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Open Access
Volume: 35(1), Article ID: e2020004, 10 pages
https://doi.org/10.5620/eaht.e2020004

Environmental Analysis Health and Toxicology

eISSN: 2671-9525• Systemic Review

Microplastics Exposure Routes and Toxicity Studies to
Ecosystems: An Overview
Christian Ebere Enyoh1 , Leila Shafea2, Andrew Wirnkor Verla 1, Evelyn Ngozi Verla 3, Wang Qingyue4, Tanzin Chowdhury4,Marcel Paredes5
1Group Research in Analytical Chemistry, Environment and Climate Change (GRACE&CC), Department of Chemistry, Imo State University (IMSU), PMB
2000 Owerri, Imo State, Nigeria; 2Department of Soil Science, Universität Kassel, Kassel, Germany; 3Department of Environmental Technology, School ofEnvironmental Science, Federal University of Technology, Owerri, P.M.B.1526, Imo State, Nigeria; 4Graduate School of Science and Engineering, Saitama
University, Saitama, Japan; 5Department of Civil Engineering, National University of Chimborazo, Riobamba, Ecuador

Microplastics (MPs) are now ubiquitous in global ecosystem, therefore all biota is at risk of exposure and potential toxicity. In this study,
we presented an overview of information based on literature concerning exposure to MPs and the toxicity of such exposure. Currently,
four major routes of exposure have been identified including entanglement, contact, ingestion and inhalation. Humans maybe the most
exposed organism because they are at the peak of the food chain. Toxicology effect to marine and freshwater organisms are classified
based on exposure dosage as either high (mortality, decreased reproductive output, organ damage) or low (changes in behavior withtime). On plants, reports have shown that MPs exposure can affect negatively the growth and depending on exposure concentration andtypes of MPs and oxidative activities. However, effects on plants maybe short-term and transient. Although, toxicity studies regarding
human are still ongoing as per reports, plants and animals are still scantly studied. Animal toxicity studies have widely used D. magna asmodel specie. MPs pollution may have a knock-on effect on trophic structure and functioning of ecosystems by affecting the base of thefood chain. We concluded by identifying the gap in knowledge and give recommendations for future research.
Keywords: Air, Soil, Water, Food, Human health, Toxicity, Exposure, Micropollutants

INTRODUCTION
Plastic products with tremendous consumption are ubiquitous in our daily lives and the annual production of plastics isdrastically increasing [1]. It is now expected to reach 33 billion
tonnes by 2050 [2] with plastic waste in the environment projected to reach 67.8 million metric tons by the same year [3].
The pollution of plastic cum microplastics (MPs) from degradation in the environment is currently a hot issue attracting
the global attention of many scientists, governmental andnon-governmental organization and the public media [3-7].
Although few studies have investigated the chemical behavior
and toxicity of MPs in the natural environment, studies focusing on its toxicity in tandem with toxic chemicals to ecosystems are scarce [1,4,8]. There are many toxicology reports that
MPs are harmful to ecosystems. Terrestrial organisms such as
Received: January 18, 2020 Accepted: March 4, 2020
Corresponding author: Christian Ebere Enyoh

Imo State University, PMB 2000, Owerri, Nigeria
E-mail: cenyoh@gmail.com

This article is available from: https://eaht.org/

earthworms, soil collembolans, and other animals, as well as
plants have been affected by MPs [1,4,9-13]. Also, aquatic organisms such fish, sandhoppers, sea turtle, crustacean andmussel have also been affected by MPs [14-15]. In addition,
humans are exposed to MPs via trophic transfer or by direct
ingestion, contact and inhalation and plausible effects include
lung inflammation and genotoxicity may occur [1,3,16,17].
Aside from the innate toxicity of MPs, they can carry different
toxic chemicals such as heavy metals and organic pollutant byadsorption process, and double the effect of such pollutants[1,8,18].
Succinctly, plastic products are made up of mixtures of polymers, fillers, and multiple additives to improve its usability.
Also, there are other chemicals including unreacted monomers, starting substances and non-intentionally added substances (NIAS; impurities, side or breakdown products) thatare also present in plastic. However, most of these chemicals
are not covalently bound to the polymer, so they can be released at all stages of the plastics’ life cycle via migration to liquids or solids or via volatilization [1]. Therefore, plastic materials are an important source of human exposure to chemicals.

