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Ecotoxicology and Public Health

Solving the Nonalignment of Methods and Approaches Used in Microplastic Research to Consistently Characterize Risk
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Environmental Science & Technology

Cite this: Environ. Sci. Technol. 2020, 54, 19, 12307–12315
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https://doi.org/10.1021/acs.est.0c02982
Published September 4, 2020

Copyright © 2020 American Chemical Society. This publication is licensed under CC-BY-NC-ND.

Abstract

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The lack of standard approaches in microplastic research limits progress in the abatement of plastic pollution. Here, we propose and test rescaling methods that are able to improve the alignment of methods used in microplastic research. We describe a method to correct for the differences in size ranges as used by studies reporting microplastic concentrations and demonstrate how this reduces the variation in aqueous-phase concentrations caused by method differences. We provide a method to interchange between number, volume, and mass concentrations using probability density functions that represent environmental microplastic. Finally, we use this method to correct for the incompatibility of data as used in current species sensitivity distributions (SSDs), caused by differences in the microplastic types used in effect studies and those in nature. We derived threshold effect concentrations from such a corrected SSD for freshwater species. Comparison of the rescaled exposure concentrations and threshold effect concentrations reveals that the latter would be exceeded for 1.5% of the known surface water exposure concentrations worldwide. Altogether, this toolset allows us to correct for the diversity of microplastic, to address it in a common language, and to assess its risks as one environmental material.

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Introduction

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The literature portrays environmental microplastic as a diverse and complex material. (1−5) This diversity follows from the fact that microplastic originates from many different types of products, (2,3) and from how the material is defined: all plastic particles smaller than 5 mm. (2,6−9) Because of the unspecific nature of this definition, microplastics (MPs) constitute a heterogeneous mixture of particles represented by a range of polymers, sizes, and shapes, and associated with all kinds of chemicals. (2,3,10) Microplastics interact with natural particles and with organisms under a wide range of environmental conditions, which are even more diverse in space and time. (11,12) This explains the challenges in assessing the risks these particles pose to the environment and to human health. It has even been argued to not consider “microplastic” as one contaminant, but instead focus on its components and address specific (classes of) microplastic for a range of types, sizes, and shapes. (5)
Assessing whether microplastic particles are a risk for human health and the environment is considered one of today’s major challenges in the environmental sciences. (2) A consistent risk assessment for microplastic particles requires alignment of exposure and effect data. This means that whatever metric or unit is used to characterize exposure also is used for the effect assessment. (1,13) For instance, ideally, one would measure exposure via all possible pathways for the entire size range that makes up microplastic, i.e., from 1 μm to 5 mm, according to an environmentally realistic size distribution. For the effect assessment, one would need to test particle effects for the same size distribution and boundaries, such that exposure and effect threshold data are aligned, and can be used in the risk characterization in a meaningful way. (1)
Unfortunately, the microplastic community is still far away from this situation. First, the literature uses different definitions of microplastics, with differences especially relating to the size ranges included in the definition. (3,14) Second, methods to detect microplastic numbers or mass concentrations have different size detection limits. (15) Since particles with smaller sizes typically occur at much higher number concentrations, (4,16) methods that use finer sieve or filter mesh sizes and/or accurate spectroscopic methods would hence be able to capture most of these particles and yield orders of magnitude higher concentrations. (15) Sieves or nets used for sampling often have either, e.g., 20, 100, 300, or 333 μm as the smallest size limit, whereas detection methods such as visual inspection, attenuated total reflection Fourier transform infrared (ATR-FTIR), focal-plane-array FTIR (FPA-FTIR), or Raman typically are associated with lower size detection limits of either 1000, 300, 20, or 1 μm, respectively. (17) At the other end of the scale, sampling volume determines the detection limit for large particles, where larger particles with a low frequency of occurrence are simply missed when the volume is too small. (15,18) Therefore, the aforementioned differences in methods have resulted in data incomparability. Third, conversion from number to mass concentrations (or vice versa) is usually done assuming microplastic particles to have a certain fixed shape (e.g., spherical) with the density of a discrete polymer. (19,20) This, however, is not correct, because microplastic particles have a wide range of shapes and densities. More accurate conversions can be done by taking actual shape and density distributions into account. Fourth, there is nonalignment of microplastic particles used in effect tests. Tests often use particles of one size, or very limited size range, one polymer type, or one shape category, whereas other tests use particles with a wider range of properties. (2) This renders these studies incomparable. Nevertheless, several recent publications have compared effect thresholds obtained for such highly different microplastic particle types in species sensitivity distributions (SSDs). (19−24) A key feature for a meaningful SSD is that the threshold effect concentrations relate to the same stressor, which is not the case for these SSDs because the individual data points relate to very different particle types. This renders the hazard data obtained from these SSDs fundamentally flawed. Fifth, the exposure data do not align with the effect threshold data. This follows logically from the first two reasons. Exposure calculated from environmental concentration data automatically relates to environmental microplastic, a diverse mixture of sizes, polymer types, and shapes. This does not match with the types and sizes of microplastic particles used in effect tests, as mentioned. To have exposure and effect test data aligned such that a meaningful risk characterization can be done, the effect assessment should also be done with environmentally realistic, that is, diverse microplastic. (1,4)
One logical and ongoing strategy to deal with these issues is harmonization of methods, and when all is harmonized, use these methods to collect all necessary data to do risk assessments of sufficient quality. This, however, will take very long. There are several ongoing harmonization processes, (9,25−27) but even they differ and there seems to be a lack of harmonization of these efforts in itself. This means that there is an urgent need for pragmatic workarounds to allow for the translation of all of these different types of data into one common currency, such that risk assessment for microplastic becomes feasible.
Here, we propose and test rescaling methods that are able to substantially reduce the nonalignment of methods used in microplastic research. First, we provide a simple method to correct for differences in size ranges targeted by studies reporting microplastic concentrations. This results in conversion of data obtained for any size range to default size ranges, like, for instance, 1 μm–5 mm, 20 μm–5 mm, or 1 μm–1 mm for microplastic. (1−7) We apply the method to previously published concentration data to test the hypothesis that their variation decreases due to rescaling. Second, we provide a method to convert number to volume and mass concentration (or vice versa) taking the full heterogeneity of environmental microplastic into account. Third, we provide a method to correct for differences in the particle sizes, shapes, and densities in ecotoxicological particle effect studies. We apply the method to a traditionally constructed and flawed microplastic SSD for freshwater species to obtain a more meaningful SSD and hazardous microplastic threshold effect concentration (HC5). Finally, the latter microplastic threshold effect concentrations (HC5) for 1–5000 μm of microplastic are evaluated against the aforementioned rescaled 1–5000 μm microplastic concentrations for surface waters, to illustrate how a consistent characterization of risk can be done.

Methods

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Aligning Data Sets That Target Different Size Ranges

The literature uses different size ranges to define microplastic, here referred to as default microplastic size ranges. An often-used pragmatic range is between approximately 20 and 5000 μm, where 20 μm relates to the practical detection limit of common FTIR spectroscopic methods. A formally more correct default range would include all particles between 1 and 5000 μm, with 1 μm being the size below which the material would be referred to as nanoplastic. Recently, it was proposed to use a range between 1 and 1000 μm as default; however, this range is only rarely used in the literature. (3) We argue, however, that these differences are trivial because number concentration measured within each of these ranges can be translated into any other range as soon as the particle size distribution is known. (28) Kooi and Koelmans (4) demonstrated that microplastic size distributions typically follow a power law according to
(1)
in which y and x are the relative abundance and size (i.e., length), respectively, and α and b are the fitting parameters. 4 This means that the number of particles found within any size range can be translated into the number expected for any other size range, by using a correction factor (CF) that scales the integral of eq 1 for the measured size range, against the integral of eq 1 for the default range.
(2)
Here, subscripts 1 and 2 relate to the minimum and maximum values of the range (μm), and D and M denote default and measured ranges, respectively. For example, if a number concentration measured for a range from 30 to 2000 μm needs to be rescaled to the default definition range from 1 to 5000 μm, then the values for x1D, x2D, x1M, and x2M would be 1, 5000, 30, and 2000, respectively. With these boundary values and α = 1.6 (see below), the CF would be 8.32 (eq 2). If, for instance, a measured number concentration for the range from 30 to 2000 μm is 100 #/L, the extrapolated number concentration for the default definition range from 1 to 5000 μm would be 8.32 × 100 = 832 #/L. The parameter α may vary a bit among data sets and therefore can be best obtained by fitting the log-transformed version of eq 1 to the measured size distribution. When, however, no data on the distribution is available, then a default value for α of 1.6 is recommended, which is an average value based on 14 environmental microplastic size distributions. (4) For the proof-of-principle calculations in the present paper, the value of α = 1.6 was used.
Using eq 2 (with α = 1.6), (4) standard correction factors were calculated to translate size ranges commonly reported (e.g., 333–5000, 300–5000, 100–5000, 20–1000, 20–5000 μm) to the ranges suggested as preferred definitions of microplastic in the literature (1–5000, 20–5000, 1–1000 μm). (2,3,14)
Furthermore, as a case study, data on surface water number concentrations reported by Koelmans et al. (15) were rescaled. These data were compiled from a large number of studies targeting different size ranges, with studies able to detect small, i.e. <100 μm particles reporting much higher number concentrations. Here, our hypothesis was that rescaling would result in a lower variation in the number concentrations.

Aligning Number and Mass Concentration Data for Microplastic

Currently, conversions of number to volume and mass concentration (or vice versa) based on the assumption of spherical shape are often inaccurate because the particles have a wide range of shapes and densities. More accurate conversions can be done by taking the actual shapes and densities into account. For instance, Kooi and Koelmans (4) defined generic environmental microplastic via probability density functions for size (1 to 5000 μm), shape, and density. Shape was quantified by considering length/width/height (L/W/H) ratio distributions for common microplastic shape categories (e.g., spheres, fragments, films, fibers) and their relative abundances in the environment. These were then combined into overall L/W and L/H distributions (Figure 1). Subsequently, the latter distributions were further simplified into one Corey shape factor (CSF) distribution (). (29,30)L/W/H or CSF distributions can be used to translate number to volume and mass concentrations, as follows: First, Monte Carlo (MC) simulations are used to create a large number (n) of imaginary particles “i”, by randomly sampling their length from the length distribution (eq 1), and their L/W and L/H ratios from these known L/W/H distributions (Figure 1), keeping L/H ≥ L/W. Subsequently, the volume of an ellipsoid, as the best “one shape fits all” approximation for all possible microplastic particles, was calculated using
(3)
Depending on L, W, and H, an ellipsoid can take the shape of an extremely elongated fiber, a thin film, a fragment, a microbead (oblate spheroid), or a sphere. The volume of the ellipsoid VEL,i for imaginary particle i can then be calculated from the sampled length (Li) distribution and the sampled L/W and L/H distributions. (4) If desired, eq 3 can be further simplified by assuming that the length to width ratio equates to the width to height ratio (L/W = W/H), (31,32) and by writing it as a function of CSF by implementing the CSF equation
(4)
The simplified ellipsoid volume (VEL,iS) can thus be calculated from the lengths (Li) and CSFi values sampled from their respective distributions. (4) Again, for 0 < CSF < 1, this results in ellipsoids that can be extremely elongated when CSF approaches zero, to the complete opposite: spherical shape when CSF = 1. For CSF = 1, eq 4 reduces to the volume equation for a sphere (with L = 2r). The resulting VEL,i or VEL,iS (eqs 3 or 4) is then converted into particle mass MPS,i using a density selected from the density distribution. (4)VPS,i values and MPS,i values from all iterations are summed. This results in total volume and total mass for the total number of simulated particles, from which number–volume–mass conversion factors can be calculated. The factors will be more accurate if the number of simulated particles i (i.e., the number of MC iterations, n) is high enough to capture the variation in the probability density functions. Here, we used n = 105 in the MC simulations.

Figure 1

Figure 1. Length to width (panel A) and length to height (panel B) ratios for polydisperse microplastic, obtained from Monte Carlo simulations for the main microplastic shape categories, and their relative abundance in the environment (4) for n = 1 × 106 iterations, with fitted bi- and trimodal distributions. The main difference is an extra peak for sheets at a low L/H ratio.

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Aligning Threshold Effect Data Used in Species Sensitivity Distributions for Microplastic

Currently published SSDs need two corrections to make them consistent with environmental MP exposure concentrations: one aligning threshold effect concentrations among studies that use different types of MP and one aligning these threshold effect concentrations with actual MP exposure concentrations found in the environment.

Aligning Threshold Effect Concentrations Obtained from Microplastic Effect Studies Using Different and Nonrealistic Particle Types

Microplastic effect tests use different types of microplastic particles, widely varying in sizes, shapes, and densities, which makes the data incomparable and limits their use in, for instance, SSDs or risk characterizations. (1,20) Furthermore, the types of particles used in effect tests are usually less diverse compared to those found in the environment. Here, we provide a calculation method to correct for this nonalignment. The approach is based on a method to convert published threshold effect concentrations into volume equivalent threshold effect concentrations for environmental microplastic, in combination with the method to convert number concentrations into volume and mass concentrations for environmental microplastic as described above.
Recent reports have identified that the most demonstrated and consistently reported effect mechanisms across both marine and freshwater taxa are inhibition of food assimilation and/or decreased nutritional value of food (“food dilution”), and internal physical damage due to microplastic ingestion. (33−35) Ingestion of the low-caloric particles inevitably leads to loss of energy intake, causing growth inhibition and eventual mortality. (2,12,19,34) If the effect mechanism is based on food dilution only, then the actual volume of ingested MP is of primary relevance, rather than the number or size of the MP particles. This implies that a reported threshold effect (number) concentration from an effect test with ingestible monodisperse MP (ECX,mono, with X being the affected fraction of the population) can be converted into a threshold effect concentration for an ingestible size range of particles from an environmentally realistic polydisperse MP suspension (ECX,poly), as long as the total volume of what is ingested remains the same. Information on polydispersity can be obtained either from experimental data or from MC simulations where the ingestible polydisperse distribution is based on selected parts of distributions that are parameterized using environmental data within the ingestible size range, e.g., from Kooi and Koelmans. (4) The volume preservation equation is
(5)
in which ECX,mono is the reported threshold effect number concentration (#/L), Vmono is the average volume per particle as used in the reported monodisperse MP effect test, ECX,poly is the volume equivalent threshold effect concentration of the ingestible size range of polydisperse MP particles (#/L), and Vpoly is the average volume per particle that is ingestible by the species under consideration. The latter Vpoly can be calculated as the ratio of the total volume of all bioavailable, i.e., ingestible (polydisperse) particles and the number of ingestible particles (kingestible) needed to make that volume:
(6)
This can be generated with an MC simulation of particle volumes based on the aforementioned approach (previous section, eqs 3 or 4), however with k representing the number of ingestible particles. The volume equivalent effect threshold concentration corrected for the bioavailable fraction of environmental microplastic is then
(7)
This threshold concentration is also referred to as the environmentally relevant threshold effect number concentration. An example calculation is provided in the Supporting Information.

