1. The case for microplastics mitigation
This chapter sets out the motivation for the development of this report. It summarises current knowledge on the sources and flows of microplastics into the environment and on the adverse impacts on the health of humans and other living organisms. The chapter concludes by making the case for policy action on microplastics pollution and for further consideration of mitigation measures for two sources of microplastics: textile products and vehicle tyres.
1.1.1. Growth in plastics use and environmental consequences
Since the invention of plastic materials, their production, use and disposal has continued to increase. Due to their numerous desirable properties (e.g. durability, resistance, lightness), versatility and low costs of production, plastics have gradually penetrated almost all sectors and substituted traditional materials such as concrete, glass and wood in countless industrial applications. From 0.5 Mt in 1950, global annual plastics production soared to 465 Mt in 2019, as presented in Figure 1.1. Trends indicate that demand for plastic materials will continue to increase in future years, mainly driven by increasing shares of plastics consumption in emerging economies (IEA, 2018[1]).
Source: Update by the authors of (Geyer, Jambeck and Law, 2017[2])
The large production and use of plastic materials comes with several negative consequences for the environment and climate. Plastics production is a fossil fuel-intensive activity, consuming 4-8% of global oil production (by mass) (World Economic Forum, 2016[3]). Approximately 400 Mt of CO2 were released in the year 2012 during the production, transport and disposal of plastics (EU, 2018[4]). The large use of plastics, especially in products with short life spans (e.g. single-use plastics), also puts significant pressure on waste management systems (Geyer, Jambeck and Law, 2017[2]). In 2010, failure to channel waste to the adequate disposal systems resulted in the discharge into the oceans of 4.8-12.7 Mt of the 275 Mt of plastics waste generated on land (Jambeck et al., 2015[5]). Approximately 14 Mt of microplastics have accumulated on the ocean floor only (Barrett et al., 2020[6]).
Plastic materials are generally very resistant to degradation: they can last for prolonged periods of time if released into the environment, leading to several adverse consequences to our environment and economy (Geyer, Jambeck and Law, 2017[2]). Detrimental consequences on freshwater and marine ecosystems include ingestion by marine species and impediment of food acquisition, entanglement of wildlife in lost or discarded fishing nets and damage to coral reefs (Derraik, 2002[7]; Macfadyen, Huntington and Cappell, 2009[8]). Adverse economic consequences of plastics pollution may in particular affect coastal communities relying on tourism (e.g. due to the detrimental effects of marine and coastal litter on tourism, which also increase beach maintenance costs) and those affected by plastic pollution of internal water streams (e.g. as a consequence of the blockage of road drains).
1.1.2. Microplastics: framing the challenge
While the environmental consequences of plastic items dispersed in the environment have long been scrutinised by the scientific and media communities, concerns are now rising over microplastics (MP), plastic particles smaller than 5 mm (see Box 1.1).
Microplastics are generally defined as solid synthetic polymer particulates with a size < 5 mm (Arthur, Baker and Bamford, 2009[9]; ECHA, 2019[10]; GESAMP, 2015[11]). Because the definition of microplastics has not yet been standardised, existing studies have been employing different definitions and cut-off sizes. The upper size limit is generally set at 5 mm (although a cut-off at 1 mm is also employed by some studies), while there is some discrepancy with regards to the lower cut-off sizes employed, which range between 1 and 100 nm (ECHA, 2019[10]; GESAMP, 2015[11]; California State Water Resources Control Board, 2020[12]). Usually, plastics with a size below this lower bound are referred to as nanoplastics (SAPEA, 2019[13]). In general, the lack of consensus over the definition of microplastics has been an issue as it renders results of microplastics surveys and toxicological studies difficult to compare and aggregate in order to draw general conclusions, respectively on occurrence and risks. Work is underway to standardise definitions for microplastics in the environment (see CEN prEN 17615 Plastics – Environmental Aspects - Vocabulary).
Microplastics can be considered an array of contaminants of diverse sizes, shapes, colours and physico-chemical composition (Rochman et al., 2019[14]). They are usually found in the environment as fragments, fibres, pellets, or beads. Plastic polymers commonly employed in the manufacture of plastic products (e.g. polypropylene, polyethylene, polyethylene terephthalate) account for a large share of microplastics, although particles from other synthetic polymers such as rubber (e.g. in vehicle tyres) are also considered microplastics (SAPEA, 2019[13]). Microplastics are also always found in the environment as a complex mixture or diverse suite of chemicals, either those which have added during manufacturing or which have been adsorbed from the environment (see Section 1.5).
Note: despite the commonly employed upper size limit for microplastics, microfibres with a diameter larger than 5 mm can also be considered microplastics. For instance, the ECHA restriction on the intentional use of microplastics sets limits as follows: i) for particles, all dimensions between 1 nm and 5 mm, and (ii) for fibres, a length between 3 nm and 15 mm and length to diameter ratio of >3 (ECHA, 2019[10]).
Microplastics are stock pollutants, i.e. pollutants with a long lifetime and for which the ecosystem has little or no absorptive capacity. Biodegradation is the process of complete destruction of the polymer chain and its conversion into small molecules such as carbon dioxide, water, or methane by the action of microorganisms (UNEP, 2015[15]). Full biodegradation of microplastics requires a set of conditions (e.g. the presence of microorganisms capable of breaking down plastic polymers, appropriate temperature, adequate pH and salinity in case of the aquatic media) which are not typically present in the natural environment (UNEP, 2015[15]; Wagner and Lambert, 2018[16]). Current evidence of microplastics biodegradation in the natural environment is limited and the majority of microplastics is believed to persist and accumulate in the environment, only slowly degrading into smaller microplastics and potentially nanoplastics.
1.1.3. Objectives of this report
The continuous use and disposal of plastic materials around the globe has led to the pollution of all types of marine and freshwater environments with microplastics (Andrady, 2011[17]; Thompson et al., 2004[18]). Given projected trends of plastics production, use and disposal, concentrations of microplastics are likely to continue to increase, raising concerns over the potential environmental and human health hazards posed.
This report assesses mitigation options for microplastics pollution of marine and freshwater environments, with a focus on microplastics originating from textile products and vehicle tyres. It presents mitigation solutions implementable throughout the lifecycle of products, from manufacturing processes to the use phase and the end-of-life stages of textiles and tyres. Besides OECD countries, this report also assesses the perspective of major textile and tyre manufacturing countries, in particular China and India.
