Koelmansetal. 2022 RiskassessmentofMPparticles

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Risk assessment of microplastic

particles
Albert A. Koelmans ✉, Paula E. Redondo-Hasselerharm , Nur Hazimah Mohamed Nor ,
Vera N. de Ruijter , Svenja M. Mintenig and Merel Kooi
Abstract | Microplastic particles are ubiquitous in the environment, from the air we breathe to the
food we eat. The key question with respect to these particles is to what extent they cause risks
for the environment and human health. There is no risk assessment framework that takes into
account the multidimensionality of microplastic particles against the background of numerous
natural particles, which together encompass an infinite combination of sizes, shapes, densities and
chemical signatures. We review the current tenets in defining microplastic characteristics
and effects, emphasizing advances in the analysis of the diversity of microplastic particles. We
summarize the unique characteristics of microplastic compared with those of other environmental
particles, the main mechanisms of microplastic particle effects and the relevant dose metrics for
these effects. To characterize risks consistently, we propose how exposure and effect thresholds
can be aligned and quantified using probability density functions describing microplastic
particle diversity.
Plastic debris is a contaminant of emerging concern another25–27. These challenges have been addressed in a
that is often discussed in society, science, the media and growing body of literature1,2,4,8–11,15–18,25,27–31.
policy1,2. It is visible to the naked eye and easily linked However, the way scientists currently look at
to our daily lives, which explains part of the public microplastic is missing a critical nuance. Microplastic
concern2,3. One size fraction of plastic debris is called is generally viewed as an environmental contaminant
microplastics, arbitrarily defined as particles smaller with unique properties, one of which is its unparal-
than 5 mm (refs1,4,5). Plastic particles smaller than 0.1 μm leled complexity9,10. However, when each of its prop-
or 1 μm are often referred to as nanoplastics, although erties is considered individually, microplastic particles
some recent reports put the lower limit for microplastics do not differ much from their natural counterparts.
at 1 nm (refs6,7). Nevertheless, researchers studying microplastics have
The vast majority of microplastics come from the focused mainly on the persistence profiles, fate processes
breakdown of larger plastic waste. The diversity of and effect mechanisms of microplastic particles. By con-
sources is reflected in the heterogeneity of microplastic sidering the complexity of microplastics in addition to
properties (shape, size, density and polymer type)8–10, that of natural particles, a more balanced picture of the
transport characteristics1,11–13, and in vivo and in vitro risks posed by microplastics can emerge.
biological effects — and therefore also in its risks14. The No risk assessment framework adequately examines
presence of contaminants in microplastic adds to this the multidimensionality of microplastics while also tak-
diversity15. Together with a high probability of being ing into account the numerous types of natural particles
ingested and absorbed by a large range of species, this present in the background, which together can encom-
diversity in multiple dimensions has contributed to pass an enormous number of combinations of sizes,
the concern that microplastics may constitute a risk shapes, densities and chemical characteristics1,2,17,25.
to humans and the environment1,16–22. Natural particles should also be considered because
Aquatic Ecology and Water Assessing such risks is challenging. Microplastics have they are much more ubiquitous than microplastics. Most
Quality Management Group, been detected in air, soil, fresh water, drinking water, the organisms are therefore equipped to deal with natural
Wageningen University and oceans, aquatic and terrestrial biota, food products, and particles32. Natural and microplastic particles that share
Research, Wageningen,
Netherlands.
human placenta and stools1,19,23,24. Microplastic is both characteristics can cause similar adverse effects, and dif-
✉e-mail: bart.koelmans@ diverse and ill-defined. Even more uncertainties exist for ferences in properties can help to explain differences in
wur.nl assessing the risks of microplastics owing to the diver- effects32.
https://doi.org/10.1038/ sity of exposure and impact assessment methods used In this Review, we survey the current tenets in
s41578-021-00411-y by scientists, and their inability to be compared with one microplastic research and emphasize progress in efficiently
138 | February 2022 | volume 7 www.nature.com/natrevmats
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describing the diversity of microplastic particles. We a result of the small proportion of primary microplastic44
compare the characteristics of microplastics to those of and effects of weathering and ageing. Nevertheless, the
natural particles, and discuss the main mechanisms diversity and complexity of sources continues to be
of their effects on organisms and the relevant dose reflected in the diversity of the material once it reaches
metrics for these mechanisms. We describe how expo- the microplastic scale33.
sure and effect thresholds can be tuned and quantified
using probability density functions (PDFs) that cap- Polymer composition and density
ture the diversity of microplastic particles in order to Microplastic polymer composition and thus particle
characterize risks consistently. Finally, we propose a density are a function of the polymers used in products,
framework to assess these risks through multiple simul- how much those polymers emit into the environment45,
taneous effect mechanisms, while accounting for the and the alteration of polymers during ageing and weath-
multidimensionality of the particles. ering in the environment42. Environmental altering
involves processes such as photo-oxidation, embrittle-
The multidimensionality of microplastic ment, crack propagation, abrasion, erosion, biodegrada-
Scientists have coined the term ‘environmentally rele- tion, biofouling and aggregation. These processes affect
vant microplastic’ to refer to the plastic particles found such polymer properties as crystallinity, density and
in nature, including in the human diet. These differ tensile strength42,46–48.
substantially from the virgin plastic particles ‘from the The most abundant polymers in microplastic are
shelf ’ that are used in many laboratory experiments28,33. polyethylene (PE), polyethylene terephthalate (PET),
The difference between laboratory and nature needs to polyamide (PA), polypropylene (PP), polystyrene
be addressed to understand the risks of microplastic (PS), polyvinyl alcohol (PVA) and polyvinyl chloride
for humans and the environment9,28. Scientists often (PVC)17,31. However, in the environment, each of these
portray environmental microplastic as a diverse and discrete polymers has a certain range of densities, owing
complex material, given its origins from a wide variety to differences in manufacture, crystallinity, additives,
of materials and products1,2,4,8,9,29, and define it simply age and level of weathering and biofouling. This range
as ‘all plastic particles smaller than 5 mm’. Because this ensures that the density distribution is essentially con-
definition is vague, microplastic constitutes a hetero- tinuous for large numbers of particles9. Overall, the
geneous mixture of polymers, sizes and shapes, and is densities of microplastic polymers found in nature
associated with all kinds of chemicals1,4,8,10,17,29,34,35. For range between roughly 0.8 and 2 g cm–3, with a density
instance, polybrominated diphenyl ethers (PBDEs), of around 1 g cm–3 being most abundant9. An environ-
phthalates, nonylphenols (NPs), bisphenol A (BPA) and mentally relevant microplastic particle on average has a
antioxidants, all common additives in plastic products, weight of 12.5 μg, a volume of 0.011 mm3 and a density
slowly desorb into the environment when plastic items of 1.14 g cm–3 (ref.49).
age and fragment36. At the same time, microplastic par-
ticles are passive samplers37,38, which means that any Size
contaminant that has higher fugacity in the environ- Among microplastic characteristics, size spans the wid-
ment than in the plastic, will adsorb to microplastic until est range. Microplastic particles include nanometre to
chemical equilibrium is reached39,40. Leached additives millimetre sizes, a range of more than six orders of mag-
also re-adsorb to other microplastic particles in the nitude. The size distribution of microplastic particles has
environment, whenever their aqueous concentration is been shown often to follow a power law with a negative
higher than the chemical equilibrium concentration41. exponent, whose magnitude is determined by processes
Therefore, the often postulated distinction between des- through which the particles are formed (by fragmen-
orbing additive chemicals and adsorbing environmental tation) or removed (by erosion, size-selective trans-
chemicals as separate categories36 does not truly exist. As port or settling) from environmental media, including
for the particles themselves, once released into or formed air9,19,33,50–52 (Fig. 1a). This implies that number concen-
in the environment, they undergo biofouling, weathering trations increase dramatically with smaller size, which
and ageing, and interact with chemicals, organisms and may have serious implications for the abundance of
natural particles under a wide range of environmental as-yet-undetectable nanometre-sized plastic particles53.
conditions that are highly variable in time and space30,42. Microplastic components in the human diet follow this
size trend as well19.
Source
The sources of microplastic vary widely, as plastics Shape
have a broad range of uses and applications. Primary Fragments, fibres and films are the most frequently
microplastic refers to the micrometre-sized particles reported microplastic shape categories9,10,29,54. The occur-
deliberately manufactured for specific applications or rence of these shapes follows in part from product or
products, such as pellets for industrial production (for material categories such as fibres, beads and films9,10,29.
example, for drink bottles) and microbeads (such as Primary dimensions such as length, width and height are
those used in personal care products), that enter the similar for spherical particles, but can also differ greatly
environment43. Secondary microplastic refers to parti- from each other, with very small heights for films, or
cles formed from the fragmentation and breakdown of very small heights and widths for fibres. Once in the
larger plastic debris1,4,8. In the environment, these two environment, the categories remain recognizable as
classes ultimately become virtually indistinguishable as such, although fibre length or film area may reduce over
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a b
Higher abundance 2.5 Ingested Exposure
of smaller particles
Particle abundance
2.0
log[width (μm)]
1.5
Fragmentation, erosion
1.0
1 nm 1 μm 1 mm 1.0 1.5 2.0 2.5

