Sources and Pathways of Microplastics to Habitats
Identifying and eliminating the sources of microplastic to habitats is crucial to reducing the social, environmental and economic impacts of this form of debris. Although eliminating sources of pollution is a fundamental component of environmental policy in the U.S.A. and Europe, the sources of microplastic and their pathways into habitats remain poorly understood compared to other persistent, bioaccumulative and/or toxic substances (i.e. priority pollutants; EPA in U.S. Environmental Protection Agency 2010–2014 Pollution Prevention (P2) Program Strategic Plan. Washington, USA, pp. 1–34, 2010; EU in Official J Eur Union L334:17–119, 2010). This chapter reviews our understanding of sources and pathways of microplastic, appraises terminology, and outlines future directions for meaningfully integrating research, managerial actions and policy to understand and reduce the infiltration of microplastic to habitats.
KeywordsHypothesis Micrometre Emissions Sewage Storm-water Textile Exfoliants
9.1 Defining Sources and Pathways of Microplastic
Since the first review of microplastic (Browne et al. 2007), a number of terms have been used to describe and categorize sources of microplastic. Some authors have used the terms “primary” and “secondary” to distinguish between sources of microplastic, in which they borrow terminology from atmospheric sciences (Arthur et al. 2009; Cole et al. 2011). In these cases, “primary sources” are those in which microplastic is intentionally produced through extrusion or grinding, either as precursors to other products (e.g. plastic pellets; Costa et al. 2010) or for direct use (e.g. abrasives in cleaning products or roto-milling), whilst “secondary sources” of microplastics are those formed in the environment from the fragmentation of larger plastic material into ever-smaller pieces (Arthur et al. 2009; Cole et al. 2011). Using similar ideas, Andrady (2011) described runoff as a “direct source” (sewage or storm water) whilst fragmentation of existing plastic debris was described as “indirect source” of microplastic to the environment.
Although using adjectives to categorize sources may be helpful, these terms introduce jargon without clearly identifying the actual sources and conflate sources with the pathway by which microplastic enter habitats, which may in turn confuse scientists, public, industry and government. In this chapter, I argue that these problems can be overcome if we choose alternative terms for sources that identify the place, person, company, or product where the microplastic originates and use separate terms to describe the pathways of microplastic from its source to a habitat.
Based upon our current understanding there are four types of sources (i) larger plastic litter, (ii) cleaning products (Zitko and Hanlon 1991; Gregory 1996; Derraik 2002); (iii) medicines; and (iv) textiles. For the latter, I have chosen to use a global case-study to illustrate how one can gain a more meaningful and scientific understanding of the sources and pathways of microplastic through developing better programs of research and monitoring that integrate advances in forensics (e.g. vibrational spectroscopy to identify the shape and type of microplastic), logic and experimental design (i.e. making observations, developing explanatory models, testing explicit hypotheses about composition and spatial patterns; Underwood 1997) and statistics. Throughout the chapter, I have chosen to define microplastic as micrometre-sized particles of plastic because this is consistent with previous work on this topic (Browne et al. 2007, 2008, 2010, 2011, 2013; Costa et al. 2009; Claessens et al. 2011; Rochman et al. 2013; Van Cauwenberghe 2013) and the globally ratified use of the prefix “micro” for measures of length under the International System of Units. Other authors have chosen to ignore the International System of Units definition of “micro” and have instead chosen to use <5 mm to define microplastic, a philosophical discussion about which people should use is beyond the scope of this review.
9.2 Larger Plastic Litter
Pathways. Fragmentation is the pathway, by which plastic debris breaks into smaller pieces, which is distinct from the more subtle processes of degradation that reduces the molecular mass of plastic debris (Andrady 2011, 2015). These processes occur through the action of light (photolysis), heat and oxygen (thermal-oxidation), water (hydrolysis), organisms (see review by Andrady (2011) for more details about weathering experiments with pieces of plastic >1 mm) and physical abrasion by particles of sediment. Laboratory and field experiments are required to determine the relative importance of these processes in generating the sizes and shapes of micrometre-sized plastic that we find in habitats. One such laboratory experiment by Davidson (2012) showed that each time a single isopod (Sphaeroma quoianum) burrows into a floating dock made of expanded polystyrene they can produce and release between 4900 and 6300 micrometre-sized fragments of polystyrene. There is also a modest literature on plastic degradation in marine habitats (Andrady and Pegram 1989; Andrady 2003; Gregory and Andrady 2003; Corcoran et al. 2009; Cooper and Corcoran 2010; O’Brine and Thompson 2010). But the rigour of these studies varies with some lacking many of the ecological developments in designing, implementing and analysing manipulative field experiments (Underwood 1997). So far, no one has experimentally deployed micrometre-sized plastic (or millimetre-sized pellets) in habitats to determine the rates at which it fragments into smaller pieces. Thus, the role of pellets as major sources of microplastic is unclear.
