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Wear and tear from tyres significantly contributes to the flow of (micro-)plastics into the environment. This paper compiles the fragmented knowledge on tyre wear and tear characteristics, amounts of particles emitted, pathways in the environment, and the possible effects on humans. The estimated per capita emission ranges from 0.23 to 4.7 kg/year, with a global average of 0.81 kg/year. The emissions from car tyres (100%) are substantially higher than those of other sources of microplastics, e.g., airplane tyres (2%), artificial turf (12–50%), brake wear (8%) and road markings (5%). Emissions and pathways depend on local factors like road type or sewage systems. The relative contribution of tyre wear and tear to the total global amount of plastics ending up in our oceans is estimated to be 5–10%. In air, 3–7% of the particulate matter (PM2.5) is estimated to consist of tyre wear and tear, indicating that it may contribute to the global health burden of air pollution which has been projected by the World Health Organization (WHO) at 3 million deaths in 2012. The wear and tear also enters our food chain, but further research is needed to assess human health risks. It is concluded here that tyre wear and tear is a stealthy source of microplastics in our environment, which can only be addressed effectively if awareness increases, knowledge gaps on quantities and effects are being closed, and creative technical solutions are being sought. This requires a global effort from all stakeholders; consumers, regulators, industry and researchers alike.
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International Journal of
Environmental Research
and Public Health
Review
Wear and Tear of Tyres: A Stealthy Source
of Microplastics in the Environment
Pieter Jan Kole 1, Ansje J. Löhr 1, Frank G. A. J. Van Belleghem 1,2 and Ad M. J. Ragas 1 ,3,*
1
Department of Science, Faculty of Management, Science & Technology, Open University of The Netherlands,
6419 AT Heerlen, The Netherlands; PJ.Kole@studie.ou.nl (P.J.K.); Ansje.Lohr@ou.nl (A.J.L.);
Frank.vanBelleghem@ou.nl (F.G.A.J.V.B.)
2Zoology: Biodiversity and Toxicology, Centre for Environmental Sciences, Hasselt University,
BE 3590 Diepenbeek, Belgium
3Institute of Water and Wetland Research, Faculty of Science, Radboud University Nijmegen,
6525 AJ Nijmegen, The Netherlands
*Correspondence: Ad.Ragas@ou.nl; Tel.: +31-24-365-3284
Academic Editor: A. Dick Vethaak
Received: 31 July 2017; Accepted: 16 October 2017; Published: 20 October 2017
Abstract:
Wear and tear from tyres significantly contributes to the flow of (micro-)plastics into the
environment. This paper compiles the fragmented knowledge on tyre wear and tear characteristics,
amounts of particles emitted, pathways in the environment, and the possible effects on humans.
The estimated per capita emission ranges from 0.23 to 4.7 kg/year, with a global average of
0.81 kg/year. The emissions from car tyres (100%) are substantially higher than those of other
sources of microplastics, e.g., airplane tyres (2%), artificial turf (12–50%), brake wear (8%) and road
markings (5%). Emissions and pathways depend on local factors like road type or sewage systems.
The relative contribution of tyre wear and tear to the total global amount of plastics ending up in
our oceans is estimated to be 5–10%. In air, 3–7% of the particulate matter (PM
2.5
) is estimated to
consist of tyre wear and tear, indicating that it may contribute to the global health burden of air
pollution which has been projected by the World Health Organization (WHO) at 3 million deaths in
2012. The wear and tear also enters our food chain, but further research is needed to assess human
health risks. It is concluded here that tyre wear and tear is a stealthy source of microplastics in our
environment, which can only be addressed effectively if awareness increases, knowledge gaps on
quantities and effects are being closed, and creative technical solutions are being sought. This requires
a global effort from all stakeholders; consumers, regulators, industry and researchers alike.
Keywords: tyre wear and tear; microplastics; particulate matter; tyre rubber
1. Introduction
The global production of thermoplastics has grown rapidly since the start of its large-scale
production around the 1950s, reaching 322 million tonnes/year in 2015 [
1
]. The different varieties of
polymers produced have unique characteristics when compared to traditional materials, in particular in
terms of durability, production costs, weight, strength, flexibility and limited electric conductivity. As a
result, plastics are used increasingly in many sectors such as construction, transportation, household
goods and packaging. Nowadays, the market of thermoplastics is dominated by four main classes of
plastics, being polyethylene (PE; 73 million tonnes in 2010), polyethylene terephthalate (PET; 53 million
tonnes in 2010), polypropylene (PP; 50 million tonnes in 2010) and polyvinyl chloride (PVC; 35 million
tonnes in 2010) [
2
]. Besides thermoplastics, rubber is also considered a class of plastic. The 26.9 million
tonnes rubber market sells two main classes: natural rubber (12.3 million tonnes in 2016) and synthetic
rubber (14.6 million tonnes in 2016) [3].
Int. J. Environ. Res. Public Health 2017,14, 1265; doi:10.3390/ijerph14101265 www.mdpi.com/journal/ijerph
Int. J. Environ. Res. Public Health 2017,14, 1265 2 of 31
As a result of the growing production of plastics, their widespread use and the mismanagement
of waste, the amount of plastics in the environment is increasing rapidly. It has been estimated that
between 4.8 and 12.7 million metric tonnes of plastic ended up in the ocean in the year 2010 [
4
].
Even on the beaches of remote areas such as Henderson Island, an uninhabited island in the South
Pacific, large amounts of plastics have been detected [
5
]. Pollution of the environment with plastics is
recognized as a serious global threat because it can negatively affect human health, aquatic organisms,
as well as the economy [2,68].
Plastics end up in the ocean either as large pieces, macroplastics, microplastics (
5 mm) or
nanoplastics (
100 nm) [
9
]. The sources of both macroplastics and microplastics are many and diverse.
However, the implications for ecological and human health and the impact on our economy are still
unknown. More research is needed to pinpoint these sources in order to enable the identification and
implementation of cost-effective measures to reduce plastic pollution sources.
In this paper, the whole family of synthetic polymers, including modified natural bio-polymers, are
considered to be a potential source of pollution. From an environmental point of view, thermoplastics,
thermosets and elastomers all are potential sources of microplastics [9].
Car tyres release wear particles through mechanical abrasion. Several studies have suggested
that wear and tear from car tyres is an important source of microplastics in the environment [
10
13
].
However, many questions remain. Which factors determine the release of wear and tear from car tyres,
and how much is actually being released? What is the fate of these particles, once released into the
environment? What impacts do the particles have on human health and on aquatic ecosystems? And,
how can the emission of wear and tear from car tyres be reduced effectively? Although some of these
issues have been addressed in specific scientific studies, the available knowledge is largely fragmented
and often localized.
The aim of the present review is to bring together the fragmented knowledge on wear and tear
of car tyres emitted into the environment and provide a global assessment of the implications for
human health of this emerging source of microplastics. The review: (1) describes the characteristics of
the tyre and its wear and tear; (2) summarises the amount released into the environment in different
countries; (3) describes the different pathways of the wear and tear into the environment; (4) presents
an estimate of the total amount of tyre wear and tear to total emissions of microplastics to the oceans;
(5) discusses possible effects on humans; and (6) evaluates mitigation options. The numbers calculated
in the present paper are rough estimates and should be considered as such. These numbers have been
produced in a first attempt to explore the extent of the problem.
2. Emissions
2.1. Tyres
When driving a vehicle, particles are being released into the environment from its tyres.
This section summarises the available knowledge on the composition of tyres, the particle generation
process and the sizes of the particles generated.
2.1.1. Tyre Composition
Tyres were initially only made of natural rubber, often derived from the Brazilian rubber tree
(Hevea brasiliensis). Nowadays, a mixture of natural and synthetic rubbers is being used. Synthetic
rubbers are polymers made from petroleum. About 1–4% of sulphur is added in order to vulcanise
the rubber compounds, transforming them into highly elastic material, in which 1% zinc oxide
serves as a catalyst. Furthermore, 22–40% carbon black is added as a filler and to make the tyre
UV-resistant. In recent years, carbon is sometimes partially replaced by silica (nanoscale glass balls) [
14
].
Silica reduces the road resistance but it’s more difficult to form a proper bond to the rubber. In a final
stage, oil is added to make the tyre less stiff and to improve its wet grip performance. Traditionally,
the oil used is aromatic because of its low price and its compatibility with rubber.
Int. J. Environ. Res. Public Health 2017,14, 1265 3 of 31
The specific gravity of a microplastic influences the floating ability of the particle in water [
15
].
According to the United States (US) Federal Highway Administration [
16
], the specific gravity of tyre
rubber is approximately 1.15 [
16
]. Banerjee and colleagues [
17
] mention a specific gravity of 1.17,
while Dumne [
18
] mentions a specific gravity of 1.18. The average density of ocean waters at the
surface is 1.025.
2.1.2. Particle Generation
The release of wear and tear from tyres results from the contact between the road surface and the
tyre. The amount and size of the particles released depend on factors such as climate (temperature),
composition and structure of the tyre, the road surface, driving speed and style and the nature of the
contact (e.g., rolling versus slipping) [19].
The contact between tyre and road surface causes shear and heat in the tyre; both of these
processes result in the generation of wear particles. Shear forces result in the emission of comparatively
large tyre particles. Heat accumulates, creating hot spots on the tyre’s surface, reaching temperatures
that cause the volatile content to evaporate, which results in the subsequent release of relatively small,
submicrometer, particles. Additional to the tyre wear and tear, the shear forces and the heat in the
rubber also cause road wear particles to stick to the rubber wear and tear. Some researchers report that
most tyre wear and tear are conglomerates with road wear [20,21].
2.1.3. Size of Wear and Tear
The tyre wear and tear particle size (distribution) is dependent on many factors such as type of
pavement, temperature, speed, age and composition of the tyre. However, the particles sizes reported
in any particular experimental study also depend on the experimental setup.
Particles can be collected while driving a car on the road or in laboratory tests with road simulators.
The samples collected typically consist of a mixture of tyre particles and particles from the road or
simulator surface. Airborne particles are typically drawn by suction and measured real-time, while the
coarser particles are typically collected after the test run, e.g., from the contact surface or from its
direct environment like the road run off. By adjusting the suction flow, a range of particle sizes can be
collected. The size ranges reported in any particular study also depend on the technical specifications
of the equipment and analytical techniques used.
Figure 1provides an overview of the size ranges covered and detected in four key studies focusing
on the size distribution of tyre wear and tear, highlighting the vast differences that can exist between
experiments. A more extensive overview of the available studies is provided by Grigoratos and Martini [
20
].
Study 1 by Kreider and colleagues [
21
] was on the size of wear and tear of car tyres and the
interaction with pavement in a road simulator using asphalt concrete pavement, i.e., a mixture of
sand, gravel, crushed stone and recycled concrete bound together with asphalt. Their sampling
device, consisting of a suction system located close to the tyre’s contact surface, only collected particles
>0.3 µm
; an upper size limit was not specified. They found particles sizes ranging from 4 to 350
µ
m
with most particles having a size around 5 µm and 25 µm.
Study 2 by Aatmeeyata and colleagues [
22
] was on wear and tear on a specially constructed road
simulator using concrete pavement, i.e., a mixture of sand and granite stone bound together with
cement. The air was withdrawn by suction and continuously analysed by a particle size analyser
on particle number and size within a 0.3 to 20
µ
m range. Samples were also taken from the walls
and the equipment afterwards, which were considered to represent run-off material. The emission
of particulate matter with particles of 10
µ
m or less (PM
10
) to ambient air was compared to the total
weight of the run-off of particles, and was found to be less than 0.1% by weight. Almost 50% of the
PM
10
mass had a size between 0.3 and 1
µ
m. No size distribution was given on the course particles,
i.e., particles >PM10.
In study 3 Dahl and colleagues [
23
] tested tyres in a road simulator of the Swedish National Road
and Transport Research Institute. They focussed on fine air particles in the size range of 15–700 nm
Int. J. Environ. Res. Public Health 2017,14, 1265 4 of 31
from both road and tyre wear and tear. The measured wear and tear was between 15 nm and 50 nm
and had a distinct mean particle diameter of 27 nm. Based on transmission electron microscopy studies
of the collected ultrafine wear and tear particles and on-line thermal treatment using a thermodesorber,
they concluded that the particles consisted most likely of mineral oils from the softening filler and
fragments of carbon black. Carbon black, which is added to tyres as a filler material and to make them
UV resistant, is thought to form aggregates of 1–100
µ
m held together by Van der Waals bonds [
24
].