Copyright © 2020 The Korean Society of Environmental Health and Toxicology/Korea Society for Environmental Analysis
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (https://creativecommons.org/
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Environmental Analysis Health and Toxicology 2020;35(1):e2020004

The well-known examples are the plastic monomer bisphenol
A (BPA) and phthalate esters used as plasticizers [19]. Moreroute of attachment of environmental chemicals to plastics isthrough formed biofilm on the microplastic surface and hydrophobic adsorption (for adsorption and desorption mechanism see ref. [1]).
According to the recent studies around toxicities of plastics
and their chemical releases, more analyses of the dangers of
MP pollution to terrestrial biodiversity are required because
the abundance, composition and physicochemical surface
properties of particulate material follow typical patterns in terrestrial and continental environments [20, 21]. Recently Verlaet al. presented a comprehensive review of the interaction between MP and toxic chemicals as well as the implications ofsuch interaction to ecosystems [1]. In their review, they reported that twelve metals - aluminum (Al), arsenic (As), cadmium(Cd), chromium (Cr), cobalt (Co), copper (Cu), iron (Fe),manganese (Mn), nickel (Ni), lead (Pb), titanium (Ti), andzinc (Zn); one halogen - bromine (Br); and fifteen groups oforganic pollutants - polycyclic aromatic hydrocarbons (PAHs),
Polychlorinated biphenyls (PCBs), dichloro-diphenyl-trichloroethane (DDTs), hexachlorocyclohexane (HCHs), chlordane,
mirex, hexachlorobenzene, hopanes, polybrominated diphenyl ether (PBDEs), perfluorinated compound/acid (PFC/
PFA), aliphatic hydrocarbons, BPA, nonylphenols (NPs) andoctylphenols (OPs) have generally been quantified from different locations globally up till now. These chemicals are ubiquitous just as MPs and are abundant in areas of high anthropogenic activities [1]. Further, they can adversely affect humanhealth with potential carcinogenic, teratogenic and mutagenic

effects [22]. The study aimed at presenting a summary of studies conducted on exposure and toxicities of MPs to man andenvironment. Understanding the route of exposure to MPs
may help preventing lethal effects they may pose to biota.

EXPOSURE ROUTE TO MICROPLASTICS
MPs are ubiquitous in the environment since they have beenfound in all components of the ecosystems [4]. Therefore, biota interact each other either from the soil, water or air. Sizes,
density, abundance, shape, and color of MPs control its bioavailability [1] and therefore its toxicological effect. In the last
five years, research have focused on the transfer and the effects of MPs through the food chain. Diverse species are currently being investigated to find out more susceptible species
to MPs exposure [23]. Cunningham and Sigwart presented ameta-analysis of experimental exposure studies (number ofstudies = 128) on marine and freshwater species to MPs [24].
Exposure impact of MPs was classified as either high (decreased reproductive output, damage to organ and mortality)
when dosage is greater than 100 MPs/L for water and greater
100 MPs/kg for sediment and low (transient behavioral changes such as increased respiration, reduced feeding, and reduced energy) when dosage is ≤ 100 MPs/L for water and ≤100 MPs/kg for sediment. The threshold for a cut-off between
the two classes was selected based on the highest recorded
environmental surface water concentrations [25]. Althoughconcentrations of MPs can vary with time, the threshold of 100
MPs/L is intended to represent the highest values of MPs reported at a given time [24]. Therefore, different cut-off size will

Contaminated Source(s) of exposure

Route of exposure

Microplastics

Contact

Ingestion

Plant• Soil• Air• Water

Inhalation

Entanglement

Human

Animal• Soil/dust• Air• Water• Plant• Animal• Food• Personalcare product• Soil/dust• Sediment• Air• Water