The remainder of Chapter 1 provides a justification for the development of this report. Chapter 2 traces a typology for microplastics released from textile products and vehicle tyres and outlines entry points for mitigation action. Chapter 3 provides an assessment of the available mitigation best practices and technologies implementable at different stages of the lifecycle of products. Chapter 4 argues that policy action to tackle unintentional releases of microplastics is emerging but remains limited and discusses selected policy instruments which could be employed to mandate, incentivise, or encourage the uptake of mitigation best practices and technologies. Chapter 5 presents key messages to guide mitigation policy action on microplastics released from textiles and tyres.
1.2.1. Types and sources of microplastics
Microplastics are typically categorised into primary and secondary. Primary microplastics are manufactured at the micro scale to be used in particular applications, while secondary microplastics stem from the fragmentation of larger plastics (GESAMP, 2016[19]; UNEP, 2018[20]).
Table 1.1 summarises the main types of microplastics and the relative sources and modes of emissions. Pre-production plastics are primary microplastics which may be unintentionally released into the environment, mainly due to accidental spills or run-off from processing facilities. Plastic pellets, the intermediary good between the polymers and plastic products, are known to regularly leak into the environment during production, transport and storage (GESAMP, 2015[11]). During use, numerous products may release microplastics intentionally-added during manufacturing. Microbeads used in personal care and cosmetic products (PCCPs) such as scrubs and toothpastes are examples of primary microplastics intentionally discharged into sewage waters or into the surrounding environment during the consumption stage.
Secondary microplastics can be further categorised into two groups. Use-based secondary microplastics are generated unintentionally due to abrasion occurring during the use of products containing synthetic polymers. Common examples are microfibres released from synthetic textiles during washing, tyre and road wear particles emitted during road transport activity, and paint flakes worn off from the surface of buildings, roads and ships. Degradation-based secondary microplastics are those originating from the fragmentation of larger plastic items discarded in the environment after their useful life, mainly as a consequence of exposure to solar UV radiation (GESAMP, 2015[11]).
1.2.2. Entry-pathways into the natural environment
Microplastics enter the natural environment via a number of pathways: i) direct discharge, ii) wastewater networks, iii) dry and wet deposition and surface run-off and iv) fragmentation of plastic waste that has leaked into the environment. These are summarised in Table 1.2 and further discussed in the following sections.
Transport via the municipal wastewater network (point-source microplastics)
Urban wastewater networks collect used water resources originating from households and/or industries. They receive microplastics originating from several sources, such as synthetic microfibers emitted during laundering (and manufacturing) and plastic microbeads contained in rinse-off consumer products. Additionally, the wastewater network may also collect diffuse-source microplastics contained in urban runoff, i.e. the flow of excess water occurring in urbanised areas containing a variety of pollutants washed off during precipitation events.1
Currently, urban wastewaters constitute a significant pathway for microplastics to enter aquatic environments. Over 80% of global used water resources are released into the environment without treatment, meaning that large amounts of microplastics and other pollutants are discharged directly into aquatic environments (WWAP, 2017[24]). The lack of wastewater treatment is an issue in several upper-middle and low-middle income countries (especially in Sub-Saharan Africa and in the Asia-Pacific region), where respectively only 38% and 28% of wastewaters are treated (Sato et al., 2013[25]; WWAP, 2017[24]). Overall, several non-OECD still lack the wastewater infrastructure required to guarantee adequate water supply and sanitation and to preserve water quality (OECD, 2015[26]).
In OECD countries, more than 80% of the population is connected to wastewater treatment facilities and the presence of at least secondary level treatment is prominent (OECD, 2015[26]). The treatment technologies already in place can significantly reduce microplastics concentrations of the wastewater influent (e.g. microplastics retention rates of up to 99% for wastewater treatment plants located in Finland) (Lares et al., 2018[27]; Talvitie et al., 2017[28]). However, the vast volumes of wastewaters treated imply that, in absolute terms, substantial amounts of microplastics are continuously being discharged from WWTPs into receiving water bodies.
The discharge of untreated wastewater from the waste water infrastructure can have a significant impact on the release of microplastics and on the overall water quality of surface waters. Discharge may occur due to technical faults at wastewater treatment plants (WWTPs) or sewer overflows occurring when the hydraulic capacity of the wastewater system is exceeded (Baresel and Olshammar, 2019[29]). Wastewater systems can either convey sewage only to the wastewater treatment plant (Separate Sewer Systems, SSSs) or sewage combined with storm water through a single pipe (Combined Sewer Systems, CSSs). Where CSSs are employed, periods of heavy rainfall may overload the sewer management system with storm water runoff, causing untreated domestic and industrial waste to be discharged directly into receiving waters in order to prevent flooding in the system. As discussed in Box 1.2, combined sewer overflows events are expected to become more frequent in future years due to climate change and continued urbanisation, unless infrastructure is adapted.
Furthermore, a share of microplastics retained by WWTPs can enter the environment via applications of wastewater sludge. Wastewater sludge is the waste by-product of wastewater treatment containing water pollutants removed from the influent. Sludge reuse for agricultural applications (via “landspreading”, i.e. the application to agricultural soil or in fertiliser production) is encouraged in several countries, mainly due to the high nutrient content and its beneficial effects on crops, as well as to reduce the need for landfilling or incineration (WWAP, 2017[24]). Although common national and regional regulations require that sludge undergoes stabilising treatment prior to its disposal or safe reuse, these do not include restrictions on microplastic concentrations, which usually vary between 1 000 and 170 900 particles per kg of dry sludge (Iyare, Ouki and Bond, 2020[30]). Evidence indicates that large amounts of microplastics are likely being directly discharged onto terrestrial environments via sludge use in agriculture. An estimated 63 000−430 000 and 44 000−300 000 tonnes of microplastics are applied every year onto farmlands in Europe and North America respectively (Nizzetto, Futter and Langaas, 2016[31]). Sludge disposal practices vary widely by country, as presented for selected OECD countries in Figure 1.2.
Transport via dry and wet deposition, stormwater and road runoff (diffuse entry)
The most significant entry pathway for diffuse-source microplastics is likely to be the action of rain events washing off particles and fibres suspended in air or deposited on outdoor surfaces (Dris et al., 2016[32]). Road runoff, i.e. the portion of precipitation which flows from road surfaces, is a known transport pathway for a variety of pollutants originating from diffuse sources (e.g. heavy metals, hydrocarbons, urban pesticides, litter), as well as for microplastics suspended in air or deposited on roads. Significant quantities of diffuse pollutants are washed off especially during the first minutes of intense rainfall.
