Sources of microplastics and their distribution in the environment

Widespread application of plastics in everyday life leads to micropollution and its accumulation in the environment. Micropollution has significant negative effects on nature and human health. This article focuses on one of the main sources of micropollution – microplastics, and their distribution in the environment.  

Sources of microplastics

Microfibres which shed from textiles are the main contributor to micropollution. Although only synthetic microfibers would be considered microplastics, micro fragments from all types of fibres - including natural ones such as cotton and wool, also contribute to pollution. You can learn more about microfibres here.

Vehicle tyres (10-20%) - Tyre wear is an unavoidable consequence of their use and the rate of abrasion is influenced by the tyre composition, design, vehicle speed/acceleration, use of brakes, and road surface texture¹. Brake pads are also a source of microplastic pollution, although a quantitative estimate for the pollution is still not available². Although tyres are generally thought to consist of natural rubber, the natural rubber contents in a tyre can sometimes be as little as 20% with the rest of the materials being synthetic, including plastics³. 

The total microplastics generated from the wear of automotive tyres in the European Union is around 0.5 million metric tonnes (MMT) per year⁴. A simple extrapolation of this estimate to the global vehicle fleet indicates annual microplastic release of around 2.5MMT, which is 4% of the total weight of tyres on the roads globally⁵. 

The expected increase in the proportion of electric vehicles (EVs) on the road is expected to increase total microplastic emissions from vehicle tyres, in part because current EVs typically weigh 20-30% more than their internal combustion engine (ICE) counterparts due to the weight of the battery⁶. 

City dust (10-20%) - City dust refers to a wide range of microplastic sources originating from urban areas - artificial turf, building paints, and industrial abrasives constitute the largest and most well-understood sources of city dust. 

• Artificial turf is largely used in contact sports as a means to absorb impact and thus prevent athlete injury. The shock-absorbing impact of the turf is achieved through the use of polymeric infill comprised of plastic particles, typically manufactured at least partially from recycled vehicle tyres⁷. This infill is located just beneath the artificial grass and on top of a stabilising infill such as sand. While the artificial grass fibres are eventually worn down into microfibres, the majority of microplastics from artificial turf come from the polymeric infill, which can be accidentally removed by athletes or during maintenance.
• Interior and exterior building paints often contain microplastics in the form of microspheres or microfibres, which thicken paint and increase elasticity and resilience⁸. When dried paint is removed, when paint cracks or degrades, or when paintbrushes and rollers are washed (for water-based paints), these paints can release microplastics into the environment. 
• Plastic pellets are also used as an abrasive material in industrial processes and are common in blasting media and scrubbers for machinery and other metallic surfaces, particularly when there is a need to gently clean surfaces⁹. For example, sandblasting is used in Denmark to sanitise and remove graffiti from buildings, remove paint from and clean airplanes, and clean moulds, tanks, and turbine blades¹⁰.

Road markings (3-5%) - Similarly to road covers and tyres, road markings wear. Hot-melt paints, which are commonly used for road markings, consist of 15-25% polymer binders, which contribute to microplasitc pollution when worn away. While not all road markings are plastic-based, thermoplastic bases are the most commonly used material in road markings in certain places in the EU¹¹.

Marine coatings (3.7%) - Many types of marine coatings applied to the hulls of marine vessels include polymers such as polyurethane, epoxy coatings, vinyl and/or lacquers, as well as other compounds such as metals¹². When these coatings are weathered, scraped, sanded, disposed of, or spilt during the application, they contribute to microplastic load in the environment.

Personal care products and cosmetics (1-2%) - Personal care products, such as exfoliants contain microbeads, which make up a relatively small but well-recognised facet of microplastic pollution. Several countries addressed this by introducing bans on the manufacture and use of plastic microbeads in personal care products. Despite these regulations, personal care products continue to generate microplastic pollution. In addition, different types of solid insoluble plastics (typically polyethene and polyurethane) are commonly added to leave-on cosmetics, resulting in an annual 540-1,120 tonnes of plastic in the EU¹³. 

Plastic pellets (0.3%) - Plastic resin pellets (also known as ‘nibs’ or ‘nurdles’) are used as feedstock for the manufacture of most plastic products¹⁴. They are typically spherical or cylindrical with a diameter of 5mm and can be made out of a variety of polymers¹⁵. These pellets contribute to microplastic pollution through accidental losses occurring throughout the value chain, including during transport to the converters where they are processed, and the plastic manufacturing process¹⁶. While pellets may enter the environment as relatively large pieces compared to other microplastics, their size decreases progressively as they are worn down. While pre-production pellet loss has been estimated in specific regions, the total amount of pre-production plastics entering the marine environment is difficult to estimate given the uncertainty in the rate of pellet loss during handling globally¹⁷. IUCN estimates suggest that plastic pellet losses constitute <1% of total microplastic pollution.

