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Plastic and Human Health: A Micro Issue?

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Microplastics are a pollutant of environmental concern. Their presence in food destined for human consumption and in air samples has been reported. Thus, microplastic exposure via diet or inhalation could occur, the human health effects of which are unknown. The current review article draws upon cross-disciplinary scientific literature to discuss and evaluate the potential human health impacts of microplastics and outlines urgent areas for future research. Key literature up to September 2016 relating to bioaccumulation, particle toxicity, and chemical and microbial contaminants were critically examined. Whilst this is an emerging field, complimentary existing fields indicate potential particle, chemical and microbial hazards. If inhaled or ingested, microplastics may bioaccumulate and exert localised particle toxicity by inducing or enhancing an immune response. Chemical toxicity could occur due to the localised leaching of component monomers, endogenous additives, and adsorbed environmental pollutants. Chronic exposure is anticipated to be of greater concern due to the accumulative effect which could occur. This is expected to be dose-dependent, and a robust evidence-base of exposure levels is currently lacking. Whilst there is potential for microplastics to impact human health, assessing current exposure levels and burdens is key. This information will guide future research into the potential mechanisms of toxicity and hence therein possible health effects.
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Plastic and Human Health: A Micro Issue?
Stephanie L. Wright*
,
and Frank J. Kelly
MRC-PHE Centre for Environment and Health, Analytical and Environmental Sciences, Kings College London, London SE1 9NH,
United Kingdom
*
SSupporting Information
ABSTRACT: Microplastics are a pollutant of environmental
concern. Their presence in food destined for human consumption
and in air samples has been reported. Thus, microplastic exposure
via diet or inhalation could occur, the human health eects of
which are unknown. The current review article draws upon cross-
disciplinary scientic literature to discuss and evaluate the
potential human health impacts of microplastics and outlines
urgent areas for future research. Key literature up to September
2016 relating to accumulation, particle toxicity, and chemical and
microbial contaminants was critically examined. Although micro-
plastics and human health is an emerging eld, complementary
existing elds indicate potential particle, chemical and microbial
hazards. If inhaled or ingested, microplastics may accumulate and exert localized particle toxicity by inducing or enhancing an
immune response. Chemical toxicity could occur due to the localized leaching of component monomers, endogenous additives,
and adsorbed environmental pollutants. Chronic exposure is anticipated to be of greater concern due to the accumulative eect
that could occur. This is expected to be dose-dependent, and a robust evidence-base of exposure levels is currently lacking.
Although there is potential for microplastics to impact human health, assessing current exposure levels and burdens is key. This
information will guide future research into the potential mechanisms of toxicity and hence therein possible health eects.
INTRODUCTION
Plastic is a material that provides enormous societal benet.
Global production currently exceeds 320 million tonnes (Mt)
per year, over 40% of which is used as single-use packaging,
resulting in plastic waste.
1
A substantial proportion of the
plastic produced each year is lost to and persists in the marine
environment, with an estimated accumulative potential of 250
Mt by 2025.
2
Consequently, plastic debris is a critical
environmental issue. Exposure to ultraviolet (UV) radiation
catalyzes the photo-oxidation of plastic, causing it to become
brittle. In combination with wind, wave action and abrasion,
degraded plastic fragments into micro- (0.11000 μm)
3
and
potentially nanosized (0.1 μm)
4
particles, referred to from
herein as micro- and nanoplastics, respectively.
Microplastics are also purposefully manufactured for various
applications, such as exfoliants (microbeads) in personal care
products.
5
This material, along with plastic microbers from
machine-washed clothing,
6
is directly released to the environ-
ment in municipal euent.
7
Recently, it was reported that
although a wastewater treatment plant (WWTP) reduced the
microplastic concentration of euent by >98%, an estimated 65
million microplastics were still released into the receiving water
daily.
8
Furthermore, in the United States, it was conservatively
estimated that up to 8 trillion microbeads enter aquatic habitats
each day via WWTPs, presenting a notable source.
9
Marine debris, including glass, metals, paper, textiles, wood
and rubber, is dominated by plastic. Of this, microplastics are
often most common.
10
They occur in a variety of shapes; bers
are the most commonly reported form,
6
followed by frag-
ments.
11
Microplastics are ubiquitous, having been reported in
aquatic habitats worldwide from the poles
12
to the Equator.
13
An estimated 5.25 trillion plastic particles contaminate the
global sea surface,
14
whereas approximately 4 billion bers
km2contaminate the deep Indian Ocean oor.
15
Even Arctic
Sea ice represents a sink for microplastics, indicated by their
presence in ice cores from remote locations.
16
Nanoplastics are also increasingly being manufactured.
Paints, adhesives, drug delivery vehicles, and electronics are
some of the products that may contain nanoplastics.
17
3D
printing, for example, can emit polymeric nanoparticles.
18
The
reduction in size, both purposefully and due to environmental
degradation, may induce unique particle characteristics, which
could inuence their potential toxicity.
Because of their hydrophobic surface, microplastics can
adsorb and concentrate hydrophobic organic contaminants
(HOCs) such as polycyclic aromatic hydrocarbons (PAHs),
organochlorine pesticides and polychlorinated biphenyls
(PCBs) to a high degree.
19,20
They also accumulate heavy
metals such as cadmium, zinc, nickel, and lead.
21,22
Micro-
plastics are thus considered as vectors for these priority
Received: February 7, 2017
Revised: May 15, 2017
Accepted: May 22, 2017
Published: May 22, 2017
Critical Review
pubs.acs.org/est
© XXXX American Chemical Society ADOI: 10.1021/acs.est.7b00423
Environ. Sci. Technol. XXXX, XXX, XXXXXX
pollutants,
23
which are listed in the Stockholm Convention for
their potential adverse health eects.
24
Microplastics may harbor endogenous chemical additives,
due to their incorporation during the manufacture of plastic
products. Because these additives are not chemically bound to
the plastic polymer matrix, they are susceptible to leaching to
the external medium.
25
There is potential for the constant
migration of intrinsic chemicals along a concentration gradient
to the surface of microplastics as they continue to fragment.
Such pollutants can be released upon ingestion and transfer to
surrounding tissue.
26,27
If microplastics have the capacity to
accumulate, they potentially present a source of chemicals to
tissues and uids, if there is any additive remaining to leach.
Emerging evidence suggests that human exposure to
microplastics is plausible. Microplastics have been reported in
seafood,
2830
and in processed food and beverages such as
sugar,
31
beer,
32
and salt.
33
In addition, the sludge byproducts of
WWTPs that are applied to agricultural land have been found
to contain synthetic (plastic) clothing bers, which persist up to
5 years postapplication.
34
The wind-driven transport of
microplastics from sludge-based fertilizer, in addition to other
sources such as the degradation of agricultural polyethylene
(PE) sheets or the release of bers from drying clothes outside,
could also result in airborne microplastics.
35
The atmospheric
fallout of microplastics has recently been reported,
36,37
representing a possible inhalation exposure pathway. Whether
microplastics and their associated chemicals are transferred to
humans via diet and/or inhalation is unknown.
The quantity of microplastics in the environment is likely to
increase due to the legacy of plastic items that contaminate the
planet. Given the evidence suggesting human exposure to
microplastics and their associated pollutants is possible, it is
important to assess the risk they pose to human health. To our
knowledge, there are two peer-reviewed articles that review this
subject;
35,38
however, neither article considers inhalation as a
potential exposure pathway, and the subsequent toxicity this
could exert on the respiratory tract. We build on these two
publications to incorporate the marine environment, diet, and
inhalation as pathways to microplastic exposure. This review
therefore aims to assess the evidence for this new potential
environmental challenge by addressing the following issues: (1)
dietary exposure pathways; (2) inhalation exposure pathway;
(3) microplastic uptake and translocation; and (4) potential
human health risks of microplastics.
EVIDENCE FOR DIETARY EXPOSURE PATHWAYS
Seafood. Given the prevalence of microplastics in the
marine environment, an anticipated route of human exposure is
via seafood, which forms an essential dietary component.
Seafood provides almost 3 billion people worldwide with
approximately 20% of their animal protein intake.
39
It is
therefore one of the most important food commodities
consumed globally; however, it can also be a source of
environmental contaminants such as PCBs and dioxins. If
seafood were to exceed regulatory levels of contaminants, there
could be negative health impacts following consumption;
however, these regulations are only in place for specic
contaminants, e.g., mercury, not for contaminants of emerging
concern such as microplastics.
Fish. Globally, sh provides approximately 4.3 billion people
with 15% of their animal protein intake.
40
The capacity for sh
to ingest microplastics has been demonstrated in laboratory
studies,
41,42
although these employed substantially higher
concentrations of microplastics than those found in nature.
43,44
Importantly, the ingestion of microplastics by sh in situ has
been widely reported, including by commercial species,
although the quantity of ingested microplastics is low (Table
S1).