Figure 1. Summary of exposure route to environmental microplastics (MPs)
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Enyoh, et al. | Microplastics exposure routes and toxicity
lead to different values, since number concentration of MPs issize-dependent. However, many species either by selective
targeting of plastic items, or accidental ingestion by filtrationor predation are exposed directly to MPs [26]. Based on current information, all categories of exposure fall under the four
major routes to MPs: entanglement, contact, ingestion and inhalation (Figure 1). Entanglement and contact are similar, but
this study regarded them as different based on the mode of
exposure. Contact involves attachment and/or penetration
through skin pores while entanglement is defined when MPs
remain attached/packed on the surface/body of the organism,
which can cause the organism to sink if in water.
Plant
Plants are exposed to MPs through contact (from MPs fallout
or through contaminated water or irrigation water) and ingestion by uptake through the rhizosphere in the plastic-soil matrix. Contact exposure was reported for aquatic plant e.g.
Duckweed (lemma minor) and moss plant (Sphagnum palustre L). MPs made of polyethylene (PE) are adhering to whole L.
minor colonies by exposing to 50,000 MPs/mL of size dimension of 10–45 μm [27]. Capozzi et. al. [79] also made similar
observation for polystyrene (PS) on S. palustre L in freshwater.
Overall, prolong exposure to the treatment showed increasing
adherence to MPs in water by aquatic plant [27,79], thus they
can be used for monitoring MPs contamination of freshwater
environments.
Recent studies have also suggested that plants which are exposed to MPs in the soil matrix have the potential to uptake
them [4]. Studies have shown nanoplastics (<100 nm) and MPs(0.2 µm) can uptake by a plant [28,29,80,82]. This is hypothetically possible because small-sized MPs can bypass plant cellwall and membrane barriers or block cell pores [4,18,82].
AnimalThe exposure route of animals, including marine and soil organisms, to MPs is by entanglement and ingestion [1,15]. Oneof the major reasons why some marine organisms get entangled or ingest MPs is due to the formation of biofilm on the
microplastic surface [1]. Biofilm forming processes on virginmicroplastic particles begin within seconds of the first contact
with ambient water [1,30].
The number of marine species reported to have interactedwith plastic debris has increased over time with current counts
over 1,000, while about 800 of these have shown interaction
with MPs [31]. Previous reports showed that by ingestion 220
species were contaminated and through entanglement 580
species were contaminated and these organisms include ma-

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rine mammals, fish, invertebrates and fish-eating birds[1,32,33]. However, soil animals such as nematodes, collembolan, oligochaeta (e.g. earthworms) and isopods are also
known to interact within different ways and ingest soil plastic
debris [1].
Land or terrestrial animals are mainly herbivorous, e.g. goat,cow, etc., and they may be exposed indirectly by consuming
MP-contaminated plant and/or directly from consuming contaminated animal feed or water. Carnivores, e.g. lion, tiger, etc.,
may then be exposed through food-chain process or trophic
transfer by consuming or feeding on herbivores tissues andbio-system. The food chain exposure processes are also often
exhibited in the aquatic ecosystem, for example, whale or
sharks feeding on smaller marine animals may have ingested
MPs or amphipod Gammarus duebeni eating PE laden L. minor [27]. Generally, data on the presence of MPs in freshwater
macroinvertebrates are scarce, as most studies have focusedon fish and birds [34].
Human
The routes of exposure of humans to MPs are diverse. The
summary and mechanism of exposure are presented in Figure 1.
The ingestion may be by oral route which involves consumption of contaminated water, food products (honey and beverages), through use of personal care products (toothpaste, face
wash, scrubs, soap; also dermal route), marine product (foodchain), plant (food chain), contact (dermal) from soil, water or
fallout of airborne MPs, from particulate fallout from air during open meal and inhalation.
Humans may also be directly exposed to MPs through theactual ingestion of these particles from drinking water, honey,
beers and table salt [35-38]. MPs have been found in drinking
water sources but in low concentrations [40,78]. Analysis ofbeer samples in Germany reported about 2 to 79 fibers/L andfrom 12 to 109 fragments/L [36]. Recently, Karami et al. analyzed different salt brands (n = 17) from 8 countries for MPlike particles and observed that the salts were predominantly
contaminated by fragments of polypropylene (PP) and PE [37].
Through salt consumption, it was estimated that at most 37
particles per individual were ingested [1]. Although, theseroutes of exposure to MPs through consumption of salts poseno/negligible health risks as concluded by researchers [38,39].
Another route of direct ingestion of MPs is from atmosphericfallout of indoor airborne MPs during open meals [3]. Cantarino et al. demonstrated this exposure route in their study andextrapolated that humans may inject MPs which ranged from13,731 to 68,415 particles per year from this scenario [4,41].
MPs can also be ingested indirectly through personal care
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Environmental Analysis Health and Toxicology 2020;35(1):e2020004