Particle size log[length (μm)]
Microplastics
7.5
Black carbon
Detritus
log[lifetime (days)]
Minerals
Natural fibres
5.0
2.5
−2 0 2 4
log[size (μm)]
Fig. 1 | Relationships between processes and characteristics of environmental microplastic particles. a | Relation
between size distribution and particle-formation processes. The formation of small particles through fragmentation
and erosion of larger particles, in combination with the removal of large particles by erosion, size-dependent transport and
settling, produce a higher abundance of smaller particles. b | Contour plot for width and length of environmentally
realistic microplastic, showing how uptake of bioavailable microplastic (ingested) represents a fraction of the total
exposure to environmentally realistic microplastic (exposure). Data are for the uptake of microplastic from sediment by
Gammarus pulex9,13. c | Contour plots of longevity and size for microplastics compared with several categories of natural
particles. Contours are plotted to guide the eye based on triangular distributions using reported median, minimum and
maximum values (natural particles, as detailed in Table 1) or a power-law distribution (microplastics), based on refs9,33.
time. Shapes that are irregular in all three dimensions, half-lives ranging from 58 years for bottles to 1,200 years
such as fragments, also remain in the same category for pipes55. Under laboratory conditions, a particle of
upon further fragmentation, although change is random 1 mm in diameter would require about 320 years to
in each of the three dimensions, and fragments are far reach a nanoscale diameter of 100 nm, based on parti-
more difficult to trace to the original material. cle shrinking from photo-oxidation and biodegradation
at the polymer surface53. In the environment, however,
Longevity degradation can be assumed to proceed much more
Plastic is designed to be durable, and therefore the per- slowly owing to the limited availability of oxygen, light
sistence and longevity of environmental plastic are very and bacteria. Modelling of fragmentation and calibra-
high. As the timescales of degradation of some poly- tion of experimental weathering data found that frag-
mers are much longer than what can be simulated in ments at the ocean surface and beaches have a 176-year
laboratory or field experiments, lifetime estimates must lifespan52. It was calculated in 2015 that about 50% of
rely on extrapolations with considerable uncertainties. the plastic in the oceans had been present for more than
Extrapolated persistence data have been used to estimate 13 years, whereas 80% and 90% of the plastic were older
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than 4 and 2 years, respectively41. Given these half-lives comparable transport and fate processes — and thus
and timescales of 100 to 1,000 years, a major fraction toxicological effects — are also to be expected. Here we
of present-day environmental plastic is thus still in the look at microplastic characteristics from the perspec-
early degradation stage. On the other hand, the fact that tive of several major categories of natural particles, and
microplastics are widespread in the environment despite compare their key features.
the production of plastics having started only 70 years
ago implies that the formation of microplastics is quite Categories of natural particles
rapid, with a timescale of at most decades. Inert minerals. The mineral particles occurring in sedi
ments, soils, suspended solids or desert dust are sand,
Microplastic characteristics follow a continuous silt, silica, and clay, in order of decreasing size. They orig-
distribution inate from prolonged chemical weathering of silicate-
There are two approaches to define the characteristics bearing rocks or locally from hydrothermal activity58.
of microplastic particles and their diversity. As in the These minerals have a size range of about 60 nm to 2 mm
discussion above, characteristics have traditionally been (refs59–62), a density of 1.1 to 2.8 g cm–3 (refs60,63) and an
described using discrete categories, such as polymer environmental longevity of 104 to 109 days64–66. The
type, shape category and size class10,17,29. This categori- concentrations in soils, sediment or suspended solids
zation is useful for tracking sources and origins of parti- can range up to 99% by weight67,68. Minerals have a low
cles, and may be useful from a regulatory perspective29. affinity for hydrophobic organic contaminants69.
However, it also makes microplastics look complex in
an artificial way, owing to the high number of descrip- Organic matter. Non-living organic-matter particles in
tors required. For instance, the category of polymer sediments, soils and aquatic systems include decom-
type would easily require more than 20 descriptors or posing algae, detritus and natural fibres. Detritus is
parameters, shape would need more than 10, and size composed of organic compounds originating from the
would also need more than 10, the latter depending on remains of organisms such as animals and plants and
the accuracy required. their waste products. Detritus particles have a size range
These predetermined categories are simplifications, of roughly 200 nm to 2.5 mm (refs60,70), a density of 0.8 to
because microplastic particles actually represent a 1.2 g cm–3 (refs60,71) and an environmental longevity of
continuum9. Although two microplastic particles will about 20 days up to 10,000 days72. Organic-matter con-
never be the same, the distributions of characteristics of tents of soils, sediment or suspended solids can range
large populations of particles will converge and become from very low (<1%) to several tens of per cent67,73,74.
similar9,19,33,49,52. A PDF is a mathematical function that Studies suggest that 80–90% of oceanic fibres are actu-
describes the actual distribution of a certain microplastic ally of natural origin75–77. This includes purely natural
characteristic. A PDF is created by fitting the function fibres as well as man-made (textile) cellulosic fibres,
to empirical data measured for a large number of which are difficult to differentiate78. Data that include
microplastic particles in an environmental compartment both marine and freshwater samples suggest that 75%
of interest33. Size, shape and density can be captured in of the fibres are of natural origin33 and show that natural
three PDFs with a total of twelve parameters describing and microplastic fibres have a similar length distribu-
the full diversity of environmental microplastics9. PDFs tion (Fig. 2). The affinity of hydrophobic organic con-
can be used to probabilistically quantify microplas- taminants for organic matter is high, with partition
tic in the context of transport and fate modelling56; to coefficients of up to 107 litres kg–1 (ref.69).
rescale number concentrations obtained for limited
size ranges to the full 1 to 5,000 μm microplastic size Black carbon. Environmental condensed carbon or
range; to convert number into mass concentrations; black carbon is the collective term for a range of car-
to assess exposure, effect and risk; and to quantify and bonaceous substances, encompassing partially charred
visualize the bioavailability of microplastic19,33,49 (Fig. 1b). plant residues (char, charcoal) to highly graphitized soot,
PDFs have been calibrated based on a meta-analysis produced from incomplete combustion of biomass and