Aligning Environmentally Relevant Threshold Effect Concentrations and the Actual Environmental Microplastic Exposure Concentration

The threshold effect concentration for environmentally relevant polydisperse microplastic ECX,poly (eqs 5 and 7) resembles the effect concentration while accounting for the full diversity of ingestible environmental microplastic in terms of size, shape, and density. However, this ECX,poly relates to ingestible particles and still is only a fraction of the total concentration of environmental microplastic that aquatic species are exposed to. Quantification of this species-specific fraction makes it possible to convert all ECX,poly threshold effect concentrations into threshold concentrations of “total” environmental microplastic (ECX,env). For example, in the present paper, n = 105 imaginary particles were created using Monte Carlo simulations. From these particles, a selection was made based on the species-specific ingestibility as described above (Table S1 and Table S2), resulting in the ingestible subset of particles, based on size constraints. The resulting number of selected particles is then expressed as a fraction (favailable) of the total number generated by the Monte Carlo simulations. Threshold concentrations of total environmental microplastic (ECX,env) were then calculated as
(8)

Construction of Species Sensitivity Distributions

A traditional freshwater species sensitivity distribution (SSD) for microplastic was constructed by combining data from the SSDs published by Adam et al. (22) and Besseling et al. (20,36) These SSDs included data for nanoplastic and marine species, respectively, data that would be irrelevant for a microplastic SSD for freshwater species and thus were omitted. The remaining freshwater MP data were updated with effect threshold data from studies published in 2018 and 2019 (Table S2). To obtain an SSD consistent with the concept defined in the previous section (eqs 3–8), only invertebrate species for which ingestion was demonstrated (37) (Table S1) and food dilution was suggested as the effect mechanism (35) were retained, leading to 54 data points for 11 species (Table S2). Following Adam et al., (22) reported dose descriptors like EC50, EC10, and lowest observed effect concentration (LOEC) were converted into chronic no observed effect concentration (NOEC) values using an assessment factor (AF) to convert acute into chronic values (AFtime) and an AF to convert dose descriptors into NOECs (AFdescriptor), according to ECHA guidelines. (38) Reported NOEC values, however, were considered too uncertain (22) and were therefore omitted. To construct our new rescaled SSD for each of the species, an ingestible MP size range was defined based on literature data (Tables S1 and S2). Particle width (W) was taken as the relevant MP size dimension for ingestion, assuming that elongated particles can be ingested along their longest dimension (L). For instance, fibers often are found to be ingested, even though the size of the mouth is less than the fiber length. (39) Using the US-EPA’s species sensitivity distribution generator, (20) two SSDs were constructed: one using the original untransformed effect threshold data as reported (ECX,mono) and one using effect threshold concentrations rescaled according to eq 8 (ECX,env). By converting the original as well as the rescaled number concentration data into mass (eq 3), SSDs for mass concentration were obtained as well. Mass conversion of the original number concentration data was based on reported particles’ size and density. Rescaled data number to mass conversion was based on the polymer density distribution for polydisperse environmental MP as reported by Kooi and Koelmans. (4)

Results and Discussion

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Aligning Data Sets That Target Different Size Ranges

The calculated correction factors can be used as standard factors to translate commonly used ranges into the default microplastic size ranges of either 1–5000, 1–1000, or 20–5000 μm (Table 1). For instance, microplastic number concentrations obtained with standard 333 μm trawling nets would require multiplying by a factor of 40.4 to obtain an estimate of the full 1–5000 μm microplastic size range. If one adopts a 20–5000 μm range for default microplastic, the correction factor would be 6.49. Table 1 also illustrates the sensitivity of the correction factor to some of the different approaches used in microplastic research. For instance, it makes only a 1% (40.37/39.97) difference normalizing to 1–5000 μm versus normalizing to 1–1000 μm. This is caused by the fact that sizes between 1000 and 5000 μm have very low contribution to the overall number concentration that reaches all the way down to 1 μm. This marginal difference in number concentrations, which are the units most frequently used in the literature, may reduce the urgency of revisiting the current habit of using 5 mm as the upper boundary in the definition of microplastic. (3)
Table 1. Correction Factors (CF) to Convert Number Concentrations Observed for Five Common Size Ranges (Measured Range) to Full Microplastic Size Ranges (1–5000, 1–1000, 20–5000 μm)
 measured range
CF to default size range of333–5000 μm300–5000 μm100–5000 μm20–1000 μm20–5000 μm
1–5000 μm40.3737.3617.426.636.22
1–1000 μm39.9736.9917.256.576.16
20–5000 μm6.496.002.801.071
We further explored the sensitivity of the correction factor with respect to variability in the value of the exponent α (eq 1). It appears that this sensitivity is quite large; a 10% increase or decrease in the default value of 1.6 results in a factor of two increase or decrease in the value of the correction factor needed to convert number concentration data from 333–5000 to 1–5000 μm (Figure S1). This sensitivity is much lower when the correction concerns a smaller difference in the size range. For instance, the 20–5000/333–5000 correction factor would change only by a factor of 1.4. Consequently, it is recommended to always correct a data set with a calibration of eq 1 that is based on that same data set. If this is not possible, for instance, when it concerns generic corrections, the generic value of α = 1.6 still is the best value available.

Aligning Microplastic Concentrations in Surface Waters

Microplastic number concentration data in water samples from either groundwater, surface water, wastewater, or drinking water show an enormous range. (15) This can be attributed to many factors such as differences in water types, geographical locations, analytical methods, and target size ranges. Because of the latter, we expect that rescaling the different size ranges to one identical range would reduce the range of variation in the data. To test this hypothesis, surface water microplastic number concentration data from our previous review (15) was rescaled to a standard microplastic range of 1–5000 μm using eq 2 with the minimum and maximum size boundaries as reported in the original studies, (15) and α = 1.6. A convenient way to compare the rescaled number concentrations with the original values is via their cumulative frequency distributions (Figure 2). The comparison shows that the original data span 7.5 orders of magnitude (from 10–5 to 200 #/L), whereas the rescaled data span six orders of magnitude (from 10–3 to 800 #/L). Rescaling thus leads to a narrower frequency distribution. The difference in concentration is a factor of 100 at the lowest concentrations (10–5 versus 10–3), whereas it is only a factor of 4 (200 versus 800 #/L) at the highest concentrations (Figure 2). Low concentrations often were measured with methods that have a high size detection limit, for which thus a higher rescaling factor is used. In contrast, the high concentrations often were obtained with methods able to detect the smallest particle sizes, which therefore were already closer to the 1 to 5000 μm size range defined for environmental microplastic.

Figure 2

Figure 2. Cumulative frequency distribution of original global microplastic concentrations in surface freshwater taken from (15) (black curve), the same data consistently rescaled to 1–5000 μm, (blue curve), compared to the concentration (ECX,env; eq 8) protecting 95% of species obtained from threshold effect concentrations corrected for bioavailability and polydispersity as defined by probability density distributions for environmental microplastic (HC5, vertical orange line, obtained from the SSD in Figure 3B).

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Instead of this visual approach, the variation in the data also can be explored using interquartile ranges. There were 634 surface water concentration values in the original data set (see Koelmans et al. (15)), which had an interquartile range of 6.16 #/L, whereas the corrected data set had a higher interquartile range of 16.7 #/L. Thus, the correction leads to a higher number concentration (Figure 2). To quantify the relative variation in the data, we calculated the ratio of the quartile for 75 and 25% of the data (IQR75%/IQR25%). For the original data, this IQR75%/IQR25% ratio is 2200, whereas for the rescaled data set, this ratio is only 223, thus an order of magnitude lower. Rescaling thus indeed helps to substantially reduce the artifactual variation as expected. When rescaled to a standard microplastic range of 20–5000 μm, the IQR75%/IQR25% ratio still is 10 times smaller than that for the original data, showing that the improvement is not very sensitive to the definition of the microplastic size range. The remaining IQR75%/IQR25% variability, i.e., 223, can be seen as a better estimate of the actual variability of microplastic concentrations in and across surface waters. Although it remains a correction method based on extrapolation, rescaling has demonstrated the ability to reduce the artifact of different size ranges used when comparing number concentration data across studies.

Rescaling Species Sensitivity Distributions for Environmental Microplastic

Equations 3–8 were used to rescale and align the threshold effect concentrations of a traditional SSD (Figure 3A). This SSD uses effect concentrations for ingestible microplastics, which were thus 100% bioavailable in the laboratory tests. However, in nature, a much wider environmental microplastic continuum is present, part of which is not bioavailable for organisms that ingest microplastic, for instance, because it would have a size larger than the organisms’ mouth opening. (40) The traditional SSD (Figure 3A) further used the threshold effect number concentrations from the literature as reported, neglecting that two identical number concentrations do not mean the same thing if they relate to microplastic particles of different sizes, shapes, and densities, which in fact are different stressors. The effect threshold concentrations reported so far are thus incomparable. (19−24) Therefore, our rescaling concerned two corrections. First, all reported threshold effect number concentrations from effect tests largely performed with monodisperse MP suspensions were converted into “environmentally realistic” threshold effect number concentrations for the ingestible size range of particles, with the size distributions of these particles matching those of the ingestible polydisperse environmental microplastic, and assuming preservation of ingested volume (eq 5). Second, the bio-unavailable fraction of environmental microplastic was corrected. Threshold effect concentrations were calculated so they relate to the fraction of environmental microplastic that is ingestible by the species based on size and density. To this end, ingestible size ranges were defined for each of the species, based on literature data (Tables S1 and S2). The ratio of species size and maximum ingestible MP size is 50.5 ± 14.9 (n = 8) on average, which agrees well with the range of 40 to 50 recently reported for small invertebrate species. (41) Autotrophic phytoplankton and macrophyte species were not supposed to ingest microplastic particles, in which case the bio-unavailable fraction of environmental microplastic was assumed to be negligible. From the MC simulations, it appeared that the ingestible percentages ranged from 93 (Ceriodaphnia dubia) to 99% (Haworthiopsis attenuata) based on particle numbers (Figure S2A), and from 0.0054 to 1.7% based on mass (Figure S2B). The difference in these percentages is explained by high numbers of small particles calculated to be ingestible, which, however, represent very little mass. Microplastic is thus highly bioavailable when expressed in number concentrations, yet marginally bioavailable when expressed in mass concentrations. As for density, all polymer types are sometimes reported at the water surface, in the water column, or in sediments. (4) Therefore, for the current implementation, we assigned all polymer densities to be available for all species.

Figure 3

Figure 3. Species sensitivity distributions for microplastic based on laboratory data for different types of particles (panel A, R2 = 0.896, HC5 = 251 #/L), and based on the same data after corrections for bioavailability and polydispersity at the threshold effect concentration resulting in (ECX,env) x-axis values in particles/L (panel B, R2 = 0.940, HC5 = 75.6 #/L) or mg/L (panel C, R2 = 0.940, HC5 = 0.94 mg/L). Gray curves relate to 95% confidence intervals.

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As a result of these corrections, the recalculated x-axis values now consistently relate to the environmental microplastic concentrations (ECX,env) needed to provide a bioavailable fraction with a volume equal to the ingested volume at the threshold effect concentration in the laboratory tests (Figure 3B). Consequently, the order of the species in the SSD changes because differences in bioavailability and ingestibility are now taken into account. Furthermore, the hazardous concentration for 5% of the species (HC5) changes from 251 #/L (Figure 3A) to an ingestible volume as well as a bioavailability-corrected effect threshold of 75.6 #/L (Figure 3B, R2 = 0.940, 95% confidence interval: 11–521 #/L). Here, it is emphasized that the two HC5 values 251 and 75.6 #/L cannot be compared directly because they relate to different things. In Figure 3A, each x-axis value relates to a concentration of another stressor (i.e., microplastic particles of different sizes, shapes, densities), which means that the HC5 value is fundamentally flawed and largely meaningless. In Figure 3B, x-axis values all relate to the same variable: the concentration of “1–5000 μm polydisperse environmental MP”. In this example, they are defined by the probability distributions provided by Kooi and Koelmans. (4) The effect threshold of 75.6 #/L can thus be compared with exposure concentrations as long as these are also measured or rescaled to cover the same size range of 1–5000 μm, e.g., like those calculated in the previous section.
The MC simulations provided an accurate number to mass conversion, as for each individual simulated particle, size, shape, and density were sampled from their respective probability density functions, from which particle weight was calculated. This yields estimates of the characteristics of a single, average environmentally relevant microplastic particle: a weight of 12.5 μg, a volume of 0.011 mm3, and a density of 1.141 g cm3. For the ingestible particles, these simulations were used to construct a mass concentration-based SSD with an effect HC5 threshold of 0.95 mg/L (Figure 3C, R2 = 0.940, 95% confidence interval: 0.14–6.5 mg/L). Note that this concentration equates to the product of the number-based threshold (75.6 #/L) and the average weight of an individual particle (12.5 μg).

Risk Characterization Using Rescaled Exposure and Effect Threshold Data

Risk assessment implies a comparison of predicted environmental concentration (PEC) with species sensitivity, often expressed as predicted no effect concentration (PNEC). (1,2) Risk is indicated when the PEC/PNEC ratio is larger than 1. (1) Previous assessments for microplastic suffered from the fact that PEC and PNEC data were incomparable. For instance, exposure data were often reported for particles larger than 100 or 300 μm, whereas laboratory tests usually used much smaller particles, e.g., <20 μm. (2) The aforementioned correction methods allow for a more consistent PEC/PNEC comparison for 1–5000 μm microplastic, that is, with all data recalculated to match an environmentally relevant size distribution for this size range, and with correction for those parts of the microplastic size, shape, and density continuum that are not available for species. The latter depends on species traits (Table S1). Here, we used the cumulative frequency distribution of 1–5000 μm global microplastic concentrations in surface freshwater (Figure 2). (15) They can be compared to the concentration protecting 95% of species (HC5) obtained from threshold effect concentrations also rescaled for bioavailable fraction, size, shape, and density as defined by probability density distributions for environmental microplastic in water (Figure 2, vertical line). The comparison shows that based on these available data, risk would be indicated for only a very small percentage (1.5%) of the locations in the data set. For the ten locations at the right-hand side of the risk threshold, PEC/PNEC ranges from 1 to 10, with an average of 3.7. This quantitative assessment supports the conclusions of two recent reports stating that at present, risks of microplastic are not widespread, but could occur at (rare) hotspot locations. (2,6) The comparison is still surrounded by uncertainty as can be seen from the rather high uncertainty interval for the SSD (Figure 3B), which ranges from 11 to 521 #/L for the HC5. In other words, if we take the lower limit of the 95% confidence interval, i.e., 11 #/L, risk would be indicated for 28% of the locations. Conversely, we can say risk is really unlikely for 72% of the locations in the data set. Obviously, these percentages are not representative for “all locations in the world,” given that the data set we used (15) is not necessarily representative.

Implications and Prospects

Because microplastic is perceived as a diverse and complex mixture, researchers have suggested focusing on its components. (5) Although this may seem reasonable at first sight, the question is whether this strategy of complexifying microplastic is the best way forward. This view may lead to fragmentation and delay of research effort. After all, testing all of the possible combinations of particles that make up environmental microplastic with all possible endpoints in biota is a daunting, if not, impossible task. From an ecological or human health point of view, assessing the risk of all of the components of microplastics is insufficient. For exposure, the bioavailable fraction of the whole plastic continuum is relevant, whereas that bioavailable fraction in itself is a continuum across all biota present in the biosphere. Therefore, innovative concepts and tools need to be developed to understand the joint behavior and risks of the plastic continuum within the biological continuum. We propose that by using such tools, the complexity of microplastic can be reduced to the extent required to assess their risk. To some extent, this is similar to the previous development of quantitative structure–activity relationships (QSARs) (42,43) to understand the toxicity of wide ranges of different organic chemicals.
A rescaling methodology to increase the comparability of concentration data obtained with different measurement methods as well as a methodology to align exposure and effect data to get a consistent characterization of risk have been presented. The essence of these methodologies is that the measured diversity of environmental microplastic is captured via continuous probability density functions, which subsequently allow for quantitative rescaling and corrections while fully preserving this diversity. This is not necessarily limited to microplastics because the same concepts are applicable to macroplastics and nanoplastics as well. Providing, explaining, and testing the method was the primary aim of this paper, not necessarily providing the final answers with respect to concentrations and risks of microplastic in the environment. After all, the data used in the present paper may be the best available but still need to be renewed and expanded for several reasons. For instance, the most recent chemometric procedures to analyze microplastic spectroscopic data are able to provide particle number, size, shape, polymer type, and therefore weight, in one go. (32,44) This will inevitably lead to more accurate data and possibly refined and different parameterizations of the probability density functions. It is thus recommended to use automated analysis and always report and interpret analytical data also in the form of such functions. This will make it possible to assess how accurate parametrizations for average microplastic are, and whether they need to become compartment specific. A second reason is that the concentration data for surface waters used in the present analysis (Figures 2 and 3) partly relate to less reliable methodologies. (15) We expect future studies to deploy higher levels of quality assurance during sampling, laboratory handling, and detection of microplastics, which will thus also affect the outcomes of the example calculations provided here. A third reason relates to SSDs, which already have been qualified as sophisticated, (45) high-level risk assessments, (6) and have been used to inform the risk assessment for microplastic particles as done by several international expert groups. (2,6,24) However, they have conceptual flaws as explained and they still have limited data. A new SSD based on best available data for freshwater species was provided here. The SSD focuses on food dilution by low-caloric plastic particles as the effect mechanism. Other mechanisms may also play a role but data for these are still insufficient to build SSDs on. (35) Generating new experimental data to build new SSDs was beyond the scope of the present study. However, higher quality and uniformity in future microplastic effects tests are expected. (35,46)
The applicability of our methodologies goes beyond the aforementioned examples. For instance, imagine an effect mechanism where membrane translocation of <3 μm particles (47−49) is followed by distribution in the body tissues and subsequent inflammation responses, which could be relevant for human health as well as for ecological risks. (2) In such a case, exposure would benefit from the method provided to align concentration data, whereas alignment of effect threshold data still would benefit substantially from the representation of environmental microplastic via probability density functions. (4) The relevant particle size fractions were now quantified by sampling from the MP parameter space for particles that fit in an organism’s mouth opening, which after all is an important prerequisite for food dilution. However, for translocation, followed by inflammation, one should sample the 10 nm to 3 μm size fraction, while further only selecting particles with an aspect ratio considered to be relevant for the specific inflammatory response.
In conclusion, the presented toolkit would be useful for any type of microplastic exposure and effect studies, from ecological to human health, and would lead to more meaningful risk assessments in the future.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.est.0c02982.