Depending on the local context, road runoff can either be channelled into the wastewater network or be discharged directly into nearby surface waters or soil. In urban areas, runoff collected by drainage systems may be conveyed with sewage via CSSs to wastewater treatment plants to be treated. However, the majority of microplastics washed off by rain events likely end up directly in the environment (NIVA, 2018[33]). Especially in non-urban areas, road and stormwater runoff is commonly discharged directly into surrounding water streams, contributing to the deterioration of water quality. As outlined in Box 1.2, continued urbanisation and climate change are projected to magnify the incidence of diffuse pollution on the quality of surface waters, mainly as a consequence of the higher frequency of extreme precipitation patterns, increasing road traffic and sealing of surfaces, and the higher propensity for flooding and combined sewer overflows (OECD, 2017[34]).
Climate change puts increasing pressure on existing water quality challenges. Altered precipitation and flow regimes may result in higher volumes of runoff containing diffuse pollutants to water bodies, mainly due to a combination of increased pollutant loadings during heavy rainfall and an increased build-up of pollutants on catchment surface areas during long dry weather periods (OECD, 2017[34]). Furthermore, unless mitigation measures are taken, continued urbanisation and climate change are projected to worsen existing drainage problems and pose significant risks to the performance of sewer systems, potentially exacerbating the frequency and intensity of CSOs.
As climate change intensifies and extreme weather events become more frequent, this may also create higher pressures on aquatic plastic pollution. As indicated by recent studies, extreme weather events (such as heavy rainfall, flooding, droughts) discharge significant quantities of plastics and microplastics from land and river catchments into the oceans or river estuaries, either due to an effective flushing out of microplastics from rivers, or the transport of larger quantities of mismanaged plastic waste from land sources. For instance, a study conducted in Northern England found that flood events flush out 70% of the microplastics stored on the beds of rivers discharging into the Irish Sea (Hurley, Woodward and Rothwell, 2018[35]). Similar studies conducted in South Korea and India found that microplastics concentrations were three times greater following heavy rains and floods, respectively (Lee et al., 2013[36]; Veerasingam et al., 2016[37]).
Generation of secondary microplastics from macro plastic waste
The fragmentation of leaked macro plastics is likely to be a major contributor to microplastics pollution. Fragmentation into microplastics results from the degradation of ageing plastics, i.e. a change in the mechanical and chemical properties of materials (e.g. strength, colour, shape) causing the breakdown of plastic polymers.2 This typically occurs via photo degradation under exposure to UV radiation, via thermo-oxidative degradation under the effect of oxygen exposure and moderate temperatures, via physical degradation caused by abrasive forces, or via a combination of these processes (GESAMP, 2015[11]; UNEP, 2015[15]; Fraunhofer Umsicht, 2018[21]).
The rate of generation of degradation-based microplastics is poorly understood and difficult to predict due to the complexity of factors which influence it, as well as the time variability in degradation patterns. In general, the rate of degradation is mainly influenced by:
Plastics composition and age. Prodegradants are additives aimed at accelerating degradation processes and their utilisation (e.g. in oxo-degradable plastics) enhances the generation of microplastics (EC, 2018[38]). Conversely, the presence of additives aimed at preventing ageing and oxidation, such as UV stabilisers and anti-oxidants, generally slows down the degradation of plastics. While certain polymer structures may generate microplastics at a faster rate, this has not been extensively studied yet. In general, plastic polymers of items designed to last may be more resistant to weathering and degradation.
Environmental fate of plastics waste. Since the degradation and fragmentation of plastic s is mainly driven by the exposure to certain environmental factors, the rate of generation of microplastics is highly dependent on the environmental fate of plastics. Table 1.3 presents estimated rates of degradation and fragmentation of plastics in different environmental media. In general, plastic degradation is assumed to occur slowly in aquatic environments. Temperatures are generally too low to prompt thermo-oxidative degradation processes and UV light may only reach plastics floating on the surface of water bodies. Also, the development of biofilms on the surface of floating plastics may shield away UV light or cause the plastics to sink, impeding degradation (Gregory and Andrady, 2003[39]; Fraunhofer Umsicht, 2018[21]). Fragmentation into microplastics may occur relatively quickly on beaches, due to exposure to moderate temperatures, UV light and oxygen, as well as the abrasive action of sand and sea waves on plastic debris (Cooper and Corcoran, 2010[40]; UNEP, 2016[41]).
Modelling the environmental fate of marine plastic litter and its exposure to environmental conditions enhancing degradation remains a challenge due to the complex set of factors which may influence the horizontal and vertical transport of plastics in water, such as marine currents, the density of plastics compared to that of seawater and the creation of biofilms. Recent studies indicate that over 90% of mismanaged plastics entering the oceans end up in sediments and in the lower levels of the oceanic water column, where they may take a long time to degrade (Eunomia, 2016[42]; GESAMP, 2015[11]). The generation of microplastics may be relatively high on beaches and coastal areas, which receive approximately 5% of all plastic litter (mainly packaging and other single use-plastics) entering the oceans every year from land-based sources (Eunomia, 2016[42]).
The decomposition of landfilled plastic waste may also be contributing to microplastic pollution of the water cycle, through leachate from both active and closed landfills (He et al., 2019[43]; Praagh, Hartman and Brandmyr, 2019[44]). Landfill leachate is the liquid that has seeped through solid waste in a landfill and has been contaminated with pollutants originating from decomposing waste. It contains contaminants which are toxic for the environment and so it generally undergoes specialised treatment before being discharged. However, microplastics generated in sanitary landfills could leak into soil and groundwater where there are defects in landfill liners (He et al., 2019[43]). Further, it may also be the case that microplastics persist in leachate for a long time after the post-closure monitoring period (usually 30 years)3 and that these are released directly into water streams. Overall, the occurrence of microplastics in leachate remains largely unknown and more research is required to estimate the contribution of landfilled waste to microplastics pollution of soils and water streams (Magnusson et al., 2016[45]).
As illustrated in Figure 1.3, OECD and non-OECD countries tend to face different waste management and plastic pollution challenges. Even though the mismanagement of plastic waste is mainly an issue outside of the OECD area, microplastics leakage is an emerging reason of concern in most OECD countries. North America, Western Europe and Japan alone account for almost one third of global microplastics emissions and for 45% of total microplastics losses from tyre abrasion. This section discusses major trends in macro and micro plastics pollution in OECD countries and beyond.
1.3.1. Flows of mismanaged macro plastic waste
Human activities on land contribute to approximately 80% of the pollution of aquatic environments with plastic debris (Li, Tse and Fok, 2016[47]). Plastic waste generated on land enters the environment mainly where collective waste management systems are lacking or unable to manage waste effectively. An estimated 2 billion people do not have access to solid waste collection and tend to resort to independent disposal practices such as open dumping, open burning and direct disposal in the environment (World Bank, 2018[48]). Dumped plastics as well as waste disposed of in uncontrolled landfills may easily disperse in the environment, for instance by the action of wind or currents in waterways (UNEP, 2016[49]).