The particles detected by Dahl and colleagues [
23
] fall in the size ranges of the carbon black that is
added to the tyres during the production process [
25
]. For example, Continental Carbon produces
carbon black grade N120 with a primary particle size of 1–10 nm, grade N234 of 20–25 nm and grades
N330, N339 and N351 of 26–30 nm [25,26].
In study 4 Mathissen and colleagues [
27
] measured air borne particle concentrations inside the
car’s wheel housing while driving on an existing road. The car drove on asphalt concrete roads and
the instrument was capable of measuring particles from 6 to 562 nm. When braking, accelerating and
cornering, particles were measured with sizes between 30 and 60 nm. Normal driving did not result in
an increase in particle number concentration.
Based on these four studies, it can be concluded that literature data show a considerable variation
in the size distribution of tyre wear and tear particles. Interpretation of the experimental results is
furthermore complicated by the use of different metrics (e.g., particle mass versus particle numbers),
analytical difficulties to separate tyre from road particles, and the enormous variety in experimental
conditions and analytical equipment. More research is needed to create a more univocal picture of
the numbers and sizes of the particles generated under realistic driving conditions. Nonetheless,
all studies show that tyre wear and tear will be a source of microplastics in the environment not to be
ignored, covering the range from 10 nm to several 100 µm.
Int. J. Environ. Res. Public Health 2017, 14, 1265 4 of 30
using a thermodesorber, they concluded that the particles consisted most likely of mineral oils from
the softening filler and fragments of carbon black. Carbon black, which is added to tyres as a filler
material and to make them UV resistant, is thought to form aggregates of 1–100 μm held together by
Van der Waals bonds [24]. The particles detected by Dahl and colleagues [23] fall in the size ranges
of the carbon black that is added to the tyres during the production process [25]. For example,
Continental Carbon produces carbon black grade N120 with a primary particle size of 1–10 nm,
grade N234 of 20–25 nm and grades N330, N339 and N351 of 26–30 nm [25,26].
In study 4 Mathissen and colleagues [27] measured air borne particle concentrations inside the
car’s wheel housing while driving on an existing road. The car drove on asphalt concrete roads and
the instrument was capable of measuring particles from 6 to 562 nm. When braking, accelerating and
cornering, particles were measured with sizes between 30 and 60 nm. Normal driving did not result
in an increase in particle number concentration.
Based on these four studies, it can be concluded that literature data show a considerable
variation in the size distribution of tyre wear and tear particles. Interpretation of the experimental
results is furthermore complicated by the use of different metrics (e.g., particle mass versus particle
numbers), analytical difficulties to separate tyre from road particles, and the enormous variety in
experimental conditions and analytical equipment. More research is needed to create a more
univocal picture of the numbers and sizes of the particles generated under realistic driving
conditions. Nonetheless, all studies show that tyre wear and tear will be a source of microplastics in
the environment not to be ignored, covering the range from 10 nm to several 100 μm.
Figure 1. Size ranges of tyre wear and tear covered (blue bars) and detected (red bars) in four
different studies (see text). Dark red suggests the size of the major number of particles [21–23,27].
2.2. National Estimates on the Amount of Wear and Tear from Tyres
Two different approaches are typically used to estimate the amount of wear and tear from tyres.
One approach uses emission factors per vehicle-km multiplied by the total mileage, and the other
uses the number of tyres multiplied by the weight loss of these tyres during use. Tyres in Europe
must be collected after use and processed by the manufacturer or importer [28]. Therefore, almost all
used tyres will be handed in and, hence, the numbers are known.
We performed a literature search to collate national estimates on the amount of wear and tear
from tyres, resulting in estimates for eight countries. Apart from Japan, the available studies on wear
and tear are dominated by Western European countries. In Sweden, Norway, Denmark and
1 10 100 1000 10000 100000
range
(4) Mathissen and colleagues [27] size
range
(3) Dahl and colleagues [23] size
range
(2) Aatmeeyata & Sharma [22] size
range
(1) Kreider and colleagues [21] size
size (nm)
Figure 1.
Size ranges of tyre wear and tear covered (blue bars) and detected (red bars) in four different
studies (see text). Dark red suggests the size of the major number of particles [2123,27].
2.2. National Estimates on the Amount of Wear and Tear from Tyres
Two different approaches are typically used to estimate the amount of wear and tear from tyres.
One approach uses emission factors per vehicle-km multiplied by the total mileage, and the other uses
the number of tyres multiplied by the weight loss of these tyres during use. Tyres in Europe must be
collected after use and processed by the manufacturer or importer [
28
]. Therefore, almost all used
tyres will be handed in and, hence, the numbers are known.
We performed a literature search to collate national estimates on the amount of wear and tear
from tyres, resulting in estimates for eight countries. Apart from Japan, the available studies on wear
Int. J. Environ. Res. Public Health 2017,14, 1265 5 of 31
and tear are dominated by Western European countries. In Sweden, Norway, Denmark and Germany
both emission estimation approaches have been used. The tyre number weight loss method has been
used in the United Kingdom, Italy and Japan. In The Netherlands the emission factor per vehicle-km
approach was used. To obtain a global estimate on the amount of wear and tear emitted into the
environment, data on mileage and number of vehicles were gathered for countries for which national
emission estimates were lacking. These data were found for China, India, Australia, the USA and
Brazil. The emission factor method based on data from Japan and a number of European countries
was used to estimate the national emission of tyre wear and tear in these countries. In this way,
we calculated emissions from countries on all continents, except Africa, covering half the world’s
population. Here, we first discuss the emissions factor per country, followed by the estimation of the
total wear and tear on our planet.
2.2.1. The Netherlands
Kole and colleagues [
13
] estimated the emissions of wear and tear from car tyres in The
Netherlands (Table 1). They used emission factors per vehicle category and mileage data, provided by
the institutes Deltares and TNO, The Netherlands Organisation for applied scientific research [29].
Table 1.
Calculation of the amount of tyre wear and tear in The Netherlands by Kole and colleagues [
13
].
Wear
mg/km
Mileage in 2012
Built-Up Area
(×106km)
Mileage in 2012
Rural Roads
(×106km)
Mileage in 2012
Motorways
(×106km)
Total Wear 2012
tonnes/year
Corrected for
95% Trapped in
Motorways
Passenger car 100 20,876 36,472 45,349 10,270 6263
Articulated lorry
495 274 867 3418 2257 762
Lorry 600 406 525 1434 1419 659
Other 1084 1084
Total 15,030 8768
Of the Dutch motorways, 95% is paved with very open asphalt concrete, consisting of rock, sand,
filler and bitumen. Contrary to standard asphalt, very open asphalt concrete has 15% to 25% hollow
space and is used because of its capabilities to drain rainwater and to reduce noise. Of the wear
and tear of tyres, 95% is considered to be captured in the pores of this very open asphalt concrete to
remain trapped [
29
]. To maintain the draining, reducing and trapping capacities of the very open
asphalt concrete, its pores have to be cleaned approximately twice per year. The washing water is
processed and the dirt disposed properly [
30
]. If the total amount listed in Table 1is corrected for
the amount trapped in very open asphalt concrete on motorways, still 8768 tonnes will end up in the
environment [13].
Verschoor and colleagues [
31
] calculated the wear and tear using the specific emission factors per
vehicle-km method for urban, rural and highway roads (Table 2). The capturing of wear and tear in the
pores of the very open asphalt concrete was taken into consideration. The total estimated amount of
wear and tear for the three road types was 17,300 tonnes/year [
31
]. If we take the 95% capturing in the
very open asphalt concrete into consideration, and subtract this from the amount of 17,300 tonnes/year
we end up with 8900 tonnes/year that is released into the environment. The average of the estimated
amounts (8768 and 8900 tonnes) is 8834 tonnes/year.
2.2.2. Sweden
Magnusson and colleagues [
32
] estimated the wear and tear in Sweden based on emission per
vehicle kilometre (Table 3). In Sweden, a special emission pathway exists; snow is taken from the streets
and dumped into the waters. The snow will contain particles already present on the pavement even
before precipitation started. Stockholm alone has permission to dump 800,000 m
3
of snow annually
from the streets into the waters around the city [32].
Int. J. Environ. Res. Public Health 2017,14, 1265 6 of 31
Table 2. Calculation of the amount of tyre wear and tear for urban, rural and highway roads in The Netherlands by Verschoor and colleagues [31].
Urban Roads Rural Roads Highway Roads
Wear
mg/km
Total Mileage in
2012 (×106km)
Total Wear 2012
tonnes/year
Wear
mg/km
Total Mileage in
2012 (×106km)
Total Wear 2012
tonnes/year
Wear
(mg/km)
Total Mileage in
2012 (×106km)
Total Wear 2012
tonnes/year
Moped 13 1608 21 9 690 6 10 0 0
Motorcycle 60 393 24 39 1100 43 47 1089 51
Passenger car 132 20,959 2767 85 36,622 3113 104 45,541 4736
Van 159 2670 425 102 5331 544 125 8649 1081
Lorry 850 412 350 546 533 291 668 1453 971
Truck 658 277 182 423 876 371 517 3455 1786
Bus 415 354 147 267 207 55 326 82 27
Special
vehicle light 159 22 3 102 44 4 125 72 9
Special
vehicle heavy 850 59 50 546 76 41 668 210 140
Total 26,754 3969 45,479 4468 60,551 8801
Table 3. Calculation of the amount of tyre wear and tear in Sweden [32,33].
Based on Annual Mileage: Magnusson and Colleagues [32] Based on Tyres Sold: Swedish National Chemicals Inspectorate [33]
Wear mg/km Total Mileage in
2012 (×106km)
Total Wear 2012
Tonnes/Year
Mass of Tyres
Consumed Annually kg Weight Loss (17%) kg Total Wear 2002
tonnes/year
Passenger car 50 62,940 3147
Bus/lorry 700 14,416 10,091
Total 77,356 13,238 60,000,000 10,000,000 10,000
Int. J. Environ. Res. Public Health 2017,14, 1265 7 of 31
The Swedish National Chemicals Inspectorate [
33
] estimated the amount of wear and tear using
the total weight of the tyres consumed in Sweden and 17% weight loss during use (Table 3).
On average the annual amount is (13,238 + 10,000)/2 = 11,619 tonnes. Here the two approaches,
emission factors per vehicle-km multiplied by the total mileage, and number of tyres multiplied by the
weight loss of these tyres during use provide similar results.
2.2.3. Norway
Sundt and colleagues [
10
] used several methods to calculate the amount of wear and tear in
Norway. First, they used data by the United Nations Economic Commission for Europe (UNECE) on
tyre wear and tear per km, based on Russian research [
26
]. UNECE advises to use an emission factor of
0.033 g/tyre km for passenger cars and 0.178 g/tyre km for commercial vehicles. In their calculations,
Sundt and colleagues [10] assumed all vehicles to have four wheels (Table 4).
Second Sundt and colleagues [
10
] calculated the wear and tear using the wear/km for passenger
cars by Luhana and colleagues [34], who used an emission factor of 0.1 g/vehicle km [27] (Table 4).
Third Sundt and colleagues [
10
] estimated the amount of wear and tear in Norway using figures
by Norsk Dekkretur (Norwegian Tyre Recycling) on disposed tyres, assuming tyres will wear 12.5%
on average before being disposed (Table 4). Norsk Dekkretur organises the collection and recycling of
disposed tyres in Norway.
The estimated emissions for Norway vary from 6560 to 9571 tonnes/year, with an average of
7884 tonnes/year from all the studies. As the outcomes calculated by emission/km and by disposed
tyre weight loss are relatively tantamount, they are considered to be reliable. As mentioned in
Section 2.2.2 the differences between the two calculation approaches could be used to improve the
figures. Here the disposed tyres approach gives a higher wear and tear number. Contrary to the results
the weight loss in Norway was assumed to be 12.5%; in Sweden 17%.
2.2.4. Denmark
In Denmark Lassen and colleagues [
12
] used two different sources of emission factors to estimate
the amount of wear and tear. First, they used the emission factors advised by the United Nations
Economic Commission for Europe (UNECE) [
35
]; 0.033 g/km per passenger car tyre, 0.051 g/km per
light commercial vehicle tyre and 0.178 g/km per other commercial vehicle tyre. Using these emission
factors and considering 35,800, 7400 and 2000 million kilometres for passenger, light commercial
and other commercial cars, respectively, the total emission of tyre wear and tear was estimated to be
1915 tonnes/year [
12
]. However, instead of being quoted by car, the UNECE data are given per tyre.