  • Detailed explanation and example calculation for ECX,poly and the effect concentration for environmental microplastic ECX,env; ingestible size ranges (Table S1); original and rescaled effect threshold concentrations as used in Figures 3A,B, respectively (Table S2); sensitivity analysis for exponent α (Figure S1); and bioavailable fractions of number concentrations and mass concentrations for 11 studied species (Figure S2) (PDF)

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Author Information

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  • Corresponding Author
  • Authors
    • Paula E Redondo-Hasselerharm - Aquatic Ecology and Water Quality Management Group, Wageningen University, P.O. Box 47, 6700 DD Wageningen, The Netherlands;  Orcidhttp://orcid.org/0000-0002-8055-6847
    • Nur Hazimah Mohamed Nor - Aquatic Ecology and Water Quality Management Group, Wageningen University, P.O. Box 47, 6700 DD Wageningen, The Netherlands;  Orcidhttp://orcid.org/0000-0002-8086-1833
    • Merel Kooi - Aquatic Ecology and Water Quality Management Group, Wageningen University, P.O. Box 47, 6700 DD Wageningen, The Netherlands
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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A.A.K., M.K., and P.E.R.H. acknowledge financial support from the NWO-TT project “Technologies for the Risk Assessment of Microplastics” (TRAMP).

References

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This article references 49 other publications.

