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RESEARCH ARTICLE
Microplastic pollution in deposited urban dust,
Tehran metropolis, Iran
Sharareh Dehghani
1
&Farid Moore
1
&Razegheh Akhbarizadeh
1
Received: 13 April 2017 /Accepted: 27 June 2017
#Springer-Verlag GmbH Germany 2017
Abstract Environmental pollutants such as microplastics
have become a major concern over the last few decades. We
investigated the presence, characteristics, and potential health
risks of microplastic dust ingestion. The plastic load of 88 to
605 microplastics per 30 g dry dust with a dominance of black
and yellow granule microplastics ranging in size from 250 to
500 μm was determined in 10 street dust samples using a
binocular microscope. Fluorescence microscopy was found
to be ineffective for detecting and counting plastic debris.
Scanning electron microscopy, however, was useful for accu-
rate detection of microplastic particles of different sizes,
colors, and shapes (e.g., fiber, spherule, hexagonal, irregular
polyhedron). Trace amounts of Al, Na, Ca, Mg, and Si, de-
tected using energy dispersive X-ray spectroscopy, revealed
additives of plastic polymers or adsorbed debris on
microplastic surfaces. As a first step to estimate the adverse
health effects of microplastics in street dust, the frequency of
microplastic ingestion per day/year via ingestion of street dust
was calculated. Considering exposure during outdoor activi-
ties and workspaces with high abundant microplastics as acute
exposure, a mean of 3223 and 1063 microplastic particles per
year is ingested by children and adults, respectively.
Consequently, street dust is a potentially important source of
microplastic contamination in the urban environment and con-
trol measures are required.
Keywords Microplastic .Potential health risk .Street dust .
Ingestion .Tehr a n
Introduction
Plastic production has been growing for more than
60 years, as the durable, primarily petroleum-based mate-
rial gradually replaces materials like glass and metal.
Nowadays, plastics are used extensively in a growing
range of applications such as packaging industry, building
and construction, automotive industry, textiles, electrical
and electronics, agriculture, household applications,
health, and safety equipment (Björkner, 1995; Gallagher
et al., 2016). Correspondingly, in recent decades, great
attention has been paid to pollution resulted by
microplastics—to plastic debris smaller than 5 mm—
whichisanemergingconcern(Arthuretal.2008). Cole
et al. (2011) defined primary and secondary microplastics.
Primary microplastics (e.g., personal care, cosmetic and
cleaning products, pre-production pellets, pharmaceuti-
cals, and air blasting media) are initially manufactured
in microscopic size, while secondary microplastics are
originated from the fragmentation of coarse plastic debris
(Akhbarizadeh et al., 2017).
Responsible editor: Philippe Garrigues
Capsule: Street dust is a potential source of microplastic contamination in
the urban environment and poses a potential health risk to inhabitants via
inadvertent ingestion.
Electronic supplementary material The online version of this article
(doi:10.1007/s11356-017-9674-1) contains supplementary material,
which is available to authorized users.
*Sharareh Dehghani
Sh.Dehghani@shirazu.ac.ir
Farid Moore
Moore@susc.ac.ir
Razegheh Akhbarizadeh
akhbarizade@shirazu.ac.ir
1
Department of Earth Sciences, College of Sciences, Shiraz
University, Shiraz 71454, Iran
Environ Sci Pollut Res
DOI 10.1007/s11356-017-9674-1
Microplastics are widespread in marine environment from
the coastal areas to the deep seas and oceans (Thompson
2015). Microplastics are small enough to be ingested by ma-
rine fauna (Thompson 2016). This micro-sized debris can also
absorb pollutants (organic and inorganic) from water or soil
solution on their surfaces and intensify the potential health
risks due to their ingestion or inhalation (Rillig, 2012).
Hence, the majority of researches have focused on
microplastics occurrence, abundances, and toxic effects on
aquatic biota in sediment and water samples using different
sampling methods and analysis techniques (Imhof et al., 2012;
Dubaish and Liebezeit, 2013; Van Cauwenberghe et al., 2013;
Barasarathi et al., 2014).