of >60,000 microplastic particles, whose characteris- fossil fuel in the absence of oxygen79–83. Black carbon
tics were measured with Fourier transform infrared is of mixed natural (natural wildfires, vulcanism) and
(FTIR) imaging33. In this way, polymer-specific and anthropogenic (industry, traffic, domestic fires) origin.
environmental-compartment-specific PDFs can be The particles have a size range of about 1 nm to 200 μm
obtained, which lend themselves to site-specific risk (refs84–86), a density of 0.13 to 2.1 g cm–3 (refs87,88) and an
assessments. environmental longevity of 105.4 to 107.6 days89,90. Soots,
comprising the smallest size fraction of black carbon,
Microplastic and natural particles can contain aggregated fractions of single-walled carbon
Natural particles are ubiquitous in our living nanotubes, multi-walled carbon nanotubes and multi-
environment 32. Their numbers, compositions and concentric fullerenes, rendering engineered organic
characteristics are highly diverse and exceed those nanomaterials part of the black carbon space91,92. Black
of microplastic particles (Table 1; Fig. 1c). Differences carbon particles are abundant in the air and make up
between natural particles relate to their chemistry, on average 3% and 9% of organic carbon in soils and
reactivity, physical density, geometry, size, persistence freshwater sediments, respectively80,86,88, with extremes
and abundance57. If particles of natural versus plastic of up to 40% at specific locations80. They can also be a
origin nevertheless have similar characteristics, then significant component (up to 20%) of sediment organic
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Table 1 | Indicative medians and ranges of characteristics of microplastic and natural particles taken from
various studies
Characteristic Microplastic Clay minerals Detritus and Black carbon
organic matter
Origin Plastic products Weathering Remains of plants, Incomplete combustion
of rocks animals and bacteria of biomass and fossil fuel
Size median and range 20 (1 – 5,000); 30 (0.06–2000); 5 (0.2–2,500); refs60,70 0.2 (0.001–200); refs84–86
(μm) refs1,10,33 refs59,60
Longevitya median 105 (101.8–107.2); ref.55 106.3 (104.3–109.0); 103.1 (101.2–104.5); ref.72 107.2 (105.4–107.6)b; refs89,90
and range (days) refs64–66
Particle density range 0.8–2; ref. 9
1.1–2.8; refs60,63 0.8–1.2; refs60,71 0.13–2.1; refs87,88
(g cm–3)
Concentration range <0.1–3.6; ref.95 <0.1–99; ref.67 <0.1 to about 40; 0.002–3; refs80,86,88
sediment (% dry weight) refs67,73,74
Concentration range <0.0001–1c; refs49,54 <0.1–360; ref.67 <0.1–60; ref.74 <0.0001–0.35; ref.80
freshwater water
(mg per litre)
Numbers are based on the literature; they are indicative and illustrative and we used them to define the qualitative boundary lines
in Fig. 1c. aLongevity, defined as five times the half-life, that is, loss of 97% of initial mass. bMaximum value, based on reported 64%
degradation in 10,000–20,000 years90. cMaximum value based on reported highest concentration of 105 particles per litre and an
average weight of 12.5 μg (refs49,54).
carbon in the remote ocean93. The affinity of hydropho- from that of a fully natural floc or aggregate. The extent
bic organic contaminants for black carbon is high, with to which the aggregation affects the fate and transport
partition coefficients two to three orders of magnitude of microplastic or microplastic-embedded composite
higher than those for organic matter69,80,81,85. particles is an open question94.
Many transport modelling studies assume spherical
Microplastics as a phase in particulate matter, particles or aggregates11. A sphere has the smallest pos-
sediments, suspended solids or soils sible surface area for a given particle volume or size, and
The above natural particles exist in nature in the form thus constitutes only a very small fraction of the actual
of mixtures and aggregates. They are composite particles shape distribution of microplastic and natural particles.
that we often refer to with functional names such as aero- The PDFs discussed above provide opportunities for
sols, dust, sediments, soils, particulate matter, suspended more realism in transport models with respect to shape.
solids or settling solids. The physical characteristics of
these natural particles are somewhere in between those Microplastics have a distinctive combination
of their individual components, with the range of the of characteristics
characteristics determined by the physical characteristics Here we compare the key characteristics of environ-
and relative proportion of the components. Like other mental microplastics with other categories of particles11
particles of anthropogenic origin, microplastics can be (Fig. 1c). It appears that some particle categories can
considered as a new artificial phase of composite nat- have a similar size to microplastics, but then have a
ural particles94,95. This raises the semantic question of higher density (minerals, sand, silt, clay, metal-based
whether one should speak of ‘the sediment plastic frac- nanoparticles and colloids). Other particle categories
tion’, or of ‘the extent to which the actual sediment is can have similar density, but then are far less persistent
diluted by plastic’. (organic-matter flocs, detritus, algae, detritus or organic
colloids) than microplastics. Yet other particle categories
Natural particles as a proxy for microplastics do not exist in a nanometre-to-centimetre size range,
Microplastic particles can be transported in air and or in a fibre-to-sphere range of shapes, with all other
water, are subject to vertical mixing owing to their small properties being similar to those of microplastics.
size and can settle in a manner similar to that of these In summary, it is the combination of low density
natural solids1,4,8,11,18,19,96,97. Because the continuum of (often close to that of water)4,8–10,29, high persistence52,55,
characteristics for natural particles overlaps with that wide size range4,8–10,29,51,53 and variable shape4,8–10,29,33,54
of microplastic9,33 (Fig. 1c), natural particles may serve that makes microplastic particles unique. However, apart
as a proxy for microplastics, and models simulating the from their manufactured polymer composition, none
transport of such natural particles can form the basis of the individual features of microplastic is unique by
of transport models for microplastic11. In natural waters, itself. Microplastic sizes overlap with those of minerals,
for instance, several processes drive the transport of organic matter and black carbon particles, and densities
natural and microplastic particles11, including turbulent are similar to those of organic matter and black carbon.
transport, aggregation, biofouling, settling, resuspen- Microplastic longevity in the environment is high, of
sion and burial. Microplastics rapidly acquire a biofilm an order of magnitude similar to that of inert persistent
and become captured in low-density aggregates or flocs. minerals such as clays and black carbon (Fig. 1c). Natural
This implies that the transport of the plastic-inclusive fibres, spherical soot black carbon particles and film-like
floc or aggregate becomes virtually indistinguishable flake graphite exist, so these shape categories are not
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with natural colloids, surface area and reactivity104,105,