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    Water Research (2019), 155 (), 410-422CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
    Microplastics have recently been detected in drinking water as well as in drinking water sources. This presence has triggered discussions on possible implications for human health. However, there have been questions regarding the quality of these occurrence studies since there are no std. sampling, extn. and identification methods for microplastics. Accordingly, we assessed the quality of fifty studies researching microplastics in drinking water and in its major freshwater sources. This includes an assessment of microplastic occurrence data from river and lake water, groundwater, tap water and bottled drinking water. Studies of occurrence in wastewater were also reviewed. We review and propose best practices to sample, ext. and detect microplastics and provide a quant. quality assessment of studies reporting microplastic concns. Further, we summarize the findings related to microplastic concns., polymer types and particle shapes. Microplastics are frequently present in freshwaters and drinking water, and no. concns. spanned ten orders of magnitude (1 × 10-2 to 108 #/m3) across individual samples and water types. However, only four out of 50 studies received pos. scores for all proposed quality criteria, implying there is a significant need to improve quality assurance of microplastic sampling and anal. in water samples. The order in globally detected polymers in these studies is PE ≈ PP > PS > PVC > PET, which probably reflects the global plastic demand and a higher tendency for PVC and PET to settle as a result of their higher densities. Fragments, fibers, film, foam and pellets were the most frequently reported shapes. We conclude that more high quality data is needed on the occurrence of microplastics in drinking water, to better understand potential exposure and to inform human health risk assessments.
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  16. 16
    Lenz, R.; Enders, K.; Nielsen, T. G. Microplastic exposure studies should be environmentally realistic. Proc. Natl. Acad. Sci. U.S.A. 2016, 113, E4121– E4122,  DOI: 10.1073/pnas.1606615113
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    Microplastic exposure studies should be environmentally realistic
    Lenz, Robin; Enders, Kristina; Nielsen, Torkel Gissel
    Proceedings of the National Academy of Sciences of the United States of America (2016), 113 (29), E4121-E4122CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    There is no expanded citation for this reference.
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  17. 17
    Mintenig, S. M.; Bäuerlein, P. S.; Koelmans, A. A.; Dekker, S. C.; van Wezel, A. P. Closing the gap between small and smaller: Towards a framework to analyse nano- and microplastics in aqueous environmental samples. Environ. Sci.: Nano 2018, 5, 1640– 1649,  DOI: 10.1039/C8EN00186C
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    17
    Closing the gap between small and smaller: towards a framework to analyse nano- and microplastics in aqueous environmental samples
    Mintenig, S. M.; Baeuerlein, P. S.; Koelmans, A. A.; Dekker, S. C.; van Wezel, A. P.
    Environmental Science: Nano (2018), 5 (7), 1640-1649CODEN: ESNNA4; ISSN:2051-8161. (Royal Society of Chemistry)
    Measuring concns. and sizes of micro- and nanoplastics in the environment is essential to assess the risks plastic particles could pose. Microplastics have been detected globally in a variety of aquatic ecosystems. The detn. of nanoplastics, however, is lagging behind due to higher methodol. challenges. Here, we propose a framework that can consistently det. a broad spectrum of plastic particle sizes in aquatic environmental samples. Anal. evidence is provided as proof of principle. FTIR microscopy is applied to detect microplastics. Nanoplastics are studied using field-flow-fractionation and pyrolysis GC-MS that gives information on the particle sizes and polymer types. Pyrolysis GC-MS is shown to be promising for the detection of nanoplastics in environmental samples as a mass of approx. 100 ng is required to identify polystyrene. Pre-concg. nanoplastics by crossflow ultrafiltration enables polystyrene to be identified when the original concn. in an aq. sample is >20μg L-1. Finally, we present an approach to est. polymer masses based on the two-dimensional microplastic shapes recorded during the anal. with FTIR microscopy. Our suite of techniques demonstrates that anal. of the entire size spectrum of plastic debris is feasible.
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  18. 18
    Karlsson, T. M.; Kärrman, A.; Rotander, A.; Hassellöv, M. Comparison between manta trawl and in situ pump filtration methods, and guidance for visual identification of microplastics in surface waters. Environ. Sci. Pollut. Res. 2020, 27, 5559– 5571,  DOI: 10.1007/s11356-019-07274-5
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    18
    Comparison between manta trawl and in situ pump filtration methods, and guidance for visual identification of microplastics in surface waters
    Karlsson Therese M; Hassellov Martin; Karrman Anna; Rotander Anna
    Environmental science and pollution research international (2020), 27 (5), 5559-5571 ISSN:.
    Owing to the development and adoption of a variety of methods for sampling and identifying microplastics, there is now data showing the presence of microplastics in surface waters from all over the world. The difference between the methods, however, hampers comparisons, and to date, most studies are qualitative rather than quantitative. In order to allow for a quantitative comparison of microplastics abundance, it is crucial to understand the differences between sampling methods. Therefore, a manta trawl and an in situ filtering pump were compared during realistic, but controlled, field tests. Identical microplastic analyses of all replicates allowed the differences between the methods with respect to (1) precision, (2) concentrations, and (3) composition to be assessed. The results show that the pump gave higher accuracy with respect to volume than the trawl. The trawl, however, sampled higher concentrations, which appeared to be due to a more efficient sampling of particles on the sea surface microlayer, such as expanded polystyrene and air-filled microspheres. The trawl also sampled a higher volume, which decreased statistical counting uncertainties. A key finding in this study was that, regardless of sampling method, it is critical that a sufficiently high volume is sampled to provide enough particles for statistical evaluation. Due to the patchiness of this type of contaminant, our data indicate that a minimum of 26 particles per sample should be recorded to allow for concentration comparisons and to avoid false null values. The necessary amount of replicates to detect temporal or spatial differences is also discussed. For compositional differences and size distributions, even higher particle counts would be necessary. Quantitative measurements and comparisons would also require an unbiased approach towards both visual and spectroscopic identification. To facilitate the development of such methods, a visual protocol that can be further developed to fit different needs is introduced and discussed. Some of the challenges encountered while using FTIR microspectroscopic particle identification are also critically discussed in relation to specific compositions found.
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    Everaert, G.; Van Cauwenberghe, L.; De Rijcke, M.; Koelmans, A. A.; Mees, J.; Vandegehuchte, M.; Janssen, C. R. Risk assessment of microplastics in the ocean: modelling approach and first conclusions. Environ. Pollut. 2018, 242, 1930– 1938,  DOI: 10.1016/j.envpol.2018.07.069
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    19
    Risk assessment of microplastics in the ocean: Modelling approach and first conclusions
    Everaert, Gert; Van Cauwenberghe, Lisbeth; De Rijcke, Maarten; Koelmans, Albert A.; Mees, Jan; Vandegehuchte, Michiel; Janssen, Colin R.
    Environmental Pollution (Oxford, United Kingdom) (2018), 242 (Part_B), 1930-1938CODEN: ENPOEK; ISSN:0269-7491. (Elsevier Ltd.)
    We performed an environmental risk assessment for microplastics (<5 mm) in the marine environment by estg. the order of magnitude of the past, present and future concns. based on global plastic prodn. data. In 2100, from 9.6 to 48.8 particles m-3 are predicted to float around in the ocean, which is a 50-fold increase compared to the present-day concns. From a meta-anal. with effect data available in literature, we derived a safe concn. of 6650 buoyant particles m-3 below which adverse effects are not likely to occur. Our risk assessment (excluding the potential role of microplastics as chem. vectors) suggests that on av., no direct effects of free-floating microplastics in the marine environment are to be expected up to the year 2100. Yet, even today, the safe concn. can be exceeded in sites that are heavily polluted with buoyant microplastics. In the marine benthic compartment between 32 and 144 particles kg-1 dry sediment are predicted to be present in the beach deposition zone. Despite the scarcity of effect data, we expect adverse ecol. effects along the coast as of the second half of the 21st century. From then ambient concns. will start to outrange the safe concn. of sedimented microplastics (i.e. 540 particles kg-1 sediment). Addnl. ecotoxicol. research in which marine species are chronically exposed to realistic environmental microplastic concn. series are urgently needed to verify our findings.
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  20. 20
    Besseling, E.; Redondo-Hasselerharm, P. E.; Foekema, E. M.; Koelmans, A. A. Quantifying Ecological Risks of Aquatic Micro- and Nanoplastic. Crit. Rev. Environ. Sci. Technol. 2019, 49, 32– 80,  DOI: 10.1080/10643389.2018.1531688
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    Burns, E. E.; Boxall, A. B. A. Microplastics in the Aquatic Environment: Evidence for or against Adverse Impacts and Major Knowledge Gaps. Environ. Toxicol. Chem. 2018, 37, 2776– 2796,  DOI: 10.1002/etc.4268
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    Microplastics in the aquatic environment: Evidence for or against adverse impacts and major knowledge gaps
    Burns, Emily E.; Boxall, Alistair B. A.
    Environmental Toxicology and Chemistry (2018), 37 (11), 2776-2796CODEN: ETOCDK; ISSN:0730-7268. (Wiley-Blackwell)
    There is increasing scientific and public concern over the presence of microplastics in the natural environment. We present the results of a systematic review of the literature to assess the wt. of evidence for microplastics causing environmental harm. We conclude that microplastics do occur in surface water and sediments. Fragments and fibers predominate, with beads making up only a small proportion of the detected microplastic types. Concns. detected are orders of magnitude lower than those reported to affect endpoints such as biochem., feeding, reprodn., growth, tissue inflammation and mortality in organisms. The evidence for microplastics acting as a vector for hydrophobic org. compds. to accumulate in organisms is also weak. The available data therefore suggest that these materials are not causing harm to the environment. There is, however, a mismatch between the particle types, size ranges, and concns. of microplastics used in lab. tests and those measured in the environment. Select environmental compartments have also received limited attention. There is an urgent need for studies that address this mismatch by performing high quality and more holistic monitoring studies alongside more environmentally realistic effects studies. Only then will we be able to fully characterize risks of microplastics to the environment to support the introduction of regulatory controls that can make a real pos. difference to environmental quality. Environ Toxicol Chem 2018;9999:1-21. © 2018 SETAC.
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    Adam, V.; Yang, T.; Nowack, B. Toward an Ecotoxicological Risk Assessment of Microplastics: Comparison of Available Hazard and Exposure Data in Freshwaters. Environ. Toxicol. Chem. 2019, 38, 436– 47,  DOI: 10.1002/etc.4323
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    22
    Toward an ecotoxicological risk assessment of microplastics: Comparison of available hazard and exposure data in freshwaters
    Adam, Veronique; Yang, Tong; Nowack, Bernd
    Environmental Toxicology and Chemistry (2019), 38 (2), 436-447CODEN: ETOCDK; ISSN:0730-7268. (Wiley-Blackwell)
    A review. Microplastics have been detected in freshwaters all over the world in almost all samples, and ecotoxicol. studies have shown adverse effects of microplastics on organisms. However, no risk assessment of microplastics has been performed specifically in freshwater so far. The aim of the present study was therefore to review all exposure and ecotoxicity data available for microplastics in freshwaters and to perform a preliminary probabilistic risk assessment. The exposure probability distribution was based on 391 concns. measured in Asia, Europe, and North America. Because exposure data are mainly available in particle no.-based metrics but results from hazard studies are mostly mass-based, the hazard results were converted into particle no. concns. A statistical anal. of the hazard data showed that there was no significant influence of particle shape or type of polymer on the no-obsd.-effect concn. The predicted-no-effect concn. (PNEC) was calcd. as the fifth percentile of the probabilistic species sensitivity distribution, based on 53 values from 14 freshwater species, to have a mode of 7.4 × 105 particles · m-3 (25th and 75th quantiles of 6.1 × 105 and 1.3 × 106 particles · m-3, resp.). The exposure probability distribution was divided by the PNEC probability distribution to calc. risk characterization ratios (RCRs), with modes of 1.3 × 10-6 in North America, 3.3 × 10-6 in Europe, and 4.6 × 10-3 in Asia. Probability distributions assocd. with the RCRs showed that ecol. risks cannot be entirely excluded in Asia, where 0.4% of the RCR values were above 1. Environ Toxicol Chem 2018;9999:1-12. © 2018 SETAC.
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    Zhang, X.; Leng, Y.; Liu, X.; Huang, K.; Wang, J. Microplastics’ pollution and risk assessment in an urban river: A case study in the Yongjiang River, Nanning City, South China. Exposure Health 2020, 12, 141– 151,  DOI: 10.1007/s12403-018-00296-3
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    Microplastics' Pollution and Risk Assessment in an Urban River: A Case Study in the Yongjiang River, Nanning City, South China
    Zhang, Xin; Leng, Yifei; Liu, Xiaoning; Huang, Kai; Wang, Jun
    Exposure and Health (2020), 12 (2), 141-151CODEN: EHXEAI; ISSN:2451-9685. (Springer)
    Microplastics (MPs) have been considered as a global environmental problem threatening the ecol. security. However, studies on MPs' pollution in freshwaters and the assocd. risk assessment remain limited in the literature. In this study, the concns., distributions, and the potential ecol. risks of MPs were analyzed in Yongjiang River, which is an important drinking water source flowing through Nanning City, the mega city of China. The MPs' abundances in surface waters and sediments ranged from 500 to 7700 n/m3 and from 90 to 550 n/kg, resp. Spatial distribution highlighted the significant impact of anthropogenic activity on the MPs' accumulation. Polyethylene and polypropylene were the most common polymer compns. investigated. Shape, size, and color were examd. to analyze the characteristics of MPs in the river. To assess the ecol. risk of MPs, the predicted no-effect concn. (PNEC) values were derived from a species' sensitivity distribution model based on the toxicity data of MPs for freshwater species available in the literature. The PNEC for MPs in surface water was derived to be 4920 n/m3. Risk assessment results through risk quotient (RQ) method suggest that most of the monitored sites in Yongjiang River posed negligible risks to freshwater biota, except the two sites with high risk in the urban center. The results provided a basis for ecol. risk assessment of MPs in freshwaters.
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    Skåre, J. U.; Alexander, J.; Have, M.; Jakubowicz, I.; Knutsen, H. K.; Lusher, A. L.; Ogonowski, M.; Rakkestad, K. E.; Skaar, I.; Tvedt Sverdrup, L. E.; Wagner, M.; Agdestein, A.; Bodin, J.; Elvevoll, E.; Hemre, G. I.; Hessen, D. O.; Hofshagen, M.; Husøy, T.; Krogdahl, Å.; Nilsen, A. M.; Rafoss, T.; Skjerdal, T.; Strand, T. A.; Vandvik, V.; Wasteson, Y. Microplastics; Occurrence, Levels and Implications for Environment and Human Health Related to Food. Opinion of the Steering Committee of the Norwegian Scientific Committee for Food and Environment; Norwegian Scientific Commitee for Food and Environment (VKM): 2019.
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    Rochman, C. M.; Regan, F.; Thompson, R. C. On the harmonization of methods for measuring the occurrence, fate and effects of microplastics. Anal. Methods 2017, 9, 1324– 1325,  DOI: 10.1039/C7AY90014G
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    Michida, Y.; Chavanich, S.; Cózar Cabañas, A.; Hagmann, P.; Hinata, H.; Isobe, A.; Kershaw, P.; Kozlovskii, N.; Li, D.; Lusher, A. L.; Martí, E.; Mason, S. A.; Mu, J.; Saito, H.; Shim, W. J.; Syakti, A. D.; Takada, H.; Thompson, R. C.; Tokai, T.; Uchida, K.; Vasilenko, K.; Wang, J.. Guidelines for Harmonizing Ocean Surface Microplastic Monitoring Methods, Version 1.1; Ministry of the Environment: Japan. https://www.env.go.jp/en/water/marine_litter/method.html, 2019.
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    Guidelines or the Monitoring and Assessment of Plastic Litter and Microplastics in the Ocean; Kershaw, P. J.; Turra, A.; Galgani, F., Eds.; GESAMP Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection: 2019; p 130.
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    Roch, S.; Walter, T.; Ittner, L. D.; Friedrich, C.; Brinker, A. A systematic study of the microplastic burden in freshwater fishes of south-western Germany - Are we searching at the right scale?. Sci. Total Environ. 2019, 689, 1001– 1011,  DOI: 10.1016/j.scitotenv.2019.06.404
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    28
    A systematic study of the microplastic burden in freshwater fishes of south-western Germany - Are we searching at the right scale?
    Roch, Samuel; Walter, Thomas; Ittner, Lukas D.; Friedrich, Christian; Brinker, Alexander
    Science of the Total Environment (2019), 689 (), 1001-1011CODEN: STENDL; ISSN:0048-9697. (Elsevier B.V.)
    In a comprehensive study of microplastic contamination in southern Germany, 1167 individual fish of 22 different species were sampled from 11 rivers and 6 lakes across the state. The microplastic burden of investigated fish was analyzed on the basis of habitat type, location, and a no. of abiotic and biotic factors. A particle size distribution anal. of the detected microplastics was carried out. The results showed a relatively low plastic prevalence of 18.8%, with significant differences between rivers (20.6%) and lakes (16.5%). The no. of ingested plastic particles ranged between 1 and 4 particles per fish. The majority of abiotic and biotic factors seem to play little or no role in the ingestion of microplastics, suggesting that in most cases uptake is passive or accidental. It is notable that piscivorous fish appeared significantly less burdened, suggesting a low transfer rate and no accumulation in the food web. However, size distribution anal. identified a power law growth fit in particle nos. at the smallest end of the distribution. This carries a worrying implication, that >95% of particles are likely to be smaller than 40 μm and thereby beyond the detection range of this and most other microplastic surveys conducted so far. When the frequency development of small particles is taken into account, the likely microplastic prevalence in the present study increases to 100%, with an av. intensity of around 23 predominantly small particles per fish. A striking 70% of those particles would be smaller than 5 μm and therefore eligible for translocation into tissues, with crit. implications for fish health and consumer exposure. This raises a question as to whether current ests. of microplastic burden in fishes generally might be overlooking a majority of potential contamination within the crit. smaller particle size classes.
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    Waldschläger, K.; Schüttrumpf, H. Effects of Particle Properties on the Settling and Rise Velocities of Microplastics in Freshwater under Laboratory Conditions. Environ. Sci. Technol. 2019, 53, 1958– 1966,  DOI: 10.1021/acs.est.8b06794
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    Effects of Particle Properties on the Settling and Rise Velocities of Microplastics in Freshwater under Laboratory Conditions
    Waldschlager Kryss; Schuttrumpf Holger
    Environmental science & technology (2019), 53 (4), 1958-1966 ISSN:.
    Microplastic (MP) contaminates terrestrial, aquatic, and atmospheric environments. Although the number of river sampling studies with regard to MP concentrations is increasing, comprehension of the predominant transport processes of MP in the watercourse is still very limited. In order to gain a better process understanding, around 500 physical experiments were conducted to shed more light on the effects of particle shape, size and density on the rise and settling velocities of MP. The determined velocities ranged between 0.39 cm/s for polyamide fibers (settling) and 31.4 cm/s for expanded polystyrene pellets (rise). Subsequently, the determined velocities were compared with formulas from sediment transport and, as there were large differences between theoretically and experimentally determined velocities, own formulas were developed to describe settling and rise velocities of MP particles with a large variety of shapes, sizes and densities. This study shows that MP differs significantly from sediment in its behavior and that a transfer of common sediment transport formulas should be treated with caution. Furthermore, the established formulas can now be used in numerical simulations to describe the settling and rising of MP more precisely.
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    McNown, J. S.; Malaika, J. Effect of particle shape on settling velocity at low Reynolds numbers. Trans., Am. Geophys. Union. 1950, 31, 74– 82,  DOI: 10.1029/TR031i001p00074
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    Simon, M.; van Last, N.; Vollertsen, J. Quantification of microplastic mass and removal rates at wastewater treatment plants applying Focal Plane Array (FPA)-based Fourier Transform Infrared (FT-IR) imaging. Water Res. 2018, 142, 1– 9,  DOI: 10.1016/j.watres.2018.05.019
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    Quantification of microplastic mass and removal rates at wastewater treatment plants applying Focal Plane Array (FPA)-based Fourier Transform Infrared (FT-IR) imaging
    Simon, Marta; van Alst, Nikki; Vollertsen, Jes
    Water Research (2018), 142 (), 1-9CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
    This paper presents a method for microplastic (MP) mass quantification using a Focal Plane Array-based Fourier Transform IR imaging technique. It discusses the issue that particle no. is not a conserved base quantity and hence less suited than mass to compare independent studies on MP in the environment. It concludes that MP mass should be included when quantifying MP pollution in the environment, supplementing the conventional approach of reporting particle nos. Applying mass as the unit of MP measurement, the paper presents data showing that Danish wastewater treatment plants discharge around 3 t/yr of MP in the size range 10-500μm. This value corresponds to an annual per capita emission from these plants of 0.56 g MP/(capita year). The distribution of polymer types by mass and particle no. differed because the size of MP particles of the different material types varied.
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  32. 32
    Mintenig, S. M.; Kooi, M.; Erich, M. W.; Primpke, S.; Redondo-Hasselerharm, P. E.; Dekker, S. C.; Koelmans, A. A.; van Wezel, A. P. A systems approach to understand microplastic occurrences in Dutch riverine surface waters. Water Res. 2020, 176, 115723  DOI: 10.1016/j.watres.2020.115723
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    A systems approach to understand microplastic occurrence and variability in Dutch riverine surface waters
    Mintenig, S. M.; Kooi, M.; Erich, M. W.; Primpke, S.; Redondo-Hasselerharm, P. E.; Dekker, S. C.; Koelmans, A. A.; van Wezel, A. P.
    Water Research (2020), 176 (), 115723CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
    Assessment methods on data quality and environmental variability are lacking for microplastics (MP). Here we assess occurrence and variability of MP no. concns. in two Dutch rivers. Strict QA/QC procedures were applied to identify MP using Fourier-transform IR (FTIR) microscopy followed by state of the art automated image anal. For a series of randomly selected, yet ever smaller subareas of filters, we assessed how accurately MP nos. and polymer types are represented during partial filter anal. Levels of uncertainty were acceptable when analyzing 50% of a filter during chem. mapping, and when identifying at least a subset of 50 individual particles with attenuated total reflection (ATR)-FTIR. Applying these guidelines, MP no. concns. between 67 and 11532 MP m-3 were detected in Dutch riverine surface waters. Spatial differences caused MP no. concns. to vary by two orders of magnitude. Temporal differences were lower and induced a max. variation of one order of magnitude. In total, 26 polymer types were identified, the most common were polyethylene (23%), polypropylene (19.7%) and ethylene propylene diene monomer rubber (18.3%). The highest diversity of polymer types was found for small MPs, whereas MP larger than 1 mm was scarce and almost exclusively made of polyethylene or polypropylene. Virtually all sampling locations revealed MP no. concns. that are considerably below known effect thresholds for anticipated adverse ecol. effects.
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    Wright, S. L.; Rowe, D.; Thompson, R. C.; Galloway, T. S. Microplastic ingestion decreases energy reserves in marine worms. Curr. Biol. 2013, 23, R1031– R1033,  DOI: 10.1016/j.cub.2013.10.068
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    Microplastic ingestion decreases energy reserves in marine worms
    Wright, Stephanie L.; Rowe, Darren; Thompson, Richard C.; Galloway, Tamara S.
    Current Biology (2013), 23 (23), R1031-R1033CODEN: CUBLE2; ISSN:0960-9822. (Cell Press)
    Deposit-feeding marine worms maintained in sediments spiked with microscopic unplasticised polyvinylchloride (UPVC) at concns. overlapping those in the environment had significantly depleted energy reserves by ≤50%. Our results suggest that depleted energy reserves arise from a combination of reduced feeding activity, longer gut residence times of ingested material, and inflammation.
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    Foley, C. J.; Feiner, Z. S.; Malinich, T. D.; Höök, T. O. A meta-analysis of the effects of exposure to microplastics on fish and aquatic invertebrates. Sci. Total Environ. 2018, 631–632, 550– 559,  DOI: 10.1016/j.scitotenv.2018.03.046
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    A meta-analysis of the effects of exposure to microplastics on fish and aquatic invertebrates
    Foley, Carolyn J.; Feiner, Zachary S.; Malinich, Timothy D.; Hook, Tomas O.
    Science of the Total Environment (2018), 631-632 (), 550-559CODEN: STENDL; ISSN:0048-9697. (Elsevier B.V.)
    Microplastics are present in aquatic ecosystems the world over and may influence the feeding, growth, reprodn., and survival of freshwater and marine biota; however, the extent and magnitude of potential effects of microplastics on aquatic organisms is poorly understood. In the current study, we conducted a meta-anal. of published literature to examine impacts of exposure to microplastics on consumption (and feeding), growth, reprodn., and survival of fish and aquatic invertebrates. While we did observe within-taxa neg. effects for all four categories of responses, many of the effects summarized in our study were neutral, indicating that the effects of exposure to microplastics are highly variable across taxa. The most consistent effect was a redn. in consumption of natural prey when microplastics were present. For some taxa, neg. effects on growth, reprodn. and even survival were also evident. Organisms that serve as prey to larger predators, e.g., zooplankton, may be particularly susceptible to neg. impacts of exposure to microplastic pollution, with potential for ramifications throughout the food web. Future work should focus on whether microplastics may be affecting aquatic organisms more subtly, e.g., by influencing exposure to contaminants and pathogens, or by acting at a mol. level.
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    Microplastic particles have been obsd. in the environment and routinely detected in the stomachs and intestines of aquatic organisms over the last 50 yr. In the present review, information on the ingestion of plastic debris of varying sizes is collated, including data for >800 species representing approx. 87 000 individual organisms, for which plastic debris and microplastic particles have been obsd. in approx. 17 500, or 20%. The av. reported no. of microplastic particles/individual across all studies is estd. to be 4, with studies typically reporting avs. ranging from 0 to 10 particles/individual. A general observation is that although strong evidence exists for the biol. ingestion of microplastic particles, they do not bioaccumulate and do not appear to be subject to biomagnification as a result of trophic transfer through food webs, with >99% of observations from field-based studies reporting that microplastic particles are located within the gastrointestinal tract. Overall, there is substantial heterogeneity in how samples are collected, processed, analyzed, and reported, causing significant challenges in attempting to assess temporal and spatial trends or helping to inform a mechanistic understanding. Nevertheless, several studies suggest that the characteristics of microplastic particles ingested by organisms are generally representative of plastic debris in the vicinity where individuals are collected. Monitoring of spatial and temporal trends of ingested microplastic particles could thus potentially be useful in assessing mitigation efforts aimed at reducing the emission of plastic and microplastic particles to the environment. The development and application of standardized anal. methods are urgently needed to better understand spatial and temporal trends. Environ Toxicol Chem 2020;39:1119-1137. 2020 The Authors. Environmental Toxicol. and Chem. published by Wiley Periodicals LLC on behalf of SETAC.
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    Impact of Microplastic Beads and Fibers on Waterflea (Ceriodaphnia dubia) Survival, Growth, and Reproduction: Implications of Single and Mixture Exposures
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    Environmental Science & Technology (2017), 51 (22), 13397-13406CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    There is limited knowledge regarding the adverse effects of wastewater-derived microplastics, particularly fibers, on aquatic biota. In this study, we examd. the acute (48 h) and chronic (8 d) effects of microplastic polyester fibers and polyethylene (PE) beads on freshwater zooplankton Ceriodaphnia dubia. We also assessed the acute response of C. dubia to a binary mixt. of microplastic beads and fibers for the first time. Acute exposure to fibers and PE beads both showed a dose-dependent effect on survival. An equitoxic binary mixt. of beads and fibers resulted in a toxic unit of 1.85 indicating less than additive effects. Chronic exposure to lower concns. did not significantly affect survival of C. dubia, but a dose-dependent effect on growth and reprodn. was obsd. Fibers showed greater adverse effects than PE beads. While ingestion of fibers was not obsd., SEM showed carapace and antenna deformities after exposure to fibers, with no deformities obsd. after exposure to PE beads. While much of the current research has focused on microplastic beads, our study shows that microplastic fibers pose a greater risk to C. dubia, with reduced reproductive output obsd. at concns. within an order of magnitude of reported environmental levels.
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    Organic chemical pollutants associated with microplastic (MP) may represent an alternative exposure route for these chemicals to marine biota. However, the bioavailability of MP-sorbed organic pollutants under conditions where co-exposure occurs from the same compounds dissolved in the water phase has rarely been studied experimentally, especially where pollutant concentrations in the two phases are well characterized. Importantly, higher concentrations of organic pollutants on ingested MP may be less bioavailable to aquatic organisms than the same chemicals present in dissolved form in the surrounding water. In the current study, the sorption kinetics of two model polycyclic aromatic hydrocarbons (PAHs; fluoranthene and phenanthrene) to MP particles in natural seawater at 10 and 20 °C were studied and the bioavailability of MP-sorbed PAHs to marine copepods investigated. Polyethylene (PE) and polystyrene (PS) microbeads with mean diameters ranging from 10 to 200 μm were used to identify the role of MP polymer type and size on sorption mechanisms. Additionally, temperature dependence of sorption was investigated. Results indicated that adsorption dominated at lower temperatures and for smaller MP (10 μm), while absorption was the prevailing process for larger MP (100 μm). Monolayer sorption dominated at lower PAH concentrations, while multilayer sorption dominated at higher concentrations. PE particles representing ingestible (10 μm) and non-ingestible (100 μm) MP for the marine copepod species Acartia tonsa and Calanus finmarchicus were used to investigate the availability and toxicity of MP-sorbed PAHs. Studies were conducted under co-exposure conditions where the PAHs were also present in the dissolved phase (Cfree), thereby representing more environmentally relevant exposure scenarios. Cfree reduction through MP sorption was reflected in a corresponding reduction of lethality and bioaccumulation, with no difference observed between ingestible and non-ingestible MP. This indicates that only free dissolved PAHs are significantly bioavailable to copepods under co-exposure conditions with MP-sorbed PAHs.
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    Abstr.: The ingestion of plastics appears to be widespread throughout the animal kingdom with risks to individuals, ecosystems and human health. Despite growing information on the location, abundance and size distribution of plastics in the environment, it cannot be assumed that any given animal will ingest all sizes of plastic encountered. Here, we use published data to develop an allometric relationship between plastic consumption and animal size to est. the size distribution of plastics feasibly ingested by animals. Based on more than 2000 gut content analyses from animals ranging over three orders of magnitude in size (lengths 9 mm to 10 m), body length alone accounts for 42% of the variance in the length of plastic an animal may ingest and indicates a size ratio of roughly 20:1 between animal body length and the largest plastic the animal may ingest. We expect this work to improve global assessments of plastic pollution risk by introducing a quantifiable link between animals and the plastics they can ingest.
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    A review. Quant. structure-activity relationships (QSAR) have been applied for decades in the development of relationships between physicochem. properties of chem. substances and their biol. activities to obtain a reliable statistical model for prediction of the activities of new chem. entities. The fundamental principle underlying the formalism is that the difference in structural properties is responsible for the variations in biol. activities of the compds. In the classical QSAR studies, affinities of ligands to their binding sites, inhibition consts., rate consts., and other biol. end points, with at., group or mol. properties such as lipophilicity, polarizability, electronic and steric properties (Hansch anal.) or with certain structural features (Free-Wilson anal.) have been correlated. However such an approach has only a limited utility for designing a new mol. due to the lack of consideration of the 3D structure of the mols. 3D-QSAR has emerged as a natural extension to the classical Hansch and Free-Wilson approaches, which exploits the three-dimensional properties of the ligands to predict their biol. activities using robust chemometric techniques such as PLS, G/PLS, ANN etc. It has served as a valuable predictive tool in the design of pharmaceuticals and agrochems. Although the trial and error factor involved in the development of a new drug cannot be ignored completely, QSAR certainly decreases the no. of compds. to be synthesized by facilitating the selection of the most promising candidates. Several success stories of QSAR have attracted the medicinal chemists to investigate the relationships of structural properties with biol. activity. This review seeks to provide a bird's eye view of the different 3D-QSAR approaches employed within the current drug discovery community to construct predictive structure-activity relationships and also discusses the limitations that are fundamental to these approaches, as well as those that might be overcome with the improved strategies. The components involved in building a useful 3D-QSAR model are discussed, including the validation techniques available for this purpose.
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    A review. Quant. structure-activity relationship modeling is one of the major computational tools employed in medicinal chem. However, throughout its entire history it has drawn both praise and criticism concerning its reliability, limitations, successes, and failures. In this paper, we discuss (i) the development and evolution of QSAR; (ii) the current trends, unsolved problems, and pressing challenges; and (iii) several novel and emerging applications of QSAR modeling. Throughout this discussion, we provide guidelines for QSAR development, validation, and application, which are summarized in best practices for building rigorously validated and externally predictive QSAR models. We hope that this Perspective will help communications between computational and exptl. chemists toward collaborative development and use of QSAR models. We also believe that the guidelines presented here will help journal editors and reviewers apply more stringent scientific stds. to manuscripts reporting new QSAR studies, as well as encourage the use of high quality, validated QSARs for regulatory decision making.
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    An automated approach for microplastics analysis using focal plane array (FPA) FTIR microscopy and image analysis
    Primpke, S.; Lorenz, C.; Rascher-Friesenhausen, R.; Gerdts, G.
    Analytical Methods (2017), 9 (9), 1499-1511CODEN: AMNEGX; ISSN:1759-9679. (Royal Society of Chemistry)
    The anal. of imaging data derived from micro-Fourier transform IR (μFTIR) microscopy is a powerful tool allowing the anal. of microplastics enriched on membrane filters. In this study we present an automated approach to reduce the time demand currently needed for data analyses. We developed a novel anal. pipeline, based on the OPUS Software by Bruker, followed by image anal. with Python and Simple ITK image processing modules. By using this newly developed pipeline it was possible to analyze datasets from focal plane array (FPA) μFTIR mapping of samples contg. up to 1.8 million single spectra. All spectra were compared against a database of different synthetic and natural polymers by various routines followed by benchmark tests with focus on accuracy and quality. The spectral correlation was optimized for high quality data generation, which allowed image anal. Based on these results an image anal. approach was developed, providing information on particle nos. and sizes for each polymer detected. It was possible to collect all data with relative ease even for complex sample matrixes. This approach significantly decreases the time demand for the interpretation of complex FTIR-imaging data and significantly increases the data quality.
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    Investigations into the environmental fate and effects of microplastics have been gaining momentum. Small, insol. polymeric particles are implicated by scientists in a wide variety of studies that are used to suggest a potential for widespread impacts in freshwater and marine pelagic and sediment environments. An exponential growth in scientific publications and an increase in regulatory attention have occurred. However, despite these efforts, the environmental hazard of these particles is still unknown. To evaluate the hazard of microplastics within a risk assessment context, we need a way to evaluate the quality of exptl. studies. We performed a thorough review of the quality and focus of environmental microplastic research, to understand the methodologies employed and how this may assist or distract from the ability of environmental risk assessors to evaluate microplastics. We provide guidance to improve the reliability and relevance of ecotoxicol. studies for regulatory and broader environmental assessments. Nine areas of needed improvement are identified and discussed. Important data gaps and exptl. limitations are highlighted. Environ Toxicol Chem 2017;9999:1-7. © 2017 SETAC.
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    Uptake of ingested microparticles into small intestinal tissues and on to secondary organs has moved from being an anecdotal phenomenon to a recognised and quantifiable process, which is relevant to risk assessment of accidental exposure, treatment of multi-organ dysfunction syndrome and therapeutic uses of encapsulated drug or vaccine delivery. This review puts in context with the literature the findings of a morphological study of microparticle uptake, using two approaches. The first is a rat in vivo in situ model, appropriate to a study rooted in the exposure of human populations to microparticles. Latex microspheres 2 μm in diameter are the principal particle type used, although others are also investigated. Most data are based on microscopy, but analysis of macerated bulk tissue is also useful. Uptake occurs at early time points after a single dose and is shown to take place almost entirely at villous rather than Peyer's patch sites: however, multiple feeding and therefore a longer time-span produces a higher proportion of particles associated with Peyer's patches, albeit for very small total uptake at those later time points. Uptake is less affected by species, fasting and immunological competence than by age and reproductive status. The second approach uses in vitro methods to confirm the role of intercellular junctions in particle uptake. Particle-associated tight junction opening, in a Caco-2 monolayer, is reflected in changes in transepithelial resistance and particle uptake across the epithelial monolayer: Tight junction opening and particle uptake are both increased further by external irradiation, ethanol and sub-epithelial macrophages, but reduced by exposure to ice. An M cell model has looser tight junctions than Caco-2 cells, but a similar level of particle uptake. These results, along with the changes seen in junctional proteins after particle addition, confirm the role of tight junctions in uptake but suggest that adhering junctions are also important.
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Cite this: Environ. Sci. Technol. 2020, 54, 19, 12307–12315
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https://doi.org/10.1021/acs.est.0c02982
Published September 4, 2020