After consulting Lassen we recalculated the emissions considering an average of four tyres per vehicle,
as done by Sundt and colleagues [10], the total emission is 7660 tonnes/year (Table 5).
Second, Lassen and colleagues [
12
] also calculated the emission by passenger car using an
emission factor of 0.1 g/vehicle km, taken from Luhana and colleagues [
34
]. In this way the total
emission will be 6514 tonnes/year, see Table 5.
Lassen and colleagues [
12
] also calculated the amount of wear and tear by multiplying the number
of tyres sold with the mass difference between new and disposed tyres. The weight loss of new tyres
was estimated by several studies to vary between 10% and 15%. Considering the number of tyres
completely outworn during the car’s life and assuming the car ’s last set of tyres (i.e., before disposing
the car) lost half of what an outworn tyre would lose, their calculations resulted in an estimate of
5400 tonnes/year.
Fauser and colleagues [
36
] also calculated the wear and tear by the number of consumed tyres
multiplied by the average wear per tyre (Table 5).
On average the estimated emissions for Denmark vary from 7660, 6514, 5400 to 7310 tonnes/year,
on average 6721 tonnes/year. Here the results of the two emission factors studies provide similar
results; but the two studies on sold tyres differ: 5400 and 7310 tonnes/year. In this case further study
should provide more insight.
Int. J. Environ. Res. Public Health 2017,14, 1265 8 of 31
Table 4. Calculation of the amount of tyre wear and tear in Norway [10].
Based on Annual Mileage: Sundt and
Colleagues [10] Using United Nations Economic
Commission for Europe (UNECE) Data
Based on Annual Mileage: Sundt and Colleagues
[10] Using Data by Luhana and Colleagues [34]Based on Disposed Tyres: Sundt and Colleagues [10]
Wear
mg/km
Total Mileage in
2013 (×106km)
Total Wear 2013
tonnes/year
Wear
mg/km
Total Mileage in
2013 (×106km)
Total Wear 2013
tonnes/year
Weight of
Disposed Tyres
Times
Re-Treaded
Weight Loss
from New
Total Wear 2013
tonnes/year
Passenger car 132 30,000 3960 100 30,000 3000 42,000 0 12.5% 6000
Heavy
transport 712 5000 3560 712 5000 3560 10,000 2.5 12.5% 3571
Total 35,000 7520 35,000 6560 52,000 9571
Table 5. Calculation of the amount of tyre wear and tear in Denmark using the wear data on passenger car [12,34,36].
Based on Annual Mileage: Lassen and Colleagues
[12] Using UNECE Data [35]
Based on Annual Mileage: Lassen and Colleagues
[12] Using Data by Luhana and Colleagues [34]Based on Tyres Sold: Fauser and Colleagues [36]
Wear
(mg/km)
Total Mileage in
2014 (×106km)
Total Wear 2014
tonnes/year
Wear
(mg/km)
Total Mileage in
2014 (×106km)
Total Wear 2014
tonnes/year
Number of Tyres
Consumed
Annually
Weight Loss
per Tyre kg
Total Wear 1990
tonnes/year
Passenger car 132 35,800 4726 100 35,800 3580 1,900,000 2.4 4560
Light commercial 204 7400 1510 204 7400 1510 0
Commercial car 712 2000 1424 712 2000 1424 250,000 11 2750
Total 45,200 7660 45,200 6514 7310
Int. J. Environ. Res. Public Health 2017,14, 1265 9 of 31
2.2.5. Germany
Hillenbrand and colleagues [
37
] calculated the amount of wear and tear by using an emission per
vehicle kilometre. The figures they used are covering mileage for the year 2001 and vehicle numbers
for the year 2002 (column 3 in Table 6) [
37
]. We used the emission per vehicle kilometre data from
Hillenbrand and colleagues [
37
] to estimate the emissions for the year 2013 using data on total mileage
from the German Federal Ministry of Transport (Bundesministerium für Verkehr) [38] (Table 6).
Table 6. Calculation of the amount of tyre wear and tear in Germany [37,38].
Wear
(mg/km) [37]
Total Wear 2001/2002
tonnes/year [37]
Total Mileage in
2013 (×106km) [38]
Total Wear 2013
tonnes/year
Moped 22.5 88 4700 106
Motorcycle 45 621 12,300 689
Passenger car 90 46,017 615,100 55,359
Bus 700 2590 3300 2310
Lorry 700 43,540 64,300 45,010
Articulated lorry 1200 16,440 16,700 20,040
Other 180 2124 9300 1674
Total 111,420 725,700 125,188
In a study commissioned by the German Federal Environment Agency (Umweltbundesamt)
on sources of microplastics, Essel and colleagues [
11
] discuss two studies, i.e., the calculations by
Hillenbrand and colleagues [
37
] mentioned above and the calculations by the German rubber trade
association (Wirtschaftsverband der Deutschen Kautschukindustrie; WDK). The WDK calculated the
total annual amount of wear and tear for all vehicle categories to be 60,000 tonnes [
11
]. Hillenbrand
and colleagues [
37
] calculated 62,570 tonnes for buses, lorries and articulated lorries alone, while the
WDK calculated only 17,000 tonnes for these categories. For the category “lorry”, Hillenbrand and
colleagues [
37
] used 700 mg of wear per vehicle kilometre, whereas the WDK assumes a wear of
17,000/62,570 ×700 = 190 mg/km. This is about what UNECE advises for calculating a single tyre.
Baumann and Ismeier [
39
] calculated that the total wear and tear using amounts of wear and tear
per tyre kilometre significantly differed from that calculated by UNECE; the wear and tear per tyre for
heavy and articulated lorries is assumed to be about the same as for a passenger car (Table 7).
Table 7. Calculation of the amount of tyre wear and tear in Germany by Baumann and Ismeier [39].
Number of
Vehicles in 1995
Wear per Tyre
mg/km
Average Number
of Tyres
Average
Mileage in
1995 km
Total Wear 1995
tonnes/year
Passenger car 40,500,000 20 4 14,200 46,008
Bus 46,900 32 6 46,900 422
Lorry < 7.5 tonnes 1,961,000 36 5 25,000 8825
Lorry > 7.5 tonnes 254,000 21 9 70,000 3360
Articulated lorry 124,100 18 15 82,000 2748
Total 61,363
Comparing the results by Hillenbrand and colleagues [
37
], the WDK and Baumann and
Ismeier [
39
], the large differences in tyre wear and tear for heavy vehicles are remarkable. WDK and
Baumann and Ismeier [
39
] consider the wear and tear per vehicle km for lorries to be in the same
order of magnitude as passenger cars. Considering the fact that experiments in a road simulator have
shown a linear relationship between tyre load and tyre wear and tear [
22
], the figures by the WDK and
Baumann and Ismeier [
39
] can be considered unconvincing. Considering this, we will only use the
125,188 tonnes/year as calculated by Hillenbrand and colleagues [
37
] and adapted to 2013 mileage,
for further calculations.
Int. J. Environ. Res. Public Health 2017,14, 1265 10 of 31
2.2.6. United Kingdom
The United Kingdom (UK) Environment Agency [
40
] estimated the amount of tyre wear and
tear in the year 1996 by the weight of the 37 million tyres disposed that year, to be approximately
380,000 tonnes. The Agency assumed that a car tyre loses approximately 10–20% of its weight
during use. Calculating the amount of wear and tear this way in the year 1996 results in
38,000–76,000 tonnes/year. The tyres from 1996 were probably different from current ones because
of technological progress, technological changes and the impact of European Union (EU) legislation.
Also, mileage will be different now. In 1996, the UK population was 58 million [
41
], in 2016 the UK
population was 64 million [
42
]. Assuming the mileage per capita/year did not change, the emission
would have grown to approximately 42,000–84,000 tonnes/year, or on average 63,000 tonnes/year.
2.2.7. Italy
Milani and colleagues [
43
] considered a 10 kg passenger car tyre to lose about 1.5 kg before being
abandoned after 50,000 km; this equals approximately 0.03 g/km. They calculated the total amount of
wear in Italy to be 50,000 tonnes/year without providing their calculation on mileage and number of cars.
2.2.8. Japan
Yamashita and Yamanaka [
44
] calculated the wear and tear from tyres in Japan. In 2012 there
were 79,882,112 vehicles on the roads in Japan (Table 8). The average number of tyres per vehicle
category is listed in column 3 of Table 8. They considered the mean life expectancy of tyres to be five
years. The wear in these 5 years was calculated by considering new tyres to have an 8 mm tread depth
and 1.6 mm when disposed. By measuring the diameter and tread width of a standard tyre for each
category the loss for each tyre has been calculated by Yamashita and Yamanaka [
44
] and is given in
column 5 of Table 8.
Table 8. Calculation of the amount of tyre wear and tear in Japan generated in 5 years [44].
Number of
Vehicles Tyres/Vehicle Total Number
of Tyres
Wear and Tear
in cm3/Tyre Total Wear m3Total Wear m3as
Reported in [44]
Motorcycle 3,402,405 2 6,806,810 1136 7733 (7733)
Light vehicle 24,756,432 4 99,025,728 1780 176,266 (176,266)
Normal vehicle 43,350,396 4 173,401,584 2880 499,397 (4,993,966)
Truck/bus 2,790,562 10 27,905,620 5484 153,034 (1,666,803)
Trailer 2,463,607 14 34,490,498 5973 206,012 (1,891,459)
Total 76,763,402 1,042,442 (8,736,183)
We recalculated the total emissions for the categories “normal vehicle”, “truck/bus” and “trailer”,
as the calculated totals by Yamashita and Yamanaka [
44
] clearly had typos. The calculations of
Yamashita and Yamanaka [
44
] resulted in an unlikely result of 15 kg/year wear and tear per capita/year.
The original values reported by Yamashita and Yamanaka [
44
] have been put between brackets into
the last column of Table 8. We recalculated the totals using the “number vehicles”, “tyres/vehicle” and
“cm3/tyre” as provided by the authors.
According to these figures, in five years, a total of 1,042,442 m
3
have been released from the tyres
in Japan alone. The specific gravity of tyre rubber is approximately 1.15 [
16
]. This equates to an annual
wear and tear of 239,762 tonnes/year.
2.2.9. China
For China, no estimate on total tyre wear and tear in the literature was found. However,
some relevant input data are available, which, combined with some assumptions, we translated
to estimates of tyre wear and tear. The World Health Organization (WHO) provides the number of
registered vehicles for the year 2013 [
45
]. Huo and colleagues [
46
] provided annual mileage for the
categories “cars and 4-wheeled light vehicles”, “motorised 2- and 3-wheelers”, “heavy trucks” and
Int. J. Environ. Res. Public Health 2017,14, 1265 11 of 31
“other” for the year 2009. No data was found on the amount of wear and tear per kilometre in China.
Therefore, the amount of wear and tear per kilometre was taken from the UNECE; 0.033 g/km for cars,
0.051 g/km for light commercial vehicles and 0.178 g/km for commercial vehicles [
35
]. For 2-wheelers
Aatmeeyata and colleagues [
22
] found 0.0035 g/km. These data are per tyre; for cars, we consider 4
wheels, except for heavy trucks where a conservative 6 wheels were assumed. As 2- and 3-wheelers
are not differentiated, 2 wheels per vehicle were assumed (Table 9).
Table 9. Calculation of the amount of tyre wear and tear in China [22,35,45,46].
Number
Vehicles
Annual
Mileage
Wear and Tear
g/km
Wear and Tear
Tonnes
Cars and 4-wheeled light vehicles 137,406,846 19,400 0.132 352,000
Motorised 2- and 3-wheelers 95,326,138 5600 0.007 3740
Heavy lorries 5,069,292 60,000 1.068 325,000
Other (light duty lorries) 12,335,936 30,000 0.204 75,500
Total 250,138,212 756,240
2.2.10. India
For India, no estimate in the literature was found but, like in the case of China, data to calculate
it was available. Again, the WHO provides the number of registered vehicles for the year 2011 [
45
].