Natural
1.5 chemical affinity, and adsorption and desorption rates
Microplastics for sorbed chemicals98,106. Hydrophobic organic con-
taminant sorption affinities for nanoplastic particles are
much higher than those for microplastics and approach
those for soot and black carbon85,98. However, this infor-
mation is largely based on laboratory studies using syn-
Probability density
1.0
thesized spherical nanoplastics. Apart from polymer
identity, the actual characteristics of nanoplastic parti-
cles in the environment remain unknown107. Common
engineered nanoparticles include metal nanomaterials
0.5
such as Ag or Au, stable oxides such as TiO2 or CeO2,
iron oxides and carbon nanotubes105,108. Nanoplastics
are distinguished from these engineered nanoparti-
cles by their primary origin and particle heterogeneity,
which is likely to be higher for nanoplastics. Metal-based
engineered nanomaterials have a higher density than
0
nanoplastics, whereas densities and environmental
100 1,000 10,000 persistence of carbon-based nanoparticles (fullerenes,
carbon nanotubes) are likely to be in the same range.
Length (μm)
Some metal-based nanomaterials are subject to disso-
Fig. 2 | Comparison of lengths for natural and microplastic fibres. Minimum, median lution processes. For nanoplastic, it is not clear to what
and maximum lengths for natural fibres (55, 120 and 6,090 μm) versus microplastic fibres extent dissolution, particle shrinking, further frag-
(55, 120 and 7,470 μm), obtained from six water types. A minimum diameter of 15 μm was mentation and/or degradation processes contribute to
assumed, implying a detection limit for length of 15× 3 = 45 μm, based on a minimum removal53, which constitutes a major research gap.
aspect ratio of 3. Data from ref.33.
Interactions of microplastic with biota
unique to microplastic. The sorption affinity of hydro- Owing to the multidimensionality of microplastic,
phobic organic contaminants for microplastic is similar the mechanisms for uptake, bioaccumulation and
to that of organic matter but orders of magnitude lower adverse effects, as well as the dose metrics to quantify
than that of black carbon80,85,98,99. It is often said that chem- these effects, are diverse. Systematic research is needed
ical leaching from microplastic particles distinguishes to decipher these mechanisms28.
them from natural particles. However, black carbon and
soot particles are highly contaminated and therefore Uptake and bioaccessibility
also leach chemicals99–101. The same applies to aquatic Many effect mechanisms require microplastic to be
sediments contaminated with legacy compounds that ingested, and microplastic ingestion has been demon-
leach chemicals into a cleaner overlying water column102. strated in the laboratory and field for a wide range of
In other words, leaching is not an essential difference, species109–118. The ingestion of microplastics by aquatic
and given the abundance of contaminated sediment and biota depends on their bioaccessibility, which is mostly
black carbon compared with microplastic masses in the determined by particle size, the species-specific char-
environment, the fluxes of organic chemicals leaching acteristics of the exposed organisms, such as mouth
from natural particles will often overwhelm those of opening or resistance to translocation, and the envi-
additives leaching from plastics in many habitats15,103. ronmental conditions19,49,119. The formation of biofilms
can affect bioaccessibility by increasing the size and
Microplastics, nanoparticles and nanoplastics density of microplastics and modifying their shape.
In addition to the natural microparticles described For example, biofilms were found to be able to pro-
above, smaller nanoparticles are also present in the envi- mote particle uptake under some conditions120,121, but
ronment. Research in the field of engineered nanoparti- to decrease it when the biofilm increased particle size
cles has provided a picture of the main similarities and and promoted aggregation122. After ingestion, microplas-
differences between microparticles and nanoparticles104. tics can be transported along the digestive tract until
Here, we briefly reflect on the main differences between excretion, or they can accumulate in the gut123, the
micrometre-sized and nanometre-sized environmental digestive gland124, the gills123,124 or the liver123 of some
particles. organisms. Cell internalization of microplastics in the
Important categories of environmental nanoparticles digestive system125 and their translocation from the gut
are organic and inorganic natural colloids, soot, the sub- cavity to the bloodstream126 have been demonstrated
micrometre size fractions of polydisperse particles such in the marine mussel Mytilus edulis and the trans-
as black carbon80 and desert dust aerosols61,62, engineered port of microplastics along the circulatory system has
nanoparticles32 and nanoplastics53. At present, the rela- been shown in the marine clam Scrobicularia plana124.
tive contribution (abundance) of nanoplastic particles to Digestive processes can change microplastic character-
this complex mixture is unknown. istics: the Antarctic krill Euphausia superba127 and the
Nanoplastics can be expected to differ from microplas- amphipod Gammarus47 were found to turn microplas-
tics with respect to their transport and fate, aggregation tics into nanoplastics through digestive fragmentation.
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Besides being ingested, microplastics can adsorb to the In general, studies comparing microplastics with
surface of microalgae128, aquatic plants129, cnidarians130 natural particles (for instance, red clay, kaolin or nat-
and crustaceans131. ural sediment) under controlled settings show that
microplastic-induced adverse effects occur at lower
Adverse effects concentrations than for natural particles13,135,159,162–164,
The ingestion or adsorption of microplastics can cause although some of these studies lack confirmation of qual-
adverse effects on aquatic organisms16. At the population ity assurance and control (QA/QC)28. That microplastic
level, the presence of microplastics can reduce the num- effects occur at lower concentrations does not neces-
ber of species or their biomass132–134. At the individual sarily imply that they cause effects when mixed with
level, microplastics can affect survival134–137, reproduc- natural particles, because natural particles can be more
tion135,137–141, growth13,122,128,134,138,141–144, feeding130,131,139,145, abundant. Because most of the published comparisons
emergence146, embryonic development147, mobility148,149 between microplastic and natural particle effects concern
and photosynthetic efficiency128. At the sub-organismal clays or natural sediments, a clear research gap exists as
level, microplastics can cause increased oxygen con- to how other abundant and hazardous microparticles
sumption145,150, inflammation125,151, reduced lysosomal compare, such as micrometre-sized soot and black car-
stability in the digestive gland125, reduced antioxidant bon particles165. Thus, we argue that the implications of
capacity124, DNA damage16,124, neurotoxicity124, oxi- microplastic particles in the biosphere should not remain
dative damage124, gut dysbiosis152,153, alteration of the fully separated from those of other, natural particles.
genetic expression136, ionic exchange150 and enzymatic
activity149. The mechanisms leading to these effects are Nanoplastics
often unknown, but many studies have speculated that Although many uncertainties remain, years of particle
microplastic-induced physical damage or reduced feed- toxicity research have provided a picture of the rela-
ing may contribute13,121,122,128,130,134–142,144,148. Microplastics tionship between particle characteristics and toxicity,
can cause physical damage by adsorbing and aggregat- and of the main similarities and differences between
ing on the surface of microalgae128, restrict movement microparticles and nanoparticles32,104,166–173 (Table 2).
by accumulating in tentacles130, or promote satiation However, for nanoplastic particles, our knowledge of the
and reduce food assimilation by blocking the food pas- relationship between particle characteristics and toxicity
sage121,133–135,137,139,141,143,144,147. Other studies have attrib- is based on laboratory studies with fabricated particles,
uted the effects to specific plastic properties, such as mainly submicrometre-sized polystyrene spheres. For
the surface groups28,31,35,36,135,138,142,143, or the leaching of environmentally realistic conditions, this relationship is
toxic chemicals from the microplastics136,140,145,147. The difficult to address, because as mentioned, apart from
severity of the effects vary depending on the proper- polymer identity107, toxicologically relevant character-
ties of the microplastics, their concentrations and the istics of environmental nanoplastic particles (such as
exposure time. shape, submicrometre-size range, area, volume, surface
The weight of evidence of these reported effects and chemistry, biopersistence and zeta potential32,104,166) are
effect mechanisms has been analysed30,154, including a unknown.
quantitative analysis that also took the quality of stud-
ies into account28. The four most relevant effect mech- Risks of microplastic particles
anisms, in order of decreasing weight of evidence, were Current status of risk assessments