9.3 Cleaning Products
Sources. Another source of microplastic is from industrial and domestic cleaning products that use microplastic as an abrasive scrubber (Browne et al. 2007). For instance, surfaces of buildings, machinery and boats can be cleaned and prepared (e.g. smoothed, roughened, shaped) using ‘media blasting’, where small plastics (e.g. polystyrene, acrylic, polyester, poly-allyl-diglycol-carbonate, urea-, melamine- and phenol-formaldehyde; 0.25–1.7 mm; DOD 1992) and other types of granules (e.g. sand) are propelled onto a surface using a centrifugal wheel or pressurized fluid/gas (Wolbach and McDonald 1987; Abbott 1992; Gregory 1996; Neulicht and Shular 1997; Anonymous 1998). Although ‘media blasting’ has been suspected of being a source of microplastic to habitats there has been no scientific work to (i) characterize the number of industries using this technique, (ii) the size, shape and amount of microplastic used in the process of cleaning and (iii) the quantity of particles emitted into, or found within, the environment through this source.
Sources. Ingestible and inhalable medicines containing microplastic are used to deliver drugs to the organs of humans and farmed animals (terrestrial and aquatic) because the microplastic can translocate from their lungs or guts into their circulatory system (Thanoo et al. 1993; Dalmon et al. 1995; Curley et al. 1996; Hussain et al. 2001; Matsusaki et al. 2001; Wen et al. 2003; Kockisch et al. 2003; Corbanie et al. 2006). There has, however, been no work to synthesize information about the different types of polymers and sizes of microplastic used in medicines because this information does not seem to be readily available.
Pathways. Like microplastics in cleaning products, microplastics from medicines are likely to transfer to habitats through sewage and storm water, or more directly through treating diseased animals in aquaculture and farming. There is, however, no quantitative work evaluating how much plastic is taken up by animals and excreted compared to those retained in tissues. As such, the quantities of medical microplastic in water or sediment sampled from habitats, sewage or storm water are unknown. Research is needed to synthesize a complete inventory of these polymers so that samples from humans, sewage, storm water, wildlife and habitats can be tested for the presence of these polymers. Some of the polymers used are thought to be biodegradable (Matsusaki et al. 2001), whist others can be composed of more durable polymers such as polycarbonate and polystyrene (Thanoo et al. 1993; Dunn et al. 1994). The rates and mechanisms of degradation inside human tissues may not be the same as in wildlife or habitats. Moreover, just because a polymer degrades, does not necessarily mean that the resulting metabolites are not toxic themselves: So, research is needed to determine how safe these particles are in humans, wildlife and habitats.
A global program to sample sediment from sandy shores. Between 2004 and 2007 samples of sediment were collected from sandy beaches in Australia, Oman, United Arab Emirates, Chile, Philippines, Azores, USA, South Africa, Mozambique and the U.K. During collection (and in all work), cotton clothing was worn rather than synthetic items to prevent samples being contaminated by plastic fibres. Samples were collected by working down-wind to the particular part of the highest strandline deposited by the previous tide. Using established techniques, sediment was sampled to a depth of 1 cm and microplastic and sediment was quantified using established techniques (Browne et al. 2010, 2011). Two explanatory models for pathways of microplastic in habitats were put forward to explain spatial patterns of microplastic.
- (ii)Comparison of sub-tidal areas where sewage sludge had been discharged with reference areas. Previous observations from coastal habitats of the U.K. suggested that each year treatment plants discharge >11 km3 of sewage effluent into coastal habitats (CEFAS 1997) and for nearly 30 years, a quarter of U.K. sewage sludge was dumped at 13 designated sub-tidal disposal-sites around the coast, until this stopped in 1998 through implementation of The Urban Waste Water Treatment Regulations 1994 (Fig. 9.6; CEFAS 1997; British Government 1994). Using replicate disposal-sites and reference-sites allowed us to test Hypothesis 2.1 that sediments from disposal-sites would contain larger quantities of fibres in their sediments and that the shape and types of polymers that make up the microplastic would resemble those found on shores. For this, van Veen grabs deployed from boats collected replicate samples of sediment from two reference-sites and two disposal-sites in the English Channel and the North Sea (U.K.). Despite sewage not being added for more than a decade, disposal-sites still contained >250 % more fibres than reference sites (Fig. 9.7; ANOVA, F1,16 = 4.50, P < 0.05). Again the types of fibres were dominated by polyester (78 %) and acrylic (22 %).