The occurrence of microplastics in the gastrointestinal tract
(GIT) of sh does not provide direct evidence for human
exposure, as this organ is usually not consumed. There is
potential for the leaching and accumulation of associated
chemical contaminants in edible tissue, post-microplastic
ingestion. Dietary microplastic exposure via sh could be
possible if microplastics were able to translocate across the GIT
or gill via transcellular uptake or paracellular diusion and enter
the circulatory uid. The respiratory epithelium of the gill is
much tighter than that of mammalian lungs, decreasing the
likelihood of this route of exposure; uptake across the sh gut is
more likely.
45
There is evidence for the uptake of 1 μm latex spheres from
the surrounding water in rainbow trout, with particles localizing
and persisting in the surface and subsurface epidermal cells of
the skin and in phagocytes underlying the gill surface.
46
This
highlights the importance of sh epithelial cells in the
attachment and entry of microplastics. Additionally, con-
sumption of the skin or gill tissue could present a direct
route of human exposure to microplastics (1μm).
Shellsh. Perhaps the most important source of dietary
exposure to microplastics at present is via bivalve molluscs
(shellsh). Shellsh represents an important food source,
comprising approximately 22 Mt of world sh production from
capture and aquaculture in 2012 (almost 15 million USD).
40
Bivalves feed by pumping large volumes of water through the
pallial cavity within their shells, retaining particles from
suspension on their gills for subsequent ingestion.
47
Thus,
they are directly exposed to microplastics via the water column.
There is ample evidence for the capture and ingestion of
microplastics by bivalves in laboratory studies,
4850
and
microplastics in wild and aquaculture shellsh for human
consumption have been detected.
Bivalves are a popular seafood in China
28
where >60% of the
global aquaculture volume is produced. This coincides with
where the greatest volume of plastic enters the marine
environment from land-based sources.
2
Consequently, concen-
trations reaching 8720 microplastics/kg of sediment, including
polyethylene terephthalate (PET), polystyrene (PS) and PE
particles, have been found on beaches.
51
Nine of the most
commercially popular species of bivalves purchased from a
shing market in Shanghai, were found to be contaminated
with microplastics (Table S2).
28
Based on the abundances
observed, it was estimated that Chinese shellsh consumers
could be exposed to 100 000s of microplastics each year.
The contamination of shellsh by microplastics is not limited
to China. In Canada and Belgium, both wild and purchased
farmed mussels were contaminated by microplastic bers.
29,52
Farmed mussels are often cultured on deployed polypropylene
(PP) lines, which may present a source of microplastics as the
line degrades.
29,52
In Belgium, microplastics were recovered
from farmed mussels and shop-bought Pacic oysters, which
were subjected to a 3 day depuration period. Based on the
average recovered concentrations, it was estimated that the
average European shellsh consumer may ingest up to 11 000
microplastics per year.
30
It is of concern that following 3 days of
depuration, microplastics remained in the bivalves, suggesting
standard depuration periods may not be sucient to ensure
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microplastic clearance. Shellsh food safety is an increasingly
important issue with respect to microplastics.
Other Foods. In addition to seafood, potential microplastics
have been reported in other foods. The presence of synthetic
microbers (minimum 40 μm in length) and fragments (mostly
1020 μm in size) was reported in honey and sugar.
31
An
average of 174 (maximum of 660) bers and 8 (maximum of
38) fragments/kg honey, and an average of 217 (maximum of
388) bers and 32 (maximum of 270) fragments/kg of sugar
were found. Negative Rose Bengal staining determined which
bers and fragments were of synthetic origin; however, no
further methods were used to identify denitively whether the
particles were plastic.
31
The contamination of honey suggests synthetic micro-
particles and microplastics are airborne. In support of this
hypothesis, the authors reported nding 18 bers and 4
fragments/L of rain during precipitation events. If airborne,
microplastics may be deposited on owers and foliage, where
they could become incorporated with pollen and transported
by bees to the hive. In support of this, the authors reported
nding bers in owers.
31
Perhaps even more remarkable is that the contamination of
German beer by potential microplastics has been reported.
32
In
all 24 samples tested, contamination by potential microplastics
was found. Fragments were the most abundant, reaching up to
109 fragments/L. One suggested source was the atmospheric
deposition of microplastics while a second related to the
materials used in the production process.
32
Given the
prevalence of microplastics in freshwater systems,
53
the water
source may also be a source of contamination.
Microplastics have recently been identied in 15 brands of
shop-bought sea salt. Up to 681 microplastics/kg sea salt were
reported down to 45 μm. PET was the most common type of
plastic found, followed by PE. It is likely that the coastal waters
used to produce sea salt were the source of contamination,
33
although microplastics could also be present due to
atmospheric deposition at these sites.
Clearly, microplastics currently contaminate food destined
for human consumption, the impacts of which are unknown.
The presence of microplastics in other foods also suggests they
contaminate the atmospheric environment.
EVIDENCE FOR AN INHALATION EXPOSURE
PATHWAY
In the ocean, sea salt aerosol (SSA) formation occurs due to
bubbles bursting during white cap formation and wind stress, or
due to waves breaking in the coastal surf zone. SSAs can range
in size from <0.2 to >2000 μm diameter; the ambient mass is
typically dominated by particles in the 110 μm range. During
periods of onshore winds, they can be transported to urban
environments close to the coast. Particles <50 μm are likely to
have an extended atmospheric lifetime.
54
Because many plastics
have a specic gravity less than seawater, it is plausible that
wind action and sea spray may aerosolise sea-surface micro-
plastics of appropriate size; however, this theory remains to be
tested.
WWTP sludge byproducts applied to agricultural land have
been found to contain synthetic clothing bers, which persist in
both the sludge and soil columns up to 5 years postapplication.
Synthetic bers have even been detected in eld site soils 15
years after application.
34
This suggests that microplastics
released via municipal euent are retained in sludge, which is
then applied as fertilizer, representing a persistent terrestrial
contaminant. The wind-driven transport of microplastics from
dried sludge-based fertilizer in addition to other sources such as
the degradation of agricultural PE sheets
55
or the release of
bers from drying clothes
32
could all represent sources of
airborne microplastics.
Recently, evidence for the presence of microplastics in
atmospheric fallout has been reported.
37
The total atmospheric
fallout of microplastics was assessed in a densely populated
urban area and a less-dense suburban area in Paris. The
majority of particles observed were bers, approximately 30% of
which were conrmed plastic. Diameters varied mainly between
7 and 15 μm and almost 25% of bers were 100500 μmin
length; 50 μm was the limit of detection. Up to 355 particles/
m2/d were reported, with an average of 110 ±96 particles/m2/
d. Abundances were substantially greater in urban than
suburban areas.
37
Periods of heavy rainfall corresponded with
some of the highest concentrations observed.
37
This prelimi-
nary study highlights the potential for human exposure to
microplastics via inhalation, especially in densely populated
areas.
To quantify the level of ber exposure, a small scale study
assessing 24 h personal exposure to respirable inorganic and
organic bers was undertaken at 3 European sites. Mean
personal exposure levels to organic bers (diameter <3 μm)
were 0.0030.011 bers/mL with lengths <5 μm, 0.0090.019
bers/mL with lengths >5 μm, and 0.00080.002 bers/mL
with lengths >20 μm.
56
Although organicbers (natural and
manmade) included PE, PP, poly(vinyl alcohol), polyester,
polyamide (PA), polytetrauoroethylene, carbon, and natural
cellulose,
56
no distinction was made regarding the composition
of the bers sampled.
Tires have recently been acknowledged as a source of
microplastics. Synthetic rubber is a variation on plastic,
produced by the plastics industry. It is also a hydrocarbon-
based polymer, although it has dierent properties to plastic,
such as elasticity. Tire abrasion products are a reported
component of ambient particulate matter (PM). In Japan, an air
sample was reported to contain 0.16 μg/m3of tire wear
particles in the PM10 fraction.
57
Furthermore, the concentration
of tire and road wear particles (TRWP), tire particles with road
mineral incrustations, was low, with global (United States,
Europe, and Japan) averages ranging from 0.05 to 0.70 mg/m3.
This comprised an average PM10 contribution of 0.84%.
58
On an occupational level, indoor exposure levels can reach
0.5 and 0.8 particles/mL for polyvinyl chloride (PVC) and
nylon (PA), respectively; critical particle concentrations (aspect
ratio 3μm) were 0.06 and 0.02/mL, respectively.
59
In the
ocking area (where many small bers are deposited onto a
surface) of a ock (microbre) manufacturing plant, the highest
concentration of airborne particles reached 7 mg/m3.
60
Polyester concentrations of 700 000 (up to 1 000 000) total
bers/m3and 10 000 critical bers/m3were reported during
processing operations.
59
Size and exposure concentrations inuence the potential risk
that microplastics pose to human health. Thus, to understand
the risk, it is rst important to consolidate current exposure
concentrations to inhalable, thoracic, and respirable particles.