products such as toothpaste, face wash, scrubs and soap
[42,43]. There are reports of MPs present in diverse personalcare products. Results from a recent study showed that 50 % of
the face wash products and 67 % of the facial scrubs studied
mainly contained microbeads [44]. These microbeads cancause skin aging and dark spots on the skin by letting in bacteria through the tiny rips formed. Kaur explained that tiny rips
in the skin may occur from exposing the skin to microbeads inpersonal care products [44]. Microbeads can also injure thecornea when it gets stuck in under the eyelid from face washing. There are many reports of the presence of MPs in toothpaste products [45]. MPs in toothpaste can lead to gingivitis
and bleeding with prolonged exposure when trapped between
gum [44].
The food chain exposure process is based on human consuming MPs contaminated aquatic organisms, animals orplants, which in turn may have consumed the plastic throughMP loaded water or the feeding from other organisms. There
are many studies which reported the presence MPs in variousorganisms from the lowest levels of food chain such as zooplanktonic organisms to the highest levels in both invertebrates (Crustacea, mollusks) and vertebrates (fish) [1,15,4648]. Güven et al. studying the Mediterranean coast (Turkey)
for MPs in fishes, reported a total of 1,822 and 1,337 MP particles were recovered from stomachs and intestines respectively[49]. Furthermore, it was also reported MPs reached 92.3% of
the total number of plastic items (meso-/micro- plastics) infishes collected from coastal and freshwater in China [37].
MPs have also been recently found in the livers of fish fed plastic particles [39]. It was estimated that a top European shellfish
consumer will consume 11,000 plastic particles annually by
the aforementioned route [1]. Another aspect of food chain
exposure is by consuming contaminated plants. Previous reports have demonstrated the potentiality of plant bioaccumulating MPs and from extrapolations, they may consume about
80 g daily [4].
Dermal exposure may occur when humans interact with water or soil contaminated with MPs or from contact with particulate MPs. Contact (dermal) exposure will be through skinpores penetration. Exposure by this means is based on individual susceptibility as human skin pores vary by individual.
Very small synthetic fibers (<25 μm) can penetrate skin pores
which is as small as 40–80 μm, but they will bypassed the striatum corneum [4,51-53].
Humans may also be exposed to MPs through inhalation.
This is only possible when the MPs become airborne [4,6].
Few studies have reported the presence of MPs in indoor andoutdoor air [4]. For MPs to be inhalable, it must have a size
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that can allow it reaches the respiratory system and not conform to the criteria of fiber provided by World Health Organization (WHO), i.e. must have a length more than 5 μm, with adiameter less than 3 μm, and a length-to-diameter ratio morethan 3:1 [54]. Recently, Vianello et al. observed from their simulated experiment using a breathing thermal manikin (BTM)
that over 24 h, humans may inhale airborne MPs up to 272
particles from inhaling indoor air [55]. Although using BTM
cannot truly replicate the complexity of the branching airwaysof the human lung, the findings from the study still expose thehigh potential exposure route through inhalation [5].

TOXICITY STUDIES ON MICROPLASTICS
Chemicals can be adsorbed to MPs surfaces and may posethreat biota post-ingestion. The precise mechanisms by which
MPs adsorb toxic chemicals in the environment is yet to beunderstood fully, although the plausible mechanism involves:hydrophobic adsorption, biofilm and plastic additives [1].
MPs, therefore, serve as a carrier for toxic chemicals such as
heavy metals and organic pollutants in the environmental
matrix. Toxicity studies of MPs to marine and terrestrial animals have been well conducted whereas plants are scarcely
studied and recently started gaining increasing attention while
no reports yet concerning humans.
Plants
The effects of MPs on soil-plant and water-plant systems
have started gaining increasing attention recently. MPs in soil
matrix can affect the soil properties including soil aggregation,bulk density and water holding capacity and may also affects
water properties such as total dissolved and suspended solids,
electrical conductivity, pH and dissolved oxygen [3,5-6,56].
Plants are usually exposed to MPs from the soil matrix and theeffect or toxicity they have maybe dependent on plant, type,
shape, size and concentration of MPs [1,4]. Studies conductedso far have focused on vegetative and reproductive growth aswell as chlorophyll contents of plants. These studies are summarized in Table 1. In soil-plant systems, most studies haveadopted an approach of comparing the outcome of single andcombined exposure. Reports so far have shown that MPs exposure can affect negatively, depending on exposure concentration and types, on the growth and varying effects on oxidative activities of plants. However, effects maybe short-term andtransient.
Animals
Although using animal as biomarker are commonly used for
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Enyoh, et al. | Microplastics exposure routes and toxicity
Table 1. Summary of toxicological studies of microplastics (MPs) exposure on plants
MatrixstudiedSoil