Approximately 76 Mt of plastic waste are mismanaged every year in river catchment areas and may potentially enter rivers (Schmidt, Krauth and Wagner, 2017[50]). While rivers and river beds are important sinks of debris themselves, plastics with a lower density and higher propensity to float (e.g. bottle caps, plastic bags, plastic bottles filled with air) may be transported into marine waters by river currents and contribute to marine plastic pollution. Lebreton et al. (2017[51]) estimate that between 1.15 and 2.41 Mt of plastic waste enters the oceans every year from rivers. The top 20 polluting rivers, which account for 67% of plastic flows from the global riverine system into the oceans, are mostly located in the Asian continent (Lebreton et al., 2017[51]).
Proximity to the coast may result in large quantities of mismanaged plastic litter reaching the oceans. Assuming inputs into the sea are proportional to the amount of plastic waste that is mismanaged within 50 km of the coast, Jambeck et al. (2015[5]) estimated the amount of marine plastic debris generated in coastal countries at 4.8-12.7 Mt per year. Although the data employed does not allow for precise estimates, the study predicted that the majority of land-based emissions of plastic litter into the oceans occur in emerging economies in East and South Asia, mainly due to a combination of high intensity of human activities near the coast, high plastic waste generation and poor waste management. Especially in South Asian countries, waste collection rates are low and open dumping is common (World Bank, 2018[48]).
Oceans are also heavily polluted with plastic debris discharged directly from marine-based sources, such as commercial and fishing ships, recreational boats and offshore industrial sites. The incidence of marine-based leaked plastic litter may be particularly high in the open ocean. A recent study of plastics in the Great Pacific Garbage Patch found that marine-based sources contributed to at least 50% of all recovered plastics mass (Lebreton et al., 2018[52]). In particular, lost or discarded fishing gear has been identified as a significant source of marine pollution, including degradation-based microplastics (Macfadyen, Huntington and Cappell, 2009[8]).4 The main factors causing the loss or abandonment of fishing gear are gear conflicts and overfishing (mainly caused by illegal, unregulated and unreported fishing), adverse weather conditions and the costs of gear retrieval (Macfadyen, Huntington and Cappell, 2009[8]; Richardson et al., 2018[53]).
1.3.2. Flows of microplastics and regional distribution
The main studies modelling the releases of primary and use-based secondary microplastics on a global (or macro-regional) level have been conducted by Eunomia (2016[42]; 2018[54]), the International Union for the Conservation of Nature (2017[55]) and UNEP (2018[20]). Table 1.4. presents a summary of available estimates of annual microplastics releases into the environment by source. Up to 3.01 Mt of primary and use-based secondary microplastics enter the environment annually (UNEP, 2018[20]). The wear of synthetic textiles and vehicle tyres alone accounts for between one-half and two-thirds of all microplastics releases (Eunomia, 2016[42]; IUCN, 2017[55]; UNEP, 2018[20]).
There are significant differences in model estimates both in terms of total releases and in terms of the relative contribution of sources. Available models are largely based on national-level estimates of microplastics releases for a number of countries located in Northern and Western Europe, which may not be representative of emissions in other regions (Essel et al., 2015[56]; Lassen et al., 2016[57]; Magnusson et al., 2016[45]; MEPEX, 2014[58]). Further, differences in the methodologies employed, the environmental sinks considered and in the assumptions taken on the transport and fate of microplastics may also partially explain the divergences.5 Overall, better and more representative geographical coverage of the source data as well as more industry-derived data (e.g. on fibre shedding from textiles, tyre abrasion, industrial emissions) are needed in order to improve the quality of estimates of the flows of microplastics from source to sink.
While all macro regions contribute to the release of microplastics, there are significant regional differences. Figure 1.4 presents microplastics losses to the environment by source and by macro region. In Western Europe, North America and Japan the primary sources of microplastics releases are the abrasion of tyres and road markings. For synthetic microfibres, the majority (72%) of losses occur in four macro regions: China, India, the rest of Asia and North America. Broadly, while in emerging economies microplastics releases tend to be high primarily due to the low rates of connectedness to wastewater treatment plants and large population sizes, in OECD countries diffuse sources of microplastics constitute the majority of emissions. With regards to synthetic microfibres, there are concerns that other stages of the use phase (wearing) as well as the production phase might also significantly contribute to microplastics pollution. While models so far have only considered the washing of textiles due to a lack of data and monitoring, the high volumes of microfibre emitted during washing in countries where the majority of global fibre and textile production takes place (e.g. China, India) suggests that microfibre releases into waterways from production could also be substantial. Further research is required in order to evaluate the contribution of industrial emissions to microplastics pollution.
1.4.1. Marine environments
The presence of microplastics has been documented in every habitat of the major ocean basins, including semi-enclosed seas, coastal environments and beaches and polar ice (Browne et al., 2011[59]; Desforges et al., 2014[60]; Eriksen et al., 2013[61]; Lusher et al., 2015[62]; Obbard et al., 2014[63]; Wessel et al., 2016[64]). Microplastics are also present at all ocean depths, from the sea surface to the ocean floor (Barrett et al., 2020[6]; Browne et al., 2011[59]; Choy et al., 2019[65]).
Available surveys of marine microplastics are fairly recent and limited in number, but can provide a good knowledge base of the trends of accumulation of microplastics in marine waters. Coastal environments are particularly vulnerable to microplastics pollution, likely due to proximity to the point of emission (e.g. river and sewage effluents, coastal human activity) (Browne et al., 2011[59]; Cole et al., 2011[66]). A map of indicative microplastics abundance on the surface of coastal marine waters, based on data from Lebreton et al. (2012[67]), is presented in Figure 1.5. Concentrations of floating microplastic are especially high in semi-enclosed seas (e.g. the Mediterranean Sea) and eastern seas (GESAMP, 2015[11]). With regards to microplastics in the open ocean, hotspots of floating microplastics have been identified in gyres, i.e. areas where marine currents concentrate floating debris (Eriksen et al., 2013[61]; Cózar et al., 2014[68]; Van Sebille et al., 2015[69]).
Source: Adapted from Large Marine Ecosystems Database, Estimated relative distribution of microplastic abundance in Large Marine Ecosystems, based on (Lebreton, Greer and Borrero, 2012[67]), http://bit.ly/2LK4ucM.