Airborne bers are ubiquitous and some of these bers are
likely to be inhaled. Once they gain entry to the respiratory
tract, most bers are likely to be trapped by the lung lining
uid. However, some bers may avoid the mucociliary
clearance mechanisms of the lung, especially in individuals
whose clearance mechanisms have been impaired. Occupational
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health literature specic to the synthetic textile industry
provides a good indication of the anticipated hazards that
microplastics, particularly bers, may incur to human health.
MICROPLASTIC UPTAKE AND TRANSLOCATION
Fibers and Occupational Health. Studies among nylon
ock workers suggest there is no evidence of increased cancer
risk, although workers had a higher prevalence of respiratory
irritation.
61
Interstitial lung disease, a work-related condition
that induces coughing, dyspnea (breathlessness), and reduced
lung capacity, has been identied in 4% of workers from nylon
ock plants in the US and Canada.
62,63
Workers processing
para-aramid, polyester, and PA bers in the Netherlands
presented similar symptoms, including coughing, dyspnoea,
wheezing, and increased phlegm production.
64
Prick tests and
nasal and inhalation provocation tests in nylon workers also
found synthetic bers, such as nylon, may act as haptens,
causing an allergic reaction leading to occupational asthma.
65
Histopathological analysis of lung biopsies from workers in
the textile (nylon, polyester, polyolen, and acrylic) industry
showed interstitial brosis and foreign-body-containing gran-
ulomatous lesions, postulated to be acrylic, polyester, and/or
nylon dust. The clinical symptoms presented were similar to
allergic alveolitis (a form of inammation in the lung).
66
Although occupational exposure likely occurs at levels higher
than those in the environment, the health outcomes evidence
the potential for microplastics to trigger localized biological
responses, given their uptake and persistence.
Both cellulosic and plastic microbers have been observed in
non-neoplastic and malignant lung tissue taken from patients
with dierent types of lung cancer.
67
The bers exhibited little
deterioration, supporting the notion that they are biopersistent.
Additionally, these observations suggest that the human airway
is of a sucient size for plastic bers to penetrate the deep
lung; one ber found was 135 μm in length, approximately one-
quarter of the diameter of a respiratory bronchiole of
generation 17 (540 μm diameter, 1410 μm length).
67
These
observations conrm that some bers avoid clearance
mechanisms and, as they persist, these foreign bodies may
induce acute or chronic inammation. Importantly, rigorous
sterile methods were employed throughout the sample
processing in this study to prevent contamination by
environmental bers.
In addition to biopersistence, ber dimensions play a role in
toxicity. Thinner bers are respirable, whereas longer bers are
more persistent and toxic to pulmonary cells; bers 1520 μm
cannot be eciently cleared from the lung by alveolar
macrophages and the mucociliary escalator,
61
and bers <0.3
μm thick and >10 μm long are most carcinogenic.
68
The use of
ne-diameter (15μm) plastic bers has increased, such as in
the sports clothing industry.
61
Nylon bers of a respirable size
(2 μm diameter, 14 μm length on average) interacted with the
alveolar macrophages of exposed rats and were retained up to at
least 29 days postexposure, causing an acute inammatory
response.
69
Shorter (9.8 μm) but wider (1.6 μm diameter)
nish-freenylon respirable bers, however, showed no
signicant impact on lung weights, pulmonary inammation
or macrophage function in male rats up to the highest
concentration tested (57 bers/cm3) compared to control
animals.
70
The burden of bers, site of deposition and the
potential for chemicals to desorb from the ber surface also
contribute to toxicity,
67
e.g., the anity of PAHs for the
hydrophobic surface of plastic
23,71
may present a route of
carcinogenicity.
Potential for and Factors That May Aect Bioaccu-
mulation. An essential factor determining whether micro-
plastics present a physical threat or act as a vector for chemical
transfer is the ability for these particles to accumulate.
Throughout evolution, it is likely that both the lungs and
GIT have been exposed to nondegradable, exogenous nano-
and microparticles, and endogenous nanoparticles.
72,73
Con-
sequently, the body has evolved mechanisms to respond to
particle exposure (Figures 1 and 2). Recently, there has been an
increased dietary inux of nondegradable microparticles,
approximately 40 mg/person/day, primarily due to their
inclusion as additives in processed foods.
73,74
The contribution
of microplastics to exogenous microparticle exposure is
unknown, however the biological response to microplastics in
comparison to other nondegradable microparticles could dier
due to their unique chemical composition and properties.
Microplastics are resistant to chemical degradation in vivo.If
inhaled or ingested, they may also resist mechanical clearance,
becoming lodged or embedded. Their biopersistence is an
essential factor contributing to their risk, along with dose. The
uptake and toxicity of several types of polymeric nano- and
Figure 1. Potential microplastic (0.1 > 10 μm) uptake and clearance
mechanisms in the lung. (A) The chance of microplastic displacement
by the lung lining uid (surfactant and mucus) is reduced in the upper
airway, where the lining is thick (central lung). Here mucociliary
clearance is likely for particles >1 μm. For particles <1 μm, uptake
across the epithelium is possible.
107
(B) If the aerodynamic diameter
of a microplastic permits deposition deeper in the lung, it may
penetrate the thinner lung lining uid and contact the epithelium,
translocating via diusion or active cellular uptake (adapted from ref
162). Reprinted from Ruge, C. A.; Kirch, J.; Lehr, C. M. Pulmonary
drug delivery: From generating aerosols to overcoming biological
barriers-therapeutic possibilities and technological challenges. Lancet.
Respir. Med. 2013, 1(5), 402413.
162
Copyright 2013 Elsevier.
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microparticles have been studied in model mammalian systems.
The ndings suggest they can translocate across living cells to
the lymphatic and/or circulatory system,
72,75
potentially
accumulating in secondary organs,
7678
or impacting the
immune system and health of cells.
79,80
Retention time, and therefore the likelihood of uptake and
clearance, is inuenced by particle characteristics such as size,
shape, solubility, and surface chemistry; by biological factors
such as the anatomical site of deposition and structure; and by
the nature of particle interaction with dierent biological
structures, including the airliquid interface, aqueous phase
and free cells (e.g., macrophages, dendritic cells, epithelial
cells).
81
Uptake of inhaled microplastics will depend on their
wettability; it is possible that inhaled microplastics deposited on
the airway will not be immersed in the lung lining uid due to
their hydrophobicity, and may therefore be subjected to
mucociliary clearance leading to exposure via the gut (Figure
1). Shape also aects displacement at the airliquid interface;
shapes with sharper edges are less likely to be displaced in
liquid.
82
However, the histological prevalence of plastic
microbers in ock worker
66
and lung cancer
67
tissue biopsies
implies that uptake and embedment of at least plastic
microbers is possible.
As with lining uid in the lung, mucus is the rst layer in the
GIT that foreign particles interact with. Here, mucus can cause
particles to aggregate; surfactants reduce mucus viscosity,
increasing the uptake of particles.
83
Size and surface charge also
inuence the ability for microplastics to cross the GIT mucus
gel layer and contact the underlying epithelial cells;
84,85
smaller
sizes and negative surface charge are most likely to lead to
increased uptake.
If a microplastic contacts the airway or gastrointestinal
epithelium, there are several routes of uptake and translocation
that may occur. This is primarily via endocytic pathways in the
lung and GIT, and also via persorption in the GIT (Figures 1
and 2). Paracellular transfer of nanoparticles through the tight
junctions of the epithelium has been postulated for the GIT.
Although tight junctions are extremely ecient at preventing
such permeation, their integrity can be aected, potentially
allowing for particles to pass through.
73
Uptake Pathways. Endocytosis: Airway Surface. If an
inhaled microplastic encounters the respiratory epithelium, it
may translocate via diusion, direct cellular penetration or
active cellular uptake, as has been reported for other
nonbiological micro- and nanoparticles.
86
The active uptake
of nano- and microparticles by epithelial and endothelial cells
occurs via energy-dependent endocytic and phagocytic
processes.
87
Phagocytosis is the primary clearance pathway
for particles 13μm from the alveoli.
88
PS microparticles (1
μm) were phagocytosed by porcine pulmonary macrophages,
whereas smaller PS microparticles and nanoparticles (0.2 and
0.078 μm) seemed to be passively transported via diusion
across membrane pores, as endocytic particles were not
membrane-bound (Figure 1).
88
Endocytosis: Gastrointestinal Tract. In the GIT, the Peyers
patches of the ileum (third portion of the small intestine) are
considered the major sites of uptake and translocation of
particles.
73,89
These domed regions are characterized by an
epithelial layer of M cells, so-called due to their specialized
luminal surface microfolds,
90
and enterocytes. Beneath this
layer is the subepithelial dome; a cavity containing lymphocytes
and/or macrophages (Figure 2A). Peyers patches form part of
the gut-associated lymphoid tissues; M cells sample and
transport particles (0.1 < 10 μm) from the intestinal lumen
to the mucosal lymphoid tissues,
76,91
playing a key role in
immune homeostasis.