In contrast to the marine ecosystem, little attention has been
paid to microplastics in various compartments of the terrestrial
ecosystems, such as the urban environment. On the other
hand, it contains major potential sources of microplastics such
as vehicles abrasion (mainly tire treads), construction activi-
ties, synthetic clothes washing, and eolian transport of plastic
debris (Magnusson et al., 2016). Moreover, road channelizing
devices (traffic cones, drums, and barrels) as well as speed
bumps, which are manufactured using recycled PVCs from
bottles, intensify microplastics pollution in urban areas.
Plastics constitute 10% of worldwide municipal waste
(Barnes et al., 2009). In developing countries, a small propor-
tion of plastic waste is recycled and the majority of waste is
dumped in open soil waste landfills where plastic waste is
exposed to sunlight, microbes, and atmosphere.
Furthermore, plastic wastes experience the combination of
biodegradation, photo-degradation, and thermo-oxidative
and thermal degradation processes (Andrady, 2011;Cole
et al., 2011)aswellasmechanicalabrasioninlandfillsthat
enhance the chance of microplastic production by crumbling
coarse plastic fragments. These parameters also influence
microplastics settled in the road-deposited dust and urban sur-
face soil. Children may ingest deposited dust through
mouthing dirty toys and hands and consuming soil directly
(Ljung et al., 2006). Moreover, microplastics are light enough
to re-suspend from landfills and urban surfaces by wind cur-
rents and traffic activities and eventually contribute to atmo-
spheric particulate load (Ho et al., 2003). Consequently, par-
ticular concerns are microplastics in the road-deposited dust
that enters the human body via ingestion. A few studies have
been made on microplastic contamination of urban compart-
ments. Dris et al. (2015) highlighted the presence of
microplastics in atmospheric fallout (29–280 particles
m
−2
day
−1
) for the first time. They used a funnel in a glass
bottle on the rooftop of buildings to collect samples. Yang
et al. (2015) emphasized the presence, abundances, sources,
pathways, and health effects of microplastics in the urban
environment in an issue of a series titled Bcontaminants in
the urban environment.^Dris et al. (2017) investigated textile
fibers, including microplastics, in 3 indoor and 1 outdoor air
samples of sampling stations about 10 km far from Paris city.
The deposition rate of the fibers and their concentration in
deposited dust collected from vacuum cleaner bags were also
estimated. They found that outdoor concentration of
microplastics ranged between 0.3 and 1.5 fibers per cubic
meter. However, the estimation of the potential risks to human
health posed by inhaled and ingested dust containing
microplastics are not provided yet.
It is hypothesized that abundant sources of microplastics in
urban areas result in high microplastic abundance in urban
sinks such as surface soil, deposited road dust, and rivers. In
addition, denser population generates more plastic waste and
litter (Eriksen et al., 2013). Tehran metropolis, the capital of
Iran, with 8.2 M population, includes more than 7000 indus-
trial units (Tehran Municipality 2015). Packaging industry is
the greatest consumer of plastic polymers in Iran. According
to statistics of 2015 reported by Iranian Department of
Environment (IDoE), 7500 t of waste is generated daily in
the Tehran metropolis and 1 t of the generated waste per day
include plastic materials. Hence, the IDoE is working on de-
veloping bylaws to ban usage of plastic bags in the next 3 years
and cut down on the plastic litter in Iran. Recycling of poly-
meric materials is quite limited nowadays due to a limited
participation of inhabitants (<20%) to the source separation
of waste materials. Therefore, our study was designed to (1)
quantify the accumulation of microplastics in the deposited
dust on street surfaces of the central district of the Tehran
metropolis—densely populated, (2) investigate the character-
istics of microplastics in investigated street dusts, and (3) es-
timate microplastic intake to the human body through inges-
tion route.