The data on annual mileage for the year 2013 was taken from Baidya and Borken-Kleefeld [
47
] who
studied mileage in India, published from the year 1999 up to 2006. They published data for “Megacities”
and for “Rest of India” and we used the average of these two figures. The amount of wear and tear
was taken from UNECE [
35
]. For the category “motorised 2- and 3-wheelers”, a conservative emission
factor of 0.007 g per vehicle km for 2 wheelers as estimated by Aatmeeyata and colleagues [
22
] was
used (Table 10).
Table 10. Calculation of the amount of tyre wear and tear in India [22,35,45,47].
Number Vehicles
[45]
Annual Mileage
[47]
Wear and Tear
g/km [22,35]
Wear and Tear
Tonnes
Cars and 4-wheeled light vehicles 38,338,015 10,275 0.132 51,998
Motorised 2- and 3-wheelers 115,419,175 6600 0.007 5332
Heavy trucks 4,056,885 50,075 1.068 216,963
Buses (light duty trucks) 1,676,503 53,745 0.204 18,381
Total 159,490,578 292,674
2.2.11. Australia
Milani and colleagues [
43
] calculated the total amount of wear in Australia to be 20,000 tonnes/year
without providing their calculation details.
2.2.12. USA
For the USA, we calculated the amount of wear and tear by using WHO data on the number of
registered vehicles for the year 2011 [
45
], while the data on annual mileage for the year 2013 was taken
from the US Department of Energy [
48
]. The wear and tear emission per vehicle kilometre was, like for
China, taken from UNECE [35] and Aatmeeyata and colleagues [22] (Table 11).
Int. J. Environ. Res. Public Health 2017,14, 1265 12 of 31
Table 11. Calculation of the amount of tyre wear and tear in the USA [22,35,45,48].
Number Vehicles
[45]
Annual Mileage
km [48]
Wear and Tear
g/km [22,35]
Wear and Tear
Tonnes
Cars and 4-wheeled light vehicles 245,669,103 18,095 0.132 586,800
Motorised 2- and 3-wheelers 8,437,502 3899 0.007 230
Heavy lorries 10,270,693 109,685 1.068 1,203,000
Buses (light duty lorries) 666,064 54,803 0.204 7450
Total 265,043,362 1,797,480
Councell and colleagues [
49
] calculated the total amount of wear and tear in the USA by using
both approaches, i.e., emission factors per vehicle-km and weight loss of abandoned tyres. They used a
universal wear rate of 0.050 g/km, assuming 4 tyres for a passenger car, 6 for busses and lorries, and 18
for articulated lorries. Using mileage data from the US Federal Highway Administration but without
showing the figures, they calculated the amount of wear and tear for 1999 to be 1,000,000 tonnes/year.
Using the weight loss method they arrived at an estimate of 1,110,000 tonnes/year; so on average
1,055,000 tonnes/year.
According to the US Census Bureau, in 1999 the US population was 273 million people [
50
],
in 2016 the US population was 324 million [
42
]. Assuming the mileage per capita/year did not
change, the actual emissions would be 1,252,000 tonnes/year. The estimated emissions for the USA are
1,797,480 tonnes/year and 1,252,000 tonnes/year; on average 1,524,740 tonnes/year.
2.2.13. Brazil
For Brazil, no estimate was found in the literature, but again data to calculate the amount is
available. For Brazil annual mileage was found in a study by Tadano and colleagues [
51
], while again
the WHO provides the number of registered vehicles for the year 2011 [
45
]. The mileage was based on
different studies over the years 1994–2008. Again, the wear and tear was taken from UNECE [
35
] and
Aatmeeyata and colleagues [
22
]. For Brazil, the WHO did provide a number of vehicles in a category
“other”; as no mileage could be assigned, no wear and tear was calculated for this category. The results
of our calculation can be found in Table 12.
Table 12. Calculation of the amount of tyre wear and tear in Brazil [22,35,45,51].
Number Vehicles
[45]
Annual Mileage
[51]
Wear and Tear
g/km [22,35]
Wear and Tear
Tonnes
Cars and 4-wheeled light vehicles 54,175,378 20,000 0.132 143,023
Motorised 2- and 3-wheelers 21,597,261 5200 0.007 786
Heavy trucks 2,488,680 51,500 1.068 136,882
Buses 888,393 73,500 0.204 13,320
Other 2,451,017 ? ? -
Total 81,600,729 294,011
2.3. Global per Capita Tyre Wear and Tear
In the previous paragraphs, the amount of tyre wear and tear from cars was estimated for different
countries. Table 13 lists the estimates of the amount of wear and tear of car tyres per capita per year.
The emission per capita is in the same order of magnitude for all countries, i.e., between 0.23 and
1.9 kg/year, but 4.7 kg/year for the USA.
Int. J. Environ. Res. Public Health 2017,14, 1265 13 of 31
Table 13.
The amount of wear and tear of car tyres per capita per year (Number of capita as per July
2016 [42], Number of cars as per 2013 [52]).
Number of Capita
[42]
Number of Cars
[52]
Total Emission from
Tyres (tonnes/year)
Emission per
Capita/year (kg)
The Netherlands 17,016,967 9,612,273 8834 0.52
Norway 5,265,158 3,671,885 7884 1.5
Sweden 9,880,604 5,755,952 13,238 1.3
Denmark 5,593,785 2,911,147 6721 1.2
Germany 80,722,792 52,391,000 92,594 1.1
United Kingdom 64,430,428 35,582,650 63,000 0.98
Italy 62,007,540 51,269,218 50,000 0.81
Japan 126,702,133 76,763,402 239,762 1.9
China 1,373,541,278 250,138,212 756,240 0.55
India 1,266,883,598 159,490,578 292,674 0.23
Australia 22,992,654 17,180,596 20,000 0.87
USA 323,995,528 265,043,362 1,524,740 4.7
Brazil 205,823,665 81,600,729 294,011 1.4
Total 3,564,856,130 1,011,411,004 3,369,698 0.95
India has the lowest wear and tear estimate, i.e., 0.23 kg/capita/year, while the USA has the
highest, i.e., 4.7 kg/capita/year. The 20-fold difference can partly be explained by the fact that the USA
has 0.82 cars per capita, while in India there are 0.13 cars per capita. So the car density in India is only
16% of that in the USA. The amount of wear and tear per vehicle in the USA is 6.8 kg/year compared to
1.8 kg/year for India, a 3.8-fold difference. Americans are leading in wear and tear emissions because
they have more vehicles while they also travel longer distances per vehicle, especially with their lorries.
In India and China the number of vehicles per capita can explain the low emission per capita per year.
In The Netherlands, the capturing of wear and tear in the very open asphalt concrete explains the
relatively low emission per capita per year. For Japan, the assumptions of a five-year tyre lifespan in
the calculations, without considering mileage, could be a cause of the high emission per capita per year.
For the rest of the countries the estimates are roughly the same; between 0.81 and 1.5 kg/capita/year.
In Table 13 the wear and tear for roughly half of the world’s population and 57% of the world’s
vehicles has been estimated. The total amount of emitted tyre wear and tear from 1,011,411,004 vehicles
was estimated to be 3,369,698 tonnes/year (Table 13). If the mileage from these 1,011,411,004 vehicles
is considered representative for all the world’s 1,776,136,357 vehicles [
52
], the world total amount
of emitted tyre wear and tear is 1,776,136,357/1,011,411,004
×
3,369,698 tonnes/year = 5,917,518
tonnes/year. This amount is enough to fill thirty-one of the world’s largest container ships, i.e.,
the 399 m Maersk Triple E with a deadweight tonnage of 194,153 [
53
]. On a global population of
7,323,187,457 people [42], the amount of emitted tyre dust per person equals 0.81 kg/year.
2.4. Airplane Tyres
Apart from road vehicle tyres, similar wear and tear is released from planes, diggers, shovels,
bikes, conveyor belts, V-belts, etc. Here we make an educated guess for the amount of wear and
tear by airplane tyres. To understand the order of magnitude of the wear and tear of airplane tyres
we consider the Boeing 737-300 to be the average plane. The Boeing 737-300 has four main tyres,
lasting about 295 start/landing cycles. The two nose wheels will last about 210 cycles. Each cycle the
tyres wear approximately 0.05 mm [
54
]. The nose wheel diameter is 686 mm, its tread width 197 mm.
The main wheel diameter is 1016 mm, the tread width 368 mm [
55
]. The surface of a nose wheel
is 425,000 mm
2×
0.05 mm which translates to 21 g wear per start/landing cycle. The surface of a
main wheel is 1,170,000 mm
2×
0.05 mm means 59 g wear per start/landing cycle. In total this means
2×21 + 4 ×59 = 278 g
. On Dutch airports the total number of start/landings in 2016 was 571,000 [
56
].
A rough estimate on the annual wear and tear from airplanes released in The Netherlands is therefore
158 tonnes. Compared to the 8834 tonnes emitted tyre wear and tear in The Netherlands the 158 tonnes
Int. J. Environ. Res. Public Health 2017,14, 1265 14 of 31
wear and tear by planes is 158/8834 or approximately 2%. Assuming the number of flights people
travel is proportional with the distance they travel by car, the 2% can be used to make a rough estimate
of the global wear and tear by aviation tyres.
2.5. Artificial Turf as a Secondary Source of Tyre Rubber to the Environment
The disposal of tyres is regulated in the EU under directive 2000/53/EC End-of life vehicles [
28
].
Tyres must be collected after use and processed by the manufacturer or the importer. Part of these
tyres get a second life in Africa where requirements to profile depth are less strict. Another part of the
disposed tyres is ground up to pieces between 0.7 and 3 mm and used as infill in artificial turfs [
12
].
Infill was considered as a good example of recycling until concerns arose about possible adverse
health effects.
In Denmark building an artificial football field needs 100–120 tonnes of rubber infill [
12
]. After initial
infilling, a field needs 3–5 tonnes of infill each year for maintenance. Lassen and colleagues [
12
] assume
1.5–2.5 tonnes of infill leave the field each year, ending up in the soil and sewers next to the field,
in clothes of players etc. The extra 1.5–2.5 tonnes/year are needed to correct for compaction. There are
about 254 registered artificial football fields in Denmark, meaning a release of 380–640 tonnes/year.
Apart from football fields, the ground up tyres are also used for running lanes, rubber mats used for
playgrounds, rugby-tennis-and golf fields etc. Therefore, another 380–640 tonnes are estimated to enter
the environment; bringing the total to 760–1280 tonnes/year [
12
]. Comparing to the 6524 tonnes/year
from wear and tear this makes up 12–20%.
In Sweden the surface covered with artificial turfs is estimated at 6,117,600 m
2
equalling 776
football fields of 7881 m
2
. The amount of infill can vary between 59 and 140 tonnes. About 90% of
the artificial turfs is filled with ground up tyres. It is assumed 3–5 tonnes/year are lost from each
7881 m
2
field; the same as the amount added each year. The loss of tyre granulate from artificial turfs
is estimated at about 2300–3900 tonnes/year [
32
]. Comparing to the 13,000 tonnes/year from wear
and tear this makes up 18–30%.
In The Netherlands there were 1800 artificial football fields in 2015, each containing about
120 tonnes infill [
57
]. Assuming the same 1.5–2.5 tonnes/year loss per field as in Denmark, the annual
loss would be 2700–4500 tonnes/year. Comparing to the released average 8834 tonnes/year from wear
and tear this makes up 30–50%.
The loss of infill to the environment is significant when compared to the amount of wear and tear
from tyres. However, the particles are larger and their spreading can be easily prevented by changing
the infill from tyre rubber to for example cork.
2.6. Other Plastic Emissions Related to Vehicle Transport
Vehicle use will inevitably be accompanied by brake and road wear. A rough estimate of the
emitted amounts of brake and road particles, relative to the wear and tear of tyres, is stated below.
2.6.1. Brakes
At the moment, apart from regenerative braking in electric vehicles, cars are stopped by pressing
a brake pad against a rotating part of the wheel. During this action, both brake pad and counterpart
will experience wear. Brake pads contain binders (phenol-formaldehyde resins), fibres (copper, steel,
brass, potassium titanate, glass, organic material and Kevlar), fillers (barium and antimony sulphate,
magnesium and chromium oxides, silicates, ground slag, stone and metal powders), lubricants
(graphite, ground rubber, metallic particles, carbon black, cashew nut dust and antimony trisulphide)
and abrasives (aluminium oxide, iron oxides, quartz and zircon). Counterparts can be cast iron and
sometimes composites [58].