food dilution (inhibited food assimilation or decreased Although assessments of exposure to microplastics for
nutritional value); internal physical damage; external humans exist19,174,175, there has not been an assessment of
physical damage; and, with lower certainty, oxida- the risk posed by such exposure, nor of the risk to ter-
tive stress28. Consequently, these mechanisms should restrial ecosystems. However, risk assessments have been
take priority when assessing the risks of microplastic performed for microplastic particles in the aquatic envi-
particles. ronment, based on a comparison of exposure concen-
trations and threshold effect concentrations14,16,17,49,176–181.
Differences between microplastic and natural Exposure concentrations were taken from either meas-
particle effects ured literature data or modelled data176,177. Because
Because microplastic particles are present among numer- of the different analytical methods used, these expo-
ous inert and degradable natural particles, microplastic sure data are often difficult to compare and have lim-
and natural particle effects are likely to occur together. ited quality25,49,54. Similar limitations with respect to
Although there are calls to address the complexity of comparability and quality exist for effect assessment
microplastic as a mixture of plastic-based particles, methods28,49, which in combination seriously limits the
conceptually, the broader array of particle types present reliability of the resulting risk characterizations.
should also be considered. After all, for protecting the The reported risk assessments used species sensitivity
habitat quality of both aquatic and terrestrial organisms, distributions (SSDs), which aim to determine the affected
it is relevant to address risks from mixtures that also con- fraction of a series of species at a given concentration182.
tain other man-made and low-calorific-value natural SSDs increase the relevance of the assessment for the
particles. Often these particles can have similar effect community level, and are used to obtain the hazard con-
mechanisms and toxicological profiles, given that they centration for 5% of the species (HC5) (Fig. 3). Ideally,
have similar combinations of particle, particle-associated SSDs use the effect threshold values for one well-defined
chemical, surface area or size-related toxicities155–161. type of stressor and end point (one type of harm) for more
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Table 2 | Key differences between microparticles and nanoparticles in the context of toxicity
Particle type Particle characteristics relevant for effect Possible effects
Microparticles (1–1,000 μm)
Microparticles that Organic matter Chemical composition, digestibility Chemical toxicity
do not exist in the
nanometre size Microplastic Size, volume, area, aspect ratio, shape, sorbed Chemical toxicity Food dilutiona,
range chemicals mechanical
irritation,
Coal Size, area, chemical composition Pneumoconiosis, inflammation,
fibrosis, cancer oxidative stress
Particles that exist Asbestos Fibre length, aspect ratio, type, persistenceb, sorbed Asbestosis, pleural
in the micrometre chemicals disease, lung cancer,
and the nanometre mesothelioma
size ranges
Desert dust aerosols Size, area, shape Respiratory distress
Quartz (silica) Size, area, shape Silicosis, silicic acid
release, cancer
Nanoparticles (1–1,000 nm)
Particles that exist Asbestos Fibre length, aspect ratio, type, persistence Asbestosis, pleural Translocationc,
in the micrometre disease, lung cancer, biodistribution,
and the nanometre mesothelioma mechanical
size ranges irritation,
Desert dust aerosols Size, area, shape Respiratory distress oxidative stress
Quartz (silica) Size, area, shape Silicosis, silicic acid
release, cancer
Nanoparticles that Black carbon (soot) Size, surface area, sorbed chemicals Respiratory and
do not exist in the cardiovascular disease,
micrometre size cancer
range
Nanoplastic Size, surface area, charge, length, aspect ratio, Unknownd
aggregation, sorbed chemicals
Carbon nanotubes Size, surface area, length, aspect ratio, aggregation, Fibrosis, inflammation,
sorbed chemicals cancer
Metal-based engineered Size, surface area, charge, zeta potential, solubility, Inflammation,
nanomaterials aggregation mitochondrial damage,
DNA damage
Organic matter colloids Digestibility, sorbed chemicals Chemical toxicity
Summary of some of the main features of known effects or diseases of micrometre-sized versus nanometre-sized particles on biota, mostly related to inhalation.
Effects are dose-dependent. For details, see refs18,20,28,30,32,33,80,85,108,154,165–173,188,189. aThe ‘food dilution effect’ is specifically relevant for small invertebrates28.
All inert particles may cause this effect upon ingestion. bPersistence and low clearance rates are often mentioned as an important factor in fibre toxicity32. cFor
nanometre-sized particles it is generally assumed that they are subject to translocation and distribution between organs. dActual effects of nanoplastic particles
residing in the environment or in the human diet are not yet known.
than ten different species while environmental variables threshold concentration data, including those used in
are kept constant. However, such effect threshold data are SSDs9,19,33,49. The ambiguity of multiple causation — that
not yet available for microplastic particles. Even though is, effects originating from a range of mechanisms —
‘real’ microplastic represents a continuum of particle can be addressed by first identifying the effect mech-
types, laboratory tests have either used monodisperse anism, and then aligning and quantifying the relevant
particles that are all ingested28, or particles with wider size exposure and effect metrics. For instance, upon iden-
distributions, where larger particles may have disrupted tifying ‘food dilution’ as a relevant microplastic effect
the intake or effects of the smaller particles or had more mechanism, the corresponding dose metric (ingested
limited bioavailability. Microplastic particles can also trig- volume), as well as the exposure and effect threshold
ger responses through different modes of action (different data with respect to that dose metric, can be aligned and
types of harm)28, suggesting that the HC5 values obtained quantified49. This mechanism-specific quantification of
from SSDs remain ambiguous with respect to identifying effects is a well established concept in toxicology and
the relevant particles and their associated effects. should be applied to microplastic particles, including to
distinguish between the particles’ physical effects and
Strategies to improve risk assessment particle-associated chemical effects2,183. Besides applica-
Harmonizing the setups between studies, strictly tion in risk characterizations based on SSDs, this con-
adhering to quality criteria and using environmentally cept can be used in prospective risk assessments based
realistic microplastic mixtures in tests may solve these on food-web modelling184.
problems of limited comparability and quality. This,
however, also implies that much of the work has yet to Risks from chemicals
be done. As an alternative to redoing all of these tests Plastic- a ssociated chemicals. In addition to the
from scratch, microplastic PDFs can be applied to res- physical effects and risks of microplastic particles,
cale and align currently available exposure and effect microplastics can simultaneously cause chemical
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a Plastic continuum Erosion and Erosion and Fig. 3 | Interactions of microplastic with biota. a | Plastic
fragmentation fragmentation continuum. Erosion and fragmentation of plastic objects
leads to an increasing number of ever smaller microplastic
Plastic products Plastic debris Microplastics particles that are bioavailable to an increasing group of
species. b | Species continuum. As soon as microplastic
particles become bioavailable, they may trigger interactions
with a variety of species. For instance, larger particles may
be bioavailable through ingestion, but not to the smallest
organisms. Smaller particles can also be bioavailable to
b Species continuum Ingestion Ingestion and translocation small organisms, through ingestion but also through
translocation. c | Species sensitivity distribution (SSD).
The interactions between the bioavailable part of the
plastic continuum, and the species continuum, may lead
to species-specific effects. Threshold effect concentrations
for these species-specific effects can be assessed for a range
Fish Daphnia Zooplankton
of species and combined in an SSD for environmentally
relevant microplastic. Hazardous dose metric concentration
for 5% of the species (HC5) provides input for risk assessment.
d | Hazardous concentration in particles per litre for 5%
c SSD of the species (HC5) present in an aquatic community
with error bars relating to the 95% confidence intervals or
25–75 interquartile ranges14,180, according to Everaert et al.
(2018)176, Besseling et al. (2019)16, Skåre et al. (2019)179, Adam
et al. (2019)14, Zhang et al. (2020)178, Everaert et al. (2020)177,
Koelmans et al. (2020)49, Adam et al. (2021)180 and Jung et al.
(2021)181. Later studies take more data points into account.
Median HC5 for these nine studies is 75.6 particles per litre.
Affected species (%)
These terms are imprecise because phase distribution