Materials and methods
Sample collection
Ten street dusts were collected from the central district of
Tehran (Fig. 1). A 2-month dry spell preceded the sampling
campaign. Street sweeping operation runs every night to re-
move litters but not deposited dust. There is no scheduled
program for street washing in Tehran. Since sampling depos-
ited street dust using passive dust collectors is not possible,
each street dust sample was collected by gently sweeping an
area of about 30 m
2
adjacent to the curb of the two sides of the
road. This method has been used frequently for street dust
sampling by many authors (e.g., Zheng et al., 2015; Patinha
et al., 2015). A local anti-static wooden brush, which is made
from dry stems of Sorghum bicolor plant species, and a me-
tallic pan were used to sweep and collect samples to avoid
plastic contamination as well as their ease of cleaning after
collecting each sample. Then, the sub-samples were mixed
thoroughly to obtain a bulk representative sample. At each
Environ Sci Pollut Res
sampling station, more than 1 kg composite bulk sample was
collected. Each collected sample was then transferred to the
laboratory, and extraneous matter such as paving stone and
asphalt, small pieces of brick and concrete, cigarette filters,
leaves, and other debris were removed from the samples. The
samples were sieved through a 5-mm sieve, to remove other
coarse debris. To prevent errors resulted by remained particles
on road surface and disturbance of particles to the ambient air
throughout the sweeping operation, the concentration of
microplastics was presented by the number of microplastic
debris per 30 g mass of dust instead of the number of
microplastics per square meter of the street surface.
Sample pre-treatment
Nuelle et al. (2014) found that 30% H
2
O
2
solution is an ideal
reagent for removing about 50% of biogenic organic matter in
1 week. Street dusts (<5 mm) were analyzed for organic mat-
ter (OM) content gravimetrically by weight loss of the oven-
dried samples (105 °C) after ignition at 550° (Heiri et al.,
2001). Organic matter content of the selected samples varied
from 6.7 to 20.5% with the average of 11.4%. As a conse-
quence, 30 ± 0.5 g of each composite dust sample was treated
for 8 days with 100 ± 0.5 ml of 30% H
2
O
2
to remove organic
debris before microplastic separation. Five hundred-milliliter
glass beakers were used to avoid losing microplastics due to
the reaction. Twenty minutes after H
2
O
2
addition, a relatively
intense reaction occurred, and bubbles were formed and im-
ploded continuously until the end of the seventh day.
Afterwards, samples were vacuum-filtered through S&S
filter paper (blueband, grade 589/3, 2 μm pore size) and
washed using deionized water to eliminate remained H
2
O
2
on dust particles and to collect particles that are adhered
to the beaker wall. Samples were dried properly in a sand
bath at 50 °C for 8 h.
Microplastic separation
Flotation with the saturated salt solution is a common method
of microplastic separation from mineralogical dust with higher
density (2.65 g cm
−3
). Löder and Gerdts (2015)highlyrecom-
mended ZnCl
2
solution to float microplastics due to its high
efficiency of dense microplastic separation as well as financial
and environmental reasons. The zinc chloride solution with a
density of 1.78 ± 0.2 kg L
−1
was used in this study as the
separation fluid. To prepare ZnCl
2
solution, 1120 ± 0.5 g of
powder anhydrous Zncl
2
(Sigma Aldrich, ACS reagent grade,
≥97%) was dissolved in 700 ± 0.5 ml deionized water through
extreme exothermic reaction and its volume increased up to
1050 ml. The solution had to be gently agitated and after
cooling down to room temperature, it was filtered through
an S&S filter paper (blueband, grade 589/3, 2 μmporesize)
to remove undissolved salt minerals and impurities.
One hundred milliliters of prepared ZnCl
2
solution was
mixed with each sample and agitated properly for at least
5 min to separate particles that stick together. Afterwards,
samples were kept untouched overnight. The supernatant,
which includes floating microplastic particles, was poured
on filter papers gently and ZnCl
2
solution was recycled by
pressure filtration and was used for the next separation step
of the same sample as recommended by Löder and Gerdts
(2015). The described first step was repeated three times on
the remaining dust to achieve the highest possible recovery
rate (Browne et al., 2011). Filter papers were air dried for 24 h
Fig. 1 Spatial distribution of street dust samples (black dots) within Tehran metropolis as well as Tehran regions
Environ Sci Pollut Res
and microplastics were transferred to glass petri dish using a
clean non-plastic brush.
All glassware and equipment were properly cleaned, acid
washed, and rinsed with deionized water prior to use. The use
of plastic material and vessels were avoided. Cotton laborato-
ry coats and gloves were worn during all experiments. Two
samples were done in duplicate. A blank sample was prepared
similarly to the experimental samples. Moreover, an empty
petri dish was fixed on the laboratory bench at the step to
detect airborne microplastic contamination in the laboratory.
The results showed no contamination in the blank samplesand
control petri dish.