Hillenbrand and colleagues [
37
] estimated the annual amount of brake wear in Germany to be
12,350 tonnes/year. The estimated amount of brake wear is 11% of the estimated amount of tyre wear
and tear in Germany. Grigoratos and Martini [
58
] reviewed brake wear particle emissions without
Int. J. Environ. Res. Public Health 2017,14, 1265 15 of 31
considering Hillenbrand and colleagues [
37
]. They concluded that about 50% of total brake wear mass
is PM
10
. The particle number distributions varied from bimodal with peaks at 10 and 40 nm up to
unimodal with a peak at 1
µ
m. Generally, emitted particle sizes became smaller with increased braking
power. The measured emission per vehicle for cars and 4-wheeled light vehicles was in the range of
3–8 mg/km PM
10
and 2.1–5.5 mg PM
2.5
[
58
]. If we consider the 3–8 mg/km PM
10
to be half of the total
emission from brakes and compare this to the 132 mg/km emission of tyre wear and tear [
35
], then the
emission of brake wear is about 8% of the tyre wear and tear. Brake wear will exist all over the globe,
but will depend on driving behaviour and the type of road surface [20].
2.6.2. Road Markings
In Norway, 320 tonnes/year of road paint are used on the roads. Wear is heavy because of the
use of salt and spikes in winter. Although markings are sometimes removed, it is assumed that all
paint will wear and becomes part of the flow of microplastics [
10
]. In relation to the annual tyre wear
and tear of 7040 tonnes/year, the 350 tonnes/year is 5%. The wear of road markings from Norway
cannot be projected on the global scale since different conditions may apply, e.g., a substantial amount
of unpaved roads, lacking road markings or the absence of spiked tyres.
2.7. Historic Increase of Tyre Wear and Tear
To provide a basic insight on the historic figures on wear and tear of tyres, the annual growth
in the world number of vehicles was used to extrapolate wear and tear figures. Possible changes in
annual mileage, fleet composition and wear resistance were neglected. The US Department of Energy
estimated the world’s total amount of cars, busses and trucks for the year 1950 on 70,400,000. For 2014
the same amount was estimated at 1,208,005,000 [
59
,
60
]. Walsh [
61
] provided the figures on the amount
of cars for the years 1930 and 1940 (Figure 2).
Int. J. Environ. Res. Public Health 2017, 14, 1265 15 of 30
mass is PM10. The particle number distributions varied from bimodal with peaks at 10 and 40 nm up
to unimodal with a peak at 1 μm. Generally, emitted particle sizes became smaller with increased
braking power. The measured emission per vehicle for cars and 4-wheeled light vehicles was in the
range of 3–8 mg/km PM10 and 2.1–5.5 mg PM2.5 [58]. If we consider the 3–8 mg/km PM10 to be half of
the total emission from brakes and compare this to the 132 mg/km emission of tyre wear and tear
[35], then the emission of brake wear is about 8% of the tyre wear and tear. Brake wear will exist all
over the globe, but will depend on driving behaviour and the type of road surface [20].
2.6.2. Road Markings
In Norway, 320 tonnes/year of road paint are used on the roads. Wear is heavy because of the
use of salt and spikes in winter. Although markings are sometimes removed, it is assumed that all
paint will wear and becomes part of the flow of microplastics [10]. In relation to the annual tyre wear
and tear of 7040 tonnes/year, the 350 tonnes/year is 5%. The wear of road markings from Norway
cannot be projected on the global scale since different conditions may apply, e.g., a substantial
amount of unpaved roads, lacking road markings or the absence of spiked tyres.
2.7. Historic Increase of Tyre Wear and Tear
To provide a basic insight on the historic figures on wear and tear of tyres, the annual growth in
the world number of vehicles was used to extrapolate wear and tear figures. Possible changes in
annual mileage, fleet composition and wear resistance were neglected. The US Department of
Energy estimated the world’s total amount of cars, busses and trucks for the year 1950 on 70,400,000.
For 2014 the same amount was estimated at 1,208,005,000 [59,60]. Walsh [61] provided the figures on
the amount of cars for the years 1930 and 1940 (Figure 2).
Figure 2. Historic increase of the global number of cars and busses and trucks [59–61].
3. Pathways into the Environment
Tyre wear and tear particles emitted on roads can be dispersed in the environment via different
pathways. Small particles are typically emitted into the air and prone to air dispersal, whereas large
particles will get deposited on the road surface where some parts will get trapped and other parts
will be transported by rainwater runoff into soils, sewers and/or surface waters. These two most
important dispersal pathways of tyre wear and tear in the environment, i.e., transport by air and by
runoff, are discussed in more detail below and depicted in Figure 3.
0
200
400
600
800
1,000
1,200
1,400
1920 1940 1960 1980 2000 2020
number of vehicles (x 10)
year
Cars
Bus & Truck
Sum
Figure 2. Historic increase of the global number of cars and busses and trucks [5961].
3. Pathways into the Environment
Tyre wear and tear particles emitted on roads can be dispersed in the environment via different
pathways. Small particles are typically emitted into the air and prone to air dispersal, whereas large
particles will get deposited on the road surface where some parts will get trapped and other parts
will be transported by rainwater runoff into soils, sewers and/or surface waters. These two most
Int. J. Environ. Res. Public Health 2017,14, 1265 16 of 31
important dispersal pathways of tyre wear and tear in the environment, i.e., transport by air and by
runoff, are discussed in more detail below and depicted in Figure 3.
In most studies on microplastics tyre wear and tear is not dealt with separately and we therefore
use the data on microplastics as an indicator to describe the possible pathways of tyre wear and
tear. None of the environmental studies in waste water treatment plants (WWTPs) or surface waters
have actually identified tyre wear and tear particles, let alone its contribution to the total amount of
environmental microplastics.
Int. J. Environ. Res. Public Health 2017, 14, 1265 16 of 30
In most studies on microplastics tyre wear and tear is not dealt with separately and we
therefore use the data on microplastics as an indicator to describe the possible pathways of tyre wear
and tear. None of the environmental studies in
waste water treatment plants
(WWTPs) or surface
waters have actually identified tyre wear and tear particles, let alone its contribution to the total
amount of environmental microplastics.
Figure 3. Distribution of the tyre wear and tear over the compartments. WWTP:
waste water
treatment plants; NL: The Netherlands.
3.1. Transport by Runoff
Depending on the local situation, rainwater will flow directly into surface waters or into a
sewer. In countries like Denmark and The Netherlands, two main types of sewer systems exist, i.e.,
combined systems leading all inflow into the WWTP, and separated systems leading rainwater
directly into surface waters and just the wastewater into a WWTP. Climate change is a driver for
expanding separated systems because of the expected increase and intensity of rainfall. Separated
systems are built to minimise the load of relative clean rainwater to the WWTPs. Separate sewer
systems have the advantage of treating undiluted wastewater, but have the disadvantage that they
do not capture tyre wear and tear from runoff. For example, the length of
Dutch sewers consists for
35% of separated systems and 27% of Dutch houses is connected to a separated sewer system.
This implies that about 30% of the rainwater with the microplastics is discharged untreated
into surface waters [62].
3.1.1. Waste Water Treatment Plants
Several studies have been performed on the removal of microplastics in WWTPs (Table 14). In
Sweden, Magnusson and Wahlberg [63] measured the efficiency of WWTPs with a total capacity of
1,502,000 population equivalents in the cities of Stockholm, Göteborg and Lysekil. The influent and
the effluent was filtered by 20 μm and by 300 μm filters and the number of microplastics was
counted by use of a microscope. On average 19.8% of the microplastics > 20 μm and 0.6% > 300 μm
passed the WWTP.
In Norway, Magnusson [64] studied the WWTPs in Oslo, Tönsberg and Fuglevik, together
having a capacity of 970,000 population equivalents. The method was the same as used by
Figure 3.
Distribution of the tyre wear and tear over the compartments. WWTP: waste water treatment
plants; NL: The Netherlands.
3.1. Transport by Runoff
Depending on the local situation, rainwater will flow directly into surface waters or into a sewer.
In countries like Denmark and The Netherlands, two main types of sewer systems exist, i.e., combined
systems leading all inflow into the WWTP, and separated systems leading rainwater directly into
surface waters and just the wastewater into a WWTP. Climate change is a driver for expanding
separated systems because of the expected increase and intensity of rainfall. Separated systems are
built to minimise the load of relative clean rainwater to the WWTPs. Separate sewer systems have
the advantage of treating undiluted wastewater, but have the disadvantage that they do not capture
tyre wear and tear from runoff. For example, the length of Dutch sewers consists for 35% of separated
systems and 27% of Dutch houses is connected to a separated sewer system. This implies that about
30% of the rainwater with the microplastics is discharged untreated into surface waters [62].
3.1.1. Waste Water Treatment Plants
Several studies have been performed on the removal of microplastics in WWTPs (Table 14).
In Sweden, Magnusson and Wahlberg [
63
] measured the efficiency of WWTPs with a total capacity of
1,502,000 population equivalents in the cities of Stockholm, Göteborg and Lysekil. The influent and
the effluent was filtered by 20
µ
m and by 300
µ
m filters and the number of microplastics was counted
by use of a microscope. On average 19.8% of the microplastics > 20
µ
m and 0.6% > 300
µ
m passed
the WWTP.
Int. J. Environ. Res. Public Health 2017,14, 1265 17 of 31
In Norway, Magnusson [
64
] studied the WWTPs in Oslo, Tönsberg and Fuglevik, together having
a capacity of 970,000 population equivalents. The method was the same as used by Magnusson and
Wahlberg [
63
] in Sweden. On average 5.3% of the microplastics > 20
µ
m and 0.6% > 300
µ
m passed the
WWTP (Table 14).
Leslie and colleagues [
65
] measured the efficiency of WWTPs in The Netherlands. They compared
the number of microplastics in the influent and effluent of five WWTPs. The data showed that on
average 28% of the microplastics between 10 µm and 5000 µm passes the WWTP.
Table 14. The amount of microplastics, including tyre wear and tear, passing the WWTP.
Particles Passing the Sewers
Sweden 19.8%
Norway 5.3%
The Netherlands 28%
3.1.2. Amounts of Wear Particles Reaching Surface Waters
Kole and colleagues [
13
] combined different data on the emission and fate of tyre wear and
tear from The Netherlands to arrive at an overall estimate of the amount reaching surface waters.
Of the 8768 tonnes of wear and tear ending up in the environment (see Section 2), 5871 tonnes (67%)
is estimated to end up in soil, 1040 tonnes (12%) in air, 520 tonnes (6%) directly in surface waters,
and 1337 tonnes (15%) in sewers. From the 1337 tonnes entering sewers, 814 tonnes are estimated to
remain in the WWTP and 523 tonnes to pass. So, in total 1043 tonnes, or 12% will eventually end up in
surface waters [29].
Nizzetto and colleagues [
66
], estimated that 50% of the WWTP sludge in Europe and North
America is used as a fertiliser on farmland. In European and US regulations microplastics are not
named as a harmful when present in sludge to be used as fertiliser. Nizzetto and colleagues [
66
] used
the INCA-contaminants model to study the transport of microplastics from the soil to the aquatic
environment. INCA is a processed based dynamic model representation of plant/soil system dynamics
and instream biogeochemical and hydrological dynamics. About 16–38% of the microplastics spread
with the WWTP sludge on the land remain in the soil. Calculating with the 814 tonnes remaining in
Dutch WWTP, this would imply an extra 252–342 tonnes/year will be taken by wind and rainwater to
the aquatic environment. This just as an example; we do not know the Dutch percentage of WWTP
sludge used as fertiliser.
3.1.3. Transport by Rivers
Most microplastics will float in the water column, while lighter particles will drift on the water
surface. Depending on the flow rate of the river, heavier particles may migrate along the riverbed [
67
].
In the river, microplastics can get covered by micro-organisms forming a biofilm that may cause the
particles to sink to the riverbed [15].
Schuchardt and colleagues [
68
] measured microplastics concentrations in the Unterweser,
a German river flowing into the North Sea. They counted 25 particles per litre in the water column,
whereas 2260 particles per kg dry matter were counted in the sediment. Considering realistic river
flow rates, these numbers suggest that most of the particles will remain mobile and will ultimately
flow with the water into the North Sea. The Unterweser is a tidal river. The flow in the ebb stream
can reach 1.4 m/s, and in the flood stream 1.2 m/s [
69
]. In slower flowing waters, transport might be
different and heavier particles might sink to the river bed and remain in the sediment.