Copyright © 2020 American Chemical Society. This publication is licensed under CC-BY-NC-ND.

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  • Abstract

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    Figure 1

    Figure 1. Length to width (panel A) and length to height (panel B) ratios for polydisperse microplastic, obtained from Monte Carlo simulations for the main microplastic shape categories, and their relative abundance in the environment (4) for n = 1 × 106 iterations, with fitted bi- and trimodal distributions. The main difference is an extra peak for sheets at a low L/H ratio.

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    Figure 2

    Figure 2. Cumulative frequency distribution of original global microplastic concentrations in surface freshwater taken from (15) (black curve), the same data consistently rescaled to 1–5000 μm, (blue curve), compared to the concentration (ECX,env; eq 8) protecting 95% of species obtained from threshold effect concentrations corrected for bioavailability and polydispersity as defined by probability density distributions for environmental microplastic (HC5, vertical orange line, obtained from the SSD in Figure 3B).

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    Figure 3

    Figure 3. Species sensitivity distributions for microplastic based on laboratory data for different types of particles (panel A, R2 = 0.896, HC5 = 251 #/L), and based on the same data after corrections for bioavailability and polydispersity at the threshold effect concentration resulting in (ECX,env) x-axis values in particles/L (panel B, R2 = 0.940, HC5 = 75.6 #/L) or mg/L (panel C, R2 = 0.940, HC5 = 0.94 mg/L). Gray curves relate to 95% confidence intervals.