Microplastic detection and quantification
Fluorescent pigments, dyes, and optical brightening or whit-
ening agents are widely used in plastic and synthetic textile
industries (Christie, 1994). Hence, in the current study, fluo-
rescence as an identifying fingerprint for extracted
microplastics was tested using Fluorescence Microscope
model Olympus CX31 with ×100 magnification under the
ultra violet filter. The images were taken by Olympus Pen E-
PL 1 manual digital camera which was installed on the micro-
scope lens.
Microplastics were also detected and described based on
their visual properties such as shape, size, color, and degrada-
tion stage (Hidalgo-Ruz et al., 2012) using a binocular micro-
scope with up to ×200 magnification. All observations were
performed by the same operator, and features such as equal
thickness, non-organic surface structure, homogenous color,
non-shininess as well as presenting specific elasticity, and
hardness—tested by applying tweezers and probe—were used
to recognize microplastics (Hidalgo-Ruz et al., 2012; Dris
et al., 2015). Several studies described the morphological
characteristics and elemental composition of microplastics
by assessing scanning electron microscopy (SEM), energy
dispersive X-ray spectroscopy (EDX), and environmental
SEM-EDX technique (Eriksen et al., 2013; Vianello et al.,
2013; Van Cauwenberghe et al., 2013 and Fries et al., 2013).
Quantitative measurements of particle surface areas through
image-processing programs could be misleading (Nuelle
et al., 2014). Since testing all suspected particles is time con-
suming and expensive, the topography of a total of 20 proba-
ble microplastic particles from typical representatives of dif-
ferent shapes, sizes, and colors were chosen to analyze quan-
titatively through SEM. These samples were also examined
for the composition of particles using an EDX detector.
Samples were fixed on a 10-mm diameter cylindrical SEM
stub and coated with a thin conductive layer of gold before
analysis and viewed with an accelerating voltage up to 8.7 kV.
Afterwards, the microplastic counting was performed with
the aid of visual and physical properties of confirmed
microplastic particles using a binocular microscope with up
to ×200 magnification. Furthermore, a graph paper was fixed
under the petri dishes including samples and counting was
performed row by row, for 2 days for each sample. The length
of microplastics was estimated using scale bars and probes
with certain width. They were classified into five categories:
very long (1 mm ≤L), long (500 μm≤L<1mm),middle
(250 μm≤L < 500 μm), short (100 μm≤L < 250 μm), and
very short (L ≤100 μm) class. The microplastics of less than
50 μm in size were difficult to be counted and tested by
probes. Microplastic particles were categorized into three
groups based on their shape: fragment, fiber, and sphere/
pellet.
The primary estimation of microplastic intake
via ingestion
The number of ingested microplastics per day/year was calcu-
lated considering the recommended 200 and 100 mg day
−1
values by USEPA (2000) as the mean particle ingestion rate
for children (between 1 and 6 years) and adults, respectively.
These rates were established for a normal exposure scenario,
whereas for acute exposure to children, 1 g per Boutdoor^day
was suggested. Soil screening guidance recommended
330 mg day
−1
ingested soil to construction or outdoor workers
(Harris and Harper, 2004). This data along with mean value of
200 work days per year was used to calculate intake of
microplastics in a year for occupational exposure.
Results and discussion
Microplastic detection, characteristics, and abundance
in street dusts
Fluorescent particles were detected in all samples. The major-
ity of visible particles included fluorescent fragments and fi-
bers, followed by rare fluorescent spherules and pellets. They
were displayed in blue luminous color and a wide range of
shininess (Fig. 2) depending on the intensity of weathering
and degradation they have been through as well as the content
of fluorescent agents in their composition. No fluorescent par-
ticle was found in the blank sample and fixed petri dish on
laboratory bench.