Nizzetto and colleagues [
70
] used the INCA-contaminants model to study the distribution of
microplastics in the 217 km non-tidal part of the Thames from source to Teddington. Microplastics
have been modelled as pure particles by their dimensions and specific mass; the formation of biofilms
and the possibility of aggregation have not been incorporated in the model. They found that particles
Int. J. Environ. Res. Public Health 2017,14, 1265 18 of 31
size 1–5
µ
m are effectively transported by the water in depended of their specific mass. Size seems to
be the dominant parameter in transport by water. For sizes
100
µ
m the model predicts a retention
rate 40% for the whole stretch.
Besselink and colleagues [
15
] built a model to simulate the retention of microplastics in a 40 km
stretch of a small Dutch river (De Dommel) with an average flow rate of 0.2 m/s. There is a sediment
settling area after 14 km and there are several weirs. The model included particle size and density, burial
to sediments, aggregation to suspended solids and biofilm formation. The model was parameterized
based on literature data. Aggregation was identified as the main retention mechanism for 100 nm
particles, i.e., the concentration did not significantly decrease at the settling area but decreased
gradually over the 40 km stretch. The simulation over the 40 km stretch showed a 60% retention rate
for particles
1
µ
m and a 100% retention rate for particles
50
µ
m. Retention rate showed a minimum
of 18% for 4 µm particles [15].
Combining these figures, it is possible to roughly estimate the fraction of microplastics entering
surface waters to reach the ocean. According to the UNEP about 50% of the world’s population is
living within 60 km from the coast [
2
], implying that 50% of the world’s population lives on average
30 km from the shore. This distance is quite similar to the length of the river stretch studied by
Besselink and colleagues [
15
]. The retention rates reported by Besselink and colleagues [
15
] could thus
be used as a starting point for estimating the average worldwide retention in rivers, especially the 60%
retention reported for submicron particles (
1
µ
m). For larger particles (i.e.,
50
µ
m) the retention
rate reported by Besselink and colleagues [
15
], i.e., 100%, may not be representative, since they studied
a river with a relatively low flow rate, hence high sedimentation rate, and did not consider possible
resuspension of sedimented particles due to extreme flooding events [
2
]. To cover for these processes,
a 90% retention could be used as a first best guess for particles larger than 50
µ
m. The retention of
particles in the 1–50
µ
m size range could then be derived by linear interpolation from the 60% retention
for the submicron fraction and 90% retention for the fraction larger than 50
µ
m. If particle size is
unspecified, an average retention of 75% seems a reasonable assumption.
3.2. Contribution of Tyre Wear and Tear to Plastic in the Oceans J
Jambeck and colleagues [
4
] estimated the amount of plastic from mismanaged waste entering
from land into the oceans for almost every country. First, the amount of mismanaged waste within
50 km of the coast was estimated. Mismanaged waste is defined as either littered waste or inadequately
disposed waste. Primary microplastics as tyre wear and tear are not included in these figures. Next,
the percentage of plastics in the mismanaged waste was estimated. On average 11% of the mismanaged
waste consists of plastics. Finally, to estimate the amount of mismanaged plastic waste entering the
ocean, the San Francisco Bay watershed was studied. The unmanaged amount was compared to the
amount collected by street sweeping, in storm water catchments and pump stations. The percentage
of uncollected plastic waste available to enter the ocean was estimated at 61% on average with a
minimum of 36% and a maximum of 95% [4].
Table 15 compares the total amount of plastics estimated to enter the ocean in Norway and The
Netherlands to the amount of tyre wear and tear. In Norway, 7884 tonnes/year of tyre wear and tear
is being released to the environment, including both synthetic and natural rubbers (Section 2.2.3).
About half this amount, i.e., 3942 tonnes/year, is expected to end up in the ocean [
10
]. In The
Netherlands, 1043 tonnes of wear and tear is estimated to enter surface waters every year (Section 3.1.2).
Assuming an average retention of 75% (Section 3.1.3), 261 tonnes will reach the oceans. Both estimates
do not include the contribution from atmospheric deposition, since insufficient data are available to
estimate this source reliably.
The relative contribution of tyre wear and tear to the overall load of plastics in the oceans varies
considerably between these two countries, i.e., 0.9% for The Netherlands and 31.9% for Norway.
Considering The Netherlands a “best case” for emission of tyre wear and tear into the oceans,
and Norway a “worst case”, the relative contribution of tyre wear and tear to the global loading
Int. J. Environ. Res. Public Health 2017,14, 1265 19 of 31
of the ocean with plastics can be estimated to be in the range of 5–10%. Important uncertainties and
variable factors in this estimate include (1) the amount of tyre wear and tear retained in sewers and
WWTPs; (2) the river retention of tyre wear and tear; and (3) the amount of tyre wear and tear directly
discharged to the ocean.
Table 15.
The amount of wear and tear of car tyres compared to the total amount of plastics entering
the oceans by land (pop. = population as per July 2016 [42]).
A: Total Plastic
into Oceans
(tonnes/year) [4]
B: Tyre Wear and
Tear into Oceans
(tonnes/year)
% Tyre Wear and Tear
of Total (B/(A + B))
Per Capita Tyre
Wear and Tear
Emission
(kg/year/person)
The Netherlands
(pop. 17,016,967) 27,700 261 (0.9%) 0.9% 0.015
Norway
(pop. 5,265,158) 8400 3942 (31.9%) 31.9% 0.75
3.3. Transport by Air
In terms of mass, only a small fraction of the tyre wear and tear particles generated become
airborne. This is because larger particles (>10
µ
m) tend to deposit on the road or close to it and
these particles constitute the major part of the mass being released. Based on a review of the available
literature, Grigoratos and Marini [
20
] conclude that the mass fraction becoming airborne varies between
0.1% and 10%, although some studies report fractions up to 30% [20].
The fate of airborne wear particles strongly depends on size. Distinction can be made between
particles >10
µ
m, 1–10
µ
m, 0.1–1
µ
m and <0.1
µ
m. The behaviour of particles larger than 10
µ
m in
diameter is governed by gravitational forces and will typically deposit close to the source. Hence,
these particles typically constitute only a small fraction of the airborne particles. The behaviour
of particles in the 1–10
µ
m range strongly depends on particle characteristics and local conditions.
These particles can stay in the air for minutes to hours and typically travel distances varying from
hundred meters to as much as 50 km. Specific transport data on the 0.1–1
µ
m fraction are lacking, but it
is well known that PM
2.5
particles (i.e., particles <2.5
µ
m) can stay in the air for days or weeks and
travel more than a thousand kilometres. Particles in the nano range (i.e., <0.1
µ
m) are subject to various
processes influencing their fate. Due to their small size, electrostatic forces may result in adsorption of
nanoparticles to the road surface or vehicle carcass [
71
]. Furthermore, Dall’Osto and colleagues [
72
]
demonstrated that tyre wear and tear nanoparticles may be encapsulated by road wear resulting in
larger particles of mixed composition [
72
]. This may provide an explanation for the fact that several
studies have detected nanosized tyre particles under laboratory conditions, whereas such particles are
less often detected under more realistic road driving conditions. Nonetheless, a few studies also report
the emission of nanosized particles under realistic road driving conditions [
23
,
27
]. These conflicting
results make it difficult to assess whether and how many nanosized tyre wear and tear particles are
being released to air. However, even if nanoparticles are being released their transportation range
seems limited because these particles are subject to sorption and aggregation processes.
The contribution of tyre wear and tear to airborne PM
10
has been estimated in several studies,
mostly focusing on quantifying the contribution of non-exhaust PM emissions relative to exhaust PM
emissions. Based on data from several European countries, Ketzel and colleagues [
73
] estimated that
50–85% of the total traffic PM
10
emissions originates from non-exhaust sources [
73
]. The large variation
is due to factors such as the degree of precipitation (i.e., resuspension is less under wet conditions),
road surface characteristics and the type of tyres (i.e., studded tyres result in a substantial increase
of non-exhaust PM
10
emissions). Tyre wear and tear is typically expected to contribute least of the
non-exhaust sources, i.e., resuspension, road wear, brake wear and tyre wear and tear. Estimations
range from 0.1 to 10% for airborne PM
10
and 3–7% for airborne PM
2.5
[
20
]. However, one should keep in
Int. J. Environ. Res. Public Health 2017,14, 1265 20 of 31
mind that the contribution of tyre wear and tear to traffic PM10 may have been underestimated in these
studies due to the encapsulation of nanosized particles as reported by Dall’Osto and colleagues [72].
We were unable to identify studies that explicitly quantify the amount of tyre wear and tear
that ends up in the ocean after transportation by air. Although the fraction of tyre wear and tear in
airborne PM
10
is generally considered low (i.e., <1%), deposition of marine aerosols may still contribute
significantly to the overall load in our oceans since 70% of the earth’s surface is covered by oceans.
The sources and composition of marine aerosols have been extensively studied, but remarkably few
studies have looked at the presence of microplastics in marine aerosols, let alone tyre wear and tear.
The few studies that are available show conflicting results. Dall’Osto and colleagues [
72
] quantified
tyre dust in aerosol samples taken at different European monitoring sites, including three marine
sites [
72
]. For each of these sites at least 100,000 particles were analysed and less than 5 tyre wear and
tear particles per site were detected (Dall’Osto, personal communication, 19 July 2017) [
74
], implying
a negligible amount of tyre dust in marine aerosol. Fu and colleagues [
75
] analysed the organic
molecular composition of marine aerosol samples collected during at the Arctic Ocean. They analysed
more than 110 individual organic compounds which were grouped into different classes based on
the functionality and sources. One class was labelled “plastic emission” based on the detection of
phthalate esters. This group was reported to be the fourth in terms of source strength with a mean
relative abundance of 8.3%. Although tyre wear and tear by no means is the only source of phthalates
esters [
76
], these results suggest that airborne plastic particles can be transported over long distances
and may ultimately be deposited into our oceans.
Unfortunately, data are lacking to reliably estimate the fraction of airborne tyre wear and tear
ultimately reaching our oceans. One reason is the complete lack of data on the (photo)degradation
of wear and tear particles in ambient air. It is therefore recommended to perform more research
on the degradation of tyre wear and tear particles in air and the presence of tyre wear and tear in
marine aerosols, e.g., by focussing on the use of distinct tracers such as hydrogenated resin acids and
benzothiazoles [77,78].
4. Health Effects
Ultimately, humans and ecosystems can be exposed to the tyre wear and tear released into the
environment. For humans, the most relevant exposure route is inhalation of airborne particles [
20
,
79
]
Marine and other aquatic organisms may be exposed to tyre wear and tear through ventilation
(gills) and feeding [
80
]. Filter feeders and sediment dwelling organisms can be expected to have
the highest exposure because their feeding strategy involves the direct uptake of food particles from
the water and/or sediment. Many of these organisms, e.g., mussels and oysters, are important
commercial seafood species. Hence, the question arises whether human health may be at risk due to
the consumption of polluted seafood. These human health issues are discussed in more detail below.
4.1. Health Effects from Inhalation
It is well-known that inhalation exposure to airborne particles can trigger a wide range of adverse
health effects [
81
]. The effects depend on factors such as the particle concentration in the air, the size
distribution of the particles, their shapes, their chemical composition and ventilation intensity. From a
mechanistic viewpoint, distinction is often made between physical effects of particles (i.e., resulting
from the physical interaction between particle and tissue) and toxicological effects of particle leachates.
This distinction is not always easily maintained for tyre wear and tear particles since the samples
collected and tested in practice often represent a heterogeneous mixture of many different chemicals
and structures, including rubber, synthetic polymers, Zn, carbon, other additives, road wear, brake
wear and exhaust [
72
]. Here, we first briefly review the available literature on the physical effects of
plastic particles, before discussing the toxicity of leachates from tyre wear and tear. We then review
the available toxicity studies with tyre wear and tear and discuss its toxic potential based on its
contribution to PM2.5 and the global health burden attributed to air pollution.
Int. J. Environ. Res. Public Health 2017,14, 1265 21 of 31
Larger particles (>1–10
µ
m) penetrate less deep in the lung and are more likely to be subject to
mucociliary clearance [
82
]. Particles <1
µ
m can get deposited deeper in the lung and for these particles
uptake across the epithelium is possible, e.g., by means of diffusion, passive cellular penetration or
active uptake (endocytosis) [
83
]. Studies with model mammalian systems suggest that submicron
particles can translocate to the lymphatic and circulatory systems, but it is not yet clear to what extent
this phenomenon results in accumulation in secondary organs and poses a threat to the immune
system or cell health [84].