Agricultural uses for primary microplastics - Polymer-based products potentially containing microplastics have a variety of applications in agriculture, including mulches for temperature and moisture control, silage and fumigation films, and anti-bird and weed protection¹⁸. The main agricultural application is in nutrient prills, which are polymer-coated nutrient mixtures that allow for the diffusion of nutrients into the surrounding soil, increasing yields while reducing the need for constant fertiliser application¹⁹. In the EU alone, up to 8,000 tonnes of polymers are estimated to be used in fertiliser prills²⁰. While the proportion of microplastics in these polymers is unknown, they could emerge as one of the largest contributors to microplastics pollution when quantified. 

Other sources of primary microplastics - In addition to the well-understood sources discussed above, primary microplastics have hundreds of other sources across many sectors:

• Dish detergents can contain microplastics such as polyurethane particles that are used to clean surfaces and are subsequently disposed of in wastewater²¹;
• Plastic bio-beads used as filter media in wastewater treatment plants (WWTPs) can be unintentionally released due to accidents and leaks at plants²²; 
• Microplastics are used in the healthcare and pharmaceutical sectors, including as vectors for drugs and dentist polish²³;
• Microplastics are also commonly employed in the oil and gas sector as additives to drilling fluids, although it has proved difficult to estimate precisely the tonnage of microplastics used for this purpose²⁴;
• Other common uses for microplastis incude: packaging, textile printing and automotive moulding, biomedical research insulation, furniture, pillows, buoys, 3D printing, ceramics, and adhesives²⁵.

Secondary microplastics - Sources of secondary microplastics include macro-sized terrestrial and marine-based refuse (e.g. fishing gear and shipping waste and losses). While it is almost impossible to measure the rate of secondary microplastics entering the environment, there are many estimates of macro-sized plastic waste, for example:

• The Pew Charitable Trust and SYSTEMIQ estimate that the total plastic waste generation in 2016 was 215 million tonnes;
• Eunomia and ICF estimate the total amount of microplastics entering the ocean from fishing nets to be between 478 and 4,780 metric tonnes/year. 

While only a fraction of total plastic debris ends up in the ocean, the sources and fate of terrestrial microplastics are relatively unknown.

How microplastics enter the environment

Since the vast majority of plastic has a terrestrial origin, terrestrial ecosystems and wastewater infrastructure are the major pathways of microplastics into the environment. The main channels by which primary microplastics enter the environment are: 

Road runoff (66%)– As a lot of the micropollution is generated outside, roadside runoff is one of the major carriers of microplastics into the environment²⁶.

Wastewater treatment plants (WWTPs) (25%) - Some pollution goes through wastewater treatment. WWTPs with tertiary treatment processes have been found to capture over 90% of microplastics within the effluent sludge. However, even with microplastic retention of >90%, the sheer volume of wastewater being processed, still results in a significant number of microplastics bypassing filtration²⁷ . Further, less than one-third of the human population is connected to the wastewater management infrastructure²⁸ . And even when wastewater is properly treated, in many countries (e.g. in North American and Europe) it is common practice to apply the sludge as agricultural fertiliser, which adds the captured microplastics directly to agricultural soils and back into the environment²⁹.

Wind transfer (7%) - certain microplastics (for example, city dust) can also be transported via wind. In urban areas, the deposition of plastic fibres has been recorded to reach 355 particles/m2/day, which highlights the ability of microplastics to travel long distances and contaminate air, food and beverages³⁰ . 

Marine activities (2%) - marine activities such as shipping, fishing, and tourism, are responsible for directly sweeping microplastics into the marine environment³¹.

As research regarding microplastic pollution advances, it will be important to improve our knowledge of the major contributors to this pollution. Understanding all sources and drivers of pollution, including their relative contributions, will facilitate the identification and adoption of effective pollution prevention.

References

1 Eunomia and ICF (2018) Investigating options for reducing releases in the aquatic environment of microplastics emitted by (but not intentionally added in) products.

2 Kazimirova, A. et al. (2016) ‘Automotive airborne brake wear debris nanoparticles and cytokinesis-block micronucleus assay in peripheral blood lymphocytes: A pilot study’, Environmental Research. Elsevier, 148, pp. 443–449. doi: 10.1016/j.envres.2016.04.022.

3Friends of the Earth, Tyres and microplastics: time to reinvent the wheel?