Fluorescence properties of microplastic particles have been
widely used to detect ingested microplastics in gut contents
and excretions of marine and freshwater fauna as well as
transparent planktonic organisms (e.g., Cole et al., 2013;
Imhof et al., 2013). The only research that quantified abun-
dance of microplastics in inorganic and non-biological phase
using fluorescence microscopy was performed by Qiu
et al.(2015) on sediments of the Beibu Gulf, China. In contrast
to Qiu et al. (2015) research, fluorescent microscopy did not
provide satisfactory results of microplastic abundance in this
Environ Sci Pollut Res
study, due to the high interface of other materials to
microplastics. Fluorescence agents are frequently used in pa-
per that may cause interference with microplastic identifica-
tion in street dust samples. Moreover, fluorescence agents are
not added to all plastic products (Managaki and Takada,
2005). The fluorescence property of several minerals specifi-
cally calcite, albite, and hemimorphite (Robbins, 2013)—
which were abundant in Tehran street dusts (Dehghani et al.,
2017)—and glow blue under the ultraviolet light, was another
source of error to detect microplastics using fluorescence mi-
croscopy. Furthermore, splitting and transferring samples to
thin glass slides as a preparation step for fluorescence micros-
copy can cause loss of sample. As a consequence, SEM anal-
ysis was used to detect microplastic particles accurately.
Microplastic particles displayed smooth surface textures
(Fig. 3b–d), same as silicate glass spherules (Fig. 3a). High
reflection and refraction of light together with shininess and
transparency of glass micro-beads aid to distinguish them
from microplastic spherules under the binocular light micro-
scope. These glass beads are widely used in road signs, traffic
paint, and pavement marking to provide reflectivity.
Hexagonal microplastic fragments (Fig. 3c) observed in a va-
riety of colors such as light to dark blue, yellowish white, and
transparent (Fig. 4e, f). The EDX analysis indicated trace
amount of Al, Ca, Mg, Na, and Si indicated additives of plas-
tic polymers or adsorbed debris on the microplastic surface
(Fig. 3). Bolgar et al. (2015) has provided a list of all polymer
adjunct materials and described their occurrence, toxicity, and
analytical methods. According to those researches, many
types of complex blend of materials are introduced to essential
polymers to provide special characteristics. For example, an-
tioxidants including Al, Ca, Mg, Na, and Si are used in most
hydrocarbon polymers (e.g., polyethylene, polypropylene,
polystyrene) to slow down the oxidation cycle.
Figure 4illustrates the optical microscopic image of a few
microplastic samples in different shapes, sizes, and colors.
The total of 2649 microplastic particles was detected in 10
street dust samples. The minimum and maximum concentra-
tion of microplastics ranged from 83 ± 10 particles/30 g dry
dust (sample 6) to 605 ± 10 particles/30 g dry dust (sample 1),
respectively (Table S1). Samples 1, 9, 3, 2, and 10 included
more than 200 microplastics/30 g dust (605, 577, 387, 271,
and 213 microplastics/30 g dust, respectively), whereas less
than 200 microplastics/30 g dust was detected in samples 5, 4,
8, 7, and 6 (153, 148, 112, 95, and 88 microplastics/30 g dust,
respectively). Relative standard deviation of counted
microplastics was 73.4%, indicating high heterogeneity
among street dust samples. Furthermore, two sub-samples
were derived from random composite samples (n= 2) and
their microplastics were counted. The calculated coefficient
of variation for duplicate samples was 9–11%, indicating the
heterogenic nature of each dust sample.
The correlation analysis between the total abundance of
microplastics and number of microplastics in different shape
classes was conducted using a Pearson correlation.
Microplastic abundance showed significant positive
200
µ
m
(a)
200
µ
m
(d)
200
µ
m
(e)
200
µ
m
(b)
200
µ
m
(c)
200
µ
m
(f)
Fig. 2 Fluorescence micrographs of atwo fluorescent fiber, ba fluorescent fragment, and cfluorescent bead or sphere. dA fiber with low fluorescence,
ea fiber with medium fluorescence, and fa high fluorescence granule. These particles are probable microplastics
Environ Sci Pollut Res
correlation with granule numbers (r=1.00,p< 0.01), which
implied that the granule number increase in a linear relation-
ship with increasing total microplastic abundance. Abundance
of microplastic fibers were also positively correlated with the
total number of microplastics (r=0.69,p<0.01),whereasno
significant correlation was found between microplastic and
spherule/pellet abundance.
Microplastic size ranged from less than 100 up to 5 mm.