- (iii)Experiments with washing machines. Here, experimental work counted the number of fibres discharged into waste water from domestic washing machines used to launder clothing. To estimate the number of fibres entering wastewater from washing clothes and garments, three replicate washing machines were used with and without cloth (polyester blankets, fleeces, shirts). Effluent was filtered and microplastic counted. The experiments showed all garments released >100 fibres per litre of effluent, with >180 % more from fleeces (>1900 fibres per wash; Fig. 9.9), demonstrating that a large proportion of microplastic fibres found in marine habitats may be derived from sewage as a consequence of washing of clothes.
9.6 Outlook and Conclusion
In the future, contamination by microplastic is likely to continue to increase. Populations of humans are predicted to double in the next 40 years (UN 2008) and further concentrate in large coastal cities that will discharge larger volumes of sewage into marine habitats. The last case study provides a useful approach for identifying and quantifying sources and pathways of microplastic that should be extended to other sources, including medical and cleaning products, by screening sewage, storm water, habitats, wildlife and humans for the types of microplastic found in these products.
- (i)Establishing and controlling inventories that detail the use and emissions of microplastics in products. Inventories are frequently used by European (EA 2012) and U.S. government agencies (EPA 2010, 2012) to control emissions of pollutants. An open-access online inventory is urgently required for textiles, medicines and cleaning products containing microplastic so that we have accurate information about emissions of microplastic during their production, use and disposal. This should include information about the use and emissions of microplastic, in terms of dimensions of size (i.e. minimum, maximum, median, mode and mean) shape, numbers, mass, types of polymers and sales figures. Because industry has, on occasion, been unwilling to provide this information when requested (Rosner 2008), this will probably require a change in policy and specific funding for representative sampling so that measures are accurate and precise (Figs. 9.10 and 9.11).
Modifying the process of production and redesigning products so that they contain less hazardous substances. Currently there are no published data on the effectiveness of modifying the process of production of products to reduce emissions of microplastic, since microplastic is not currently considered hazardous by policy-makers (Rochman et al. 2013). In response to advocacy from scientists and activists, several companies who make domestic cleaning products (e.g. Unilever, Johnson & Johnson) have agreed to replace microplastic with non-plastic particles. It is, however, unclear what alternatives they will use (Alumina; pumice; seeds of strawberries, blueberries, cranberry, evening primrose, grapes, kiwi or raspberry; stones of apricots, avocados, olives or peaches; peel of oranges or mandarin; castor or jojoba beads, shells of cocoa, coconuts, almonds or walnuts; coir; corn cob; salt; sugar; luffa, rice; macadamia nuts) or whether they will be more or less toxic to humans and wildlife, so scientific research is needed to find the most cost-effective alternative. Similar research is needed within the textile and clothing industry so that they produce cost-effective clothing that sheds fewer and less toxic fibres. These types of research are important to ensure decisions by policy-makers are based on robust scientific information as opposed to untested perceptions of the hazards of synthetic materials over those made from natural products.
Novel equipment and technology to reduce pathways of microplastic. Filters for washing machines are a promising prospect for reducing emissions of fibres to sewage (www.environmentalenhancements.com); however, their effectiveness in reducing emissions has not been tested as yet. Work is also needed to determine how effective different types of sewage treatment are at removing the different sizes, shapes and types of polymers that represent the microplastic found in sewage. However, unless the microplastic can be isolated from the sludge or effluent there are still likely to be problems because sewage is added to soil as a fertilizer. To identify the place, company or product where the microplastic originates requires government, industry and scientists to work together and share information. For this to happen there needs to be policies that (i) provide funded frameworks to measure (and if necessary manage) sources and pathways of microplastic into the environment; (ii) balance the needs of industries, society and the environment. In the U.S., 16 persistent, bioaccumulative and toxic chemicals (i.e. aldrin, benzoperylene, chlordane, heptachlor, hexachlorobenzene, isodrin, lead, mercury, methoxychlor, octachlorostyrene, pendimethalin, pentachlorobenzene, polychlorinated biphenyl, tetrabromobisphenol A, toxaphene and trifluralin) are controlled in this way using the Emergency Planning and Community Right-to-Know Act 1986. The Community Right-to-Know provisions help to increase public’s knowledge and access to information on chemicals at individual facilities, their uses, and releases into the environment. Through this facilities are obliged to share information with government, scientists and public to improve chemical safety and protect public health and the environment. Similar requirements are required in Europe under article 5 of the Directive 2008/105/EC where member states are obliged to establish an inventory of emissions, discharges and losses of priority pollutants listed in part A of Annex I (EU 2012). For microplastic, several other solutions could also help. For instance, polymers could be designed with unique chemical fingerprints (that identify particular sites of production or use), which remain even after the polymer has been physically or chemically degraded during its time in the environment. Alternatively, prior to a product being licensed for sale on the market place, information on the composition of polymers (and additives) in commercial applications could be made available to environmental scientists so that the environmental sources and pathways of materials can be quantified and managed if needed. Whatever developments take place, hypothesis-driven frameworks are required to identify and falsify sources and pathways.