92
The subepithelial dome of the Peyers
patches act as sinks, safely storing nondegradable particles.
M cells have a high transcytic capacity.
93,94
An estimated 60%
of PS nanoparticle (60 nm) uptake occurred via the Peyers
patches in rats following a 5 day oral dosing.
95
The uptake of
plastic microspheres (12.2 μm) by the Peyers patches has
been reported in other mammalian models.
9698
Other
nondegradable microparticles, such as aluminosilicates and
titanium dioxide (TiO2), are retained in the basal phagocytes of
Figure 2. Predicted pathways of microplastic uptake from the
gastrointestinal tract (GIT). (A) Microplastic (0.1 > 10 μm) uptake
from the GIT lumen via endocytosis by the M cells of the Peyers
patches. M cells sample and transport particles from the intestinal
lumen to the mucosal lymphoid tissues (adapted from ref 163). (B)
Microplastic uptake from the GIT lumen via paracellular persorption.
Nondegradable particles, such as microplastics, may be mechanically
kneaded through loose junctions in the single-cell epithelial layer into
the tissue below. Dendritic cells are able to phagocytose such particles,
transporting them to the underlying lymphatic vessels and veins.
Distribution to secondary tissues, including the liver, muscle and brain,
could occur (adapted from ref 163). Reprinted from Mowat, A. M. I.
Anatomical basis of tolerance and immunity to intestinal antigens. Nat.
Rev. Immunol. 2003, 3 (4), 331341.
163
Copyright 2003 Macmillan
Publishers Ltd.
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the Peyers patch, where they can occur in large numbers.
73
If
microplastics also accumulate in this compartment, they could
hijack the route for endogenous microparticle uptake and
consequently interfere with immunosensing and surveillance,
compromising local immunity.
Persorption. Another route of uptake in the GIT, and
perhaps the most applicable to microplastics due to the size
range it covers, is via a phenomenon known as persorption.
Persorption describes the mechanical kneading of solid particles
(up to 130 μm diameter) through gaps in the single-layer
epithelium at the villus tips of the GIT (desquamation zones),
99
and into the circulatory system (Figure 2B).
100,101
PVC particles (5110 μm) have been used as model
nondegradable microparticles, along with starch, to study this
phenomenon.
78
Following exposure via feeding or rectal
administration, the microplastics were observed to pass
between enterocytes in a paracellular manner, especially in
desquamation zones and between the villi. The transportation
of PVC particles occurred via two routes. First by the chyle
(lumen) of the underlying lymph vessels, seen in rats, guinea
pigs, rabbits, chickens, dogs and pigs. Second by portal
circulation, suggested by the increased occurrence of particles
in blood taken from the mesenteric veins of intestinal segments
of dogs fed PVC particles.
78
The appearance of PVC particles
in the blood of dogs occurred rapidly postingestion; however,
exposure concentrations were high 200 g of PVC powder,
resulting in 1015 PVC particles/mL of venous blood 12h
postingestion. PVC particles were subsequently found in bile,
urine, and cerebrospinal uid.
78
Larger particles are found in
tissues and organs; PVC microparticles appeared in the liver of
exposed rats, peaking 23 and 10 min postesophageal
administration.
78
The reason for this multipeak curve has not
been claried but the study suggests that if ingested,
microplastics may persorb across the intestinal wall and be
transported to secondary tissues by the lymphatic and portal
systems. Cerebral softening, micronecroses and scarring were
observed in the brains of dogs postexposure via femoral artery
catheterization into the left ventricular cavity.
100
Persorption has been reported in human subjects. The
ingestion of starch particles (200 g) led to granules being
observed in urine, bile, cerebrospinal uid, peritoneal uid, and
breast milk.
102
Particles peaked in the blood at 10 min (70
particles/10 mL) and at 110 min (90 particles/10 mL)
postingestion.
102
However, the same authors found the rate
(particles recovered in the blood over 24 h postexposure) of
persorption to be low (0.002%).
99
Persorption is inuenced by
both rigidity of the particle and the level of motor activity in the
GIT.
99
The rigidity of microplastics combined with a likely
exposure pathway via diet suggests persorption of microplastics
during the consumption of contaminated food could occur.
Factors Aecting Uptake. Following uptake, translocation
can occur via macrophages to the thoracic lymph nodes, and
through systemic circulation, reaching secondary target organs
including the liver, kidneys, spleen, heart, and brain.
103105
The
uptake pathway largely depends on the properties of both the
cell type and the target particle, including its surface chemistry
and size (Figure 3).
106
Size. Size inuences the likelihood and eciency of uptake
and clearance as it governs the processes involved. Very small
inhaled particles (i.e., <1 μm), or those that persist on the
epithelial surface, will be taken up by cells and potentially cross
the epithelium.
107
Translocation eciency increases with
decreasing size
105
and dierent clearance mechanisms are
involved for dierent size fractions.
108
Based on the potential
for uptake, it can be anticipated that inhaled nanoplastics would
reach the deep lung and cross the lung epithelial lining, whereas
microplastics may be subjected to mucociliary clearance,
entering the GIT.
In the GIT, smaller particles are also postulated to
translocate across the gut at a greater eciency than larger
particles; a higher abundance (34%) of 50 nm PS particles,
administered (1.25 mg/kg) by gavage to rats, were taken up in
comparison to larger PS nanoparticles. These smaller particles
distributed to the liver, spleen, and bone marrow, whereas
particles >100 nm did not reach bone marrow, and particles
>300 nm were not detected in blood.
109
This contrasts to the
observations of Volkheimer,
101
which suggested persorption of
particles up to 130 μm across the GIT occurred. Doyle-
McCullough et al.
110
reported a low uptake eciency (0.1
0.3% of the dose) of PS microparticles (2 μm) primarily by
nonlymphoid tissue, i.e., via the villi, contrasting with other
studies that emphasize the role of the lymphatic Peyers patches
in the uptake of microparticles.
73,93,94
Surface Chemistry and Hydrophobicity. Microplastic
uptake and translocation will also likely be related to surface
chemistry and hydrophobicity. Surface functionalization greatly
inuences particle recognition and uptake. Following a 2 h
incubation to quartz nanoparticles with modied surfaces, most
A549 human lung epithelial cells had endocytosed noncoated
quartz particles whereas 15% of cells exposed to poly(2-
vinylpyridine-1-oxide)-coated quartz had ingested particles.
111
Surface charge also inuences the uptake pathway. There is
evidence that internalization of negatively charged PS nano-
particles is via clathrin- and dynamin-dependent endocytosis;
the uptake of carboxylated PS nanoparticles by macrophages
was inhibited by the presence of onodansyl cadaverine and
dynasore (inhibitors of clathrin-mediated endocytosis and
dynamin-dependent endocytosis, respectively). Alternatively,
positively charged PS nanoparticles are internalized through
micropinocytosis.
106
Surface chemistry, not charge, has been
found to have a greater inuence on translocation; there was a
30-fold dierence in uptake between two types of negatively
charged PS nanoparticles.
112
In an in vitro model of human Peyers patches, a greater
proportion of PS nanoparticles (200 and 500 nm) with cationic
sites were transported across than carboxylated nanoparticles.
This was linked to the hydrophobicity of particles,
75
which may
be attributed to better transport through the mucus layer.
81
Hydrophobicity also inuences the adsorption of proteins to
the particle surface, resulting in a unique protein pattern known
as a corona.
Protein corona formation is strongly dependent on the
chemical composition of the particle. Size is also important, as
nanoparticles have a relatively large surface area per unit of
mass for the adsorption of organic compounds from the
Figure 3. Particle characteristics predicted to inuence micro- and
nanoplastic uptake.
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surrounding environment.
88
It has been shown that non-
biological microparticles form biomolecule conjugates during
incubation in the GIT lumen in the presence of calcium
precipitates.
113
Components of the intestinal secretion,
primarily endoproteins, whole and partially digested bacteria
and nonabsorbed food antigens, adsorb onto the microparticle
surface. Consequently, cells are exposed to conjugates of the
nonbiological particle and biomolecules. Given the hydro-
phobic surface of microplastics, a unique assemblage of lumen
components are likely to accumulate. In turn, this will
encourage uptake via M-cells overlying the Peyers patches.
113
A factor that will aect microplastic surface chemistry is the
digestive environment, where the pH changes dramatically, e.g.,
acidic stomach to the neutral small intestine. The action of
digestive enzymes will also likely alter the chemical character-
istics of microplastics as they are transported along the GIT.
73
In addition to cellular uptake, the surface charge of
microplastics may also inuence the extent and pathway of
translocation to secondary organs. Following an oral single dose
exposure to PS nanoparticles, rats had accumulated a greater
amount of negatively charged particles in almost all organs
observed than positively charged particles. PS nanoparticles
accumulated in the kidney, heart, stomach wall, and small
intestine and the estimated bioavailability of the particles was
0.21.7%.
