MPFs may inadvertently influence experimental endpoints. There needs to be consistent and detailed reporting on the characterization and preparation techniques used to generate MPFs to allow for a more accurate comparison of experimental results across studies.

3.3. Polymer type and composition

In 2021, polyester, or polyethylene terephthalate (PET), accounted for more than half of the total global textile fibers produced (60.5 million metric tons) (Surana et al., 2024). Out of the synthetic textiles, polyamide was the second most produced polymer type (5.9 million metric tons) during that same year (Surana et al., 2024). The identified studies from our review assessed the toxicity of eight different MPF polymer types using several experimental designs that spanned in situ, in vitro and in vivo approaches. The most studied polymer was polyester (n = 15), followed by polyethylene terephthalate (PET) (n = 13) (we note that polyester and PET are made up of the same chemical composition), nylon/polyacrylamide (n = 8), polypropylene (PP) (n = 6), polyethylene (PE) (n = 3), polyacrylonitrile (n = 3), polyurethane (n = 1), and acrylic (n = 1) (Fig. 3a). These polymer types are consistent with the most common being found in the environment and some of the most dominant synthetic polymers being produced in the textile industry (Klein et al., 2015; Ross et al., 2021; Salvador Cesa et al., 2017). Despite the prevalence of polyamide production, less than ten papers identified in this review assessed polyamide fiber toxicity, highlighting a need for its assessment in future studies (Fig. 3a).

Fig. 3.

Categorical overview of the studies (n = 41) reviewed. (A) Number of papers categorized by the polymer type of microplastic fibers (MPFs) investigated across all included studies. Polyester and polyethylene terephthalate were the most frequently used polymers. (B) Distribution of fiber lengths reported across studies, binned into 50 μm intervals. Most studies used fibers between 50 and 450 μm in length. (C) Breakdown of studies according to the type of comparison made; most studies assessed the effects of MPFs in isolation (n = 24), while others compared MPF effects to those of natural items (i.e., fibers or particles) (n = 6), to other microplastic morphologies (e.g., fragments, beads) (n = 9), or to both natural items and other microplastic morphologies (n = 2) in the same study.

3.4. Microplastic fiber dimensions

The size of MPFs identified from our review ranged from ~5 μm to 5 mm in length with the majority of studies utilizing MPFs smaller than 250 μm (Fig. 3b). This agrees with the range (i.e., length) of MPFs being found in human lung tissue samples (Pauly et al., 1998), in indoor and outdoor air (Dris et al., 2017), and in marine organisms (Alfaro-Núñez et al., 2021). Over the years, different morphological criteria have been used to determine a fiber, such as a length-to-diameter ratio of >3 (Boulanger et al., 2014), or “a length substantially longer than its width” (Kershaw, 2019). Of the identified studies, some report both diameter and length (Bunge et al., 2022; Cheng et al., 2021) of the experimental fiber, while others (30 % (12/41)) only report the largest dimension (DiBona et al., 2021; Kim et al., 2023; Missawi et al., 2024), introducing confusion about the aspect ratio and true shape of the studied MP. These ambiguous fiber definitions and inconsistent disclosure of fiber dimensions across studies are problematic due to the long-standing general recognition that toxicity is strongly related to fiber geometry or aspect ratio (Boulanger et al., 2014). For instance, longer and thinner fibers are associated with greater toxicity in humans (Lippmann, 1990; Hill et al., 1995; Timbrell, 1982; Vattanasit et al., 2023) and have been linked to increased gut blockages and morbidities associated with intestinal lesions in fish (Ahrendt et al., 2020; MacAulay et al., 2023). We recommend studies report both the length and diameter of the MPFs used, as these aspects can greatly influence fiber deposition, downstream toxicity, and general interpretations of findings based on designated endpoints.