4 Eunomia and ICF (2018) Investigating options for reducing releases in the aquatic environment of microplastics emitted by (but not intentionally added in) products.

5 2.5MMT global release = 0.5 MMT estimate for Europe (Eunomia and ICF, 2018) x 5 (scaling up for share of European vehicle fleet within global fleet); European vehicle fleet is estimated to be around 280 million vehicles, including vans, commercial and busses (ACEA, Vehicles in use in Europe, 2021) and 1.45 billion vehicles globally (Whichcar?, How many cars are in the world, 2022); 4% abrasion = 2.5MMT microplastic release / c.60 MMT of tyres weight on the roads globally; 60MMT is estimated based on 19.2 MMT annual production in 2019 (Smithers, Global tire manufacturing output to grow 3.4% year- on -year to 2024) scaled up by 3 to account for tyres on the road –this is based on annual tyre sales vs number of tyres on the road for the UK. 

6 Eunomia and ICF (2018) Investigating options for reducing releases in the aquatic environment of microplastics emitted by (but not intentionally added in) products.

7 Fleming, P. R., Forrester, S. E. and McLaren, N. J. (2015) ‘Understanding the effects of decompaction maintenance on the infill state and play performance of third-generation artificial grass pitches’, Proceedings of the Institution of Mechanical Engineers, Part P: Journal of Sports Engineering and Technology, 229(3), pp. 169–182. doi: 10.1177/1754337114566480; Eunomia and ICF (2018) Investigating options for reducing releases in the aquatic environment of microplastics emitted by (but not intentionally added in) products.

8 Scudo, A. (2017) Intentionally added microplastics in products.

9 Cole, M. et al. (2011) ‘Microplastics as contaminants in the marine environment: A review’, Marine Pollution Bulletin. Elsevier Ltd, 62, pp. 2588–2597. doi: 10.1016/j.marpolbul.2011.09.025; Sundt, P., Schulze, P.-E. and Syversen, F. (2014) Sources of microplastic- pollution to the marine environment. Report no: M-321|2015, Norwegian Environment Agency (Miljødirektoratet); Environment Canada (2015) Microbeads – A Science Summary.

10 Sundt, P., Schulze, P.-E. and Syversen, F. (2014) Sources of microplastic- pollution to the marine environment. Report no: M-321|2015, Norwegian Environment Agency (Miljødirektoratet). 

11 Lassen, C. et al. (2015) Microplastics: Occurrence, effects and sources of releases to the environment in Denmark.

12 OECD (2009) Emission Scenario Document on Coating Industry (Paints, Lacquers and Varnishes). Paris, France; Eunomia and ICF (2018) Investigating options for reducing releases in the aquatic environment of microplastics emitted by (but not intentionally added in) products.

13 Scudo, A. (2017) Intentionally added microplastics in products.

14 Sundt, P., Schulze, P.-E. and Syversen, F. (2014) Sources of microplastic- pollution to the marine environment. Report no: M-321|2015, Norwegian Environment Agency (Miljødirektoratet).

15 Eunomia and ICF (2018) Investigating options for reducing releases in the aquatic environment of microplastics emitted by (but not intentionally added in) products.

16 Cole, M. et al. (2011) ‘Microplastics as contaminants in the marine environment: A review’, Marine Pollution Bulletin. Elsevier Ltd, 62, pp. 2588–2597. doi: 10.1016/j.marpolbul.2011.09.025; Ivar Do Sul, J. A. and Costa, M. F. (2014) ‘The present and future of microplastic pollution in the marine environment’, Environmental Pollution. Elsevier Ltd, 185, pp. 352–364. doi: 10.1016/j.envpol.2013.10.036; Essel, R. et al. (2015) Sources of microplastics relevant to marine protection in Germany; Nizzetto, L., Futter, M. and Langaas, S. (2016) ‘Are Agricultural Soils Dumps for Microplastics of Urban Origin?’, Environmental Science and Technology, 50, pp. 10777–10779. doi: 10.1021/acs.est.6b04140; Eunomia and ICF (2018) Investigating options for reducing releases in the aquatic environment of microplastics emitted by (but not intentionally added in) products.

17 Eunomia (2016a) Study to Quantify Pellet Emissions in the UK: Report to Fidra. Bristol, UK.

18 Horton, A. A. et al. (2017) ‘Microplastics in freshwater and terrestrial environments: Evaluating the current understanding to identify the knowledge gaps and future research priorities’, Science of the Total Environment. Elsevier B.V., 586, pp. 127–141. doi: 10.1016/j.scitotenv.2017.01.190; Karbalaei, S. et al. (2018) ‘Occurrence, sources, human health impacts and mitigation of microplastic pollution’, Environmental Science and Pollution Research. Environmental Science and Pollution Research, 25, pp. 36046–36063. doi: 10.1007/s11356-018-3508-7.