112
Despite having a low bioavailability, the PS
nanoparticles still spread to secondary organs.
Elimination. Microplastics are likely resistant to degradation
and will therefore persist unless eliminated. Elimination of
ingested nondegradable microparticles has been observed
following persoprtion across the GIT. Elimination via the bile
begins several minutes postoral application, whereas elimi-
nation via urine occurs within 8 h of exposure, most of which is
during the rst 4 h.
101
Particles are also eliminated via urine,
pulmonary alveoli, peritoneal cavity, cerebrospinal uid, and the
milk in animals and lactating women. Moreover, the passage of
PVC particles via the placenta into fetal circulation has been
reported.
78
This is clearly an important observation and one
that deserves further investigation.
The removal of inhaled microplastics is likely to be
inuenced by size and surface properties. Microplastics
deposited in the upper airway are likely to be cleared by
mucociliary transport, and thus enter the GIT, whereas in the
alveolar, macrophages are responsible for clearance.
POTENTIAL HUMAN HEALTH RISKS OF
MICROPLASTICS
Potential Toxicological Pathways. Plastic is considered
an inert material; however, there are pathways through which
microplastics could cause harm, such as the deposition of PVC
granules causing embolization of small vessels in animals
following long-term oral administration.
78
Size, shape,
solubility, and surface charge all inuence the cytotoxicity of
particles to cells and tissues in vivo.
114
Regarding physical
eects, the biopersistence of microplastics could lead to a suite
of biological responses including inammation, genotoxicity,
oxidative stress, apoptosis, and necrosis. If these conditions are
sustained, a range of outcomes can ensue including tissue
damage, brosis and carcinogenesis. Chemical eects could
establish due to the composition of the polymer itself; the
leaching of unbound chemicals and unreacted residual
monomers; or the desorption of associated hydrophobic
organic contaminants (HOCs). These are often priority
pollutants with known human health eects. The cellular
uptake of microplastics would allow adhered or endogenous
contaminants cellular entry.
115
Inhalation exposure studies have previously demonstrated
that oxidative stress and subsequent inammation presents the
best paradigm for particle toxicity (see references within 114).
Oxidative stress due to challenge with nanoparticles including
PM, quartz, and TiO2results in airway inammation and
intestinal brosis.
114
A similar mechanism of toxicity may be
observed for micro- and nanoplastics due to their small size and
therefore large surface area for functional sites.
All plastics contain reactive oxygen species (ROS) due to
their polymerization and processing history. However, the
concentration of free radicals can signicantly increase
following interaction with light or the presence of transition
metals. The weathering of plastics and microplastics leads to
free radical formation by the dissociation of the CH bonds.
(see references within 116 and 117). The free radicals continue
to react and therefore may pose danger. Termination of these
free radical reactions is achieved through the reaction of pairs of
ROS or oxidation of a target substrate, such as tissues.
116
Inammation and Immune Responses. Wear Debris
from Plastic Prosthetic Implants. There is a legacy of literature
concerning inammation due to wear particles from abraded
plastic prosthetic implants, which indicate the anticipatory
biological reactions that may occur if microplastics were to
cross the pulmonary or GIT epithelium. PE and PET wear
particles have been observed in the joint capsule, cavity and
surrounding tissue of patients in receipt of plastic endopro-
theses. These particles range in shape from granules to
spears.
118
The cellular response ranges from a few scattered
cells to extensive aggregations of macrophages.
119
PE particles
(0.550 μm) provoke a nonimmunological foreign body
response.
120
Particles locate to cells,
118
and cellular aggrega-
tions resembling foreign body granulation tissue have been
observed. PE particles also locate to neighboring vessels, where
transportation via the perivascular lymph spaces occurs.
118
In
rabbits, smaller PE particles (11 μm) were more potent than
larger particles (99 μm), as indicated by a marked inux of
histiocytes around the small particles.
121
PET particles 0.520 μm are stored in the cytoplasm of
histiocytes of the joint capsule, whereas larger particles (up to
100 μm) locate extracellularly in the tissue. The surrounding
tissue changes substantially in reaction to PET particles. Joint
cavities containing large quantities of brin exhibit necroses,
and show a strong necrotic tendency and scar formation in the
joint capsules. High numbers of PET particles can be
phagocytosed and the granulation tissue of the joint capsule
has appeared saturated, showing an incapacity to phagocytose
the inux of particles and remove them to the lymph system.
118
Similar reactions to microplastics could occur if they are
capable of crossing epithelia following exposure and uptake.
The removal of wear particles via proximal lymphatic
channels parallels the clearance of microparticles that have
crossed the GIT epithelium via persorption. In dogs, PE wear
particles were found in the para-aortic lymph nodes 18 months
after a total surface hip replacement.
122
In humans, PE wear
particles accumulate in the lymph nodes surrounding joint
replacements,
123,124
and can be so abundant that macrophages
containing PE particles almost completely replace the lymph
nodes.
119
PE particle-laden lymph nodes presented histiocytic
inltration (granulomatous inammation); the histiocytes
contained several PE wear particles, which induced a severe
macrophage response in the surrounding tissues.
124
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In dogs that had undergone a total surface hip replace-
ment,
122
small deposits of PE particles were found in the
alveolar walls of the lungs,
125
suggesting redistribution to
secondary tissue. Additionally, in humans, PE wear particles up
to 50 μm have been identied in abdominal lymph nodes. PE
particles were also detected in the liver or spleen of 14% of
patients.
119
The majority of particles were <1 um in size and
typically accumulated in the mobile macrophages of the portal
tracts of the liver, most likely by means of lymphatic
transport.
119
The inammatory response to plastic wear
particles in lymph nodes has been shown to include immune
activation of macrophages and associated production of
cytokines.
126
The above studies indicate the tendency for plastic
microparticles to disseminate around the body, if released.
Additionally, they highlight that immunological response is
dependent on the chemical composition of the plastic, with
PET being more harmful than PE.
Gastrointestinal Tract and Airway Surface. In the GIT, the
anticipated cellular impact of microplastics will likely be due to
adjuvant activity, i.e., the enhancement of an existing immune
response to surface-adsorbed biomolecules.
73
When macro-
phages were presented with antigens and toxins conjugated to a
microparticle, enhanced T cell proliferation was observed in
comparison to their soluble equivalents.
127
In addition, TiO2
nanoparticles have been shown to enhance the inammatory
response of peripheral cells to the endotoxin lipopolysacchar-
ide. This was also enhanced in cell culture medium enriched
with calcium, as the cations provide bridging potential for the
adsorption of luminal proteins and present biomolecule
conjugates with calcium precipitates.
113,128
The corona, which forms on microplastics during digestive
transit, could include toxins or antigens.
128,129
In addition to
hydrophobicity and charge,
128
shape and age (surface pits and
cracks) of the particle will impact the extent of this, as particle
morphology is linked to surface area. The corona inuences not
just particle uptake but toxicity.
129
The topic has been widely
studied from a therapeutic perspective, particularly in nano-
PS.
129
However, the development of coronas on environmental
microplastics is largely unstudied.
The ζ-potential (the potential dierence between the particle
surface and surrounding liquid media) of PS nanoparticles
(193.8344.5 nm) was associated with parameters indicative of
acute pulmonary inammation.
130
Cationic PS nanoparticles
(NH2attachment) exhibited greater toxicity in macrophages
and lung epithelial cells.
131
Surface charge has also been
associated with the destabilization of membrane potential and
destruction of the cellular membrane.
130
Thus, it is important
to determine the surface charge and ζ-potential of environ-
mental microplastics.
Size-related toxicity has been observed in PS nanoparticles.
Smaller PS nanoparticles (64 nm) induced a signicantly
greater inux of neutrophils, indicative of inammation, to the
lung compared to larger nanoparticles (202 and 535 nm),
following instillation in rats. This was also observed for other
measures of lung inammation. Given the low toxicity of PS,
the proinammatory eects observed were linked to the large
surface area of the smaller particles.
79
Whether plastic
nanoparticles also induce ROS responses in the GIT and
trigger the inammasome remains to be determined.
Chemical Transfer. Adsorbed Chemical Pollutants. The
increased surface area:volume ratio of microplastics, combined
with their surface hydrophobicity, means that a range of HOCs,
including PCBs, dichlorodiphenyltrichloroethane (DDTs) and
PAHs, avidly bind to their surface from the surrounding
environment.
132,133
For example, concentrations of HOCs were
up to 6 orders of magnitude greater on marine microplastics in
comparison to surrounding seawater.
19
Recently, up to 2.4 mg/
g PAHs and 0.1 mg/g DDT was reported for plastic pellets
sampled from beaches in China.