3.5. Exposure duration, dose or concentration

The toxicological effects of MPFs were analyzed under laboratory conditions with varying exposure periods. Of the studies reviewed, 41 % (17/41) were 21 days or longer in exposure durationwith the longest exposure reported at 84 days. The other 59 % (24/41) of studies described exposure durations that were shorter than 21 days, with the shortest reported at 20 min. The MPF concentrations and units indicated also varied in the reviewed studies. The most common reporting of study concentrations included mass of fibers/volume or number of fibers/volume. Some studies reported a dose of number or mass of fibers with a mass of food (Xie et al., 2021). For standardization purposes and easier comparison across multiple studies, it would be advantageous to report both the study concentration as number of fibers/volume in addition to the mass of fibers/volume, and dose if applicable. For invertebrates, a common exposure concentration range was between 10 fibers/L to 100 fibers/L (Collins et al., 2023; Détrée et al., 2023; J. A. Kim et al., 2024; Klasios et al., 2024; Y. H. Lee et al., 2023; Pittura et al., 2022). For vertebrates, a common exposure concentration range was between 100 fibers/L to 1000 fibers/L (Hu et al., 2020; J. A. Kim et al., 2023; M. J. Kim et al., 2023; Liang et al., 2023; MacAulay et al., 2023; Qiao et al., 2019; Zhang et al., 2023). Exposures reflect relevant and above relevant levels of MPF exposure in different aquatic matrices (Acharya et al., 2021).

4. Toxicological impact of MPFs

4.1. Aquatic invertebrates

Of the aquatic invertebrates, the effects of MPFs were mostly described in bivalves (e.g. Mytilus galloprovincialis and Mytilus edulis) (Christoforou et al., 2020; Cole et al., 2020; Choi et al., 2021, 2022; Esterhuizen et al., 2022; Pittura et al., 2022; Auguste M et al., 2023; Collins et al., 2023; Détrée et al., 2023; Mai et al., 2023) and Daphnia (e.g. Daphnia magna and Daphnia carinata) (D. Kim et al., 2021; Tourinho PS et al., 2022; Jiang et al., 2023; Lee et al., 2023; Iwalaye and Maldonado, 2024), with a variety of biomarkers linked to MPF exposure. Commonly identified biomarkers influenced by MPF exposure included gut damage, DNA damage, apoptotic activity, antioxidant responses, developmental and feeding impacts (Table 2) (Christoforou et al., 2020; Choi et al., 2021, 2022; Esterhuizen et al., 2022; Jiang et al., 2023; Lee et al., 2023; Mai et al., 2023; Iwalaye and Maldonado, 2024). Although zooplankton (e.g. Daphnia magna and Daphnia Carinata) exposed to MPFs had lower reproductive rates (Jiang et al., 2023) and microbiome changes (Lee et al., 2023), Klasios et al. (2024) reported that polyester MPFs did not affect zooplankton abundance or diversity indicating that MPF effects may be observed at the individual but not community level. Long-term MPF exposure decreased the capacity of Mytilus edulis to filter phytoplankton indicating a potential for decreased water quality in areas that receive large amounts of plastic pollution and highlights how MPF exposure to mussels in the environment could impact ecosystem processes (Christoforou et al., 2020). In Mytilus galloprovincialis, MPFs induced increased acetylcholinesterase activity (Choi et al., 2021, 2022), increased reactive oxygen species (Choi et al., 2021), DNA damage (Choi et al., 2022; Mai et al., 2023), and apoptotic and necrotic activity (Choi et al., 2021; Mai et al., 2023). Acetylcholinesterase can be an important biomarker in MP research because it has been shown to be dysregulated in other invertebrate species in response to MP exposure (Eom et al., 2020; Tlili et al., 2020), suggesting the importance of investigation into other neurotoxic effects associated with MPFs.