MPs type

Treatment

Bio and LDPE

1 % with andwithout
earthworm

Plant studied

Plant
part

ResultConclusion

References

wheat
Above- Bio-affected above ground biomassEarthworm had a positive effect on(Triticum aestivum) ground negatively reducing plant height,
the wheat growth and chiefly allevinumber of tillers, fruits and causing
ated the impairments made by plasthinner stem during the growth pro- tic residues increasing total biomass
cess while LDPE showed no clear ef- by 26.2% after 4 months. However,fect in comparison with control.
both Bio and LDPE significantly affected the total biomass.
Below- Significant decrease in below-groundground biomass
Synthetic fibre,
0.001% w/w,
perennial ryegrass Above- Shoot height was reduced especially The study provided evidence that MPs
Bio-PLA and HDPE 0.1 % w/w and(Lolium perenne) ground for synthetic fibre and Bio-PLA. Inmanufactured of HDPE. PLA, and
0.1 % w/w
creased chlorophyll a and b contents synthetic fibers can affect the develrespectively.
compared to control.
opment of L. perenne health
Below- Seed germination was reduced espeground cially for synthetic fibre and Bio-PLA.
Increased root biomass compared tocontrol except bio-PLA
LDPE, PP and PS 0.5 % w/w
Juvenile Lime Tree Above Showed more negative effects com- C.aurantium generally showed high
(LDPE, PP, PS(Citrus aurantium) ground pared to control by reducing height,
tolerance (>70%) to the different
respectively),number of branches, leaf number and treatment groups. Despite the ob1 % w/w
area. Highest negative effect shown
served toxicity symptoms as the de(for LDPE+PP,
by LDPE while least by LDPE+PP+PS crease in total plant biomass, there
LDPE+PS, PP+PS
was no unfavorable impact on therespectively) andrelative growth rate (RGR) of the
1.5 % w/w
tree.
(LDPE+PP+PS)
Green fluorescent 103 to 107
vascular plant
Below- Reduced germination after 8 hours of Increasing exposure concentration,
Microspheres
Particles/mL(Lepidium satiground exposure with no significant differsize and time reduces germinatingvum)
ences after 24 hours from the control rate and chlorophyll contents.
Sediment PS
10 % dry weightM. spicatum
Above- Shoot length reduced with increasing All effects occurred at higher than enElodea sp.
ground concentration
vironmentally realistic concentrations, suggesting no immediate implications for ecological risks.
Water
PE Microbeads
0, 10, 50,
duckweed
Root Significant decrease in root length with Overall, results showed that specificand 100 mg /L(L. minor)
increasing concentration
leaf growth rate and content of photosynthetic pigments in duckweed
leaves are not negatively affected bypolyethylene microbeads. However,investigated particles significantly
affected the root growth
Leaves After seven days treatment caused
less than 10% inhibition in comparison to control (<8%) with no significant effect. Furthermore, photosynthetic pigment concentration (chlorophyll a and b) was not significantly
affected compared to controls
PE
50,000 MPs/mL
duckweed
Root Root length increased with time from Over seven days PE MPs did not af(L. minor)
24 to 168 hours with no significant
fect photosynthetic efficiency anddifferences from the control
plant growth. However, 30-day
chronic exposure showed some
negative effect on root and leaves.
Leaves Photosynthetic pigment concentration(chlorophyll a and b) was not significantly affected compared to controls
PS-MPs5 µm with 10,
Vicia faba
Whole Reduced growth with varying effect on Showed mild genotoxic effects on the
50 and 100 mg/L
plant
enzyme activities such as catalase
plant and most probably block cell(CAT) (decreased significantly), super- connections or cell wall pores foroxide dismutase (SOD) and peroxidase transport of nutrients.
(POD) (both increased significantly).

[13]

[56]

[57]

[80]

[81]

[58]

[27]

[82]

*Bio: Biodegradable; PLA: polylactic acid, HDPE: high density polyethylene, PP: polypropylene, PS: polystyrene, LDPE: low density polyethylene.