Significant knowledge and data gaps persist with regards to the quantities and distribution of microplastics in the oceans. Estimating microplastic concentrations in marine environments remains a challenge for several reasons:
High variability. The distribution and transport of microplastics is highly variable and difficult to model, due to the complexity of factors influencing it (e.g. marine and wind currents, the density of the plastics compared to that of seawater, the creation of biofilms). Large differences in microplastics concentrations may exist across oceans areas as well as different depths (Cózar et al., 2014[68]; Claessens et al., 2011[70]; Eriksen et al., 2014[71]). High variability limits the representativeness of individual studies and poses limitations for the global scaling up of results (GESAMP, 2015[11]). Also, knowledge gaps persist with regards to the rate of degradation and fragmentation under different environmental conditions or due to interaction with living organisms.6
The lack of harmonised sampling and characterisation methodologies for microplastics also poses challenges to the extrapolation of global-level results. Surveys of marine microplastics tend to employ a variety of sampling methods, cut-off sizes and metrics to present results, rendering findings difficult to compare and aggregate.
Limited geographical coverage of sampling. In recent years, the use of manta trawls (i.e. net systems designed to collect microplastics from seawater over long distances) has allowed for the expansion of the geographical coverage of microplastics surveys (SAPEA, 2019[13]). Yet, sampling coverage remains largely limited to surface waters which are more accessible (such as the Mediterranean Sea) or which are already the focus of research on macro plastics pollution (such as subtropical gyres).
Methodological issues in sampling surface waters. A significant drawback of the net system of mantra trawl methods employed to sample the ocean surface is that they cannot retain smaller microplastics (generally below 0.333 mm). Surveys of the ocean surface have consistently found lower amounts of microplastics than previously thought and especially of small microplastics (Cózar et al., 2014[68]; Eriksen et al., 2014[71]). With current data and sampling methodologies, it is impossible to determine with confidence whether this gap exists due to methodological issues, or whether other factors (e.g. biofouling, degradation, transport via marine currents, ingestion by marine species) may be contributing to the transport of microplastics away from the ocean surface (Cózar et al., 2014[68]).
Lack of microplastics survey data for lower compartments of the sea. Microplastics surveys of the seafloor can be particularly complex and expensive to conduct. However, it has been suggested that the deep sea may be a large sink of microplastics, mainly due to the sinking effect of biofouling (SAPEA, 2019[13]). Hotspots of microplastics accumulation may form near the seafloor due to the influence of marine currents on the horizontal distribution of microplastics, potentially also overlapping with biodiversity hotspots (Kane et al., 2020[72]). Further field research is required to reliably assess the occurrence, distribution and risks of microplastics close to the ocean floor.
1.4.2. Freshwater environments
Several recent studies have also pointed to freshwaters as important microplastics sinks. Microplastics have now been observed in the surface waters and sediment of lakes and rivers, as well as in drinking water (Free et al., 2014[73]; Koelmans et al., 2019[74]; Castañeda et al., 2014[75]; Wang et al., 2017[76]; Eriksen et al., 2013[77]). Key pathways of microplastics pollution to freshwaters are terrestrial run-off and wastewater effluent, as well as mismanaged plastic waste.
Observed microplastics concentrations in freshwater environments vary widely depending on the sampling location. Table 1.5 presents microplastics concentrations from selected studies looking at microplastics in freshwaters (Koelmans et al., 2019[74]). For surface waters of lakes and rivers, some studies report concentrations significantly higher than the average observed concentrations for the ocean surface, while others report relatively low numbers. In general, the different methodologies employed (e.g. sieve sizes) render results difficult to compare and aggregate in order to draw general conclusions on the degree of MP pollution of different freshwater bodies.
Microplastics contamination of water destined for human consumption has also been reported (Kosuth, Mason and Wattenberg, 2018[78]; Mintenig et al., 2019[79]; Schymanski et al., 2018[80]). In general, groundwater resources are generally well protected from contamination and drinking water treatment removes most microplastics (Koelmans et al., 2019[74]). However, further research is required to assess potential routes of microplastics contamination of drinking water (e.g. the distribution stage) and the potential for human health risks (WHO, 2019[81]).
Non-aquatic environmental media (soil, air) can also be considered microplastics sinks.
Soil
Sludge and fertiliser application are key entry pathways into terrestrial environments, predominantly for synthetic fibres and other microplastics present in municipal wastewaters. Microplastics have been detected in agricultural fields in North America (1 microfibre per gram) (Zubris and Richards, 2005[82]), Mexico (0.87 particles per gram) (Huerta Lwanga et al., 2017[83]) and China (7.10 – 42.96 particles per gram) (Zhang and Liu, 2018[84]). Current knowledge indicates that microplastics entering soil through sludge may persist for a long time after the interruption of sludge application (Zubris and Richards, 2005[82]; Browne et al., 2011[59]). Additionally, atmospheric transport and road side deposition of microplastics, the use of intentionally-added microplastics in agriculture (e.g. seed coatings, controlled release fertilisers) and the degradation of plastic items employed in agriculture (e.g. agricultural mulch films) may also substantially contribute to terrestrial microplastics pollution.
In general, microplastics pollution of terrestrial habitats is a new field of research. Sampling methodologies are still under development, and significant data and knowledge gaps persist on the quantity, distribution and potential degradation of microplastics in soil. Further research is required in this field, especially with regards to microplastics persistence in the top layers of soils, penetration in lower levels, potential leakage into groundwater or rivers and the hazards posed to terrestrial species and ecosystems.
Air
Similarly, knowledge of airborne microplastics remains limited. Significant quantities of microplastics have been identified in atmospheric fallout (i.e. dust and particulate matter) in Paris, Hamburg and Dongguan (China) (Dris et al., 2016[32]; Klein and Fischer, 2019[85]; Cai et al., 2017[86]), as well as in protected areas of the United States (Brahney et al., 2020[87]). Commonly sampled airborne microplastics are those released during the use of garments and from road transport activity (see Chapter 2). Recent evidence indicates that airborne microplastics can travel for long distances and that air deposition may play a significant role in the pollution of remote and pristine areas (Allen et al., 2019[88]; Evangeliou et al., 2020[89]). Overall, knowledge of the abundance and concentrations of airborne microplastics remains limited, especially with regards to smaller microplastics and nanoplastics, and further research is required in this area.
Emerging knowledge on the ubiquitous environmental presence of microplastics raises concerns for the hazards that these may pose to the health of ecosystems and humans. The next sections summarise findings on microplastics exposure levels (Section 1.5.1) and the health hazards posed by the toxicity of the particles (Section 1.5.2) and present an assessment of the risk implications for ecosystem and human health (Section 1.5.3). It is important to note that a significant challenge in the assessment of risks associated with microplastics is that the word is employed as an umbrella term to describe a vast array of particles with different physico-chemical characteristics and different potential for eco-toxicological effects. A discussion of specific environmental and health concerns associated with microplastics originating from the use of textiles and tyres is also included in Chapter 2.