4.2. Aquatic vertebrates

MPF exposure was associated with multiple toxic effects in vertebrates (Table 3), including an increase in inflammation in the guts of zebrafish (Logan et al., 2023; Zhao et al., 2021), decline in mucus volume (Qiao et al., 2019), increase in intestinal epithelial cell apoptosis (Zhang et al., 2023), oxidative stress (Zhang et al., 2023; Zhao et al., 2021), disruption of lipid metabolism, and gut microbiota alterations including an increase in a plastic-related compound catabolizing bacteria Gordonia (Qiao et al., 2019). In addition to impacts on zebrafish, Japanese medaka (O. latipes) exposed to MPFs exhibited gill epithelium changes (Hu et al., 2020), delayed hatching rates (DiBona et al., 2022), alterations in Proteobacteria abundance (DiBona et al., 2021), and decreased survival (M. J. Kim et al., 2023). Since gut microbiota dysbiosis has been repeatedly observed following MPF exposure in vertebrate species (DiBona et al., 2021; J.-N. Huang et al., 2022; Qiao et al., 2019; Xie et al., 2021; Zhao et al., 2021), further assessment of intestinal microbiota may be useful in better understanding the mechanisms behind MPF toxicity in the gut. Certain bacteria (i.e., Proteobacteria) have been shown to be more sensitive to MP exposure (DiBona et al., 2021; Qiao et al., 2019) and non-spherical MPs have been shown to cause more severe effects on gut microbiota (J.-N. Huang et al., 2022; Qiao et al., 2019), bolstering the idea that microbiota taxation may be useful in the toxicological assessment of MPFs. Very few studies reported behavioral changes, carcinogenesis, or neurotoxic effects in aquatic vertebrates following MPF exposure, making these future research areas of interest. Notably, only one study assessed the respiratory behavior of fish where goldfish (Carassius auratus) exposed to MPFs exhibited increased mucus secretion and coughing behavior, warranting further investigation into the respiratory impact of MPFs (Liang et al., 2023).

4.3. Human models

Of the identified studies, only two (Table 1) focused on MPF toxicity in the context of human health despite literature showing MPs present in human tissue samples (Li and Liu, 2024; Zhu et al., 2024). The two identified studies both assessed the effect of MPFs on respiratory health, which is relevant as the lungs have been shown to accumulate the highest amounts of MPs (14.19 ± 14.57 particles/g) in comparison to other organs (i.e., small intestine, large intestine and tonsils) (Zhu et al., 2024). In the identified studies, MPFs were shown to become enveloped into the structure of human lung organoids due to cellular polarization (Winkler et al., 2022) and inhibit human airway organoid development (Song et al., 2024), suggesting that future studies should focus on the long-term effect of lung repair following MPF exposure.

4.4. Synthetic vs. natural fibers or particles

Humans have been exposed to natural fibers (i.e., those originating from plants and animals) for years, resulting in negative health effects driven by fiber composition and/or physical structure (Churg and Wright, 1994; Järvholm, 2000; Vu and Lai, 1997). Since synthetic fiber production has continued to increase, resulting in a larger consumption of synthetic over natural fibers (Surana et al., 2024), it is important to assess if synthetic and natural fibers produce similar or dissimilar health outcomes. A few studies show a link between the inhalation of synthetic fibers and respiratory diseases (Goldberg and Thériault, 1994; Zuskin et al., 1998), while suggesting that the risk of cancer may be similar for both natural and synthetic fibers (Mastrangelo et al., 2002; Facciolà et al., 2021). Overall, toxicity differences resulting from exposure to natural versus synthetic fibers remain unclear.