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Environmental Analysis Health and Toxicology 2020;35(1):e2020004

neuro- and genotoxicity, this review focused more on thestudies that related toxicity to sorption of toxic chemical byMPs [59-64]. Many animals collected from both marine andterrestrial ecosystems have shown to ingest MPs [1,15,18].
Therefore, studying the role of MPs in the toxicity of chemical
contaminants cannot be underemphasized. A review of toxicity studies on animals by MPs-toxic chemical interaction has
been conducted recently [65]. Most studies have used Daphnia magna (a small planktonic crustacean belonging to thesubclass Phyllopoda; adult length is usually 1.5–5.0 mm) as amodel specie.
Outcome comparison of single and combined effects of MPs
approach has been commonly adopted across studies. For
example, Rehse et al. using D. magna reported that the half
maximal effective concentration (EC50) for the immobilizationof BPA did not differ significantly from when the organism istreated with both BPA and polyamides (PA) [66]. Furthermore,
Horton et al. reported that the immobilization and toxicity ofpesticide (dimethoate and deltamethrin) by D. magna was notaffected when polystyrene (PS) was introduced [67]. Using thesame organism, similar observation was also made for the immobilization of phenanthrene alone and with irregular PE
MPs [68]. For predatory performance of Pomatoschistus microps, Fonte et al. reported an increase in EC50 due to combined exposure effects to cefalexin and PE spheres [69]. Davarpanah and Guilhermino showed that the EC10 (maximal
effective concentration at 10% inhibition), EC20 (maximal effective concentration at 20% inhibition) and EC50 of Cu alone
on algae (Tetraselmis chuii) growth inhibition did not differ
from the obtained ECx when PE MPs were added with Cu [70].
Therefore, it could be said that PA, PS and PE MPs did not affect the toxicity of the cefalexin, BPA, dimethoate and deltamethrin, phenanthrene and Cu to aquatic organisms [65].
For other chemicals, such as glyphosate, 17a-ethinylestradiol
and Ni, toxicity and MPs type (e.g. PVC), reports have suggested that MP-presence may decrease or increase contaminant
toxicity depending on treatment methods. Zhang et al. observed an antagonistic effect on the growth of the algae (Microcystis aeruginosa) due to the interaction of glyphosate and
PS-NH2 [64]. A gene expression study reported that the bioavailability of phenanthrene and 17a-ethinylestradiol to zebrafish in the presence of MPs decreased significantly due tosorption of these chemicals to PVC [71]. Additive effect suchas increasing bioaccumulation of phenanthrene was observed
for small plastic particles (50 nm) in combination with phenanthrene on D. magna [72]. PS and PS-COOH in water showedantagonistic and synergistic effects on Ni toxicity to D. magna[73]. The toxicity of the latter may have been enhanced by the
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presence of carboxylic acid functional group, which enabledeasy bonding with water molecules. Mateos-Cárdenas et. al.,[27], recently observed that at 24 and 48 h exposure of G. duebeni to PE MPs via trophic transfer from contaminated L. minor showed negligible effect on the amphipod mortality or
mobility. Furthermore, juveniles of a planktivorous fish (Acanthochromis polyacanthus) showed great tolerance to acuteand chronic exposure to polyethylene terephtalate (PET) withno significant effect on growth, body condition or behavior[74].
For terrestrial organisms, soil invertebrates, such as nematodes, collembolan, oligochaeta (e.g. earthworms) and isopods have been studied [1,9,56]. The toxicological effect ofMPs on soil organisms have been extensively discussed andreviewed [1,9]. Toxicity is controlled by exposure concentration, type and shape of MPs and organismal susceptibility. Effects were low ranging from MPs affecting growth or survival,
feeding behavior and energy reserve of soil animals while higheffects including death have not been reported. There is aneed for studies for MPs in the presence of chemical containment on soil animals [1]. However, as pointed by van Gestel
and Selonen, studies focusing on a full dose-response be conducted to address the precise toxicity effects of MPs on organisms is required and if such study adopted a co-exposure of
MPs and chemical contaminants treatment testing, the experimental design should allow some kind of comparison between single and combined toxicities [65,75].
Human
Currently, the toxicity of MPs to humans is still speculative as
there are no studies yet to confirm the toxic effect. Previous reviews have explained in detail the potential effects of MPs tohuman from different environmental compartments[1,4,5,16,17,39,50,76,77]. These studies critically assessed thepotential levels of exposure of particles, chemical and microbial hazards associated with MPs and also inform understanding of uptake, internalization, impacts and potential adverse
human health outcomes of MPs. Overall, the plausible impact
may include particle localization, microbial (biofilm with associated toxins) or chemical toxicity (with associated toxic
chemicals).
Some recent in vitro studies have demonstrated the potential
effects of MPs on human-derived cells [83-85]. These effects
ranged from inflammation to DNA damage. Poma et al. analyzed the effects of PS nanoparticles (PS-NPs) in the Hs27 cellline at different time windows and polymer nanoparticle(PNP) concentrations [83]. The authors adopted the cytokinesis-block micronucleus (CBMN) assay to evaluate the genohttp://eaht.org/ EAHT