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      Because of their diverse sizes, shapes, and densities, environmental microplastics are often perceived as complex. Many studies struggle with this complexity and either address only a part of this diversity or present data using discrete classifications for sizes, shapes, and densities. We argue that such classifications will never be fully satisfactory, as any definition using classes does not capture the essentially continuous nature of environmental microplastic. Therefore, we propose to simplify microplastics by fully defining them through a three-dimensional (3D) probability distribution, with size, shape, and d. as dimensions. In addn. to introducing the concept, we parametrize these probability distributions, using empirical data. This parametrization results in an approx. yet realistic representation of "true" environmental microplastic. This approach to simplifying microplastic could be applicable to exposure measurements, effect studies, and fate modeling. Furthermore, it allows for easy comparison between studies, irresp. of sampling or lab. setup. We demonstrate how the 3D probability distribution of environmental vs. ingested microplastic can be helpful in understanding the bioavailability of and exposure to microplastic. We argue that the concept of simplified microplastic will also be helpful in probabilistic risk modeling, which would greatly enhance our understanding of the risk that microplastics pose to the environment.
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      A review. Plastic pollution has become a major environmental concern due to its omnipresence and degrdn. to smaller particles. The potential toxicol. effects of micro- and nanoplastic on biota have been investigated in a growing no. of exposure studies. We have performed a comprehensive review of the main detg. factors for plastic particle toxicity in the relevant exposure systems, from publications until including the year 2018. For a focused scope, effects of additives or other pollutants accumulated by the plastic particles are not included. In summary, current literature suggests that plastic particle toxicity depends on concn., particle size, exposure time, particle condition, shape and polymer type. Furthermore, contaminant background, food availability, species, developmental stage and sex have major influence on the outcome of plastic particles exposures. Frequently reported effects were on body and population growth, energy metab., feeding, movement activity, physiol. stress, oxidative stress, inflammation, the immune system, hormonal regulation, aberrant development, cell death, general toxicity and altered lipid metab. Several times reported were increased growth and food consumption, neuro-, liver- or kidney pathol. and intestinal damage. Photosynthesis disruption was reported in studies investigating effects on phytoplankton. For the currently unquantified plastic particles below 10μm, more toxic effects were reported in all aquatic life, as compared to plastic particles of larger size.
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      13
      Toward the Development and Application of an Environmental Risk Assessment Framework for Microplastic
      Gouin, Todd; Becker, Richard A.; Collot, Anne-Gaelle; Davis, John W.; Howard, Brett; Inawaka, Kunifumi; Lampi, Mark; Ramon, Blanca Serrano; Shi, Jay; Hopp, Philipp W.
      Environmental Toxicology and Chemistry (2019), 38 (10), 2087-2100CODEN: ETOCDK; ISSN:0730-7268. (Wiley-Blackwell)
      Emissions of plastic waste to the environment and the subsequent degrdn. into microplastic particles that have the potential to interact with biol. organisms represent a concern for global society. Current understanding of the potential impacts on aquatic and terrestrial population stability and ecosystem structure and function assocd. with emissions of microplastic particles is limited and insufficient to fully assess environmental risks. Multistakeholder discussions can provide an important element in helping to identify and prioritize key knowledge gaps in assessing potential risks. In the present review, we summarize multistakeholder discussions from a 1-d International Council of Chem. Assocns.-sponsored symposium, which involved 39 scientists from 8 countries with representatives from academia, industry, and government. Participants were asked to consider the following: discuss the scientific merits and limitations of applying a proposed conceptual environmental risk assessment (ERA) framework for microplastic particles and identify and prioritize major research needs in applying ERA tools for microplastic particles. Multistakeholder consensus was obtained with respect to the interpretation of the current state of the science related to effects and exposure to microplastic particles, which implies that it is unlikely that the presence of microplastic in the environment currently represents a risk. However, the quality and quantity of existing data require substantial improvement before conclusions regarding the potential risks and impacts of microplastic particles can be fully assessed. Research that directly addresses the development and application of methods that strengthen the quality of data should thus be given the highest priority. Activities aimed at supporting the development of and access to standardized ref. material were identified as a key research need. Environ Toxicol Chem 2019;00:1-14. © 2019 The Authors. Environmental Toxicol. and Chem. published by Wiley Periodicals, Inc. on behalf of SETAC.
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      Wagner, M.; Scherer, C.; Alvarez-Muñoz, D.; Brennholt, N.; Bourrain, X.; Buchinger, S.; Fries, E.; Grosbois, C.; Klasmeier, J.; Marti, T.; Rodriguez-Mozaz, S.; Urbatzka, R.; Vethaak, A. D.; Winther-Nielsen, M.; Reifferscheid, G. Microplastics in freshwater ecosystems: what we know and what we need to know. Environ. Sci. Eur. 2014, 26, 12  DOI: 10.1186/s12302-014-0012-7
      14
      Microplastics in freshwater ecosystems: what we know and what we need to know
      Wagner Martin; Scherer Christian; Alvarez-Munoz Diana; Rodriguez-Mozaz Sara; Brennholt Nicole; Buchinger Sebastian; Reifferscheid Georg; Bourrain Xavier; Fries Elke; Grosbois Cecile; Klasmeier Jorg; Marti Teresa; Urbatzka Ralph; Vethaak A Dick; Winther-Nielsen Margrethe
      Environmental sciences Europe (2014), 26 (1), 12 ISSN:2190-4707.
      BACKGROUND: While the use of plastic materials has generated huge societal benefits, the 'plastic age' comes with downsides: One issue of emerging concern is the accumulation of plastics in the aquatic environment. Here, so-called microplastics (MP), fragments smaller than 5 mm, are of special concern because they can be ingested throughout the food web more readily than larger particles. Focusing on freshwater MP, we briefly review the state of the science to identify gaps of knowledge and deduce research needs. STATE OF THE SCIENCE: Environmental scientists started investigating marine (micro)plastics in the early 2000s. Today, a wealth of studies demonstrates that MP have ubiquitously permeated the marine ecosystem, including the polar regions and the deep sea. MP ingestion has been documented for an increasing number of marine species. However, to date, only few studies investigate their biological effects. The majority of marine plastics are considered to originate from land-based sources, including surface waters. Although they may be important transport pathways of MP, data from freshwater ecosystems is scarce. So far, only few studies provide evidence for the presence of MP in rivers and lakes. Data on MP uptake by freshwater invertebrates and fish is very limited. KNOWLEDGE GAPS: While the research on marine MP is more advanced, there are immense gaps of knowledge regarding freshwater MP. Data on their abundance is fragmentary for large and absent for small surface waters. Likewise, relevant sources and the environmental fate remain to be investigated. Data on the biological effects of MP in freshwater species is completely lacking. The accumulation of other freshwater contaminants on MP is of special interest because ingestion might increase the chemical exposure. Again, data is unavailable on this important issue. CONCLUSIONS: MP represent freshwater contaminants of emerging concern. However, to assess the environmental risk associated with MP, comprehensive data on their abundance, fate, sources, and biological effects in freshwater ecosystems are needed. Establishing such data critically depends on a collaborative effort by environmental scientists from diverse disciplines (chemistry, hydrology, ecotoxicology, etc.) and, unsurprisingly, on the allocation of sufficient public funding.
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    15. 15
      Koelmans, A. A.; Mohamed Nor, N. H.; Hermsen, E.; Kooi, M.; Mintenig, S. M.; De France, J. Microplastics in Freshwaters and Drinking Water: Critical Review and Assessment of Data Quality. Water Res. 2019, 155, 410– 422,  DOI: 10.1016/j.watres.2019.02.054
      15
      Microplastics in freshwaters and drinking water: Critical review and assessment of data quality
      Koelmans, Albert A.; Mohamed Nor, Nur Hazimah; Hermsen, Enya; Kooi, Merel; Mintenig, Svenja M.; De France, Jennifer
      Water Research (2019), 155 (), 410-422CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
      Microplastics have recently been detected in drinking water as well as in drinking water sources. This presence has triggered discussions on possible implications for human health. However, there have been questions regarding the quality of these occurrence studies since there are no std. sampling, extn. and identification methods for microplastics. Accordingly, we assessed the quality of fifty studies researching microplastics in drinking water and in its major freshwater sources. This includes an assessment of microplastic occurrence data from river and lake water, groundwater, tap water and bottled drinking water. Studies of occurrence in wastewater were also reviewed. We review and propose best practices to sample, ext. and detect microplastics and provide a quant. quality assessment of studies reporting microplastic concns. Further, we summarize the findings related to microplastic concns., polymer types and particle shapes. Microplastics are frequently present in freshwaters and drinking water, and no. concns. spanned ten orders of magnitude (1 × 10-2 to 108 #/m3) across individual samples and water types. However, only four out of 50 studies received pos. scores for all proposed quality criteria, implying there is a significant need to improve quality assurance of microplastic sampling and anal. in water samples. The order in globally detected polymers in these studies is PE ≈ PP > PS > PVC > PET, which probably reflects the global plastic demand and a higher tendency for PVC and PET to settle as a result of their higher densities. Fragments, fibers, film, foam and pellets were the most frequently reported shapes. We conclude that more high quality data is needed on the occurrence of microplastics in drinking water, to better understand potential exposure and to inform human health risk assessments.
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    16. 16
      Lenz, R.; Enders, K.; Nielsen, T. G. Microplastic exposure studies should be environmentally realistic. Proc. Natl. Acad. Sci. U.S.A. 2016, 113, E4121– E4122,  DOI: 10.1073/pnas.1606615113
      16
      Microplastic exposure studies should be environmentally realistic
      Lenz, Robin; Enders, Kristina; Nielsen, Torkel Gissel
      Proceedings of the National Academy of Sciences of the United States of America (2016), 113 (29), E4121-E4122CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
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      Mintenig, S. M.; Bäuerlein, P. S.; Koelmans, A. A.; Dekker, S. C.; van Wezel, A. P. Closing the gap between small and smaller: Towards a framework to analyse nano- and microplastics in aqueous environmental samples. Environ. Sci.: Nano 2018, 5, 1640– 1649,  DOI: 10.1039/C8EN00186C
      17
      Closing the gap between small and smaller: towards a framework to analyse nano- and microplastics in aqueous environmental samples
      Mintenig, S. M.; Baeuerlein, P. S.; Koelmans, A. A.; Dekker, S. C.; van Wezel, A. P.
      Environmental Science: Nano (2018), 5 (7), 1640-1649CODEN: ESNNA4; ISSN:2051-8161. (Royal Society of Chemistry)
      Measuring concns. and sizes of micro- and nanoplastics in the environment is essential to assess the risks plastic particles could pose. Microplastics have been detected globally in a variety of aquatic ecosystems. The detn. of nanoplastics, however, is lagging behind due to higher methodol. challenges. Here, we propose a framework that can consistently det. a broad spectrum of plastic particle sizes in aquatic environmental samples. Anal. evidence is provided as proof of principle. FTIR microscopy is applied to detect microplastics. Nanoplastics are studied using field-flow-fractionation and pyrolysis GC-MS that gives information on the particle sizes and polymer types. Pyrolysis GC-MS is shown to be promising for the detection of nanoplastics in environmental samples as a mass of approx. 100 ng is required to identify polystyrene. Pre-concg. nanoplastics by crossflow ultrafiltration enables polystyrene to be identified when the original concn. in an aq. sample is >20μg L-1. Finally, we present an approach to est. polymer masses based on the two-dimensional microplastic shapes recorded during the anal. with FTIR microscopy. Our suite of techniques demonstrates that anal. of the entire size spectrum of plastic debris is feasible.
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    18. 18
      Karlsson, T. M.; Kärrman, A.; Rotander, A.; Hassellöv, M. Comparison between manta trawl and in situ pump filtration methods, and guidance for visual identification of microplastics in surface waters. Environ. Sci. Pollut. Res. 2020, 27, 5559– 5571,  DOI: 10.1007/s11356-019-07274-5
      18
      Comparison between manta trawl and in situ pump filtration methods, and guidance for visual identification of microplastics in surface waters
      Karlsson Therese M; Hassellov Martin; Karrman Anna; Rotander Anna
      Environmental science and pollution research international (2020), 27 (5), 5559-5571 ISSN:.
      Owing to the development and adoption of a variety of methods for sampling and identifying microplastics, there is now data showing the presence of microplastics in surface waters from all over the world. The difference between the methods, however, hampers comparisons, and to date, most studies are qualitative rather than quantitative. In order to allow for a quantitative comparison of microplastics abundance, it is crucial to understand the differences between sampling methods. Therefore, a manta trawl and an in situ filtering pump were compared during realistic, but controlled, field tests. Identical microplastic analyses of all replicates allowed the differences between the methods with respect to (1) precision, (2) concentrations, and (3) composition to be assessed. The results show that the pump gave higher accuracy with respect to volume than the trawl. The trawl, however, sampled higher concentrations, which appeared to be due to a more efficient sampling of particles on the sea surface microlayer, such as expanded polystyrene and air-filled microspheres. The trawl also sampled a higher volume, which decreased statistical counting uncertainties. A key finding in this study was that, regardless of sampling method, it is critical that a sufficiently high volume is sampled to provide enough particles for statistical evaluation. Due to the patchiness of this type of contaminant, our data indicate that a minimum of 26 particles per sample should be recorded to allow for concentration comparisons and to avoid false null values. The necessary amount of replicates to detect temporal or spatial differences is also discussed. For compositional differences and size distributions, even higher particle counts would be necessary. Quantitative measurements and comparisons would also require an unbiased approach towards both visual and spectroscopic identification. To facilitate the development of such methods, a visual protocol that can be further developed to fit different needs is introduced and discussed. Some of the challenges encountered while using FTIR microspectroscopic particle identification are also critically discussed in relation to specific compositions found.
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    19. 19
      Everaert, G.; Van Cauwenberghe, L.; De Rijcke, M.; Koelmans, A. A.; Mees, J.; Vandegehuchte, M.; Janssen, C. R. Risk assessment of microplastics in the ocean: modelling approach and first conclusions. Environ. Pollut. 2018, 242, 1930– 1938,  DOI: 10.1016/j.envpol.2018.07.069
      19
      Risk assessment of microplastics in the ocean: Modelling approach and first conclusions
      Everaert, Gert; Van Cauwenberghe, Lisbeth; De Rijcke, Maarten; Koelmans, Albert A.; Mees, Jan; Vandegehuchte, Michiel; Janssen, Colin R.
      Environmental Pollution (Oxford, United Kingdom) (2018), 242 (Part_B), 1930-1938CODEN: ENPOEK; ISSN:0269-7491. (Elsevier Ltd.)
      We performed an environmental risk assessment for microplastics (<5 mm) in the marine environment by estg. the order of magnitude of the past, present and future concns. based on global plastic prodn. data. In 2100, from 9.6 to 48.8 particles m-3 are predicted to float around in the ocean, which is a 50-fold increase compared to the present-day concns. From a meta-anal. with effect data available in literature, we derived a safe concn. of 6650 buoyant particles m-3 below which adverse effects are not likely to occur. Our risk assessment (excluding the potential role of microplastics as chem. vectors) suggests that on av., no direct effects of free-floating microplastics in the marine environment are to be expected up to the year 2100. Yet, even today, the safe concn. can be exceeded in sites that are heavily polluted with buoyant microplastics. In the marine benthic compartment between 32 and 144 particles kg-1 dry sediment are predicted to be present in the beach deposition zone. Despite the scarcity of effect data, we expect adverse ecol. effects along the coast as of the second half of the 21st century. From then ambient concns. will start to outrange the safe concn. of sedimented microplastics (i.e. 540 particles kg-1 sediment). Addnl. ecotoxicol. research in which marine species are chronically exposed to realistic environmental microplastic concn. series are urgently needed to verify our findings.
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    20. 20
      Besseling, E.; Redondo-Hasselerharm, P. E.; Foekema, E. M.; Koelmans, A. A. Quantifying Ecological Risks of Aquatic Micro- and Nanoplastic. Crit. Rev. Environ. Sci. Technol. 2019, 49, 32– 80,  DOI: 10.1080/10643389.2018.1531688
      There is no corresponding record for this reference.
    21. 21
      Burns, E. E.; Boxall, A. B. A. Microplastics in the Aquatic Environment: Evidence for or against Adverse Impacts and Major Knowledge Gaps. Environ. Toxicol. Chem. 2018, 37, 2776– 2796,  DOI: 10.1002/etc.4268
      21
      Microplastics in the aquatic environment: Evidence for or against adverse impacts and major knowledge gaps
      Burns, Emily E.; Boxall, Alistair B. A.
      Environmental Toxicology and Chemistry (2018), 37 (11), 2776-2796CODEN: ETOCDK; ISSN:0730-7268. (Wiley-Blackwell)
      There is increasing scientific and public concern over the presence of microplastics in the natural environment. We present the results of a systematic review of the literature to assess the wt. of evidence for microplastics causing environmental harm. We conclude that microplastics do occur in surface water and sediments. Fragments and fibers predominate, with beads making up only a small proportion of the detected microplastic types. Concns. detected are orders of magnitude lower than those reported to affect endpoints such as biochem., feeding, reprodn., growth, tissue inflammation and mortality in organisms. The evidence for microplastics acting as a vector for hydrophobic org. compds. to accumulate in organisms is also weak. The available data therefore suggest that these materials are not causing harm to the environment. There is, however, a mismatch between the particle types, size ranges, and concns. of microplastics used in lab. tests and those measured in the environment. Select environmental compartments have also received limited attention. There is an urgent need for studies that address this mismatch by performing high quality and more holistic monitoring studies alongside more environmentally realistic effects studies. Only then will we be able to fully characterize risks of microplastics to the environment to support the introduction of regulatory controls that can make a real pos. difference to environmental quality. Environ Toxicol Chem 2018;9999:1-21. © 2018 SETAC.