While 58 % (24/41) of studies from this review assessed the toxicity of MPFs only, 15 % (6/41) of our identified studies (see Tables 2 and 3) directly assessed the differences in toxicity between natural particles or fibers and synthetic fibers as shown in Fig. 3c. The studies that did so were only in studies using aquatic organisms (Bunge et al., 2022; Collins et al., 2023; Détreé et al., 2023; Kim et al., 2021, 2023; MacAulay et al., 2023; Pittura et al., 2022). Some papers reported that synthetic fibers were found to be more toxic to aquatic organisms than natural fibers or particles. For instance, in a guppy host-parasite model system (Poecilia reticulata-Gyrodactylus turnbulli), uninfected fish exposed to polyester suffered significantly earlier mortality than bamboo or cotton exposed fish (MacAulay et al., 2023). Similarly, PET MPFs induced the highest mortality and most severe gut damage in brine shrimp (A. franciscana), while lyocell fibers caused the least detrimental effects (L. Kim et al., 2021). On the other hand, some studies found natural fibers or particles to be more toxic than synthetic fibers. Although exposure to polyester and polypropylene fibers resulted in greater immobilization of Daphnia magna compared to natural lyocell fibers, the lyocell fibers induced the strongest effects on gut damage, growth rate, and mortality compared to the synthetic fibers during depuration (D. Kim et al., 2021). Additionally, in oysters (Crassostrea gigas), natural fibers were shown to elicit higher digestive and inflammatory responses than synthetic fibers (Détrée et al., 2023). Researchers speculate this may be due to the natural fibers being less even in diameter and length (i.e., rougher in structure) than synthetic fibers, while also having a higher and larger presence of polycyclic aromatic hydrocarbons and polychlorinated biphenyl contaminants. These observations raise questions on the safety of natural or green materials for which risk perception is low (Gray et al., 2022). Natural and synthetic fibers have also been demonstrated to have null effects, where responses in laboratory studies do not differ between the two. Sticklebacks (Gasterosteus aculeatus) exposed to polyester or cotton fibers did not exhibit any significant change in growth, body condition, gonad development, total leucocyte count per mg fish, or ROS activity of head kidney leucocytes compared to control (Bunge et al., 2022). Similarly, there was no observed gut damage or significant difference in the gut microbiome of Mytilus edulis exposed to either nylon MPFs or ground marsh grass (Spartina spp.) particles although, the microbiome on the fibers themselves differed between each type (Collins et al., 2023).

Overall, there is still a limited understanding of what influences the differences seen in toxicity following exposure to either natural or synthetic fibers. The physical and chemical properties of fibers are dependent on their intended use, and these are expected to greatly influence their toxicity. Clothing of both synthetic and natural origin can influence human exposure to chemicals due to the byproducts of manufacturing and use (Overdahl et al., 2021). Therefore, more laboratory studies are needed to assess toxicological outcomes of natural versus synthetic fibers due to additional harbored chemical contaminants, while emphasizing a focus on human health in the future.

4.5. Fibers versus other microplastic shapes (beads, spheres, fragments, etc)

Of the identified studies, 22 % (9/41) (see Tables 2 and 3) compared toxicity differences between MPFs and other MP shapes (i.e., beads, spheres, particles) (Fig. 3) (Cheng et al., 2021; Cole et al., 2020; Di Natale et al., 2022; Esterhuizen et al., 2022; Huang et al., 2022; Kim et al., 2023; Mai et al., 2023; Missawi et al., 2024; Qiao et al., 2019). A few studies observed that MPF exposure resulted in heightened effects compared to the other MP shapes, demonstrating that endpoints assessed can be influenced by MP morphology (Esterhuizen et al., 2022; J.-N. Huang et al., 2022; Mai et al., 2023; Qiao et al., 2019). For instance, polypropylene fibers were shown to induce more severe intestinal toxicity in zebrafish due to their longer residence time and higher accumulation in the gut than polystyrene fragments (Qiao et al., 2019). It is suggested that sharp-edged microfibers with different surface properties can cause more pronounced mechanical injuries of the delicate gut epithelium than the round and smooth-surfaced micro/nano beads. Polyamide fibers resulted in the largest reduction in weight gain for exposed juvenile discus fish compared to polystyrene microspheres (J.-N. Huang et al., 2022); meanwhile, PET fibers induced greater toxic effects (e.g. genotoxic stress, oxidative stress, enhanced energy metabolism) than polyethylene microspheres in Mytilus galloprovincialis (Mai et al., 2023).