An increase in respiratory disorders after exposure to airborne plastic particles has been reported
in several occupational studies, i.e., workers processing nylon flock, different types of plastic fibres and
synthetic textile [
83
]. Effects detected include respiratory irritation, reduced lung capacity, coughing
and increased phlegm production. This is in line with findings of histopathological analyses of lung
biopsies reporting interstitial fibrosis and locations of inflammatory lesions. Although no evidence for
increased lung cancer was found in nylon flock workers, slightly higher levels of plastic microfibers
have been detected in malignant lung tissue taken from patients with different types of lung cancer
than in nonneoplastic lung tissue [83].
Besides effects resulting from the physical contact between particles and cells or tissues, effects
may also be triggered by chemicals leaching from wear and tear particles. Several studies have
shown that toxic effects are associated with the metals in these particles. For example, Gottipolu and
colleagues [
85
] found that the water-soluble zinc and copper fraction of tyre dust was associated with
increased levels of cardiac oxidative stress detected in rats exposed to high levels of this dust (5 mg/kg
rat) [
85
]. The presence of zinc has also been associated to the toxicity of tyre particles leachates in
studies with human lung cells [
86
]. Findings of epidemiological studies seem to confirm that airborne
Zn particles can trigger acute respiratory responses [
87
]. The toxic potential of organic components in
tyre wear and tear has been demonstrated in human lung cells [86].
In vitro
tests in which human lung cells and macrophages were exposed to tyre wear and tear
particles have reported inflammatory responses, e.g., secretion of interleukin-6, interleukin-8, tumour
necrosis-factor
α
and altered protein levels [
88
,
89
].
In vivo
tests in which animals were exposed to
samples containing tyre wear and tear show contradictory results. A study in which adverse effects
were detected in rats exposed to air PM collected at locations with high traffic density, related these
effects to different sources, one of these being tyre wear and tear (Zn) [
90
]. However, other studies
found that tyre and road wear particles generated in a road simulator laboratory triggered only
minimal lung alterations, considered insufficient in extent or severity to have an impact of pulmonary
function [9193].
It can be concluded that unambiguous toxicological data on the inhalatory effects of tyre wear and
tear particles are currently lacking. However, tyre wear and tear has been estimated to contribute 3–7%
to PM
2.5
(see Section 3.3) and the toxic potential of PM
2.5
has been well established. Using PM
2.5
as
an exposure metric, the World Health Organisation recently estimated that outdoor air pollution was
responsible for 3 million deaths globally in the year 2012 [
94
]. This suggests that tyre wear and tear
may contribute to the global health burden due to air pollution. However, unambiguous conclusions
cannot be drawn since is not yet known what components in PM
2.5
contribute most to its detrimental
effects. This stresses the urgency of identifying those components.
4.2. Health Effects from Food Intake
To what extent foodborne exposure to microplastics poses a human health risk is not
well-understood [
95
]. The risk depends on the one hand on the level of exposure and on the other
on the inherent toxic potential of the wear and tear particles. Since uptake of microplastics has been
documented for hundreds of aquatic food species at several trophic levels (see [
96
] and the references
therein), also ingestion of tyre wear and tear is to be expected. As a consequence, even though the
presence of tyre wear and tear in aquatic food has not been documented yet, human exposure to
microsized and nanosized tyre particles via the consumption of aquatic food species seems apparent,
Int. J. Environ. Res. Public Health 2017,14, 1265 22 of 31
in particular in case of aquatic animals that are consumed whole. Due to atmospheric deposition,
microplastics can also enter terrestrial systems and the soil [
97
]. For instance, synthetic fibres and
fragments have been identified in honey [
98
]. It can therefore be assumed that tyre wear and tear can
also end up in terrestrial food.
Potential toxic effects of tyre wear and tear particles via the food can either be local or systemic.
Systemic effects depend on intestinal absorption followed translocation to the target organ or site.
Currently, there are no specific studies on the intestinal uptake of tyre wear and tear, but several
intestinal uptake mechanisms are documented for microsized and nanosized particles [
99
101
]. In fact,
nanosized particles with a diameter below 200 nm could display a higher bioavailability due to the
potential uptake via receptor-mediated endocytosis [
102
]. However, bioavailability is influenced by
the interaction with proteins and other biomolecules, leading to the formation of a biomolecule corona,
a process which in turn is affected by gastrointestinal digestion processes [
103
]. Another important
factor is the interaction with mucus, the intestinal wall is known to provide an effective barrier against
micro- and nanoparticles as the majority is discarded from the intestine or trapped in mucus before
reaching the epithelium [
104
]. Yet, topical application of particles such as polystyrene and diesel
particulates is shown to disrupt the mucus barrier which could possibly increase bioavailability [
105
].
Nevertheless, in general it can be assumed that in analogy with plastic particles, tyre wear and tear of
different size classes can indeed be internalized by the intestinal epithelium, but the effective uptake
is probably low [
99
]. Tyre wear and tear particles are expected to cause local inflammatory effects
in the intestinal lumen, in a similar way as observed under respiratory exposure in mice [
101
,
106
].
In addition, the presence of microsized (tyre) particles in the intestinal lumen could also pose a threat
due to the potential leaching of toxic substances to the intestinal tissue. Zinc oxide, the main form
of zinc in tyre wear and tear, is considered relatively non-toxic, but several compounds could pose a
threat such as the carcinogenic polycyclic aromatic hydrocarbons (International Agency for Research
on Cancer (IARC) Group 1–3), carbon black (possibly carcinogenic, IARC Group 2B), non-redox-active
heavy metals like cadmium, lead, nickel and redox-active metals like copper and iron [
107
,
108
]. In fact,
toxic effects from aqueous leachates of tyre wear and tear have been documented in the green alga
Raphidocelis subcapitata, the water flea Daphnia magna and the frog embryo Xenopus laevis [
109
,
110
]
and exposure to organic tyre wear and tear extracts led to genotoxic effects due to oxidative stress
in human lung cell line A549 [
86
]. Although these results clearly suggest a potential risk, extensive
toxicological research is necessary to enable a comprehensive human effect assessment.
5. Mitigation
5.1. Wear Resistant Tyres
The European Tyre Labelling Regulation 1222/2009/EC [
111
] requires the labelling of tyres for
rolling resistance (aiming at lower fuel consumption), wet slip resistance (aiming at improved safety),
and noise (aiming at noise reduction). Wear and tear of tyres is not explicitly covered by a European
regulation. The introduction of new regulations aiming at the reduction of wear and tear could thus be
an option. However, it should be realized that the different regulatory requirements imposed on tyres
are interrelated. Within the tyre technology, this is known as the “magic triangle”, i.e., the relationship
between rolling resistance, slip resistance and wear resistance. Improving one, will deteriorate the
other (Noordermeer, personal communication, 17 May 2017) [
112
]. An improved wear resistance
would thus result in a poorer rolling resistance and slip resistance. This means that a compromise has
to be sought between fuel consumption (through rolling resistance), safety (slip resistance), durability
(wear resistance) and environmental considerations (wear resistance). On one hand this requires the
application of techniques that can reflect these different dimensions in a common denominator, e.g.,
Life Cycle Impact Assessment (LCIA), and on the other hand the involvement of different stakeholders
(e.g., tyre industry, government, environmentalists, etc.) to weigh the different values involved, e.g.,
involving the application of Multi-Criteria Analysis (MCA).
Int. J. Environ. Res. Public Health 2017,14, 1265 23 of 31
5.2. Electric Cars
Experiments with motorbike tyres in a road simulator showed a linear relationship between tyre
load and tyre wear and tear [
22
,
113
] see Figure 4. As electric cars (E-cars) are, due to their battery
pack, heavier than Internal Combustion Engine (ICE) cars, E-cars will produce more tyre wear and
tear [113,114].
Table 16 compares the weights of some common ICE-cars and their electric alternatives.
On average, the electric versions are approximately 20% heavier. Assuming a linear relationship
between weight and tyre wear and tear emission, this emission will be about 20% higher for E-cars.
It can be concluded that current electric cars will not solve the particulate matter problem. They will
reduce the PM
10
problem by eliminating exhaust emissions and reduced brake wear [
115
], but at the
same time they will increase the problem by increased emission of tyre wear and tear. Only if the
weight of batteries is substantially reduced, which seems to be likely in the near future [
116
], a net
gain in terms of human health effects seems evident.
Table 16.
Weight of Internal Combustion Engine (ICE)-cars compared to the electric version. The weight
of the petrol car includes a half full tank.
Petrol Version Weight [kg] E-Version Weight [kg] Extra Weight
E-Version
Volkswagen high Up! petrol 958 Volkswagen e-Up! 1114 16%
Volkswagen Golf 1.4 TSI 1205 Volkswagen E-Golf 1485 23%
Ford Focus 1380 Ford Focus Electric 1674 21%
Mercedes-Benz B 250 1465 Mercedes-Benz B 250 e 1725 18%
Int. J. Environ. Res. Public Health 2017, 14, 1265 23 of 30
reduce the PM10 problem by eliminating exhaust emissions and reduced brake wear [115], but at the
same time they will increase the problem by increased emission of tyre wear and tear. Only if the
weight of batteries is substantially reduced, which seems to be likely in the near future [116], a net
gain in terms of human health effects seems evident.
Table 16. Weight of Internal Combustion Engine (ICE)-cars compared to the electric version. The
weight of the petrol car includes a half full tank.
Petrol Version Weight [kg] E-Version Weight [kg] Extra Weight E-Version
Volkswagen high Up! petrol 958 Volkswagen e-Up! 1114 16%
Volkswagen Golf 1.4 TSI 1205 Volkswagen E-Golf 1485 23%
Ford Focus 1380 Ford Focus Electric 1674 21%
Mercedes-Benz B 250 1465 Mercedes-Benz B 250 e 1725 18%
Figure 4. Non-exhaust particulate matter (PM) emissions by source and car size, from Simons [113]
based on Ntziachristos and Boulter [117].
5.3. Self-Driving Cars
Self-driving cars can be programmed to reduce wear and tear. Examples include quiet
acceleration, taking bends slowly and improved anticipation to traffic circumstances resulting in
fewer intense braking events. If all cars would be computer driven, driving could also become more
intrinsically safe. This could alter the balance of the magic triangle between rolling resistance,
slipping resistance and wear resistance (see Section 5.1), resulting in a higher priority for wear
resistance and thus less wear and tear. After all, road safety of self-driving cars will be part of the
Internet of Things (IoT) they will rely for their safety more on Artificial Intelligence (AI) (i.e.,
controlled acceleration and braking; anticipation of traffic circumstances, being interconnected) than
on the slipping resistance of tyres.
5.4. Sewers and Waste Water Treatment Plant Efficiency
Sewers and WWTPs play an important role in the loading of surface waters (see Section 3.1).
One option to reduce this loading is to increase WWTP treatment efficiency. However, to our
knowledge no studies are available that systematically analysed the processes responsible for
removing wear and tear particles, or microplastics in general, from WWTPs. It seems plausible that
sedimentation plays an important role for larger particles, and potentially also for smaller particles
after aggregation. Like demonstrated for graphene nanomaterials, the addition of a suitable
Figure 4.
Non-exhaust particulate matter (PM) emissions by source and car size, from Simons [
113
]
based on Ntziachristos and Boulter [117].
5.3. Self-Driving Cars
Self-driving cars can be programmed to reduce wear and tear. Examples include quiet acceleration,
taking bends slowly and improved anticipation to traffic circumstances resulting in fewer intense
braking events. If all cars would be computer driven, driving could also become more intrinsically
safe. This could alter the balance of the magic triangle between rolling resistance, slipping resistance
and wear resistance (see Section 5.1), resulting in a higher priority for wear resistance and thus less
wear and tear. After all, road safety of self-driving cars will be part of the Internet of Things (IoT) they
Int. J. Environ. Res. Public Health 2017,14, 1265 24 of 31
will rely for their safety more on Artificial Intelligence (AI) (i.e., controlled acceleration and braking;
anticipation of traffic circumstances, being interconnected) than on the slipping resistance of tyres.