1.5.1. Exposure
Aquatic species
Microplastics contamination has been documented for several marine and freshwater species, including planktonic organisms (Cole et al., 2017[90]), mussels and crustaceans (De Witte et al., 2014[91]; Farrell and Nelson, 2013[92]), freshwater and marine fish species (Jabeen et al., 2017[93]; Lusher, McHugh and Thompson, 2013[94]), marine mammals (Fossi et al., 2016[95]; Hernandez-Milian et al., 2019[96]), marine birds (Verlis, Campbell and Wilson, 2013[97]; van Franeker et al., 2011[98]), as well as for some terrestrial species such as earthworms (Rillig, 2012[99]). The main exposure route for wildlife is through ingestion, either due to the direct ingestion of microplastics or the ingestion of contaminated species.
Exposure to direct microplastics ingestion may be highly variable across different habitats and species, mainly due to differences in feeding strategies and variation in microplastics characteristics and concentrations across different feeding habitats. Particle size may be the most important factor in determining ingestion incidence (Andrady, 2011[17]). Smaller microplastics are more likely to be mistakenly ingested as prey, especially by small invertebrates at the bottom of the food chain and by filter feeders, i.e. species which strain food from the surrounding waters indiscriminately, such as small and medium invertebrates (e.g. planktons) and certain large mammals (e.g. baleen whales) (Fossi et al., 2012[100]; Wright, Thompson and Galloway, 2013[101]; GESAMP, 2015[11])7. There is also growing concern that certain selective feeders, i.e. species which have the ability to selectively ingest food such as copepods (a type of small crustacean), may selectively ingest plastic particles containing chemicals sorbed from the surrounding environment, due to their resemblance to prey (Procter et al., 2019[102]; Lusher, Hollman and Mendoza-Hill, 2017[103]).
Microplastics ingestion can also occur indirectly via the ingestion of contaminated species. The transfer across steps of the ecosystem food chain, known as trophic transfer, amplifies the exposure risk to all species in the food chain. Although indirect microplastics ingestion has been documented only for a few species (e.g. mussels, crabs, herring, captive seals) (Diepens and Koelmans, 2018[104]; Farrell and Nelson, 2013[92]; Lusher, McHugh and Thompson, 2013[94]; Nelms et al., 2018[105]), evidence of a large occurrence of microplastics in organisms at the bottom of the food chain (e.g. planktons) and recurrent inconsistencies between the types of microplastics retrieved in organisms and those commonly found in their habitats, point to a potentially large contribution of indirect ingestion to total microplastics exposure of aquatic organisms (SAPEA, 2019[13]).
Humans
Freshwater and marine microplastics contamination may contribute to increasing human exposure to microplastics, via the ingestion of commercial seafood. Microplastics have been documented in the digestive tract of several types of mussels and fish destined for human consumption (Van Cauwenberghe and Janssen, 2014[106]). These, especially when consumed without removing the digestive tract, may constitute a significant exposure route to humans (Lusher, Hollman and Mendoza-Hill, 2017[103]). The presence of microplastics has also been documented in several other contaminated food and beverages, such as tap and bottled water (Kosuth, Mason and Wattenberg, 2018[78]; Mintenig et al., 2019[79]), beer (Liebezeit and Liebezeit, 2014[107]), sea salt (Iñiguez, Conesa and Fullana, 2017[108]) and edible fruit and vegetables (Oliveri Conti et al., 2020[109]). Further, humans may also be exposed to the inhalation of airborne microplastics present both in indoor and outdoor environments (Dris et al., 2017[110]; Gasperi et al., 2017[111]).
According to Cox et al. (2019[112]), the estimated daily intake for adult women and men in the United States is of 126 and 142 particles, respectively, for ingested microplastics, and 132 and 170 particles respectively for inhaled microplastics. Conversely, a second study concluded that the largest source of microplastics acquisition is by far the ingestion of contaminated food and beverages, while the inhalation of microplastics represents a negligible exposure route (WWF, 2019[113]). More recently, microplastics have also been sampled in the human placenta, raising concerns on the levels of human exposure to MPs and the potential impacts on foetus development (Ragusa et al., 2021[114]). Overall, further data is needed in order to produce reliable and methodologically valid assessments of human exposure to microplastics via multiple exposure routes.
1.5.2. Toxicity of microplastics
Toxicity of microplastics to species, humans and ecosystems is defined as the combination of:
the physical toxicity of the uptaken particles, i.e. the adverse health effects caused by the transition or permanence of particles in organisms;
the chemical toxicity, i.e. the adverse health effects caused by the chemicals present in the ingested or inhaled microplastics; and
the pathogen toxicity, i.e. the potential of microplastics to act as a vector of microbial communities (WHO, 2019[81]).
Physical toxicity
In aquatic species, the majority of the ingested microplastics are likely to be directly excreted. Yet, systemic exposure to microplastics ingestion may cause several physical injuries such as internal inflammation and abrasion, or blockages of the gastrointestinal tract. Laboratory experiments have shown that high exposure to microplastics may result in reduced feeding efficiency, starvation, reduced growth rates, physical deterioration and increased mortality rates (Wright, Thompson and Galloway, 2013[101]).
The physical toxicity of ingested microplastics on humans remains largely unknown: current knowledge is largely based on inference from observed impacts on marine and terrestrial organisms. Over 90% of ingested microplastics are thought to pass through the gastrointestinal system without being retained (Smith et al., 2018[115]; EFSA CONTAM Panel, 2016[116]). Factors affecting the clearance/retention rate are likely to be the size, shape and polymer and chemical composition of microplastics. It is believed that only very small (<1.5 µm) microplastics may be retained and transferred into the lymphatic system and human organs, although the mechanisms and impacts of microplastics uptake remain unknown (EFSA CONTAM Panel, 2016[116]). Still, systemic exposure to microplastics ingestion may lead to localised effects on the immune system, inflammation of the gut and intestine irritation (EFSA CONTAM Panel, 2016[116]; WHO, 2019[81]). Overall, further research is required in order to reliably assess the physical toxicity associated with the ingestion of microplastics, and especially of nanoplastics, on humans (WHO, 2019[81]).
Similarly, only a small portion of the inhaled microplastics is expected to reach the lungs (Gasperi et al., 2017[111]). The inhalation of air pollutants may be facilitated by the small size of particles, as well as by compromised clearance mechanisms or respiratory functions at the individual level (Prata, 2018[117]). Chronic exposure to high concentrations of microplastics has been shown to lead to a higher prevalence of respiratory irritation, chronic respiratory symptoms, restrictive pulmonary function abnormalities, and possibly also to reproductive toxicity and carcinogenicity (Gasperi et al., 2017[111]; Pimentel, Avila and Lourenco, 2008[118]).