Fibers are the most predominant MP shape found in the lung (Jenner et al., 2022b) and toxicity has been observed to be influenced by particle shape, emphasizing the need for fiber-focused toxicity studies. Based on what we already know about the toxicity (i.e., lack of clearance, pulmonary persistence, and modulation of pro-fibrotic and cancer pathways) of other inhaled fibers (i.e., asbestos), MPFs could contribute to similar or unique negative health outcomes; however, this remains unclear at present. Additional studies should also focus on MPF toxicity in respect to other areas of human health since MPs have been discovered in various human samples (i.e., placenta, stool, colon, sputum, liver, breast milk, arteries and blood) (Barceló et al., 2023; Marfella et al., 2024), with fibers again being a prominent MP shape detected (Abbasi and Turner, 2021; S. Huang et al., 2022; Ibrahim et al., 2021; Zhu et al., 2023).

A handful of other studies observed that MP fragments and beads resulted in more heightened effects than MPFs (J. A. Kim et al., 2023; Missawi et al., 2024). A co-exposure to PET fragments and bacteria induced an increase in zebrafish locomotor activity, while no significant effect was observed for a co-exposure of PET MPFs and bacteria (Missawi et al., 2024). Additionally, polystyrene beads induced higher DNA damage and oxidative stress in C. auratus compared to polyester fibers (J. A. Kim et al., 2023). Overall, the varying levels of toxicity elicited by different MP forms seem to be influenced by translocation within the organism and greater chemical adsorption to MP shapes with higher specific surface areas (Cheng et al., 2021; Cole et al., 2020). There seems to be enough evidence to support that toxicity outcomes differ based on particle shape; however, it remains unclear if MPFs have a larger impact than beads on the health of various organisms. Most of the identified studies compared MP shapes that were of different polymer types with potentially dissimilar chemical constituents. Therefore, it would be useful if future studies assessed bead and fiber toxicity using the same polymer type to remove additional variables that make it difficult to directly compare study outcomes.

4.6. Role of fiber length in toxicity

Only 5 % (2/41) of the identified manuscripts directly analyzed the effects of fiber aspect ratio by conducting studies utilizing fibers of the same polymer type cut at different lengths (Zhao et al., 2021; Choi et al., 2021). Zhao et al. (2021) observed that long PP MPFs (mean length of 200 ± 90 μm and diameter of 20 μm) significantly reduced the thickness of intestinal mucosa and induced a higher degree of ROS and physical damage in the intestine of exposed zebrafish compared to short fibers (mean length of 50 ± 26 μm and diameter of 20 μm). Additionally, longer PP MPFs resulted in higher ROS levels and a greater reduction in zebrafish larvae lipid stores compared to short MPFs (Zhao et al., 2021). In Mytilus galloprovincialis exposed to both long PET MPFs (mean length of 118.3 ± 66.6 μm and diameter of 13 μm) and short PET MPFs (mean length of 44.8 ± 19.9 μm and diameter of 13 μm), researchers observed short MPFs induced a greater increase in acetylcholine hydrolysis (AChE) activity, while long MPFs induced a greater increase in DNA damage (Choi et al., 2021). These studies show that short and long MPFs exert their own individual toxicity profiles, but it is still unclear if longer MPFs may be more detrimental to health overall. Additional studies should focus on evaluating the relationship between MPF length and toxicity.

5. Influence of co-contaminants and additives on microplastic fiber toxicity

5.1. Co-exposures

MPs in the environment are found to co-exist with other contaminants and external factors (i.e., weathering from sunlight), potentially influencing the bioavailability and toxicological consequences of various co-contaminants. Of the identified studies, 17 % (7/41) (see Tables 2 and 3)) explored the combined effects of MPs in the presence of another stressor (i.e., chemical or infectious disease agents) (Cheng et al., 2021; Missawi et al., 2024; MacAulay et al., 2023; Tourinho et al., 2022; Lee et al., 2023; Kim et al., 2024; Seeley et al., 2023) to address the idea that mixture exposures may result in neutralizing, additive or synergistic toxicity).