In conclusion, if we are to use terms to describe the sources of microplastic they should aim to identify the origin (e.g. larger plastic litter, cleaning products, medicines, textiles, etc.) and separate terms should be used for the pathways (e.g. storm water, sewage). As researchers use more integrated hypothesis-driven frameworks and the chemical methods and inventories improve, it may be possible to be more specific (e.g. the place, person or company).
I am grateful to Mary Sewell (University of Auckland, New Zealand) for providing pictures of microplastic found in cleaning products. I thank the reviewers for their constructive comments.
- Abbott, K. E. (1992). Plastic media blasting—state of the technology (reprinted from society of automotive engineers). Materials Performance, 31, 38–39.Google Scholar
- Andrady, A. L., & Pegram, J. E. (1989). Outdoor weathering of selected polymeric materials under marine exposure conditions. Polymer Degradation and Stability, 26, 333.Google Scholar
- Andrady, A. L. (2003). Plastics and the environment. In A.L. Andrady (Ed.). Hoboken, NJ: John Wiley and Sons.Google Scholar
- Andrady, A. L. (2015). Persistence of plastic litter in the oceans. In M. Bergmann, L. Gutow & M. Klages (Eds.), Marine anthropogenic litter (pp. 57–72), Berlin: Springer.Google Scholar
- Anonymous. (1998). Plastic media blasting of composite honeycomb sandwich structures. Aerospace Engineering, 18, 41–44.Google Scholar
- Arthur, C., Baker, J., & Bamford, H. (2009). In Proceedings of the International Research Workshop on the Occurrence, Effects and Fate of Microplastic Marine Debris, 9–11 September 2008, NOAA Technical, Memorandum NOS-OR&R30.Google Scholar
- Barnes, D. K. A. (2005). Remote Islands reveal rapid rise of Southern Hemisphere, sea debris. Scientific World Journal, 5, 915–921.Google Scholar
- Browne, M. A., Crump, P., Niven, S. J., Teuten, E. L., Tonkin, A., & Galloway, T., et al. (2011). Accumulation of microplastic on shorelines worldwide: Sources and sinks. Environmental Science and Technology, 45, 9175–9179.Google Scholar
- CEFAS. (1997). Centre for Environment, Fisheries and Science. Monitoring and surveillance of non-radioactive contaminants in the aquatic environment and activities regulating the disposal of wastes at sea. Lowestoft: CEFAS.Google Scholar
- Claessens, M., Van Cauwenberghe, L., Vandegehuchte, M. B., & Janssen, C. R. (2013). New techniques for the detection of microplastics in sediments and field collected organisms. Marine Pollution Bulletin, 70, 227–233.Google Scholar
- Collignon, A., Hecq, J. H., Galgani, F., Voisin, P., Collard, F., & Goffart, A. (2012). Neustonic microplastic and zooplankton in the North Western Mediterranean Sea. Marine Pollution Bulletin, 64, 861–864.Google Scholar
- Corbanie, E. A., Matthijs, M. G. R., van Eck, J. H. H., Remon, J. P., Landman, W. J. M., & Vervaet, C. (2006). Deposition of differently sized airborne microspheres in the respiratory tract of chickens. Avian Pathology, 35, 475–485.Google Scholar
- Corcoran, P. L., Biesinger, M. C., & Grifi, M. (2009). Plastics and beaches: A degrading relationship. Marine Pollution Bulletin, 58, 80–84.Google Scholar
- Cooper, D. A., & Corcoran, P. L. (2010). Effects of mechanical and chemical processes on the degradation of plastic beach debris on the island of Kauai, Hawaii. Marine Pollution Bulletin, 60, 650−654.Google Scholar
- Costa, M., Ivar do Sul, J., Silva-Cavalcanti, J., Araújo, M., Spengler, Â., & Tourinho, P. (2010), On the importance of size of plastic fragments and pellets on the strandline: A snapshot of a Brazilian beach. Environmental Monitoring and Assessment, 168, 299–304.Google Scholar
- Curley, J., Castillo, J., Hotz, J., Uezono, M., Hernandez, S., Lim, J. O., et al. (1996). Prolonged regional nerve blockade—injectable biodegradable bupivacaine/polyester microspheres. Anesthesiology, 84, 1401–1410.Google Scholar