Chemical toxicity
Microplastics are generally found in the environment as complex mixes of different chemicals. Several chemical additives are combined with plastic polymers during manufacturing to enhance a number of desirable properties of the final plastic product (e.g. resistance to UV, biodegradation, oxidation, heat and optical brightness and colour) (OECD, 2018[119]).8 Microplastics are also prone to sorbing persistent organic pollutants (POPs) and heavy metals from aquatic or aerial environments.
Chemicals present in microplastics may leach out following ingestion and pose hazards to the health of aquatic organisms and humans (Rochman et al., 2019[14]; SAPEA, 2019[13]). Additives such as Bisphenol A, PCBs, phthalates and some brominated flame retardants are suspected endocrine disruptors, i.e. chemicals with thyroid-disrupting effects (WHO, 2019[81]). Other known or suspected health effects of hazardous chemicals added during plastic production include carcinogenicity, reproductive health effects, developmental toxicity and mutagenicity (i.e. the induction of a transmittable change in one’s genetic material). For chemicals and metals sorbed once plastics is released into the environment, potential effects on marine biota may include altered feeding behaviour, endocrine disruption, liver toxicity, tumour promotion and reduced survival (GESAMP, 2016[19]). Generally, complex equilibria dependent on the relative concentrations of pollutants in each compartment will determine absorption/desorption rates and the exposure levels for organisms (GESAMP, 2015[11]).
Pathogen toxicity
Microplastics may also act as transfer media for invasive species and virus-bearing organisms potentially harmful to ecosystems and human health. The surface of macro- and micro- plastics in aquatic environments is an ideal habitat for diverse bacterial assemblages to attach and colonize (Frère et al., 2018[120]; WHO, 2019[81]). As microplastics and pathogens are both commonly found in wastewater treatment plants, this joint exposure may increase the potential for pathogens to colonize the surface of microplastics. Documented microbial communities include those formed by pathogens commonly present in sewage as well as microorganisms able to degrade plastic polymers (Curren and Leong, 2019[121]). These may be transferred to humans via contaminated food and beverages, potentially causing imbalances in microbial communities present in the organism or spreading antibiotic resistance (SAPEA, 2019[13]; WHO, 2019[81]).
1.5.3. Environmental and human health risks and knowledge gaps
Risk assessments have been carried out in order to scientifically evaluate the adverse ecological and health impacts resulting from microplastics pollution of aquatic media. Risks are generally assessed as a function of hazard and exposure. Humans and other living species commonly ingest particles of different types and origins, and the presence of microplastics in the environment does not necessarily imply a risk for the health of organisms. Conversely, the inherent toxicity of a particle may result in health risks only under specific conditions, such as the surpassing of certain exposure levels or the vulnerability of specific species to ingesting microplastics (WHO, 2019[81]).
Available risk assessments for microplastics in aquatic environments indicate that average concentration levels lead to limited ecological risks, although adverse effects may already be occurring in certain highly polluted coastal waters or beaches (Besseling et al., 2019[122]; Burns and Boxall, 2018[123]; Everaert et al., 2018[124]). Based on the available evidence, the Science Advice for Policy by European Academies Working Group (2019[13]) concluded that continued microplastics emissions and increases in concentration levels in different environmental media may lead to widespread ecological impacts in the near future and recommended action “to reduce, prevent and mitigate” microplastics pollution in order to reduce risks. At the same time, the group of experts highlighted the need for more data on microplastics occurrence, fate, exposure levels and modes of toxicity (including sorption mechanisms for chemicals) in order to produce higher-quality risk assessments.
Similar conclusions can be drawn with regards to risks posed specifically to human health. While physical toxicity of particles has been observed in aquatic species, research efforts have yet to determine if this may also occur in humans and what the critical exposure levels may be (SAPEA, 2019[13]). Although it is now established that microplastics can act as a vector of toxic chemicals to humans, current evidence suggests that present concentration levels of microplastics are not a major exposure pathway relative to other existing ones (SAPEA, 2019[13]). Conservative estimates of the exposure to microplastics through ingestion of a portion of seafood are 7 µg of microplastics, which would contribute to less than 0.2% of the average total dietary exposure to Bisphenol A, PCBs and PAHs (EFSA CONTAM Panel, 2016[116]; SAPEA, 2019[13]). With regards to pathogen toxicity, current evidence suggests that microplastics do not yet constitute a significant exposure route to pathogens potentially harmful to human health, relatively to other potential transfer media (e.g. contamination of water pipes, inadequate wastewater treatment) (WHO, 2019[81]). The recent WHO (2019[81]) assessment of the potential human health impacts of microplastics in drinking water concluded that there is no evidence to indicate a human health concern, advised for a reduction in plastic pollution to mitigate exposure levels and called for further research to more accurately inform risk assessments.
Overall, further research and better exposure data and toxicity assessments are required in order to adequately identify and assess risks for human health. Future research needs are summarised in Box 1.4.
To improve the quality of risk assessments, further research is required on:
Microplastics occurrence and exposure levels. The lack of high-quality estimates of microplastics concentrations in different aquatic compartments limits our understanding of exposure levels for biota and humans. More data is especially needed to adequately quantify exposure to smaller microplastics and nanoplastics, which can be ingested more easily and which are believed to pose a higher risk of health hazards to wildlife and humans (SAPEA, 2019[13]). The standardisation of microplastics definitions and analytical methods for sampling and characterisation can help to accelerate research in this area and facilitate the comparison of results.
Fate of microplastics and persistence in organisms. Laboratory experiments carried out on certain aquatic species have documented the uptake of microplastics as well as their transfer to tissues and body liquids (Van Cauwenberghe, Claessens and Janssen, 2013[125]; Van Cauwenberghe et al., 2015[126]). The presence of microplastics in organisms is thought to be temporary, however the mechanisms of uptake, accumulation, and excretion of microplastics in humans and other species remain largely unknown (GESAMP, 2015[11]).
Concentration thresholds for health risks. Substantial knowledge gaps persist also with regards to the concentration levels that lead to adverse health consequences. Laboratory experiments usually employ unrealistically high concentration levels and thus cannot reproduce the relationship between exposure levels and adverse effects as it may occur in the natural environment (GESAMP, 2016[19]).
1.6.1. Plastics production, use and disposal
Plastics production is projected to continue to increase, causing concern for the projected leakage of plastics and the potential amplification of macro- and micro- plastics pollution. This is for two reasons in particular:
1. The increase in plastics production in recent years has been mostly driven by the packaging sector, which now constitutes almost 40% of all plastics production (Geyer, Jambeck and Law, 2017[2]). Packaging and other single-use plastics are discarded soon after use, significantly contributing to the generation of plastic waste.