MPFs were shown to exacerbate viral infections, with the hypothesized mechanism involving fiber-associated tissue damage and membrane disruption leading to increased viral entry (Seeley et al., 2023). Similarly, zebrafish embryos exposed to PET fibers concomitant with bacteria experienced aggravated effects (i.e., greater delayed rate of hatching and increased heart rate) compared to PET fibers alone (Missawi et al., 2024). Some studies performing MPF and chemical co-exposures also demonstrated heightened toxicity outcomes compared to individual exposures (Kim et al., 2024) whereas other studies demonstrated the ability of MPFs to reduce the toxicity of some harmful contaminants through adsorption, leading to reduced chemical bioavailability (Cheng et al., 2021). Polymer aging has been noted to influence such sorption dynamics of chemical contaminants and is expected to further influence the outcomes of mixture exposures (Di Natale et al., 2022). Overall, studies focused on combined toxicities arising from a mixture of chemical and/or pathogenic co-contaminants with MPFs remains scarce. Additional studies should focus on hydrophobic organic pollutants that tend to be more strongly sorbed by MPs and less likely to desorb, suggesting these compounds may bioaccumulate in organisms leading to worsened effects overtime (Herzke et al., 2016; Magara et al., 2018). Overall, it is recommended that future studies further investigate the dynamics of polymer age and sorption capacity for co-contaminants to provide context to the results of toxicological studies utilizing organismal models.

5.2. Leachate

MPs are known to harbor various hazardous chemical compounds (i.e., heavy metals, polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), and per- and polyfluoroalkyl substances (PFAS)) on their surface, making it important to analyze such additives, in addition to the MPs themselves (Campanale et al., 2020). Only 12 % (5/41) of studies directly determined the chemical additives present on the experimental MPFs and/or assessed toxicity of MPF leachate (MacAulay et al., 2023; Esterhuizen et al., 2022; Song et al., 2024; Détrée et al., 2023; Harikrishnan et al., 2024). One study observed that a specific dye leached from polyester fibers more than bamboo or cotton fibers, potentially explaining the higher number of G. turnbulli parasite mortalities observed in the polyester leachate exposed group (MacAulay et al., 2023). Researchers were unable to determine the exact nature of the dye; however, they suspect reactive black 5 is a likely cause since it is one of the most used black dyes in industry (MacAulay et al., 2023). The influence of dyes on MPF toxicity was further shown in another study where yellow polyethylene MPs induced a larger reduction in glutathione S-transferase (GST) activity in Corbicula javanicus compared to red and blue MPs, implying certain colorants may be more toxic than others (Esterhuizen et al., 2022). Another study found that commonly identified plastic leachate compounds (e.g., bisphenol A and benzophenone-3) were not responsible for the negative effects observed in lung organoids exposed to nylon leachate, indicating that future research should focus on the assessment of other obscure leached plastic compounds, such as the plethora of dyes used to color synthetic textiles (Song et al., 2024). For example, more recent work showed that azobenzene disperse violet 93 (i.e., common textile dye) can alter critical biotransformation pathways in human lung cells, including induction of CYP1A1 and CYP1B1 (O’Connor et al., 2024).

Détrée et al. (2023) quantified polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) on synthetic and natural fibers. The most contaminated fibers were wool (10 PAHs, 5 PCBs) and polyester (7 PAHs, 3 PCBs), showing that both natural and synthetic fibers can carry large quantities of additives (Détrée et al., 2023). Oysters exposed to natural fibers and their leachate exhibited greater changes in their physiology compared to synthetic fiber-treated oysters (Détrée et al., 2023). Notably, hydrophobic chemicals are less likely to leach off fibers into seawater but more likely to leach off and become bioavailable in oyster tissue; therefore, we recommend quantifying the concentration of chemicals present in leachate in addition to quantifying the total concentration of chemicals present on the fiber itself via solvent extraction.