5.4. Sewers and Waste Water Treatment Plant Efficiency
Sewers and WWTPs play an important role in the loading of surface waters (see Section 3.1).
One option to reduce this loading is to increase WWTP treatment efficiency. However, to our knowledge
no studies are available that systematically analysed the processes responsible for removing wear
and tear particles, or microplastics in general, from WWTPs. It seems plausible that sedimentation
plays an important role for larger particles, and potentially also for smaller particles after aggregation.
Like demonstrated for graphene nanomaterials, the addition of a suitable coagulant may reduce the
particles from the wastewater stream [
118
]. The application of tertiary treatment techniques such as
UV radiation and oxidation techniques are also likely to remove wear and tear particles from the water
phase. Herbort and Schuhen [
119
] proposed the application of innovative inorganic-organic hybrid
silica gels which have the ability to remove stressors such as microplastics from wastewater.
A second option to reduce wear and tear loading of surface waters is to limit the use of separated
sewer systems in which the runoff from roads is discharged directly into surface waters. However,
this would require a substantial increase of WWTP capacity and thus be expensive. A more viable
option is to develop a more efficient trapping device for wear and tear particles to be applied before
the runoff enters the sewer system (e.g., in the gutter) or before it is discharged into surface water.
Collection of runoff and temporary storage in a sedimentation basin could already substantially reduce
the load, particularly for the larger particles.
5.5. Open Asphalt Concrete
The pavement material is an important factor in tyre wear and tear. The pavement could be
designed to reduce wear [
120
]. Designing roads using (very) open asphalt concrete could reduce
emissions while catching the coarse part of tyre wear and tear [13].
6. Conclusions
The present review shows that wear and tear from tyres constitutes a significant global source
of microplastics in the environment. The emission of tyre wear and tear from cars was estimated for
different countries using two different methods, i.e., using (1) emission factors per vehicle-km and total
mileage; and (2) the number of tyres used combined with their weight loss. Both methods resulted in
comparable results, forming an indication that emissions can be reliably estimated with either of both
methods. The emission per capita is in the same order of magnitude for all countries, i.e., between 0.23
and 1.9 kg/year, with a 4.7 kg/year outlier for the USA.
Although quantification of environmental pathways remains a challenge, the relative contribution
of tyre wear and tear to the total global amount of plastics ending up in our oceans was roughly
estimated to be in the range of 5–10%. This makes wear and tear from tyres at least as important as
plastic bottles, bags and fibres released from clothing during washing [
2
]. These numbers underline
that tyre wear and tear deserves a higher place on the political agenda and that emission reduction of
tyre wear and tear should be given higher priority than it currently receives.
Although the pathways and potential adverse effects of tyre wear and tear are largely known,
quantification of these pathways and the associated risks remains a tough scientific challenge. First and
foremost, it remains difficult to quantify the emission of tyre wear and tear under realistic driving
conditions and to characterize the particles that are being released in terms of numbers and sizes.
An important complicating factor is the mixing with other particles, i.e., road and brake wear.
The development and application of unique robust tracers for tyre wear and tear can substantially
improve our understanding of the amount of wear and tear being released into the environment and
its dispersal with ambient air and runoff [77,78]. For the route via runoff, important knowledge gaps
include the amount of wear and tear trapped in road surfaces and soils, the removal efficiency in
Int. J. Environ. Res. Public Health 2017,14, 1265 25 of 31
WWTPs and the fate (i.e., retention) of wear and tear particles in surface waters. For the route via
air, the most important challenge is to quantify the amount of tyre wear and tear in environmental
matrices far away from the source, e.g., in marine aerosols. This could confirm whether deposition of
tyre wear and tear constitutes a significant source of microplastics in our oceans. For all environmental
dispersal routes, quantification of degradation remains an important challenge.
In terms of human health risks, the first and foremost challenge is to quantify human exposure
in a realistic manner. Numerous studies have shown that fine particles, including tyre wear and tear,
can trigger a range of adverse health effects (see Section 4). However, without reliable information
on realistic exposure levels it remains unclear whether such effects are likely to occur in real-life.
Exposure is best characterized for the inhalation route, where it was estimated that tyre wear and tear
contributes 3–7% to the ambient PM
2.5
fraction (see Section 3.3). This suggests that tyre wear and tear
may contribute to the global health burden of air pollution which has been projected by the WHO at
3 million deaths in 2012. However, this statement should be treated with care since it is not yet known
what components in particulate matter contribute most to the effects. Exposure is much less well
characterized for the intake of tyre wear and tear with (sea)food. Although there is no acute reason for
concern, some specific target groups such as people eating large amounts of mussels, oysters and other
seafood species that are consumed without removal of the intestines, may be exposed, especially when
living in highly polluted coastal areas. Research on exposure should therefore primarily focus on these
specific types of seafood, target groups and locations.
Industry, regulators and consumers quickly undertook action when it became clear that
microbeads in cosmetics contributed to the microplastics in the environment [
121
]. This was relatively
easy since there was a simple solution, i.e., replacing the plastic microbeads by natural beads, e.g.,
ground walnut shell. Tyre wear and tear constitutes a much more important source of microplastics
in the environment, but awareness is low and currently there is no alternative for tyres. It can be
concluded that tyre wear and tear is a stealthy source of microplastics in the environment, which can
only be addressed effectively if awareness increases, knowledge gaps are being closed and creative
solutions are being sought. This requires a global effort from all stakeholders; consumers, regulators,
industry and researchers alike.
Acknowledgments:
This research was funded by the Faculty of Management, Science & Technology,
Open University (Heerlen, The Netherlands) and Radboud University (Nijmegen, The Netherlands).
Conflicts of Interest: The authors declare no conflict of interest.
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2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).
... From these tires, 35% are used in the 6.6 million European fleet of heavy goods vehicles (HGVs) [5]. Hence, it is clear that the potential amount of tire wear from HGVs could be significant, a fact proven by a survey that estimated it to be about 2 million tons in 2013 from among various countries in the world [6]. Considering the level of total wear and its environmental impact, there is an increasing need to develop more environmentally-friendly vehicle systems that can decrease tire wear and secure the EU's goal of net-zero emissions by 2050 [7] and protect human health from the hazards of heavy metal contamination included in the particulate matters from tire wear [8]. ...
... The model is governed by the following equations of motion (Eqs. [4][5][6]): ...
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... The contribution of recreational activities is discussed by Scopetani et al. (2019), who attributed the notable quantities of polyamide (PA) in the area to synthetic clothing from individuals preforming outdoor activities. MPs also enter stormwater from the weathering of artificial turfs, outdoor paints, and roof membranes (Kole et al., 2017;Lassen et al., 2015;Sundt et al., 2014). ...
... TWPs are the largest component in road runoff and are one of the dominant contributors to MP pollution, accounting for an estimated 28% of global primary MP pollution (Boucher and Friot, 2017). Kole et al. (2017) estimated that 5-10% of the global plastic load into oceans is also derived from tyres. The studies listed in Table 3 estimate that rubber contributes 4-64% of MP pollution, which further demonstrates that TWPs are a notable source of MPs in stormwater. ...
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... Commuter knowledge of high pollution routes, zones and times would allow for more informed decisions on when and where to use cars in Greater Cairo. Also community awareness would drive policymakers to adopt mitigation strategies that aim to reduce the overall volume of traffic, lower speed limits in motorways, promote driving behaviour that reduces braking, redirect road traffic away from construction sites, carry out road sweeping, street washing or apply dust suppressants to street surfaces (Amato et al., 2009;Kole et al., 2017;Monks et al., 2019). -Allaban et al., 2007;Zakey et al., 2008;Lowenthal et al., 2014). ...
... -Allaban et al., 2007;Zakey et al., 2008;Lowenthal et al., 2014). Also it is estimated that 3-7% of PM 2.5 in air globally is a result of tyre wear and tear (a byproduct of heavy traffic) contributing to the health burden of air pollution (Kole et al., 2017). exposes car users to spikes in PM concentrations caused by pollution inducing events such as construction activities, unmaintained roads (dust re-suspension), proximity to desert environments (higher PM 10 concentrations) and high traffic congestion (increased PM 2.5 levels owing to transport emissions as well as tyre and brake dust). ...
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Market research shows there is a growing demand for winter tires driven by increased legislation, anticipated harsher winters, and governments and tire manufacturers pushing for greater public awareness of winter driving safety. There is a push in the tire manufacturing industry to increase fuel efficiency and minimize noise and road wear while simultaneously improving ice, wet and dry traction in winter tires. Key advances in winter tire performance are expected to occur in the tread compound (as opposed to tread design), aided by the development of advanced silica technologies. It was shown that the dynamic properties of the tread compound influence the mechanisms of friction in dry cold weather, ice and snow. Patent literature indicates that most winter tire tread compounds contain primarily BR, due to its low Tg, as well as low-to-moderate surface area silica. The studies reported here were aimed at predicting the effects of various formulation changes on the microhysteretic properties of a model winter tire tread compound using DMA. The elastomer ratio (sSBR, BR and NR), silica surface area and loading, and silica to carbon black ratio in the tread formulation had a significant impact on the predicted winter tire performance. While high-BR content provides favorable predicted winter tire performance, NR is added to improve warm weather dry traction, sSBR is added to improve rolling resistance, and combinations of both sSBR and NR are added to improve wet traction. In addition, a higher loading of lower surface area silica (i.e., Hi-Sil EZ90G-D) was also shown to provide an overall improved predicted winter tire performance. This article is based on a paper presented at the 188th Technical Meeting of the Rubber Division, ACS, October 2015.
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The United States Microbead-Free Waters Act was signed into law in December 2015. It is a bipartisan agreement that will eliminate one preventable source of microplastic pollution in the United States. Still, the bill is criticized for being too limited in scope, and also for discouraging the development of biodegradable alternatives that ultimately are needed to solve the bigger issue of plastics in the environment. Due to a lack of an acknowledged, appropriate standard for environmentally safe microplastics, the bill banned all plastic microbeads in selected cosmetic products. Here, we review the history of the legislation and how it relates to the issue of microplastic pollution in general, and we suggest a framework for a standard (which we call “Ecocyclable”) that includes relative requirements related to toxicity, bioaccumulation, and degradation/assimilation into the natural carbon cycle. We suggest that such a standard will facilitate future regulation and legislation to reduce pollution while also encouraging innovation of sustainable technologies.
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Microplastics (MPs) are observed to be present on the seafloor ranging from coastal areas to deep seas. Because bioturbation alters the distribution of natural particles on inhabited soft bottoms, a mesocosm experiment with common benthic invertebrates was conducted to study their effect on the distribution of secondary MPs (different-sized pieces of fishing line<1mm). During the study period of three weeks, the benthic community increased MP concentration in the depth of 1.7-5.1cm in the sediment. The experiment revealed a clear vertical gradient in MP distribution with their abundance being highest in the uppermost parts of the sediment and decreasing with depth. The Baltic clam Macoma balthica was the only study animal that ingested MPs. This study highlights the need to further examine the vertical distribution of MPs in natural sediments to reliably assess their abundance on the seafloor as well as their potential impacts on benthic communities.
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Environmental contamination by plastic particles, also known as ‘microplastics’, brings synthetic materials that are non-degradable and biologically incompatible into contact with ecosystems. In this paper we present concentration data for this emerging contaminant in wastewater treatment plants (WWTPs) and freshwater and marine systems, reflecting the routes via which these particles can travel and the ecosystems they potentially impact along their path. Raw sewage influents, effluents and sewage sludge from seven municipal WWTPs in the Netherlands contained mean particle concentrations of 68–910 L−1, 51–81 L−1 and 510–760 kg−1 wet weight (ww), respectively (particle sizes between 10 and 5000 μm). Even after treatment, wastewater constitutes a source of microplastic pollution of surface waters, and via biosolids applications in farming and forestry, plastic retained in sewage sludge can be transferred to terrestrial environments. The WWTPs investigated here had a mean microplastics retention efficiency of 72% (s.d. 61%) in the sewage sludge. In the receiving waters of treated and untreated wastewaters, we detected high microplastic levels in riverine suspended particulate matter (1400–4900 kg−1 dry weight (dw)) from the Rhine and Meuse rivers. Amsterdam canal water sampled at different urban locations contained microplastic concentrations (48–187 L−1), similar to those observed in wastewater that is emitted from sewage treatment facilities in the area. At least partial settling of the particles occurs in freshwater as well, as indicated by microplastics in urban canal sediments (