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    22. 22
      Adam, V.; Yang, T.; Nowack, B. Toward an Ecotoxicological Risk Assessment of Microplastics: Comparison of Available Hazard and Exposure Data in Freshwaters. Environ. Toxicol. Chem. 2019, 38, 436– 47,  DOI: 10.1002/etc.4323
      22
      Toward an ecotoxicological risk assessment of microplastics: Comparison of available hazard and exposure data in freshwaters
      Adam, Veronique; Yang, Tong; Nowack, Bernd
      Environmental Toxicology and Chemistry (2019), 38 (2), 436-447CODEN: ETOCDK; ISSN:0730-7268. (Wiley-Blackwell)
      A review. Microplastics have been detected in freshwaters all over the world in almost all samples, and ecotoxicol. studies have shown adverse effects of microplastics on organisms. However, no risk assessment of microplastics has been performed specifically in freshwater so far. The aim of the present study was therefore to review all exposure and ecotoxicity data available for microplastics in freshwaters and to perform a preliminary probabilistic risk assessment. The exposure probability distribution was based on 391 concns. measured in Asia, Europe, and North America. Because exposure data are mainly available in particle no.-based metrics but results from hazard studies are mostly mass-based, the hazard results were converted into particle no. concns. A statistical anal. of the hazard data showed that there was no significant influence of particle shape or type of polymer on the no-obsd.-effect concn. The predicted-no-effect concn. (PNEC) was calcd. as the fifth percentile of the probabilistic species sensitivity distribution, based on 53 values from 14 freshwater species, to have a mode of 7.4 × 105 particles · m-3 (25th and 75th quantiles of 6.1 × 105 and 1.3 × 106 particles · m-3, resp.). The exposure probability distribution was divided by the PNEC probability distribution to calc. risk characterization ratios (RCRs), with modes of 1.3 × 10-6 in North America, 3.3 × 10-6 in Europe, and 4.6 × 10-3 in Asia. Probability distributions assocd. with the RCRs showed that ecol. risks cannot be entirely excluded in Asia, where 0.4% of the RCR values were above 1. Environ Toxicol Chem 2018;9999:1-12. © 2018 SETAC.
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    23. 23
      Zhang, X.; Leng, Y.; Liu, X.; Huang, K.; Wang, J. Microplastics’ pollution and risk assessment in an urban river: A case study in the Yongjiang River, Nanning City, South China. Exposure Health 2020, 12, 141– 151,  DOI: 10.1007/s12403-018-00296-3
      23
      Microplastics' Pollution and Risk Assessment in an Urban River: A Case Study in the Yongjiang River, Nanning City, South China
      Zhang, Xin; Leng, Yifei; Liu, Xiaoning; Huang, Kai; Wang, Jun
      Exposure and Health (2020), 12 (2), 141-151CODEN: EHXEAI; ISSN:2451-9685. (Springer)
      Microplastics (MPs) have been considered as a global environmental problem threatening the ecol. security. However, studies on MPs' pollution in freshwaters and the assocd. risk assessment remain limited in the literature. In this study, the concns., distributions, and the potential ecol. risks of MPs were analyzed in Yongjiang River, which is an important drinking water source flowing through Nanning City, the mega city of China. The MPs' abundances in surface waters and sediments ranged from 500 to 7700 n/m3 and from 90 to 550 n/kg, resp. Spatial distribution highlighted the significant impact of anthropogenic activity on the MPs' accumulation. Polyethylene and polypropylene were the most common polymer compns. investigated. Shape, size, and color were examd. to analyze the characteristics of MPs in the river. To assess the ecol. risk of MPs, the predicted no-effect concn. (PNEC) values were derived from a species' sensitivity distribution model based on the toxicity data of MPs for freshwater species available in the literature. The PNEC for MPs in surface water was derived to be 4920 n/m3. Risk assessment results through risk quotient (RQ) method suggest that most of the monitored sites in Yongjiang River posed negligible risks to freshwater biota, except the two sites with high risk in the urban center. The results provided a basis for ecol. risk assessment of MPs in freshwaters.
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    24. 24
      Skåre, J. U.; Alexander, J.; Have, M.; Jakubowicz, I.; Knutsen, H. K.; Lusher, A. L.; Ogonowski, M.; Rakkestad, K. E.; Skaar, I.; Tvedt Sverdrup, L. E.; Wagner, M.; Agdestein, A.; Bodin, J.; Elvevoll, E.; Hemre, G. I.; Hessen, D. O.; Hofshagen, M.; Husøy, T.; Krogdahl, Å.; Nilsen, A. M.; Rafoss, T.; Skjerdal, T.; Strand, T. A.; Vandvik, V.; Wasteson, Y. Microplastics; Occurrence, Levels and Implications for Environment and Human Health Related to Food. Opinion of the Steering Committee of the Norwegian Scientific Committee for Food and Environment; Norwegian Scientific Commitee for Food and Environment (VKM): 2019.
      There is no corresponding record for this reference.
    25. 25
      Rochman, C. M.; Regan, F.; Thompson, R. C. On the harmonization of methods for measuring the occurrence, fate and effects of microplastics. Anal. Methods 2017, 9, 1324– 1325,  DOI: 10.1039/C7AY90014G
      There is no corresponding record for this reference.
    26. 26
      Michida, Y.; Chavanich, S.; Cózar Cabañas, A.; Hagmann, P.; Hinata, H.; Isobe, A.; Kershaw, P.; Kozlovskii, N.; Li, D.; Lusher, A. L.; Martí, E.; Mason, S. A.; Mu, J.; Saito, H.; Shim, W. J.; Syakti, A. D.; Takada, H.; Thompson, R. C.; Tokai, T.; Uchida, K.; Vasilenko, K.; Wang, J.. Guidelines for Harmonizing Ocean Surface Microplastic Monitoring Methods, Version 1.1; Ministry of the Environment: Japan. https://www.env.go.jp/en/water/marine_litter/method.html, 2019.
      There is no corresponding record for this reference.
    27. 27
      Guidelines or the Monitoring and Assessment of Plastic Litter and Microplastics in the Ocean; Kershaw, P. J.; Turra, A.; Galgani, F., Eds.; GESAMP Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection: 2019; p 130.
      There is no corresponding record for this reference.
    28. 28
      Roch, S.; Walter, T.; Ittner, L. D.; Friedrich, C.; Brinker, A. A systematic study of the microplastic burden in freshwater fishes of south-western Germany - Are we searching at the right scale?. Sci. Total Environ. 2019, 689, 1001– 1011,  DOI: 10.1016/j.scitotenv.2019.06.404
      28
      A systematic study of the microplastic burden in freshwater fishes of south-western Germany - Are we searching at the right scale?
      Roch, Samuel; Walter, Thomas; Ittner, Lukas D.; Friedrich, Christian; Brinker, Alexander
      Science of the Total Environment (2019), 689 (), 1001-1011CODEN: STENDL; ISSN:0048-9697. (Elsevier B.V.)
      In a comprehensive study of microplastic contamination in southern Germany, 1167 individual fish of 22 different species were sampled from 11 rivers and 6 lakes across the state. The microplastic burden of investigated fish was analyzed on the basis of habitat type, location, and a no. of abiotic and biotic factors. A particle size distribution anal. of the detected microplastics was carried out. The results showed a relatively low plastic prevalence of 18.8%, with significant differences between rivers (20.6%) and lakes (16.5%). The no. of ingested plastic particles ranged between 1 and 4 particles per fish. The majority of abiotic and biotic factors seem to play little or no role in the ingestion of microplastics, suggesting that in most cases uptake is passive or accidental. It is notable that piscivorous fish appeared significantly less burdened, suggesting a low transfer rate and no accumulation in the food web. However, size distribution anal. identified a power law growth fit in particle nos. at the smallest end of the distribution. This carries a worrying implication, that >95% of particles are likely to be smaller than 40 μm and thereby beyond the detection range of this and most other microplastic surveys conducted so far. When the frequency development of small particles is taken into account, the likely microplastic prevalence in the present study increases to 100%, with an av. intensity of around 23 predominantly small particles per fish. A striking 70% of those particles would be smaller than 5 μm and therefore eligible for translocation into tissues, with crit. implications for fish health and consumer exposure. This raises a question as to whether current ests. of microplastic burden in fishes generally might be overlooking a majority of potential contamination within the crit. smaller particle size classes.
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    29. 29
      Waldschläger, K.; Schüttrumpf, H. Effects of Particle Properties on the Settling and Rise Velocities of Microplastics in Freshwater under Laboratory Conditions. Environ. Sci. Technol. 2019, 53, 1958– 1966,  DOI: 10.1021/acs.est.8b06794
      29
      Effects of Particle Properties on the Settling and Rise Velocities of Microplastics in Freshwater under Laboratory Conditions
      Waldschlager Kryss; Schuttrumpf Holger
      Environmental science & technology (2019), 53 (4), 1958-1966 ISSN:.
      Microplastic (MP) contaminates terrestrial, aquatic, and atmospheric environments. Although the number of river sampling studies with regard to MP concentrations is increasing, comprehension of the predominant transport processes of MP in the watercourse is still very limited. In order to gain a better process understanding, around 500 physical experiments were conducted to shed more light on the effects of particle shape, size and density on the rise and settling velocities of MP. The determined velocities ranged between 0.39 cm/s for polyamide fibers (settling) and 31.4 cm/s for expanded polystyrene pellets (rise). Subsequently, the determined velocities were compared with formulas from sediment transport and, as there were large differences between theoretically and experimentally determined velocities, own formulas were developed to describe settling and rise velocities of MP particles with a large variety of shapes, sizes and densities. This study shows that MP differs significantly from sediment in its behavior and that a transfer of common sediment transport formulas should be treated with caution. Furthermore, the established formulas can now be used in numerical simulations to describe the settling and rising of MP more precisely.
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    30. 30
      McNown, J. S.; Malaika, J. Effect of particle shape on settling velocity at low Reynolds numbers. Trans., Am. Geophys. Union. 1950, 31, 74– 82,  DOI: 10.1029/TR031i001p00074
      There is no corresponding record for this reference.
    31. 31
      Simon, M.; van Last, N.; Vollertsen, J. Quantification of microplastic mass and removal rates at wastewater treatment plants applying Focal Plane Array (FPA)-based Fourier Transform Infrared (FT-IR) imaging. Water Res. 2018, 142, 1– 9,  DOI: 10.1016/j.watres.2018.05.019
      31
      Quantification of microplastic mass and removal rates at wastewater treatment plants applying Focal Plane Array (FPA)-based Fourier Transform Infrared (FT-IR) imaging
      Simon, Marta; van Alst, Nikki; Vollertsen, Jes
      Water Research (2018), 142 (), 1-9CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
      This paper presents a method for microplastic (MP) mass quantification using a Focal Plane Array-based Fourier Transform IR imaging technique. It discusses the issue that particle no. is not a conserved base quantity and hence less suited than mass to compare independent studies on MP in the environment. It concludes that MP mass should be included when quantifying MP pollution in the environment, supplementing the conventional approach of reporting particle nos. Applying mass as the unit of MP measurement, the paper presents data showing that Danish wastewater treatment plants discharge around 3 t/yr of MP in the size range 10-500μm. This value corresponds to an annual per capita emission from these plants of 0.56 g MP/(capita year). The distribution of polymer types by mass and particle no. differed because the size of MP particles of the different material types varied.
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    32. 32
      Mintenig, S. M.; Kooi, M.; Erich, M. W.; Primpke, S.; Redondo-Hasselerharm, P. E.; Dekker, S. C.; Koelmans, A. A.; van Wezel, A. P. A systems approach to understand microplastic occurrences in Dutch riverine surface waters. Water Res. 2020, 176, 115723  DOI: 10.1016/j.watres.2020.115723
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      A systems approach to understand microplastic occurrence and variability in Dutch riverine surface waters
      Mintenig, S. M.; Kooi, M.; Erich, M. W.; Primpke, S.; Redondo-Hasselerharm, P. E.; Dekker, S. C.; Koelmans, A. A.; van Wezel, A. P.
      Water Research (2020), 176 (), 115723CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
      Assessment methods on data quality and environmental variability are lacking for microplastics (MP). Here we assess occurrence and variability of MP no. concns. in two Dutch rivers. Strict QA/QC procedures were applied to identify MP using Fourier-transform IR (FTIR) microscopy followed by state of the art automated image anal. For a series of randomly selected, yet ever smaller subareas of filters, we assessed how accurately MP nos. and polymer types are represented during partial filter anal. Levels of uncertainty were acceptable when analyzing 50% of a filter during chem. mapping, and when identifying at least a subset of 50 individual particles with attenuated total reflection (ATR)-FTIR. Applying these guidelines, MP no. concns. between 67 and 11532 MP m-3 were detected in Dutch riverine surface waters. Spatial differences caused MP no. concns. to vary by two orders of magnitude. Temporal differences were lower and induced a max. variation of one order of magnitude. In total, 26 polymer types were identified, the most common were polyethylene (23%), polypropylene (19.7%) and ethylene propylene diene monomer rubber (18.3%). The highest diversity of polymer types was found for small MPs, whereas MP larger than 1 mm was scarce and almost exclusively made of polyethylene or polypropylene. Virtually all sampling locations revealed MP no. concns. that are considerably below known effect thresholds for anticipated adverse ecol. effects.
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    33. 33
      Wright, S. L.; Rowe, D.; Thompson, R. C.; Galloway, T. S. Microplastic ingestion decreases energy reserves in marine worms. Curr. Biol. 2013, 23, R1031– R1033,  DOI: 10.1016/j.cub.2013.10.068
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      Microplastic ingestion decreases energy reserves in marine worms
      Wright, Stephanie L.; Rowe, Darren; Thompson, Richard C.; Galloway, Tamara S.
      Current Biology (2013), 23 (23), R1031-R1033CODEN: CUBLE2; ISSN:0960-9822. (Cell Press)
      Deposit-feeding marine worms maintained in sediments spiked with microscopic unplasticised polyvinylchloride (UPVC) at concns. overlapping those in the environment had significantly depleted energy reserves by ≤50%. Our results suggest that depleted energy reserves arise from a combination of reduced feeding activity, longer gut residence times of ingested material, and inflammation.
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    34. 34
      Foley, C. J.; Feiner, Z. S.; Malinich, T. D.; Höök, T. O. A meta-analysis of the effects of exposure to microplastics on fish and aquatic invertebrates. Sci. Total Environ. 2018, 631–632, 550– 559,  DOI: 10.1016/j.scitotenv.2018.03.046
      34
      A meta-analysis of the effects of exposure to microplastics on fish and aquatic invertebrates
      Foley, Carolyn J.; Feiner, Zachary S.; Malinich, Timothy D.; Hook, Tomas O.
      Science of the Total Environment (2018), 631-632 (), 550-559CODEN: STENDL; ISSN:0048-9697. (Elsevier B.V.)
      Microplastics are present in aquatic ecosystems the world over and may influence the feeding, growth, reprodn., and survival of freshwater and marine biota; however, the extent and magnitude of potential effects of microplastics on aquatic organisms is poorly understood. In the current study, we conducted a meta-anal. of published literature to examine impacts of exposure to microplastics on consumption (and feeding), growth, reprodn., and survival of fish and aquatic invertebrates. While we did observe within-taxa neg. effects for all four categories of responses, many of the effects summarized in our study were neutral, indicating that the effects of exposure to microplastics are highly variable across taxa. The most consistent effect was a redn. in consumption of natural prey when microplastics were present. For some taxa, neg. effects on growth, reprodn. and even survival were also evident. Organisms that serve as prey to larger predators, e.g., zooplankton, may be particularly susceptible to neg. impacts of exposure to microplastic pollution, with potential for ramifications throughout the food web. Future work should focus on whether microplastics may be affecting aquatic organisms more subtly, e.g., by influencing exposure to contaminants and pathogens, or by acting at a mol. level.
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    35. 35
      De Ruijter, V. N.; Redondo-Hasselerharm, P. E.; Gouin, T.; Koelmans, A. A. Quality criteria for microplastic effect studies in the context of risk assessment: A critical review. Environ. Sci.Technol. 2020,  DOI: 10.1021/acs.est.0c03057
      There is no corresponding record for this reference.
    36. 36
      Besseling, E.; Redondo-Hasselerharm, P. E.; Foekema, E. M.; Koelmans, A. A. 2019. Correction to ‘Quantifying ecological risks of aquatic micro- and nanoplastic’. Crit. Rev. Environ. Sci. Technol. 2019, 49, 32– 80,  DOI: 10.1080/10643389.2019.1595472
      There is no corresponding record for this reference.
    37. 37
      Gouin, T. Towards improved understanding of the ingestion and trophic transfer of microplastic particles - Critical review and implications for future research. Environ. Toxicol. Chem. 2020, 39, 1119– 1137,  DOI: 10.1002/etc.4718
      37
      Toward an Improved Understanding of the Ingestion and Trophic Transfer of Microplastic Particles: Critical Review and Implications for Future Research
      Gouin, Todd
      Environmental Toxicology and Chemistry (2020), 39 (6), 1119-1137CODEN: ETOCDK; ISSN:0730-7268. (Wiley-Blackwell)
      Microplastic particles have been obsd. in the environment and routinely detected in the stomachs and intestines of aquatic organisms over the last 50 yr. In the present review, information on the ingestion of plastic debris of varying sizes is collated, including data for >800 species representing approx. 87 000 individual organisms, for which plastic debris and microplastic particles have been obsd. in approx. 17 500, or 20%. The av. reported no. of microplastic particles/individual across all studies is estd. to be 4, with studies typically reporting avs. ranging from 0 to 10 particles/individual. A general observation is that although strong evidence exists for the biol. ingestion of microplastic particles, they do not bioaccumulate and do not appear to be subject to biomagnification as a result of trophic transfer through food webs, with >99% of observations from field-based studies reporting that microplastic particles are located within the gastrointestinal tract. Overall, there is substantial heterogeneity in how samples are collected, processed, analyzed, and reported, causing significant challenges in attempting to assess temporal and spatial trends or helping to inform a mechanistic understanding. Nevertheless, several studies suggest that the characteristics of microplastic particles ingested by organisms are generally representative of plastic debris in the vicinity where individuals are collected. Monitoring of spatial and temporal trends of ingested microplastic particles could thus potentially be useful in assessing mitigation efforts aimed at reducing the emission of plastic and microplastic particles to the environment. The development and application of standardized anal. methods are urgently needed to better understand spatial and temporal trends. Environ Toxicol Chem 2020;39:1119-1137. 2020 The Authors. Environmental Toxicol. and Chem. published by Wiley Periodicals LLC on behalf of SETAC.