134
Additionally, microplastics
isolated from cosmetics were able to sorb phenanthrene and
DDT from seawater,
5
highlighting the potential for primary
microplastics to transfer HOCs. Some of these HOCs are
highly toxic, recognized for their endocrine-disrupting,
carcinogenic, mutagenic, and immunotoxic eects.
Microplastic-associated HOCs have shown to desorb to
tissues in marine species upon ingestion.
26,27,135,136
Recently,
the potential for HOCs to desorb from microplastics under
simulated physiological conditions was studied.
137
Desorption
rates in conditions simulating the digestive environment of
warm blooded organisms , 38 °C, pH 4, were up to 30 times
faster than in seawater.
137
Thus, in mammals including humans,
the transfer of HOCs from ingested or inhaled microplastics is
likely to be enhanced. This raises the question as to whether the
potential uptake and biopersistence of microplastics leads to the
bioaccumulation of priority HOCs and, in turn, what overall
contribution this has to body burdens. Such a contribution
depends on the existing concentration gradient of the HOC in
question; if it is greater on the microplastics than in the
surrounding environment, e.g., inside a cell or in tissue, the
HOC will desorb. Although recent reviews conclude that the
ingestion of microplastics is unlikely to signicantly inuence
the exposure of organisms in the marine environment to
hydrophobic organic chemicals,
138,139
the importance of this
pathway in relation to others also needs addressing for humans.
Endogenous Chemical Additives. Plastic consists of a
synthetic organic polymer to which chemical additives are
incorporated during manufacture. These additives are included
to inhibit photodegradation; to improve strength, rigidity, or
exibility; and to prevent microbial growth. Because they are
not chemically bound to the plastic and are of a low molecular
weight, such additives are susceptible to leaching to the external
medium along a concentration gradient.
25
The continuous
fragmentation of microplastics will constantly expose new
surfaces, facilitating the migration of additives from the core to
the surface of the particle.
If microplastics are capable of accumulating, they present a
source of chemicals to tissues and uids. This is of concern as
many chemical additives and monomers have known human
health eects, including reproductive toxicity (e.g., bis(2-
ethylhexyl) phthalate [DEHP] and bisphenol A [BPA]),
carcinogenicity (e.g., vinyl chloride and butadiene), and
mutagenicity (e.g., benzene and phenol). Some of the most
harmful additives include brominated ame retardants,
phthalate plasticizers, and lead heat stabilizers.
140
Some plastics
are combined with greater amounts of chemical additives than
others, for example, PVC medical devices can contain up to
80% of the plasticizer DEHP by weight.
141
Phthalates are able
to bind with molecular targets in the body, disrupting
hormones.
142
Other chemicals that could leach from the plastic polymer
matrix include antioxidants, UV stabilizers, nonylphenol, and
BPA.
140
The adult population is exposed to approximately 0.2
20 ng mL1BPA, with links to adverse human health
eects.
143,144
The plasma concentration of BPA in adults
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exceed levels predicted from exposure via food and drink
alone,
145
suggesting alternate pathways of exposure.
The ingestion and inhalation of household dust is a widely
recognized human exposure pathway to ame-retarding
polybrominated diphenyl ethers (PBDEs), which can reach
>90 ng/g dust.
146,147
PBDEs released from plastic components,
such as upholstery, carpets and electronics, lead to inhalation of
ultrane particulate PBDEs associated with dust.
148
However,
the migration pathways from treated products to dust is
understudied. One postulated mechanism is the transfer of
PBDEs and other brominated ame retardants via the abrasion
of particles and/or bers from plastic products, i.e., micro-
plastics. Recently, bers and particles generated by the abrasion
of brominated-ame-retardant- (BFR) treated curtain uphols-
tery accounted for BFR concentrations in spiked ambient dust
samples.
149
Thus, the accumulation of PBDEs via house dust
may be due to leaching of PBDEs following the ingestion or
inhalation of microplastics resulting from the wear of plastic
household products and textiles.
In addition to chemical additives, plastic can also leach
hazardous unreacted residual monomers. Polyurethanes, PVC,
epoxy resins, and styrenic polymers have been identied as
plastics of the greatest concern in terms of environmental and
health eects, as their monomers are classied as carcinogenic,
mutagenic, or both.
140
Currently, there is no information
concerning the direct transfer of additives from plastic to
human tissues, although this has been suggested for seabirds.
150
Recently, in a study investigating whether peritoneal dialysis
solution (PDS) contains leached contaminants, toxic eects
were observed in mice and linked to the leaching additives of
the plastic PDS solution storage bags.
151
Microbiome. In the environment, the surface of micro-
plastics becomes rapidly colonised by microbes; well-developed
biolms establish on the surface of plastic after 7 days in water
or sediment.
152,153
Such biolms signicantly dier from the
ambient environment
154
and can include harmful human
pathogens such as strains of Vibrio spp.
154,155
The microbiome refers to the collection of microbial
communities living on or in the body, the physiological activity
of which inuences host well-being.
156
It is known that the
composition of the GIT microbiome can signicantly dier
between liquid and solid phases.
157
Thus, it can be anticipated
that, in the instance that microplastics are colonised during GIT
transit, the composition will dier to the surrounding
environment. This is emphasized by the unique microbial
assemblage that plastic attracts.
154,155
The unique coating may
inuence the bodys response to microplastics, by enhancing
bioavailability or triggering an immune response.
Environmental pollutants have shown to aect the micro-
biome, as microbes have the capacity to metabolize a range of
environmental toxicants.
158
This can have knock on eects for
the host, compromising immunity and stimulating inamma-
tion.
156
Mice exhibited changes to the composition and
function of the colonic microbiome following long-term
exposure to PM10 administered via lavage.
159
This potentially
contributed to the induction of proinammatory cytokines in
the host. However, it was unknown whether this was a direct
cause of PM10,PM
10-induced immune changes, or both.
159
The lungs also host a microbial community that is
maintained by alveolar macrophages, antibacterial surfactant,
and other environmental conditions.
160
Colonisation is low in
comparison to the GIT, although growth and community shifts
coincide with disease.
161
Oxidative stress and inammation
have a key role in the pathogenesis of inhaled pollutants, and
also modify local conditions, which potentially inuence the
microbiome.
Thus, the response to inhaled or ingested PM, including
microplastics, may cause a shift in the microbial composition
colonising the lung or GIT. Microplastics may cause
inammation or leach HOCs, the microbial metabolism of
which could lead to oxidative stress. Microplastics could carry
pathogenic species, or the additional substrate in the lung or
GIT may facilitate growth of specic groups, shifting the
assembly. Through this, alterations in the community structure
and functions of the lung or GIT microbiome could occur, with
knock on eects for host well-being and therefore human
health.
CONCLUSIONS AND RECOMMENDATIONS FOR
FUTURE RESEARCH
Although microplastics are widely studied in the context of the
marine environment where they are a prolic pollutant, we are
only just recognizing the potential human exposure pathways.
Following exposure, via diet and/or inhalation, uptake is
plausible, as evidenced by the observations of plastic micro-
bers in lung tissue biopsy samples, and by the capacity for
biopersistent particles up to >100 μm to cross the GIT
epithelium. Following uptake, particles <2.5 μm and bers are
anticipated to be of greatest concern in the lung, whereas larger
particles are of concern in the GIT due the presence of M cells
Table 1. Key Knowledge Gaps and Recommendations for
Future Research into Microplastics and Human Health
Key Knowledge Gaps
What are the overall exposure concentrations from dietary and airborne
sources?
What proportion of microparticle exposure do microplastic comprise?
Do dierent biological responses to microplastics manifest due to their unique
chemical compositions/properties?
What eect does the interchangeable gastric environment/lung lining uid
have on the surface charge and chemistry, and therefore handling of
microplastics?
What is the composition of the protein corona on microplastics?
Is there evidence of microplastic uptake in humans?
Are microplastics able to accumulate in the body? Do they become lodged or
are they engulfed by cells?
If taken up by cells, what is the cellular mechanism of uptake? Does subcellular
localization or translocation occur?
If subcellular location occurs, does this hijackthe route for endogenous
microparticle uptake or compromise immune homeostasis?
Does dissemination and/or elimination occur? Are there target secondary
organs?
Are accumulative eects the same as those observed in occupational exposures?
Are larger particles a greater issue for the GIT due to the process of
persorption?
What is the toxicological response to biopersistent microplastics? Do
inammatory responses mimic those observed in response to plastic
prosthetics wear debris?
Do size and shape inuence toxicity? Does this depend on the point of entry,
e.g., are plastic microbres of greater concern for the lung than the GIT?
Do polymer type and hydrophobicity inuence toxicity?
Does surface charge of microplastics aect toxicity and does this vary with time
in the environment (and therefore exposure to UV)?
Once uptaken, can microplastics deliver their chemical burden and does this
cause localized toxicity?
What will the addition of the novel hard surface of microplastics, for which
specic microbes and biomolecules have an anity for, have on the
microbiome?