ensures that the same chemicals are also present in
other environmental media or particles, such as soil,
sediment, water, organic particles, algae, black carbon
or detritus15,17,35,37,38,41,85,100,102,103. Depending on exposure
concentrations, any of these chemicals from any such
HC5
media can pose a risk to biota. For instance, microplas-
5%
tic can be a significant source of hazardous chemicals
through water as an exposure medium, leading to
mortality under environmentally relevant conditions.
Effect thresholds for environmentally relevant microplastics Acute mortality in coho salmon could be ascribed to
a tyre-rubber-derived chemical that was present in the
d Everaert Besseling Skåre Adam Zhang Everaert Koelmans Adam Jung water at toxic concentrations186.
2018 2019 2019 2019 2020 2020 2020 2021 2021
5
The microplastic vector effect. It has long been hypoth-
esized that desorption of chemicals from microplastic
4 after particle ingestion may increase exposure to these
chemicals, and subsequently lead to chemical risks to
log[HC5 (particles per litre)]
biota (the ‘microplastic vector effect’). However, the

3
microplastic vector effect is now thought unlikely to
play a major part in most habitats, and no risk has been
2 demonstrated to date15,17,38. There are two main reasons
for this conclusion. First, most empirical studies con-
sidered chemical exposure only through ingestion of
1
microplastics, neglecting parallel uptake pathways. This
overestimates the relative importance of microplastics
0 as a vector for transport and uptake, but underestimates
the total exposure to chemicals15,35. When all uptake pro-
cesses are taken into account, uptake through microplas-
–1
tic ingestion is seen to be minor15,41,187. This also means
that testing the chemical toxicity of microplastic parti-
effects if chemical exposure is high enough. Chemicals cles, ignoring simultaneous exposure to the same chem-
such as additives and sorbed environmental pollut- icals through other media, is less useful when trying to
ants are abundant on microplastic particles in the mimic environmental realism. Second, most studies also
environment1,17,30,34,36,39,185, and are often referred to as used an unrealistic chemical concentration gradient
‘plastic chemicals’185 or ‘plastic-associated’ chemicals15. between plastic and biological receptors. For example,
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in most environments, uptake from other media, the

