Microplastics are "synthetic solid particulate matter or polymeric matrices, with regular or irregular shape and with size ranging from 1 μm to 5 mm, of either primary or secondary manufacturing origin, which are insoluble in water." Microplastics cause pollution by entering natural ecosystems from a variety of sources, including cosmetics, clothing, construction, renovation, food packaging, and industrial processes. However, there are different microplastic remediation strategies that can help mitigate the effects of plastic pollution.
The term microplastics is used to differentiate them from larger, non-microscopic plastic waste. Two classifications of microplastics are currently recognized. Primary microplastics include any plastic fragments or particles that are already 5 mm in size or less before entering the environment. These include microfibers from clothing, microbeads, plastic glitter and plastic pellets (also known as nurdles). Secondary microplastics arise from the degradation (breakdown) of larger plastic products through natural weathering processes after entering the environment. Such sources of secondary microplastics include water and soda bottles, fishing nets, plastic bags, microwave containers, tea bags and tire wear.
Both types are recognized to persist in the environment at high levels, particularly in aquatic and marine ecosystems, where they are considered water pollution. It is estimated that by 2050, the weight of microplastics in the sea will exceed that of the fish.
Microplastics also accumulate in the air and terrestrial ecosystems. Airborne microplastics have been detected in the atmosphere, as well as indoors and outdoors.
Because some plastics degrade slowly (often over hundreds to thousands of years), See Section 3, "Environmental Degradation of Synthetic Polymers". microplastics have a high probability of ingestion, incorporation into, and accumulation in the bodies and tissues of many organisms. In terrestrial ecosystems, microplastics have been demonstrated to lower the viability of soil ecosystems.
Microplastics are likely to degrade into smaller nanoplastics through chemical weathering processes, mechanical breakdown, and even through the digestive processes of animals. Nanoplastics are a subset of microplastics and are smaller than 1μm (1micrometer, or 1000nm). Nanoplastics cannot be seen by the human eye.
Classification
The term "microplastics" has been widely attributed to a 2004 paper by Professor Richard Thompson, a marine biologist at the University of Plymouth in the United Kingdom. However, there are examples of the term being used in relation to marine pollution dating back to the early 1990s, throughout the 1990s and into the early 2000s.Faris, Jeannie, and Kathy Hart. "Seas of debris: a summary of the third international conference on marine debris." (1994).Melissa Maher, Steven M. Howdle, Matthew F. Johnson, Vincenzo Taresco, Thomas Stanton and Rachel L. Gomes (2001) Life in Plastic, it's Fantastic? University of Nottingham. https://www.nottingham.ac.uk/engineering/documents/postgraduate-research-showcase/melissamaher-fww.pdfCarr, A. and Gregory, M. (2000) 'Conduct research on the movement and effects of derelict fishing gear, and other plastics in ecosystems', in McIntosh, N., Simonds, K., Donohue, M., Brammer, C., Mason, S. and Carbajal, S. (eds.) Proceedings of the International Marine Debris Conference on Derelict Fishing Gear and the Ocean Environment, Honolulu, Hawaiʻi, 6–11 August 2000, pp. 222–224. There is also evidence that the authors who first used the term had been exploring the topic during the late 1980s.
Microplastics are common in our world today. In 2014, it was estimated that there are between 15 and 51 trillion individual pieces of microplastic in the world's oceans, which was estimated to weigh between 93,000 and 236,000 metric tons. Under the influence of sunlight, wind, waves and other factors, plastic degrades into small fragments known as microplastics, or even nanoplastics.
Primary microplastics
Primary microplastics are small pieces of plastic that are purposefully manufactured. They are usually used in facial cleansers and cosmetics, or in air blasting technology. In some cases, their use in medicine as vectors for drugs was reported. Microplastic "scrubbers", used in exfoliating hand cleansers and facial scrubs, have replaced traditionally used natural ingredients, including ground almond shells, oatmeal, and pumice. Primary microplastics have also been produced for use in air-blasting technology. This process involves blasting acrylic, melamine, or polyester microplastic scrubbers at machinery, engines, and boat hulls to remove rust and paint. As these scrubbers are used repeatedly until they diminish in size and their cutting power is lost, they often become contaminated with heavy metals such as cadmium, chromium, and lead. Although many companies have committed to reducing the production of microbeads, there are still many bioplastic microbeads that also have a long degradation life cycle, for example in cosmetics.
Secondary microplastics Secondary microplastics are small pieces of plastic derived from the physical breakdown and mechanical degradation of larger plastic debris, both at sea and on land. Over time, a combination of physical, biological, and photochemical degradation, including photo-oxidation caused by sunlight exposure, can reduce the structural integrity of plastic debris to a size that is eventually undetectable to the naked eye. This process of breaking down large plastic material into much smaller pieces is known as fragmentation. It is considered that microplastics might further degrade to be smaller in size, although the smallest microplastic reportedly detected in the oceans in 2017 was 1.6 micrometres (6.3×10−5 in) in diameter. The prevalence of microplastics with uneven shapes suggests that fragmentation is a key source. One study suggested that more microplastics might be formed from biodegradable polymer than from non-biodegradable polymer in both seawater and fresh water.