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in the Peyers Patches, capable of engulng micrometer-sized
particles, and the phenomenon of persorption. Toxicity is via
inammation due to the biopersistent nature of microplastics,
and their unique hydrophobicity and surface chemistry.
Toxicity is likely to have an accumulative eect, dependent
on dose. Key knowledge gaps are outlined in Table 1.
Exposure concentrations are predicted to be low, although
this is partly due to the present technical limitations in sampling
and identifying microplastics. Measuring and assessing true
exposure concentrations is a current scientic challenge, largely
limited by particle size. Thus, current predicted exposure levels
are also probably an underestimation. Once we have a better
understanding of human exposure levels, and whether micro-
plastics are uptaken/able to translocate, we can begin to unravel
the potential toxicological mechanisms of microplastics and
hence therein possible health eects.
ASSOCIATED CONTENT
*
SSupporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.est.7b00423.
Occurrence of microplastics in the gastrointestinal tracts
of sh in situ; Tables S1 and S2 (PDF)
AUTHOR INFORMATION
Corresponding Author
*S. L. Wright. Tel.: +4420 7848 4007. E-mail: Stephanie.
wright@kcl.ac.uk.
ORCID
Stephanie L. Wright: 0000-0003-1894-2365
Author Contributions
These authors contributed equally.
Funding
We thank the Medical Research Council for funding this
research (MR/M501669/1).
Notes
The authors declare no competing nancial interest.
REFERENCES
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... Thus, threatening assessments to organisms and humans are also highly needed. MPs are exposed to human beings by multiple routes, e.g., ingestion, inhalation, and dermal contact, that inevitably translocate into organisms and humans tissues (Sykes et al., 2014;Wright and Kelly, 2017;Mason et al., 2018). It is estimated that, the average human ingestion of MPs is about 0.1-5 g per week (Senathirajah et al., 2021). ...
... In this study, three differently functionalized polystyrene MPs (PS MPs) (e.g., aminated (NH 2 -PS MPs), carboxylate (COOH-PS MPs), and unmodified MPs) were used to facilitate eco-corona formation and to significantly decline the oxidative stress and toxic effects, which the response process depended on the MP's surface (Natarajan et al., 2020). However, the corona facilitates MP translocation to almost every organ, which may lead to their uncontrolled impact and potential health hazards (Mowat, 2003;Mohr et al., 2014;Wright and Kelly, 2017;Abihssira-Garcia et al., 2020). It may be why MP accumulation in the gut and their increased cytotoxicity (Krug and Wick, 2011;Walczak et al., 2015). ...
... The lungs and intestines are the most important organs that directly communicate with the external environment. In the lungs, large MPs are cleared by mucociliary clearance, while very small MPs (especially <1 μm) may penetrate through bronchial epithelial cells and contact endothelial cells, enter the circulatory system by passing through endothelial cells (Wright and Kelly, 2017;Zhang et al., 2021) (Figure 2). Inhaled MPs exhibit toxicity in human alveolar epithelial A549 cells (Forte et al., 2016). ...
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Most disposable plastic products are degraded slowly in the natural environment and continually turned to microplastics (MPs) and nanoplastics (NPs), posing additional environmental hazards. The toxicological assessment of MPs for marine organisms and mammals has been reported. Thus, there is an urgent need to be aware of the harm of MPs to the human immune system and more studies about immunological assessments. This review focuses on how MPs are produced and how they may interact with the environment and our body, particularly their immune responses and immunotoxicity. MPs can be taken up by cells, thus disrupting the intracellular signaling pathways, altering the immune homeostasis and finally causing damage to tissues and organs. The generation of reactive oxygen species is the mainly toxicological mechanisms after MP exposure, which may further induce the production of danger-associated molecular patterns (DAMPs) and associate with the processes of toll-like receptors (TLRs) disruption, cytokine production, and inflammatory responses in immune cells. MPs effectively interact with cell membranes or intracellular proteins to form a protein-corona, and combine with external pollutants, chemicals, and pathogens to induce greater toxicity and strong adverse effects. A comprehensive research on the immunotoxicity effects and mechanisms of MPs, including various chemical compositions, shapes, sizes, combined exposure and concentrations, is worth to be studied. Therefore, it is urgently needed to further elucidate the immunological hazards and risks of humans that exposed to MPs.
... Actualmente, la producción mundial de plástico supera los 300 millones de toneladas (Wright y Kelly 2017). De esta cantidad, menos de la mitad es reciclada (escasamente el 40 %) (Figura 1) y el resto de estos objetos son vertidos, generalmente, en tiraderos a cielo abierto (Hopewell et al. 2009). ...
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En las últimas dos décadas, los microplásticos (partículas plásticas de tamaño menor a 5 mm), un tipo de contaminante emergente, han atraído nuestra atención por ser una amenaza potencial para la salud humana, la biodiversidad y el medio ambiente. Aquí, mencionamos las diversas fuentes de origen y distribución. Asimismo, discutimos el impacto ecológico de los microplásticos en el suelo.
... Some of the living species in marine and land habitats are ingesting the leaked plastic debris and loading harmful pollutants on their ecosystems [15][16][17]. The ingestion of micro/nano plastic fragments can cause serious health hazards such as cell death, oxidative stress, and innate immune system damage [18,19]. ...
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The current trend of using plastic material in the manufacturing of packaging products raises serious environmental concerns due to waste disposal on land and in oceans and other environmental pollution. Natural polymers such as cellulose, starch, chitosan, and protein extracted from renewable resources are extensively explored as alternatives to plastics due to their biodegradability, biocompatibility, nontoxic properties, and abundant availability. The tensile and water vapor barrier properties and the environmental impacts of natural polymers played key roles in determining the eligibility of these materials for packaging applications. The brittle behavior and hydrophilic nature of natural polymers reduced the tensile and water vapor barrier properties. However, the addition of plasticizer, crosslinker, and reinforcement agents substantially improved the mechanical and water vapor resistance properties. The dispersion abilities and strong interfacial adhesion of nanocellulose with natural polymers improved the tensile strength and water vapor barrier properties of natural polymer-based packaging films. The maximum tensile stress of these composite films was about 38 to 200% more than that of films without reinforcement. The water vapor barrier properties of composite films also reduced up to 60% with nanocellulose reinforcement. The strong hydrogen bonding between natural polymer and nanocellulose reduced the polymer chain movement and decreased the percent elongation at break up to 100%. This review aims to present an overview of the mechanical and water vapor barrier properties of natural polymers and their composites along with the life cycle environmental impacts to elucidate their potential for packaging applications.
... There is also evidence of trophic transfer of microplastics in marine and estuarine food chains (Athey et al., 2020;Nelms et al., 2018). Humans are expected to be at risk, which requires investigation due to the presence of microplastics in food sources, drinking water, and the atmosphere we breathe (Wright and Kelly, 2017). ...
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A literature assessment was conducted to determine the current state of microplastics research in ASEAN countries focusing on 1) microplastics in water, sediment, and water organisms; 2) microplastics' sources and dispersion; and 3) microplastics' environmental consequences, including human toxicity. ASEAN countries contributed only about 5 % of the global scholarly papers on microplastics, with Indonesia contributing the most followed by Malaysia and Thailand. The lack of standard harmonized sampling and processing methodologies made comparisons between research difficult. ASEAN contributes the most to plastic trash ending up in the ocean, indicating a need for more work in this region to prevent plastic pollution. Microplastics are found in every environmental compartment; however, their distribution and environmental consequences have not been sufficiently investigated. There are very few studies on microplastics in the human blood system as well as respiratory organs like the lungs, indicating that more research is needed.
... Aquatic arthropods are an important protein-food source for the people in northeastern Thailand and are integral to many dishes, such as the dancing shrimp (Goong ten), crispy fried shrimp (Goong tod), and northeastern Thai spicy soup with mixed insects (Kaeng Aom Maeng), etc. Considering that these edible arthropods are widely used for human food and are the prey of fish, they are now a potential source of MPs, or environmental, contamination to humans. Although the physical effects of accumulated MPs are less understood than the distribution and storage of toxicants in the human body, a few publications reported several potentially concerning MPs accumulation impacts, including enhanced inflammatory response, size-related toxicity of plastic particles, chemical transfer of adsorbed chemical pollutants, and disruption of the gut microbiome (Wright and Kelly, 2017;Smith et al., 2018). Therefore, attention is needed to evaluate the potential synergistic effects of toxicity to freshwater organisms, as well as bioaccumulation through the food web. ...