Plastic particles of 250 to 500 μm occupied 33.7% of total
plastics by number, followed by microplastics of greater than
1000 μm with a mean occurrence of 25.7% and a variation of
15.4 to 30.4% of the total microplastics in dust samples. The
mean occurrence of the 100–250 μm fraction (15.2%) was
approximately half that of the 250 to 500 μm fraction. The
average number of microplastics in the <100 μmcategorywas
lower than other classes. It may be caused by the underesti-
mation of small microplastics (<50 μm in size) as their detec-
tion is difficult under the light microscope. Fibrous (16.9–
44.3%, 33.5 on average) and granule microplastics (54.5–
82.2%, 65.9 on average) were the most abundant shapes in
street dusts (Fig. 5). Nine spherules and 2 pellets were record-
ed in all samples. The black (29.9%, on average), yellow
(26.4%, on average), and transparent microplastics (17.3%,
on average), followed by blue (12.6%, on average), red
(10.2%, on average), and green (3.6%, on average) were the
most abundant colors. Moreover, a small number of brown,
pink, and orange microplastics were observed.
Principle component (PC) biplot was used to differentiate
samples. The first two PCs indicated 81.4% of the variance
within the dataset (Fig. 6) and revealed that the collected
microplastics at stations 1, 3, and 9 differ from other stations
as far as the shape and color are concerned. It is also notable
that stations 2, 4, 5, and 7 plot differently due to their lower
microplastic density.
While Fourier-transform infrared (FT-IR) (Hidalgo-Ruz
et al., 2012;Songetal.,2015), pyrolysis-gas chromatogra-
phy-mass spectrometry (Pyr-GC-MS) (Dekiff et al., 2014)
and Raman spectroscopy (Horton et al., 2017) are becoming
wt%
C10.82
O 35.52
Na 5.13
Mg 1.76
Si 34.97
Ca 11.80
(
a
)
wt%
C65.79
O19.52
Na 8.62
Si 6.07
(
b
)
wt%
C58.32
O31.41
Na 2.78
Al 4.72
Si 2.00
Ca 2.77
(
c
)
wt%
C62.35
O33.46
Al 4.25
Na 2.24
Ca 2.21
(
d
)
Fig. 3 SEM images and EDAX analyses result of athe smooth silicate
glass spherule with a few pits on its surface and 600 μmdiameter,b
microplastic fiber with 2 mm length, chexagonal fragment of 500 μm
diameter observed blue under binocular microscope, and dMP fragment
with longest diameter of 30 μm
Environ Sci Pollut Res
more widely used in the identification of extracted
microplastics, such facilities were not available to this study.
Dekiff et al. (2014) and Hidalgo-Ruiz et al., (2012)confirmed
chemical composition of microplastics using pyrolysis-gas
chromatography-mass spectrometry (Pyr-GC-MS) and
Fourier transform infrared spectroscopy (FT-IR), respectively.
They found that only 47% (of a subsample of 32 particles) and
30% of optically identified particles are confirmed as common
microplastic polymers. However, these techniques also in-
clude several drawbacks (Horton et al., 2017).
Rocha-Santos and Duarte (2015) listed SEM-EDX as a
useful analytical technique to assess the surface morphology
and the composition of microplastics. In this study, chemical
verification performed on representative samples of
microplastics in all classes (shape, size, and color) using
SEM-EDX. Nevertheless, the final decision on confirming a
particle as microplastic based on visual similarities to those
confirmed microplastics have been made by the operator.
Although this method was economic in comparison with test-
ing all of the potential plastic particles, it results in errors and
lead in inaccurate estimation of microplastic abundance.
Furthermore, natural organic fibers (e.g., cotton, wool), which
are used in large quantities in the production of paper products
and textiles (Järvholm 2000), as well as inorganic fibers (e.g.,
asbestos and other fibrous minerals), possibly exist in depos-
ited dust samples. These fibers are a major source of error
because it is difficult to discriminate them from plastic fibers
using their visual features. Tire dust is also a significant pol-
lutant in street dust (Adachi and Tainosho 2004)inthatits
manufactured density (~1.52 g cm
−3
) is low enough to be
200 µm
(b)
200 µm
(a)
100 µm
(e)
200 µm
(d)
200 µm
(i)
500 µm
(g)
200
µ
m
(h)
100 µm
(c)
100 µm
(f)
Fig. 4 The optical microscope image of microplastics. aTransparent
fiber, bred fiber, ctransparent sphere, dgreen granule, e,fblue
hexagonal fragments, gwhite granule covered with silver shiny film, h
blue granule, and isample of microplastics fragments with two colors. In
such cases, the observer recorded the color which covers most of the
surface area
Environ Sci Pollut Res
suspended by the prepared ZnCl
2
solution. These errors ac-
companied by the difficulty in detection of <50-μm
microplastics using probe may obtain inaccurate abundance
of microplastics especially regarding the number of fibers.