Microplastic fibers enter the environment as a by-product during wear and tear and from the washing of synthetic clothing. Tires, composed partly of synthetic styrene-butadiene rubber, erode into tiny plastic and rubber particles as they are used and become dust particles. 2–5 mm plastic pellets, used to create other plastic products, enter ecosystems due to spillages and other accidents.
A 2015 Norwegian Environment Agency review report about microplastics stated it would be beneficial to classify these sources as primary, as long as microplastics from these sources are added from human society since the "start of the pipe", and their emissions are inherently a result of human material and product use and not secondary to fragmentation in the nature.Sundt, Peter, and Schulze, Per-Erik: "Sources of microplastic-pollution to the marine environment", "Mepex for the Norwegian Environment Agency", 2015
Nanoplastics Depending on the definition used, nanoplastics are less than 1 μm (i.e. 1000 nm) or less than 100 nm in size.There is not yet a consensus on this upper limit. Speculations over nanoplastics in the environment range from it being a temporary byproduct during the fragmentation of microplastics to it being an invisible environmental threat at potentially high and continuously rising concentrations. The presence of nanoplastics in the North Atlantic Subtropical Gyre has been confirmed. Recent developments in Raman spectroscopy coupled with optical tweezers (Raman Tweezers) as well as nano-fourier-transform infrared spectroscopy (nano-FTIR), atomic force infrared (AFM-IR), and dynamic light scattering (DLS) are promising answers in the near future regarding the nanoplastic quantity in the environment. Fluorescence could represent a unique tool for the identification and quantification of nanoplastics, since it allows the development of fast, easy, cheap, and sensitive methods. A microfluidics method is also developed to aggregate nanoplastics into larger aggregates for convenient isolation, enrichment and downstream quantification via fluorescence microscopy of Nile-Red stained nanoplastics aggregates. However, the nanoplastic problem is complex and nanoscale properties as well as interaction with biomolecules need to be explored at the fundamental level with high spatial and temporal resolution.
Nanoplastics are thought to be a risk to environmental and human health. Due to their small size, nanoplastics can cross cellular membranes and affect the functioning of cells. Nanoplastics are lipophilic and models show that polyethylene nanoplastics can be incorporated into the hydrophobic core of lipid bilayers. Nanoplastics are also shown to cross the epithelial membrane of fish accumulating in various organs including the gallbladder, pancreas, and the brain. Nanoplastics are believed to cause interruptions in bone cell activities, causing improper bone formation. Little is known on adverse health effects of nanoplastics in organisms including humans. In zebrafish (Danio rerio), polystyrene nanoplastics can induce a stress response pathway altering glucose and cortisol levels, which is potentially tied to behavioral changes in stress phases. In Daphnia, polystyrene nanoplastic can be ingested by the freshwater cladoceran Daphnia pulex and affect its growth and reproduction as well as induce stress defense, including the ROS production and MAPK-HIF-1/NF-κB-mediated antioxidant system. Nanoplastics can also adsorb toxic chemical pollutants, such as antibiotics, which enables selective association with antibiotic-resistant bacteria, resulting in the dissemination of nanoplastics and antibiotic-resistant bacteria by the bacterivorous nematode Caenorhabditis elegans across the soil.
Sources of microplastics The existence of microplastics in the environment is often established through aquatic studies. These include taking plankton samples, analyzing sandy and muddy sediments, observing vertebrate and invertebrate consumption, and evaluating chemical pollutant interactions.
Microplastic is also a type of airborne particulates and is found to prevail in air. A 2017 IUCN report identified microplastics as a major contributor to marine plastic pollution. Microplastics can adsorb heavy metals from seawater.
Microplastics can also enter terrestrial systems through multiple pathways, including the degradation of plastic mulching films, the application of compost and sewage sludge, as well as atmospheric deposition, surface runoff, and irrigation.Liu, M. et al. (2020). Microplastics in terrestrial ecosystems: sources, fate, and impacts. Environmental Science & Technology. Their distribution in soils is influenced by land use, agricultural practices, and environmental conditions.
Observations have shown that microplastics can also be detected in soils without direct plastic application, indicating the importance of diffuse input pathways and long-term accumulation.Piehl, S. et al. (2018). Identification and quantification of macro- and microplastics on agricultural farmland. Scientific Reports.
Oral intake Oral intake is the main pathway of human exposure to microplastics. Microplastics exist in daily necessities like drinking water, bottled water, seafood, salt, sugar, tea bags, milk, and so on.
In 2017, more than eight million tons of plastics entered the oceans, greater than 33 times as much as that of the total plastics accumulated in the oceans by 2015. One consequence of this is marine life consumption of microplastics. It is estimated that Europeans are exposed to about 11,000 particles/person/year of microplastics due to shellfish consumption.