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    38. 38
      ECHA. Characterization of Dose [Concentration]-Response for Environment. In Guidance on Information Requirements and Chemical Safety Assessment.; European Chemicals Agency, 2008; Chapter R.8.
      There is no corresponding record for this reference.
    39. 39
      Ziajahromi, S.; Kumar, A.; Neale, P. A.; Leusch, F. D. L. Impact of microplastic beads and fibers on waterflea (Ceriodaphnia Dubia) survival, growth, and reproduction: Implications of single and mixture exposures. Environ. Sci. Technol. 2017, 51, 13397– 13406,  DOI: 10.1021/acs.est.7b03574
      39
      Impact of Microplastic Beads and Fibers on Waterflea (Ceriodaphnia dubia) Survival, Growth, and Reproduction: Implications of Single and Mixture Exposures
      Ziajahromi, Shima; Kumar, Anupama; Neale, Peta A.; Leusch, Frederic D. L.
      Environmental Science & Technology (2017), 51 (22), 13397-13406CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      There is limited knowledge regarding the adverse effects of wastewater-derived microplastics, particularly fibers, on aquatic biota. In this study, we examd. the acute (48 h) and chronic (8 d) effects of microplastic polyester fibers and polyethylene (PE) beads on freshwater zooplankton Ceriodaphnia dubia. We also assessed the acute response of C. dubia to a binary mixt. of microplastic beads and fibers for the first time. Acute exposure to fibers and PE beads both showed a dose-dependent effect on survival. An equitoxic binary mixt. of beads and fibers resulted in a toxic unit of 1.85 indicating less than additive effects. Chronic exposure to lower concns. did not significantly affect survival of C. dubia, but a dose-dependent effect on growth and reprodn. was obsd. Fibers showed greater adverse effects than PE beads. While ingestion of fibers was not obsd., SEM showed carapace and antenna deformities after exposure to fibers, with no deformities obsd. after exposure to PE beads. While much of the current research has focused on microplastic beads, our study shows that microplastic fibers pose a greater risk to C. dubia, with reduced reproductive output obsd. at concns. within an order of magnitude of reported environmental levels.
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    40. 40
      Sørensen, L.; Rogers, E.; Altin, D.; Salaberria, J.; Booth, A. M. Sorption of PAHs to microplastic and their bioavailability and toxicity to marine copepods under co-exposure conditions. Environ. Pollut. 2020, 258, 113844  DOI: 10.1016/j.envpol.2019.113844
      40
      Sorption of PAHs to microplastic and their bioavailability and toxicity to marine copepods under co-exposure conditions
      Sorensen Lisbet; Rogers Emilie; Altin Dag; Salaberria Iurgi; Booth Andy M
      Environmental pollution (Barking, Essex : 1987) (2020), 258 (), 113844 ISSN:.
      Organic chemical pollutants associated with microplastic (MP) may represent an alternative exposure route for these chemicals to marine biota. However, the bioavailability of MP-sorbed organic pollutants under conditions where co-exposure occurs from the same compounds dissolved in the water phase has rarely been studied experimentally, especially where pollutant concentrations in the two phases are well characterized. Importantly, higher concentrations of organic pollutants on ingested MP may be less bioavailable to aquatic organisms than the same chemicals present in dissolved form in the surrounding water. In the current study, the sorption kinetics of two model polycyclic aromatic hydrocarbons (PAHs; fluoranthene and phenanthrene) to MP particles in natural seawater at 10 and 20 °C were studied and the bioavailability of MP-sorbed PAHs to marine copepods investigated. Polyethylene (PE) and polystyrene (PS) microbeads with mean diameters ranging from 10 to 200 μm were used to identify the role of MP polymer type and size on sorption mechanisms. Additionally, temperature dependence of sorption was investigated. Results indicated that adsorption dominated at lower temperatures and for smaller MP (10 μm), while absorption was the prevailing process for larger MP (100 μm). Monolayer sorption dominated at lower PAH concentrations, while multilayer sorption dominated at higher concentrations. PE particles representing ingestible (10 μm) and non-ingestible (100 μm) MP for the marine copepod species Acartia tonsa and Calanus finmarchicus were used to investigate the availability and toxicity of MP-sorbed PAHs. Studies were conducted under co-exposure conditions where the PAHs were also present in the dissolved phase (Cfree), thereby representing more environmentally relevant exposure scenarios. Cfree reduction through MP sorption was reflected in a corresponding reduction of lethality and bioaccumulation, with no difference observed between ingestible and non-ingestible MP. This indicates that only free dissolved PAHs are significantly bioavailable to copepods under co-exposure conditions with MP-sorbed PAHs.
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    41. 41
      Jâms, I. B.; Windsor, F. M.; Poudevigne-Durance, T.; Ormerod, S. J.; Durance, I. Estimating the size distribution of plastics ingested by animals. Nat. Commun. 2020, 11, 1594  DOI: 10.1038/s41467-020-15406-6
      41
      Estimating the size distribution of plastics ingested by animals
      Jams, Ifan B.; Windsor, Fredric M.; Poudevigne-Durance, Thomas; Ormerod, Steve J.; Durance, Isabelle
      Nature Communications (2020), 11 (1), 1594CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
      Abstr.: The ingestion of plastics appears to be widespread throughout the animal kingdom with risks to individuals, ecosystems and human health. Despite growing information on the location, abundance and size distribution of plastics in the environment, it cannot be assumed that any given animal will ingest all sizes of plastic encountered. Here, we use published data to develop an allometric relationship between plastic consumption and animal size to est. the size distribution of plastics feasibly ingested by animals. Based on more than 2000 gut content analyses from animals ranging over three orders of magnitude in size (lengths 9 mm to 10 m), body length alone accounts for 42% of the variance in the length of plastic an animal may ingest and indicates a size ratio of roughly 20:1 between animal body length and the largest plastic the animal may ingest. We expect this work to improve global assessments of plastic pollution risk by introducing a quantifiable link between animals and the plastics they can ingest.
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    42. 42
      Verma, J.; Khedkar, V. M.; Coutinho, E. C. 3D-QSAR in drug design--a review. Curr. Top. Med. Chem. 2010, 10, 95– 115,  DOI: 10.2174/156802610790232260
      42
      3D-QSAR in drug design - a review
      Verma, Jitender; Khedkar, Vijay M.; Coutinho, Evans C.
      Current Topics in Medicinal Chemistry (Sharjah, United Arab Emirates) (2010), 10 (1), 95-115CODEN: CTMCCL; ISSN:1568-0266. (Bentham Science Publishers Ltd.)
      A review. Quant. structure-activity relationships (QSAR) have been applied for decades in the development of relationships between physicochem. properties of chem. substances and their biol. activities to obtain a reliable statistical model for prediction of the activities of new chem. entities. The fundamental principle underlying the formalism is that the difference in structural properties is responsible for the variations in biol. activities of the compds. In the classical QSAR studies, affinities of ligands to their binding sites, inhibition consts., rate consts., and other biol. end points, with at., group or mol. properties such as lipophilicity, polarizability, electronic and steric properties (Hansch anal.) or with certain structural features (Free-Wilson anal.) have been correlated. However such an approach has only a limited utility for designing a new mol. due to the lack of consideration of the 3D structure of the mols. 3D-QSAR has emerged as a natural extension to the classical Hansch and Free-Wilson approaches, which exploits the three-dimensional properties of the ligands to predict their biol. activities using robust chemometric techniques such as PLS, G/PLS, ANN etc. It has served as a valuable predictive tool in the design of pharmaceuticals and agrochems. Although the trial and error factor involved in the development of a new drug cannot be ignored completely, QSAR certainly decreases the no. of compds. to be synthesized by facilitating the selection of the most promising candidates. Several success stories of QSAR have attracted the medicinal chemists to investigate the relationships of structural properties with biol. activity. This review seeks to provide a bird's eye view of the different 3D-QSAR approaches employed within the current drug discovery community to construct predictive structure-activity relationships and also discusses the limitations that are fundamental to these approaches, as well as those that might be overcome with the improved strategies. The components involved in building a useful 3D-QSAR model are discussed, including the validation techniques available for this purpose.
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    43. 43
      Cherkasov, A.; Muratov, E. N.; Fourches, D.; Varnek, A.; Baskin, I. I.; Cronin, M.; Dearden, J.; Gramatica, P.; Martin, Y. C.; Todeschini, R.; Consonni, V.; Kuz’min, V. E.; Cramer, R.; Benigni, R.; Yang, C.; Rathman, J.; Terfloth, L.; Gasteiger, J.; Richard, A.; Tropsha, A. QSAR modeling: where have you been? Where are you going to?. J. Med. Chem. 2014, 57, 4977– 5010,  DOI: 10.1021/jm4004285
      43
      QSAR Modeling: Where Have You Been? Where Are You Going To?
      Cherkasov, Artem; Muratov, Eugene N.; Fourches, Denis; Varnek, Alexandre; Baskin, Igor I.; Cronin, Mark; Dearden, John; Gramatica, Paola; Martin, Yvonne C.; Todeschini, Roberto; Consonni, Viviana; Kuzmin, Victor E.; Cramer, Richard; Benigni, Romualdo; Yang, Chihae; Rathman, James; Terfloth, Lothar; Gasteiger, Johann; Richard, Ann; Tropsha, Alexander
      Journal of Medicinal Chemistry (2014), 57 (12), 4977-5010CODEN: JMCMAR; ISSN:0022-2623. (American Chemical Society)
      A review. Quant. structure-activity relationship modeling is one of the major computational tools employed in medicinal chem. However, throughout its entire history it has drawn both praise and criticism concerning its reliability, limitations, successes, and failures. In this paper, we discuss (i) the development and evolution of QSAR; (ii) the current trends, unsolved problems, and pressing challenges; and (iii) several novel and emerging applications of QSAR modeling. Throughout this discussion, we provide guidelines for QSAR development, validation, and application, which are summarized in best practices for building rigorously validated and externally predictive QSAR models. We hope that this Perspective will help communications between computational and exptl. chemists toward collaborative development and use of QSAR models. We also believe that the guidelines presented here will help journal editors and reviewers apply more stringent scientific stds. to manuscripts reporting new QSAR studies, as well as encourage the use of high quality, validated QSARs for regulatory decision making.
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    44. 44
      Primpke, S.; Lorenz, C.; Rascher-Friesenhausen, R.; Gerdts, G. An automated approach for microplastics analysis using focal plane array (FPA) FTIR microscopy and image analysis. Anal. Methods 2017, 9, 1499– 1511,  DOI: 10.1039/C6AY02476A
      44
      An automated approach for microplastics analysis using focal plane array (FPA) FTIR microscopy and image analysis
      Primpke, S.; Lorenz, C.; Rascher-Friesenhausen, R.; Gerdts, G.
      Analytical Methods (2017), 9 (9), 1499-1511CODEN: AMNEGX; ISSN:1759-9679. (Royal Society of Chemistry)
      The anal. of imaging data derived from micro-Fourier transform IR (μFTIR) microscopy is a powerful tool allowing the anal. of microplastics enriched on membrane filters. In this study we present an automated approach to reduce the time demand currently needed for data analyses. We developed a novel anal. pipeline, based on the OPUS Software by Bruker, followed by image anal. with Python and Simple ITK image processing modules. By using this newly developed pipeline it was possible to analyze datasets from focal plane array (FPA) μFTIR mapping of samples contg. up to 1.8 million single spectra. All spectra were compared against a database of different synthetic and natural polymers by various routines followed by benchmark tests with focus on accuracy and quality. The spectral correlation was optimized for high quality data generation, which allowed image anal. Based on these results an image anal. approach was developed, providing information on particle nos. and sizes for each polymer detected. It was possible to collect all data with relative ease even for complex sample matrixes. This approach significantly decreases the time demand for the interpretation of complex FTIR-imaging data and significantly increases the data quality.
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    45. 45
      ECHA; Annex XV Restriction Report– Microplastic, 2019. https://echa.europa.eu/documents/10162/12414bc7-6bb2-17e7-c9ec-652a20fa43fc.
      There is no corresponding record for this reference.
    46. 46
      Connors, K. A.; Dyer, S. D.; Belanger, S. E. Advancing the quality of environmental microplastic research. Environ. Toxicol. Chem. 2017, 36, 1697– 1703,  DOI: 10.1002/etc.3829
      46
      Advancing the quality of environmental microplastic research
      Connors, Kristin A.; Dyer, Scott D.; Belanger, Scott E.
      Environmental Toxicology and Chemistry (2017), 36 (7), 1697-1703CODEN: ETOCDK; ISSN:0730-7268. (Wiley-Blackwell)
      Investigations into the environmental fate and effects of microplastics have been gaining momentum. Small, insol. polymeric particles are implicated by scientists in a wide variety of studies that are used to suggest a potential for widespread impacts in freshwater and marine pelagic and sediment environments. An exponential growth in scientific publications and an increase in regulatory attention have occurred. However, despite these efforts, the environmental hazard of these particles is still unknown. To evaluate the hazard of microplastics within a risk assessment context, we need a way to evaluate the quality of exptl. studies. We performed a thorough review of the quality and focus of environmental microplastic research, to understand the methodologies employed and how this may assist or distract from the ability of environmental risk assessors to evaluate microplastics. We provide guidance to improve the reliability and relevance of ecotoxicol. studies for regulatory and broader environmental assessments. Nine areas of needed improvement are identified and discussed. Important data gaps and exptl. limitations are highlighted. Environ Toxicol Chem 2017;9999:1-7. © 2017 SETAC.
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    47. 47
      Doyle-McCullough, M.; Smyth, S. H.; Moyes, S. M.; Carr, K. E. Factors influencing intestinal microparticle uptake in vivo. Int. J. Pharm. 2007, 335, 79– 89,  DOI: 10.1016/j.ijpharm.2006.10.043
      47
      Factors influencing intestinal microparticle uptake in vivo
      Doyle-McCullough, M.; Smyth, S. H.; Moyes, S. M.; Carr, K. E.
      International Journal of Pharmaceutics (2007), 335 (1-2), 79-89CODEN: IJPHDE; ISSN:0378-5173. (Elsevier Ltd.)
      The aim of this study is to compare microparticle uptake in animals of different ages, gender and species and at different time points. The 2 μm latex/in vivo in situ model uses the observation of animal responses or post-mortem changes and also particle identification by fluorescence microscopy in nine sequential intestinal segments and secondary sites. The wide size range of animals studied requires particle nos. in tissue compartments to be related to intestinal tissue section area through a circumference measurement. Area under the curve data for particles in intestinal tissue are plotted against measurements of intestinal length, allowing comparisons to be made across different ages and species and between males and females. The percentage uptake of administered dose and particle nos. in macerated tissue are also reported. Some parameters, in particular species, do not appear to affect the extent of microparticle uptake, which ranges from 0.12 to 0.32% of the administered dose. Particle uptake does, however, vary with age, being significantly greater in young adult males (7 wk) than in younger (3 wk) and older (17 and 52 wk) age groups. It is concluded that age is more important in detg. the extent of uptake than gender or species.
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    48. 48
      Carr, K. E.; Smyth, S. H.; McCullough, M. T.; Morris, J. F.; Moyes, S. M. Morphological aspects of interactions between microparticles and mammalian cells: Intestinal uptake and onward movement. Prog. Histochem. Cytochem. 2012, 46, 185– 252,  DOI: 10.1016/j.proghi.2011.11.001
      48
      Morphological aspects of interactions between microparticles and mammalian cells: intestinal uptake and onward movement
      Carr Katharine E; Smyth Sharon H; McCullough Melissa T; Morris John F; Moyes Siobhan M
      Progress in histochemistry and cytochemistry (2012), 46 (4), 185-252 ISSN:.
      Uptake of ingested microparticles into small intestinal tissues and on to secondary organs has moved from being an anecdotal phenomenon to a recognised and quantifiable process, which is relevant to risk assessment of accidental exposure, treatment of multi-organ dysfunction syndrome and therapeutic uses of encapsulated drug or vaccine delivery. This review puts in context with the literature the findings of a morphological study of microparticle uptake, using two approaches. The first is a rat in vivo in situ model, appropriate to a study rooted in the exposure of human populations to microparticles. Latex microspheres 2 μm in diameter are the principal particle type used, although others are also investigated. Most data are based on microscopy, but analysis of macerated bulk tissue is also useful. Uptake occurs at early time points after a single dose and is shown to take place almost entirely at villous rather than Peyer's patch sites: however, multiple feeding and therefore a longer time-span produces a higher proportion of particles associated with Peyer's patches, albeit for very small total uptake at those later time points. Uptake is less affected by species, fasting and immunological competence than by age and reproductive status. The second approach uses in vitro methods to confirm the role of intercellular junctions in particle uptake. Particle-associated tight junction opening, in a Caco-2 monolayer, is reflected in changes in transepithelial resistance and particle uptake across the epithelial monolayer: Tight junction opening and particle uptake are both increased further by external irradiation, ethanol and sub-epithelial macrophages, but reduced by exposure to ice. An M cell model has looser tight junctions than Caco-2 cells, but a similar level of particle uptake. These results, along with the changes seen in junctional proteins after particle addition, confirm the role of tight junctions in uptake but suggest that adhering junctions are also important.
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    49. 49
      EFSA Panel on Contaminants in the Food Chain (CONTAM) Presence of microplastics and nanoplastics in food, with particular focus on seafood. EFSA J. 2016, 14, e04501  DOI: 10.2903/j.efsa.2016.4501
      There is no corresponding record for this reference.

  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.est.0c02982.

    • Detailed explanation and example calculation for ECX,poly and the effect concentration for environmental microplastic ECX,env; ingestible size ranges (Table S1); original and rescaled effect threshold concentrations as used in Figures 3A,B, respectively (Table S2); sensitivity analysis for exponent α (Figure S1); and bioavailable fractions of number concentrations and mass concentrations for 11 studied species (Figure S2) (PDF)


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