Estimation of microplastic intake
Risk from microplastics in the street dust usually results from
their likelihood to enter the groundwater by urban runoff and
eventually to enter the aquatic food chain. Microplastics have
lower density (0.8–1.4 g cm
−3
)incomparisonwithmineral-
ogical dust (2.65 g cm
−3
) (Hidalgo-Ruz et al., 2012). Density
of rain drops and the initial urban runoff increase significantly
after dissolving soluble constitutes of street dust such as
mineral salts. As a consequence, the majority of microplastics
floats and transports more easily than do mineralogical parti-
cles and anthropogenic metallic particles; hence, concentrated
microplastics reach aquatic basements and pose ecological
risks as well as health risks to aquatic fauna and flora.
In addition, the proximity of the deposited dusts to humans
in urban environment results in direct ingestion and inhalation
of dust particles and consequently microplastics therein to the
human body (Abrahams, 2002). This is particularly impor-
tant for those potentially vulnerable segments of the pop-
ulation, such as children, that their digestion systems are
more susceptible to negative health effects of environmen-
tal contaminants (Leotsinidis et al., 2005;Mielkeetal.,
1999; Rojas-Bracho et al., 2002). Moreover, children
0% 20% 40% 60% 80% 100%
1
2
3
4
5
6
7
8
9
10
Sample Number
L<100
100≤L<250
250≤L<500
500≤L<1000
1000≤L
Size category (μm)
0% 20% 40% 60% 80% 100%
1
2
3
4
5
6
7
8
9
10
Sample Number
Yellow, Orange
Black, Grey
White, Transparent
Green
Blue
Red, Pink
0% 20% 40% 60% 80% 100%
1
2
3
4
5
6
7
8
9
10
Sample Number
Fiber
Granule
Sphere
(a)
(b)
(c)
Fig. 5 Abundance distribution
microplastics (in % of the total
number of microplastics) within a
size, bcolor, and cshape
categories
Environ Sci Pollut Res
ingest the street dust inadvertently through mouthing dirty
hands and objects and in some cases consuming soil di-
rectly (Ljung et al., 2006). Construction workers and oth-
er people, who work in outdoor spaces, are another group
of people who are exposed to street dust directly.
To estimate the potential health risk resulted by ingested
microplastics, the number of daily and yearly ingested
microplastics should be estimated. Several attempts have been
made to estimate the amount of total dust that people ingest
per day, using calculations based on assumptions regarding
Fig. 6 Principal component (PC)
biplot of the sampling stationsand
microplastics (shape and color) in
Tehran street dusts
Tabl e 1 Estimated daily and yearly intake of MPs to children and adults in normal and acute exposure scenarios
Sample
number
Organic
matter (%)
MPs abundance
(particles/30 g street dust)
Number of ingested MPs
Adults Childs
Normal exposure
(100 mg day
−1
)
a
Acute exposure
(330 mg day
−1
)
a
Normal exposure
(200 mg day
−1
)
a
Acute exposure
(1000 mg day
−1
)
a
Per day Per year Per day Per year Per day Per year Per day Per year
1 12.8 605 2.0 736 6.7 2429 4.0 1472 20 7361
2 12.4 271 0.9 329 3.0 1088 1.8 659 9 3297
3 12.3 387 1.3 470 4.3 1553 2.6 941 13 4709
4 11.6 148 0.5 180 1.6 594 1.0 360 5 1801
5 14.2 153 0.5 186 1.7 614 1.0 372 5 1862
6 9.8 88 0.3 107 1.0 353 0.6 214 3 1071
7 11.8 95 0.3 115 1.0 381 0.6 231 3 1156
8 9.2 112 0.4 136 1.2 449 0.7 272 4 1363
9 9.8 577 1.9 702 6.3 2316 3.8 1404 19 7020
10 10.6 213 0.7 259 2.3 855 1.4 518 7 2592
a
Mean bulk dust ingestion rate
Environ Sci Pollut Res
dust loading on hands, the frequency of hand-to-mouth trans-
fer, level of moisture on hands, and efficiency of transfer
(Kissel et al., 1998; Yamamoto et al., 2006). Table 1presented
the calculated daily and yearly microplastic intake via street
dust ingestion regarding normal and acute exposure scenarios,
obtained from the MPI data in Table S1 and recommended
daily exposure rate of bulk dust. The estimated intakes of
microplastics were in the range of 107 to 736 particles per
year for adults in normal exposure scenario and somewhat
higher (353–2429 particle year
−1
with mean of 1063 particle
year
−1
) in acute exposure to adults. Six hundred forty-four and
3223 particles per year are ingested by each child in normal
and acute exposure scenarios via ingestion, respectively. The
worst case exposure for both children and adults was sample
no. 1 with 605 counted microplastics in 30 g dry dust sample.