Problem definition
low digestibility of plastic, the abovementioned ‘food
Ecological or dilution effect’ and the lack of a chemical concentration
Exposure characteristics toxicological
effect
gradient under environmentally realistic conditions, it is
plausible that an organism would starve before it would
exceed a chemical toxicity threshold.
Analytical
Quality assurance
techniques and
and control
harmonization Dealing with chemical and particle effects. The above
does not mean that chemical effects of ingested particles
will always be minor compared with physical particle
effects. After all, the chemical effects and thus potential
Dose metric for mechanism of effect
risks are highly context-dependent, influenced by the
Effect mechanism composition of the chemical mixture, chemical concen-
trations, the directions of chemical transfer, presence
and co-exposure through water, through natural organic
Exposure dose metric Effect dose metric particles or through prey organisms15. The chemical and
physical toxicity components of microplastic hazards
are best assessed separately2,15, following the principle
of separate dose metrics and effect mechanisms, to be
Surface Aspect
Mass Volume combined only at the last stage of the assessment33.
area ratio
A new microplastic risk assessment framework

Several scientists have introduced risk assessment
Exposure profile Stressor-response profile frameworks for microplastic particles2,16,20,183,190. These
provided general strategies and recognized the chal-
lenges that come with the complexity of microplastic.
However, the frameworks did not yet include the the-
Aligning dose to microplastic continuum ory and tools required to meet these challenges. They
simplified the actual diversity of microplastic by using
Environmental
microplastic Acceptable dose categories and bins for particle properties and/or chem-
continuum ical content, thereby neglecting the considerable varia-
tion that exists within categories2,16,183,190. Second, they
did not implement a strategy to account for the lim-
Risk characterization
ited quality of data or the fact that the data cannot be
compared2,16,183,190. Third, one proposed framework did
Fig. 4 | Risk assessment scheme addressing the multidimensionality of microplastics. not include an effect threshold component, neglected
Step 1: problem definition (blue box). A problem definition is given on the basis of dose dependency of particle and chemical effects, and
a protective objective (for example, protecting a population). This guides the design inaccurately distinguished between additive and envi-
of a hazard and exposure assessment, taking into account harmonization of methods ronmental chemicals190. Because scientific progress in
and quality assurance and control (QA/QC) to select input data that are fit for purpose. the past 3 years has tackled many of these challenges,
Step 2: dose metric for mechanism of effect (yellow box). Based on selected mechanisms
we can now define a generic microplastic risk assess-
of effect, ecologically or toxicologically relevant dose metrics (ERM or TRM, respectively)
are defined, such as mass, surface area, volume and aspect ratio. Subsequently, exposure ment framework that applies to either the environment
concentrations and effect threshold concentrations are assessed for these metrics, or to human health. Three new elements determine the
leading to an exposure profile and a stressor-response profile. Step 3: aligning dose to framework: use of PDFs to describe toxicologically rel-
microplastic continuum (red box). For each of the ERMs or TRMs, the profiles from step 2 evant particle characteristics, so that no simplification
apply only to the bioavailable fraction of the microplastic continuum. Thus, they must be with categories is necessary; use of QA/QC screening
expressed in terms of the full (from 1 to 5,000 μm), environmentally realistic microplastic methods to evaluate whether exposure and effect data
continuum. This conversion is done using probability density functions (PDFs). Finally, are fit for purpose; and use of a calculation framework
for each ERM or TRM, actual exposure and effects thresholds are compared in a risk to assess exposure to plastic-associated chemicals
characterization33,49. In this way, a risk is calculated for each of the individual effect
through all relevant pathways. Here, we provide the
mechanisms.
outline for a new approach to consistently account for
the multidimensionality of microplastic.
desorption of plastic-associated chemicals was studied in The risk assessment needs three levels of information
a clean animal test system, or effects were tested in vitro (Fig. 4). First, steps are taken on a process level, which
using extracts obtained at unrealistically high plastic- includes defining the problem, designing exposure and
to-water ratios and small extract volumes188,189. Because effect assessments, risk characterization and quality
additives are subject to significant dilution in the natural assurance and control for the screening of input data2,16.
environment, which is not mirrored in such tests, any Second, based on a particular problem definition, mech-
observed toxicity does not directly indicate whether or anisms of adverse effects on biota should be identified.
not a risk occurs188. It is true that at ‘hotspot locations’, Because microplastic is a diverse mixture of contami-
ingestion of high concentrations of microplastic particles nants, it causes effects simultaneously through differ-
may occur. However, given the typical chemical dilution ent effect mechanisms9,33. Each of these mechanisms
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is linked to an ecologically or toxicologically relevant the assessment, which is crucial when the assessment is
end point, and is quantified by the ecologically or toxi intended to have regulatory implications.
cologically relevant dose metric (ERM or TRM) for
that mechanism2,33,49. Third, because dose metrics are Step 2: dose metric for the mechanism of effect
relevant to only part of the continuum, they need to As mentioned above, food dilution, internal damage
be extracted from the exposure data covering the full and oxidative stress are plausible microplastic parti-
continuum33,49 (Fig. 5). cle effects28,154. Relevant dose metrics then need to be
selected for these mechanisms33. For instance, volume is
Step 1: problem definition a relevant dose metric for a food dilution mechanism28,192,
The problem to be defined — often the protection of a and aspect ratio is relevant in cases where internal
population or an individual flagship species — comes damage correlates to fibre toxicity32,170,171,193. Surface area
from the regulatory domain. From there, exposure and or specific surface area are appropriate for endpoints
effect assessments are carried out, which rely on QA/QC such as oxidative stress32,167–169. When potential risks arise
and harmonization of analytical techniques and effect from chemicals or pathogens bound to either the sur-
test methods28,54. Exposure and effect assessments have face of plastic or absorbed by its bulk, concentrations in
benefited from advances, particularly in automated FTIR terms of surface area and mass, respectively, are suitable
imaging techniques33,191, and the cost and effort required metrics15. Environmentally realistic mixtures of particles
for these measurements can be expected to drop as soon are likely to cause effects through multiple mechanisms
as they become mainstream. A crucial requirement for simultaneously, implying that there are multiple rele-
microplastic risk characterization is that both exposure vant dose metrics33. We note that some of these met-
and effect assessments relate to the same ecologically or rics have boundaries to account for bioavailability. For
toxicologically relevant dose metric2. The gold standard instance, if particles are too large to be ingested, food
would be to test with a consistent environmentally rel- dilution would apply only to the ingestible bioavailable
evant diverse microplastic mixture, but data correction fraction of the microplastic continuum28,49. Cellular-level
and alignment methods can bridge the differences in mechanisms like oxidative stress require translocation,
the current data33,49. For retrospective risk assessments, so only much smaller particles would be bioavailable33,49
quantitative quality assurance and criteria evaluation (Table 2). Bioavailability is a well-established concept in
tools have now been developed28,54. These tools allow the risk assessment for traditional chemicals, and is relevant
risk assessor to select data that are fit for the purpose of for microplastic as well.
Step 3: aligning dose to microplastic continuum