19 Salman, O. A. (1988) ‘Polymer coating on urea prills to reduce dissolution rate’, Journal of Agricultural and Food Chemistry, 36(3), pp. 616–621; GESAMP (2015) Sources, Fate and Effects of Microplastics in the Marine Environment: A Global Assessment. London, UK.

20 Scudo, A. (2017) Intentionally added microplastics in products.

21 Scudo, A. (2017) Intentionally added microplastics in products.

22 Cornish Plastic Pollution Coalition (2018) Bio-Bead pollution on our beaches; Turner, A., Wallerstein, C. and Arnold, R. (2019) ‘Identification, origin and characteristics of bio-bead microplastics from beaches in western Europe’, Science of the Total Environment. Elsevier B.V., 664, pp. 938–947. doi: 10.1016/j.scitotenv.2019.01.281.

23 Sundt, P., Schulze, P.-E. and Syversen, F. (2014) Sources of microplastic- pollution to the marine environment. Report no: M-321|2015, Norwegian Environment Agency (Miljødirektoratet); Cole, M. et al. (2011) ‘Microplastics as contaminants in the marine environment: A review’, Marine Pollution Bulletin. Elsevier Ltd, 62, pp. 2588–2597. doi: 10.1016/j.marpolbul.2011.09.025. 

24 Scudo, A. (2017) Intentionally added microplastics in products.

25 European Chemicals Agency (ECHA) (2019) Annex to the Annex XV Restriction Report – Proposal for a Restriction (intentionally added microplastics). Helsinki, Finland; Environment Canada (2015) Microbeads – A Science Summary; Lassen, C. et al. (2015) Microplastics: Occurrence, effects and sources of releases to the environment in Denmark; GESAMP (2015) Sources, Fate and Effects of Microplastics in the Marine Environment: A Global Assessment. London, UK.

26 Eunomia and ICF (2018) Investigating options for reducing releases in the aquatic environment of microplastics emitted by (but not intentionally added in) products.

27 Magnusson, K. and Norén, F. (2014) Screening of microplastic particles in and down-stream a wastewater treatment plant, Swedish Environmental Research Institute; Carr, S. A., Liu, J. and Tesoro, A. G. (2016) ‘Transport and fate of microplastic particles in wastewater treatment plants’, Water Research, 91, pp. 174–182. doi: 10.1016/j.watres.2016.01.002; Horton, A. A. et al. (2017) ‘Microplastics in freshwater and terrestrial environments: Evaluating the current understanding to identify the knowledge gaps and future research priorities’, Science of the Total Environment. Elsevier B.V., 586, pp. 127–141. doi: 10.1016/j.scitotenv.2017.01.190; Karbalaei, S. et al. (2018) ‘Occurrence, sources, human health impacts and mitigation of microplastic pollution’, Environmental Science and Pollution Research. Environmental Science and Pollution Research, 25, pp. 36046–36063. doi: 10.1007/s11356-018-3508-7.

28 IUCN (2017) Primary Microplastics in the Oceans: A Global Evaluation of Sources. Gland, Switzerland. doi: dx.doi.org/10.2305/IUCN.CH.2017.01.en.

29 Nizzetto, L., Futter, M. and Langaas, S. (2016) ‘Are Agricultural Soils Dumps for Microplastics of Urban Origin?’, Environmental Science and Technology, 50, pp. 10777–10779. doi: 10.1021/acs.est.6b04140; Horton, A. A. et al. (2017) ‘Microplastics in freshwater and terrestrial environments: Evaluating the current understanding to identify the knowledge gaps and future research priorities’, Science of the Total Environment. Elsevier B.V., 586, pp. 127–141. doi: 10.1016/j.scitotenv.2017.01.190; Eunomia and ICF (2018) Investigating options for reducing releases in the aquatic environment of microplastics emitted by (but not intentionally added in) products.

30 Dris, R. et al. (2016) ‘Synthetic fibers in atmospheric fallout: A source of microplastics in the environment?’, Marine Pollution Bulletin. Elsevier Ltd, 104, pp. 290–293. doi: 10.1016/j.marpolbul.2016.01.006.

31 IUCN (2017) Primary Microplastics in the Oceans: A Global Evaluation of Sources. Gland, Switzerland. doi: dx.doi.org/10.2305/IUCN.CH.2017.01.en.