Harikrishnan et al. (2024) showed that surgical face masks placed in natural seawater released heavy metals (e.g., Cu, Cd, Ni, Pb and Cr) and some volatile organic compounds; as the days of face mask degradation continued, the concentration of heavy metals in seawater increased. MPFs released after 120 days of face mask degradation prolonged the hatching time and decreased the hatching rate of H. elegans embryos (Harikrishnan et al., 2024). Choi et al. (2022) showed that PET MPFs decreased estradiol and testosterone in mussels; however, these PET MPFs contained BPA (278.5 ± 101.1 ng/g), which has been previously shown to decrease fertility and testosterone levels in exposed organisms (Choi et al., 2022; Wang et al., 2019; Lee, 2003; Nakamura et al., 2010; Ye et al., 2014). Therefore, it is unclear whether the effects seen in Choi et al. (2022) and Harikrishnan et al. (2024) are predominantly caused by the additives or by the fibers, but it is critical to highlight the scientific benefit of quantifying the chemical additives present on experimental MPFs, which is rarely being done at present.

Overall, research investigating the toxicity of plastic leachates, along with the identification of their chemical additives, is still in its infancy (Détrée et al., 2023). It is incredibly challenging to compare MPF toxicity results across multiple studies when the MP material and, therefore, associated additives are consistently different. This calls for greater transparency and assessment of additives on the experimental MPFs being used in toxicity studies. Non-targeted chemical profiling can be useful to assess the risk of MPs, considering the enormous diversity of additives used in the plastic industry, where the chemical identity and toxicity remain largely unknown (Détrée et al., 2023; Groh et al., 2019).

6. Knowledge gaps and future research directions

Our review highlights several critical take-home messages that underscore both the current challenges and potential future directions surrounding toxicological research on MPFs. Currently, methodological inconsistencies across studies prevent meaningful cross-comparisons from being made, hindering research progress in the field. Standardized practices for MPF generation are needed, as fibers can be produced in-house using a variety of procedures (i.e., cryotome versus blender) and starting materials (i.e., a black fleece from a local store versus commercially manufactured filaments). Moreover, our review highlights that studies reviewed failed to comprehensively report both fiber length and diameter. This lack of transparency can impact future researchers’ experimental designs when attempting to link endpoints to physical characteristics of MPFs.

This review also identifies that the respective contributions of fiber structure, polymer composition, and associated chemical additives to observed toxic effects have not yet been clearly delineated. Furthermore, the current research is largely limited to single-stressor exposures. Due to climate change, various ecosystems are facing different types of stressors, including acidification, increased temperature, and increased salinity in freshwater due to saltwater intrusion. Expanding laboratory work to investigate the impact on organisms alongside MPF exposure will help researchers understand how populations are responding to these pollutants in a time when environmental degradation and alteration are ever-present. Studies to date that have assessed these issues have concluded that organisms challenged with MPF and altered water chemistry (i.e. lower pH) can result in a negative impact on sensitive aquatic taxa (Lee et al., 2023)

MPF toxicity in the context of human health is currently understudied (as evidenced by the severe lack of studies using human models compared to aquatic organisms in this review), despite the fact that MPFs have been detected in human tissues where they can impact various physiological and homeostatic processes. The limited human health toxicological findings also remain poorly integrated with emerging epidemiological data.

To improve upon the current state of research, future studies should (1) consistently report MPF size dimensions (i.e., state both the length and diameter) and exposure concentrations (i.e., state both the number and mass per volume); (2) focus on human-centered studies, including inhalation and ingestion exposure models that will best complement the aquatic literature; (3) perform mechanistic studies that disentangle the effects of physical fiber characteristics from those of leached or sorbed additives; (4) compare the toxicity of synthetic and natural fibers of identical sizes; (5) integrate non-targeted chemical analysis into experiments to identify unknown or currently understudied additives (i.e., dyes) that may be influencing MPF toxicity; (6) assess toxicity effects related to polymer aging and the sorption capacity for certain cocontaminants; and (7) better integrate toxicity studies with epidemiological findings available, which are currently sparse, to better understand negative health risks under environmentally realistic conditions.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.envres.2025.123272.

Data availability

No data was used for the research described in the article.