Enyoh, et al. | Microplastics exposure routes and toxicity
toxic effects of the PS and the after exposure to PNPs. The results obtained in the study showed damage in the DNA at different parameters of the CBMN test, which increased the formation of micronuclei and nuclear buds. The cellular responses of secondary PP MPs ( ~20 μm and 25–200 μm) were investigated in different conditions, such as size of normal cells, immune cells, blood cells, and murine immune cells by cytokine
analysis, ROS assay, polarization assay and proliferation assay[84]. They found that PP particles showed low cytotoxicity effect in size and concentration of cells, however, a high concentration, small-sized, DMSO (dimethyl sulfoxide) method of PP
particles stimulated the immune system and enhanced potential hypersensitivity to PP particles via an increase in thelevels of cytokines and histamines in peripheral blood mononuclear cell (PBMCs), Raw 264.7 and HMC-1 cells. Wu et al.
adopted a transcriptomic analytic method, and the molecular
toxicity mechanisms of PS MPs was performed in vitro Caco-2cell model [85]. The authors observed that PS-MPs decreased
cell viability and it is dose-dependent while the responsible
genes and involved pathways that might make contribution to
PS-MBs-induced toxicity to Caco-2 cells were identified with
Illumina RNA sequences. These pathways are involved inmodulating cell inflammatory and proliferation. These studies
defined the toxicity of small-sized plastic particles to humanderived cell and the resulting damage to DNA and inflammatory diseases. Further, providing a new insight into the understanding of the toxicity effects of MPs induced intestinal inflammatory diseases. However, further studies are needed toclarify the internalization process of MPs.

compare these studies. This study, therefore, identified the following knowledge gaps that need to be filled:•F ew plants have only been studied for MPs exposure and effects on vegetative and reproductive growth. Meanwhile,
there are more than 200,000 plant species, so it is clear that
there is a need for more studies on more different plant species to better understand their response to MPs exposure.
• There is a need for study to determine if plants can accumulate toxic chemicals and MPs together from the soil.
Also determining if the combined exposure effect on plants
is synergistic, antagonistic or additive. This is a basis for
precise risk assessment.
• As pointed by van Gestel and Selonen, studies focusing ona full dose-response be conducted to address the precise
toxicity effects of MPs on organisms is required [75].
• There is a clear need for more scientific studies on the effects of MPs on human health.

CONFLICTS OF INTEREST
The authors declare no conflict of interest.

REFERENCES
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Chemical Interaction: a Review Study on Quantified Levels, Mechanism and Implications. SN Appl Sci 2019;1:1400.
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Policy: Classify plastic waste as hazardous. Nature 2013;494(7436):
169–171.
3. Enyoh CE, Verla AW, Verla EN. Uptake of microplastics by plant: a

CONCLUSION, RECOMMENDATION AND
KNOWLEDGE GAPS

reason to worry or to be happy? World Scientific News 2019;131:256267.
4. Enyoh CE, Verla AW, Verla EN, Ibe FC, Amaobi CE. Airborne microplastics: a review study on method for analysis, occurrence, move-

Currently MPs are a threat to ecosystems. Studies on exposure akin toxicity in tandem with toxic chemical is very important to understanding MPs dynamics and developing a mitigation strategy. Humans are perhaps the most exposed organism mainly because they are at the top of the food chain. Exposure and toxicology effect to marine and freshwater organisms are classified based on exposure dosage as either high involving decreased reproductive output, damages to organ
and death, or low - involving changes in behavior with time.
Although toxicity studies regarding humans are still ongoingas per reports, plants and animals are still scantly studied. It is
recommended that if a study adopted a co-exposure of MPs
and chemical contaminants treatment testing, the experimental design should allow some kind of comparison between
single and combined toxicities [65] because it is still difficult to

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