Yearly acute intake of microplastics for children was approx-
imately three times higher than that of adults, implying the
higher potential risk of microplastic ingestion to children.
In addition to microplastic concentration, particle size frac-
tions could greatly influence the results of ingestion risk as-
sessments, assuming that microplastic behavior in sticking to
hands is the same as other dust particles. Previous studies have
confirmed that dust fine particles, with a modal size around
250 μm, can adhere more likely than bulk dust to the surface
of skin. Finer particles (<50 μm) tend to be potentially
ingested involuntarily (Choate et al., 2006; Davis et al.,
1993; Siciliano et al., 2009; Yamamoto et al., 2006). In the
current study, microplastics were most frequent in 250 to
500 μm size class with a mean occurrence of 33.7%,
supporting the high probability of intake of microplastics by
inhabitants. It is also important to take into account that plastic
fragments tend to adsorb trace metals (i.e., Cu, Zn, Fe, Cd, Pb,
and Ni) and organic pollutants on their surfaces (Holmes et al.,
2012;Holmesetal.,2014; Rochman et al., 2013).
To date, there have been a few researches on the negative
health effects of selected plastic constituents on the human
body. For example, Koch and Calafat (2009) monitored hu-
man body burdens of two common chemicals used in plastic
manufacture: phthalates and bisphenol A phthalates (BPA).
Nevertheless, there is currently no available information on
toxicology and digestion of other plastic additives as well as
various plastic polymers in the human body.
Conclusion
Although plastic material has been in commerce for decades
in great cities, microplastic abundance in urban-deposited dust
has not been investigated properly. In this study, the occur-
rence and abundance of microplastics in the Tehran street dust
was investigated. Fluorescent microscopy was recognized in-
efficient to detect microplastics and obtain their abundance in
street dust samples. Zinc chloride was found to be a recyclable
solution, and its density was high enough to extract most of
the plastic debris from urban-deposited dust. Suspected repre-
sentative samples were efficiently and precisely detected
using scanning electron microscopy, though minor errors oc-
curred due to the lack of testing all samples. Since it is not
possible to detect and examine microplastics of below 50 μm
using probe, the abundance and properties of microplastics is
still inaccurate. Microplastics of below 50 μm are the most
susceptible size fraction to be ingested or re-suspended to the
atmospheric load and thereby being inhaled.
The estimated daily and yearly abundance of microplastic
intake to different age groups of humans in normal and acute
exposure scenarios were investigated in order to provide pri-
mary knowledge of health risk assessment of being exposed to
the microplastics. However, additional research is needed to
identify microplastic exposure rates, adherence to skin and
toxicokinetic information of microplastic ingestion as well
as the relative contributions of different sources and pathways
by which urban inhabitants are exposed to microplastics.
Furthermore, there is a necessity to paying special attention
to educational programs for the inhabitants in conjunction
with the replacement of plastic by cost-effective materials
and develop recycling programs to enhance plastic recycling
and reduce its usage and thereby the negative health effects of
microplastics in human population.
Acknowledgements The authors are grateful from Shiraz University
research council support. The authors also greatly thank Mrs. M.
Gholizadeh, the laboratory expert of the Department of Biology at
Shiraz University, for the assistance in fluorescence microscopy.
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