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Microplastics (MPs) are an emerging pollutant in freshwater that have become a cause for concern among researchers around the world. In this study, MPs contamination in water, sediment and edible arthropods of the upper Chi River and Pong River was studied to analyze the influence of anthropogenic activities on contamination in the river environments and arthropods. Five species of arthropods were observed to assess the impact of MPs contamination: Caridina sp, Macrobrachium sp., Aethriamanta sp., Aciagrion sp. and Sphaerodema molestum. MPs were found in water and sediment samples with an average of 141 items/m³ and 9.5 items/kg, respectively. Fibers were major MPs shape in water (63%) and sediment (81.9%). MPs with dark blue color were numerically dominant in water (28%) and sediment (39.6%). MPs in edible arthropods were in the range of 0.25 – 8.0 items/individual. A wider variety of polymer types was found in the rivers than in the edible arthropods. Overall, dark blue colored PP fibers were found to be most abundant in water, sediment and edible arthropods with MPs mostly ranging 1000–2000 μm in size. MPs concentration in water correlated with community size (p-value <0.05), and the abundance in sediment correlated with the number of roofs (p-value <0.05) and the distance of the rivers from communities (p-value <0.05). Anthropogenic activities significantly contributed to abundance of MP in water and sediment. Industrial, community and fish farming contributed to MP in water and agriculture and community directly correlated with MP in sediment with p-value <0.05 at 95% confidence. The results indicate the significant influence of anthropogenic activities on the amount of MPs in water and sediment, directly relating to contamination in edible arthropods. Domestic wastewater and plastic waste are likely to be the leading causes of existing MPs in the rivers and arthropods.
... Ingested MPs can lead to a variety of harmful effects on fish. Pure plastics can induce acute inactivity, expenditure of energy reservoirs [67], and liver inflammation, while MPs connected with consistent organic contaminants have been exhibited to provoke chronic or acute toxic influences [53]. The fish liver is the tissue most used to evaluate metabolic disorders caused by MPs in fish species because this organ is primarily responsible for the detoxification and inactivation of exogenous compounds [68]. ...
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The presence of microplastic (MP) in different fish species taken from stations in Erzurum, Erzincan and Bingöl was examined. The obtained data were classified and shared with the scientific world as the first record made in this region. In the obtained results, the most dominant color was black (39-58%) and the most prevalent forms were fragment and fiber. The sizes (0-50, 50-100 µm) of microplastics differed according to the region and species. When the number of MPs in the gas-trointestinal systems of different fish species in the Bingöl, Erzurum and Erzincan provinces was evaluated, the most microplastics were found in Squalius squalus (20.7%) and Blicca bjoerkna (18.2%) in Bingöl province from among six different species. In Erzincan province, four fish species were sampled, and the rates were (29.7%) in Capoeta umbla and (26.6%) in Blicca bjoerkna. The highest abundance in Erzurum province was determined in Cyprinus carpio (53.0%). In the analyses performed on liver tissues, the highest ROS, which is the indicator of oxidative damage, was listed as Bingöl > Erzincan > Erzurum, while MDA levels were recorded as Bingöl > Erzurum > Erzincan, from high to low. When the differences between species were examined, the highest SOD and CAT activity was determined in the Mugil cephalus species. Considering the total MP numbers in fish samples, 47 MP was determined in this species. On the other hand, in the Squalius squalus species, where the highest total MP was determined, SOD and CAT activities were found to be low in Bingöl province. Therewithal, the high levels of ROS and MDA in this species can be said to induce oxida-tive stress due to the presence of microplastics on the one hand and to reduce antioxidant levels on the other hand. When the findings were evaluated, it was concluded that MPs in freshwater are a potential stressor, and freshwater environments may represent a critical target habitat for future MP removal and remediation strategies.
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Microplastics in urban rivers provide bacterial niches and serve as dispersal vectors for antibiotic resistant genes (ARGs) dissemination, which may exacerbate risks in the aquatic systems. However, whether MPs in the river would also selectively enrich ARGs and the underlying mechanisms shaping the resistome on MPs remains largely unknown. In this study, we explored the occurrence of ARGs, bacterial communities, and mobile genetic elements (MGEs) on MPs and in waters from the Huangpu River in China. Microplastics were widely distributed in the river (1.78 ± 0.84 items/L), with overwhelming percentages of polyethylene terephthalate fibers. Although reduced ARG abundances were observed on MPs than in waters, MPs selectively enriched the ARGs resistant to Rifamycin and Vancomycin. A clear variation for ARG profiles was elucidated between water and MPs samples. Network analysis suggested that MPs created a unique niche for the genus Afipia to colonize, potentially contributing to the vertical dissemination of ARGs. Additionally, the co-occurrence between ARGs and MGEs revealed that the MPs favor the propagation of some plasmid-associated ARGs mediated by horizontal gene transfer. The null model-based stochasticity ratio and the neutral community model suggested that the ARG assembly on MPs was dominantly driven by stochastic process. The results further indicated that microbial communities and MGEs played significant roles in shaping ARG profiles and dynamics on MPs. Our findings provided new insights into the ecological processes of antibiotic resistome of the aquatic plastisphere.
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Microplastics (MPs) are emerging pollutants detected everywhere in the environment, with the potential to harm living organisms. The present study investigated the concentration, morphology, and composition of MPs, between 20 μm and 5 mm, in a drinking water treatment plant (DWTP) located close to Barcelona (Catalonia, NE Spain). The sampling included different units of the DWTP, from influent to effluent as well as sludge line. Sampling strategy, filtration, allows sampling of large volumes of water avoiding sample contamination, and during 8 hours in order to increase the representativeness of MPs collected. The pre-treatment of the samples consisted of advanced oxidation with Fenton's reagent and hydrogen peroxide, followed by density separation of the particles with zinc chloride solution. Visual identification was performed with an optical and stereoscopic microscope with final Fourier-transform infrared spectroscopic (FTIR) confirmation. MPs were found in all DWTP samples, with concentrations from 4.23 ± 1.26 MPs/L to 0.075 ± 0.019 MPs/L in the influent and effluent of the plant, respectively. The overall removal efficiency of the plant was 98.3%. The most dominant morphology was fibers followed by fragments and films. Twenty-two different polymer types were identified and synthetic cellulose, polyester, polyamide, polypropylene, polyethylene, polyurethane, and polyacrylonitrile were the most common. Although MPs could be incorporated from the distribution network, MPs intake from drinking water from this DWTP was not an important route compared to fish and seafood ingestion.
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Microplastics [MPs], now a ubiquitous pollutant in the oceans, pose a serious potential threat to marine ecology and has justifiably encouraged focused biological and ecological research attention. But, their generation, fate, fragmentation and their propensity to sorb/release persistent organic pollutants (POPs) are determined by the characteristics of the polymers that constitutes them. Yet, physico-chemical characteristics of the polymers making up the MPs have not received detailed attention in published work. This review assesses the relevance of selected characteristics of plastics that composes the microplastics, to their role as a pollutant with potentially serious ecological impacts. Fragmentation leading to secondary microplastics is also discussed underlining the likelihood of a surface-ablation mechanism that can lead to preferential formation of smaller sized MPs.
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A state-of-the-science review was conducted to examine the potential for microplastics (MPs) to sorb hydrophobic organic chemicals (HOCs) from the marine environment, for aquatic organisms to take up these HOCs from the MPs, and for this exposure to result in adverse effects to ecological and human health. Despite concentrations of HOCs associated with MPs that can be orders of magnitude greater than surrounding seawater, the relative importance of MPs as a route of exposure is difficult to quantify because aquatic organisms are typically exposed to HOCs from various compartments, including water, sediment, and food. Results of laboratory experiments and modeling studies indicate that HOCs can partition from MPs to organisms or from organisms to MPs, depending on experimental conditions. Very little information is available to evaluate ecological or human health effects from this exposure. Most of the available studies measured biomarkers that are more indicative of exposure than effects, and no studies showed effects to ecologically relevant endpoints. Therefore, evidence is weak to support the occurrence of ecologically significant adverse effects on aquatic life due to exposure to HOCs sorbed to MPs, or to wildlife populations and humans from secondary exposure via the food chain. More data are needed to fully understand the relative importance of exposure to HOCs from MPs compared to other exposure pathways. This article is protected by copyright. All rights reserved.
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Municipal effluent discharged from wastewater treatment works (WwTW) is suspected to be a significant contributor of microplastics (MP) to the environment as many personal care products contain plastic microbeads. A secondary WwTW (population equivalent 650 000) was sampled for microplastics at different stages of the treatment process to ascertain at what stage in the treatment process the MP are being removed. The influent contained on average 15.70 (±5.23) MP·L–1. This was reduced to 0.25 (±0.04) MP·L–1 in the final effluent, a decrease of 98.41%. Despite this large reduction we calculate that this WwTW is releasing 65 million microplastics into the receiving water every day. A significant proportion of the microplastic accumulated in and was removed during the grease removal stage (19.67 (±4.51) MP/2.5 g), it was only in the grease that the much publicised microbeads were found. This study shows that despite the efficient removal rates of MP achieved by this modern treatment plant when dealing with such a large volume of effluent even a modest amount of microplastics being released per liter of effluent could result in significant amounts of microplastics entering the environment. This is the first study to describe in detail the fate of microplastics during the wastewater treatment process.