Once the relevant effect mechanism and dose metrics
Heterogeneous particle mixture are known, they need to be aligned to exposure data2,16,17.
Well-established procedures are available for risk assess-
ment of chemicals, which have been extended to chemi-
cals absorbed to microplastic particles2,15,37,41,103. To align
Effect mechanisms Cytotoxicity Oxidative stress Inflammation particle effects to exposure data, exposure needs to be
expressed in parameters such as microplastic volume,
aspect ratio, (specific) surface area or mass33,49. Often
Ecologically or toxicologically Specific mass provides a fair proxy for volume, given the pro-
Surface area Aspect ratio
relevant dose metric surface area portionality between the two, as the densities of envi-
ronmental microplastic particle mixtures are close to 1
and less variable than other microplastic character-
istics such as size and shape13,49. The various relevant
Probability density function
exposure and dose metrics can be quantified using
the above-mentioned microplastic PDFs, obtained
prospectively9 or retrospectively19,33 using existing data.
However, given the diversity of microplastic, the need to
take multiple effect mechanisms and thus dose metrics
into account is the rule rather than an exception, and
Risk characterization
requires a thorough characterization of the microplas-
tic continuum. State-of-the-art focal-plane-array FTIR
imaging provides the necessary detail to obtain such
Fig. 5 | The concept of simultaneously acting effect mechanisms. A heterogeneous PDFs33,194, although otherwise existing PDFs for environ-
mixture of plastic particles can initiate effects through different mechanisms acting mental microplastic can be used in the assessment33. For
simultaneously. For each mechanism, an ecologically or toxicologically relevant dose instance, detailed PDFs for volume, area, mass, aspect
metric is defined. These are, for example, particle surface area, specific surface area,
ratio and specific surface area have been provided from
aspect ratio (length to width ratio) or particle volume (not shown). For each of the
ecologically or toxicologically relevant dose metrics, a probability density function (PDF) a meta-analysis of characteristics of individual particles
is defined, based on the best available data on the characteristics of environmentally sampled from different environmental compartments33.
relevant microplastic particles9,33,49. Finally, using these PDFs, exposure data are translated This represents a promising approach to linking
into the ecologically or toxicologically relevant dose metrics for a risk characterization for microplastic doses and characteristics to effect mech-
each of the effect mechanisms. anisms and thresholds. A risk characterization is then
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performed for each of the effect mechanisms separately wisdom regarding the risk assessment for any stressor
(Fig. 4), after which the risk assessor can use a precaution- is that a framework is needed first. Only then can the
ary approach by basing the conclusion about the risk on acquisition of empirical data and the development of
the metric for which the highest risk has been calculated. technical and theoretical instruments optimally match
the intended purpose. Loss of time and efficiency
Prospect can thus be avoided. In this context, we present the
Particles are ubiquitous in our natural environment, and framework described in this Review and hope that it can
microplastic particles are a relatively recent subcategory. lead to further alignment and efficiency in this complex
Owing to their anthropogenic origin, continued plastic area of research.
production and fragmentation of plastic items, both the Microplastic particles form a continuum within a
mass and number concentrations of microplastic are broader continuum of natural and other anthropogenic
expected to increase. Size distributions are expected to particles. Although some studies address the differ-
gradually shift towards a greater proportion of smaller ences in effects between microplastic and clay particles,
particles, increasing bioavailability and potential risk to environmental microplastics are not typically viewed
a wider range of species, including humans. Risks do not in the context of the multiple other particle types pres-
appear to be widespread at this point, but most scientists ent. We suggest that the approach proposed here to
agree that it is not a question of if, but rather when, the distinguish effect metrics associated with potentially
environmental and human health risks of microplastic co-occurring specific effect mechanisms is equally appli-
particles become apparent. Improving risk assessment cable to other particle types. An ecological risk assess-
methods for microplastics will enable us to determine ment based on a ‘food dilution’ effect mechanism from
the timeline for these risks better. the ingestion of low-calorific microplastic particles could,
Viewing environmental microplastic as a contin- in theory, also work for low-calorie clay, silt or sand
uum of characteristics rather than as a categorical phe- microparticles. Oxidative stress after translocation of
nomenon is a relatively new concept. Although newer small particles, owing to inhalation of microplastics and
frameworks address the diversity of microplastics with other particles such as PM2.5, could ideally be assessed
respect to analytical detection, exposure assessment, together. Soot and other black carbon particles are highly
impact and risk assessment, they have not been applied aromatic, with substantial amounts of polycyclic aromatic
to perform actual risk assessments in a regulatory hydrocarbons, making it logical to assess the implications
context. It is critical that these postulated frameworks of plastic-associated chemicals in the context of exposure
are tested, validated and simply tried out across a wide to other such chemically contaminated particles. Thus,
range of environmental and human health-related risk the conceptual approaches presented in this Review to
assessment problems. address the multidimensionality of microplastics may
These new frameworks provide opportunities not perhaps be extended to address the multidimensionality
only to assess the risks of microplastic particles, but also of all categories of hazardous particles.
to unify and align research approaches that have hith-
erto been fragmented and disparate. The conventional Published online 21 January 2022
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REvIEWS

Risk assessment of microplastic

138 | February 2022 | volume 7 www.nature.com/natrevmats

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1 nm 1 μm 1 mm 1.0 1.5 2.0 2.5

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with natural colloids, surface area and reactivity104,105,

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These terms are imprecise because phase distribution

biota (the ‘microplastic vector effect’). However, the

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in most environments, uptake from other media, the

A new microplastic risk assessment framework

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Step 3: aligning dose to microplastic continuum

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