2. The largest increases in plastic waste generation are expected in regions where waste management is poor. Figure 1.6 presents projections for the top twenty coastal countries by mass of mismanaged plastic waste in 2025: eighteen of them mismanaged more than 50% of their plastic waste in 2010 (Jambeck et al., 2015[5]). As their waste management systems may not develop at a sufficiently quick rate to deal with the additional amounts of plastic waste generated, this may lead to larger quantities of plastic waste dispersed into the environment.
Current trends in plastics production and disposal, combined with the lack of effective solid waste management systems in several parts of the world, suggest that flows of mismanaged plastics to the oceans will continue to contribute to the generation of secondary microplastics in years to come. Source-reduction policies (e.g. bans and taxes on single-use plastic goods) remain limited in scope, while clean-up initiatives are expensive, remove only plastics from the surface of the oceans or from beaches and can only be sufficiently effective in the presence of emission reductions (The Ocean Cleanup[127]).
Furthermore, plastics debris already present in the environment will continue to be a source of microplastics (Andrady, 2011[17]).Microplastics constitute over 90% of the 5.25 trillion plastic particles currently present in the oceans’ surface, but only a small portion of the total floating plastics in terms of mass, implying that there are still large quantities of marine plastic litter which may fragment into microplastics in future years (Eriksen et al., 2014[71]; Lebreton, Egger and Slat, 2019[128]).9 Overall, projections based on current trends in the production of plastics find that microplastics concentrations in the surface of marine waters will increase 50-fold by 2100, to 9.6-48.8 particles per m3 (Everaert et al., 2018[124]).
1.6.2. Projected trends in microplastics emissions
Recent industry and policy-led efforts may result in a reduction in selected microplastics emissions. Improved waste management practices, policies on frequently littered single use plastics and bans on the use of microbeads in industrial applications for which natural alternatives exist have been at the focus of policy action on plastics and microplastics pollution until now. Several countries have introduced legislation to restrict the manufacture, sale and/or import of personal care and cosmetic products containing microbeads (Canada, 2017[129]; France, 2017[130]; GOV.UK, 2018[131]; Italy, 2017[132]; New Zealand, 2017[133]; United States, 2015[134]). Approved national bans generally target microbeads intentionally added to rinse-off cosmetics, which roughly account for more than two thirds of all microbeads releases from products (ECHA, 2019[10]).10 The European Chemical Agency (ECHA) has also proposed an EU-wide restriction to cover a wide range of microplastics intentionally-added to products, including in PCCPs, paints, coatings, detergents, maintenance products, medical and pharmaceutical applications and products used in agriculture and horticulture. ECHA (2019[10]) estimates that the restriction could result in emission reductions of more than 400 thousand tonnes of microplastics over 20 years. With regards to plastic pellets, emerging industry-led and international initiatives to prevent spillages may also curb their discharge into the environment (Operation Clean Sweep, (PlasticsEurope, 2017[22]; Marine Litter Solutions, 2011[23])).
However, releases of use-based secondary microplastics, which remain largely outside of the scope of policy frameworks in place in OECD countries, are expected to significantly increase in future years, in line with market trends and economic growth. Trends in the textile sector indicate that textile production, consumption and disposal is likely to continue to increase in line with GDP growth. At current trends, an estimated 175 Mt of clothing could be sold in 2050 (EMF, 2017[135]). In particular, the employment of synthetic fibres is expected to continue to increase, in line with current consumption trends, practices associated with the concept of “fast fashion” and the growth in textile markets in developing countries in East and South East Asia (TextileExchange, 2020[136]). While the higher uptake of synthetic fibres in textile manufacturing offers numerous economic and environmental benefits (e.g. lower costs relative to natural fibres, a reduced need for resource-intensive cotton production), in the absence of mitigation action this will contribute significantly to the intensification of microplastics pollution. It is estimated that, at current trends, 22 Mt of synthetic microfibres will have entered the oceans by 2050 (EMF, 2017[135]).
Emissions of microplastics from vehicle tyres are also expected to increase in future years, in line with GDP growth and trends in road transport. Market data projections indicate steady increases in the production of vehicles in the next decades, mainly driven by increases in production in China and India (EC, 2017[137]). Current trends in the composition of the vehicle fleet also show a continued tendency towards a higher proportion of larger and heavier vehicles, which generally lead to higher tyre tread wear (Andersson-Sköld et al., 2020[138]). Furthermore, climate policies and stricter controls on exhaust emissions (e.g. GHG emissions) will not necessarily contribute to the reduction of non-exhaust emissions (e.g. tyre and brake particles) (OECD, 2020[139]). On the contrary, a higher uptake of electric vehicles could lead to higher emissions of microplastics, mainly due to the heavier weight of EVs relatively to their traditional counterparts.
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Notes
← 1. The fate of microplastics washed off by stormwater is discussed separately in the next section.
← 2. A list of degradability definitions is given in Annex 1.A.
← 3. See e.g. US Code of Federal Regulations 40 CFR 258.61 1991 and EU Council Directive 1999/31/EC.
← 4. In addition to being sources of degradation-based synthetic microfibres, lost or discarded fishing gear may also result in the continued catching of non-target species (including protected species) the entanglement of marine wildlife, and damage to natural habitats as well as fishing vessels (Richardson et al., 2018[53]).
← 5. Notably, some publications assume that microplastics from road transport activity are largely transported into the marine environment. However, as discussed in more detail in Chapter 2, recent studies focusing on tyre and road wear emissions suggest that the road surface, nearby soil and water streams, and air are also primary sinks of microplastics.
← 6. Digestive fragmentation has been suggested as an additional generation route for secondary microplastics, especially for smaller microplastics and nanoplastics (Dawson et al., 2018[140]).
← 7. Large filter-feeding mammals need to swallow hundreds cubic meters of seawater per day in order to capture plankton. Thus, they are suspected to ingest large quantities of microplastics both directly and via trophic transfer from preys (Fossi et al., 2012[100]; Wright, Thompson and Galloway, 2013[101]).
← 8. Additives may constitute between 1% and more than a half of the total weight content of plastics (OECD, 2018[119]).
← 9. For instance, it is estimated that microplastics larger than 0.5 mm constitute only 8% of total plastic mass in the Great Pacific Garbage Patch (about 6.4 metric tons of microplastics), the large accumulation of floating plastics located in the North Pacific Ocean (Lebreton et al., 2018[52]).
← 10. The EU ban on oxo-degradable plastics also seeks to minimise microplastics generation (EC, 2018[38]).