- DOD. (1992). MIL-P-85891A, Military specification, plastics media, for removal of organic coatings. http://www.everyspec.com/MIL-SPECS/MIL-SPECS-MIL-P/MIL-P-85891A_8399/
- Dunn, S. E., Brindley, A., Davis, S. S., Davies, M. C., & Illum, L. (1994). Polystyrene-poly (ethylene glycol) (PS-PEG2000) particles as model systems for site specific drug delivery. 2. The effect of PEG surface density on the in vitro cell interaction and in vivo biodistribution. Pharmaceutical Research, 11, 1016–1022.CrossRefPubMedGoogle Scholar
- EA [UK Environment Agency]. (2012). Pollution inventory reporting—guidance for operators of waste transfer stations Environmental Permitting (England and Wales) Regulations 2010 Regulation 60(1) Version 5 December. Bristol, U.K.: Environment Agency. https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/296990/LIT_7759_f0b081.pdf
- EPA [US Environmental Protection Agency]. (2010). U.S. Environmental Protection Agency 2010–2014 Pollution Prevention (P2) Program Strategic Plan (pp. 1–34). Washington, USA.Google Scholar
- EPA. (2012). The Emergency Planning and Community Right-to-Know Act 1986. Washington, USA. http://www2.epa.gov/sites/production/files/2013-08/documents/epcra_fact_sheet.pdf
- EU [European Union]. (2010). Part 1: List of chemicals subject to export notification procedure Directive 2010/75/EU of the European Parliament and of the council. Official Journal of the European Union, L334, 17–119.Google Scholar
- EU. (2012). Part 1: List of chemicals subject to export notification procedure. http://edexim.jrc.ec.europa.eu/list_annex_i_chemicals.php?id_part=1
- Galgani, F., Hanke, G., & Maes, T. (2015). Global distribution, composition and abundance of marine litter. In M. Bergmann, L. Gutow & M. Klages (Eds.), Marine anthropogenic litter (pp. 29–56). Berlin: Springer.Google Scholar
- Gouin, T., Roche, N., Lohmann, R., & Hodges, G. A. (2011). Thermodynamic approach for assessing the environmental exposure of chemicals absorbed to microplastic. Environmental Science and Technology, 45, 1466–1472.Google Scholar
- Gregory, M. R., & Andrady, A. L. (2003). Plastics in the marine environment. In A. L. Andrady (Ed.), Plastics and the Environment. Hoboken, NJ: John Wiley and Sons.Google Scholar
- Merrell, T. R. Jr. (1980). Accumulation of plastic litter on beaches of Amchitka Island, Alaska. Marine Environmental Research, 3, 171–184.Google Scholar
- Neulicht, R., & Shular, J. (1997). Emission factor documentation for AP-42. Section 13.2.6. Abrasive blasting final report (pp. 1–38). Washington, USA: EPA. http://www.epa.gov/ttnchie1/ap42/ch13/bgdocs/b13s02-6.pdf
- O’Brine, T., & Thompson, R. C. (2010). Degradation of plastic carrier bags in the marine environment. Marine Pollution Bulletin, 60, 2279–2283.Google Scholar
- Oerlikon. (2009). The Fiber Year 2008/09: A world-survey on textile and nonwovens industry. Switzerland: Oerlikon.Google Scholar
- Ramirez-Llodra, E., Company, J. B., Sard, F., De Mol, B., Coll, M., & Sardà, F. (2013). Effects of natural and anthropogenic processes in the distribution of marine litter in the deep Mediterranean Sea. Progress in Oceanography, 118, 273–287.Google Scholar
- Rosner, H. (2008). Scrubbing out sea life. http://www.slate.com/articles/business/moneybox/2008/06/scrubbing_out_sea_life.html
- Thompson, R. C., Olsen, Y., Mitchell, R. P., Davis, A., Rowland, S. J., John, A. W. G., et al. (2004). Lost at sea: Where is all the plastic? Science, 304(5672), 838.Google Scholar
- Underwood, A. J. (1997). Experiments in ecology—Their logical design and interpretation using analysis of variance. Cambridge: Cambridge University Press.Google Scholar
- UN [United Nation]. (2008). World Population Prospects: The 2008 Revision Population Database, New York, 2008. www.esa.un.org/unpp. Accessed 1 Oct 2008.
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