Microplastics

Polyethylene based microspherules in toothpaste

a) Artificial turf football field with ground tyre rubber (GTR) used for cushioning. b) Microplastics from the same field, washed away by rain, found in nature close to a stream.

Primary microplastics are small pieces of plastic that are purposefully manufactured.[18] 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.[19] Microplastic "scrubbers", used in exfoliating hand cleansers and facial scrubs, have replaced traditionally used natural ingredients, including ground almonds, 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.[20] 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 similar to normal plastic.

Secondary plastics are small pieces of plastic derived from the breakdown of larger plastic debris, both at sea and on land. Over time, a culmination of physical, biological, and chemphotodegradation, including photodegradation caused by sunlight exposure, can reduce the structural integrity of plastic debris to a size that is eventually undetectable to the naked eye.[21] This process of breaking down large plastic material into much smaller pieces is known as fragmentation.[20] It is considered that microplastics might further degrade to be smaller in size, although the smallest microplastic reportedly detected in the oceans at present is 1.6 micrometres (6.3×10⁻⁵ in) in diameter.[22] The prevalence of microplastics with uneven shapes suggests that fragmentation is a key source.[10]

There are countless sources of both primary and secondary microplastics. Microplastic fibers enter the environment from the washing of synthetic clothing.[23][8] Tires, composed partly of synthetic styrene-butadiene rubber, will erode into tiny plastic and rubber particles as they are used. Furthermore, 2.0-5.0 mm plastic pellets, used to create other plastic products, often enter ecosystems due to spillages and other accidents.[7] A Norwegian Environment Agency review report about microplastics published in early 2015[24] states it would be beneficial to classify these sources as primary, as long as microplastics from these sources are added from human society at the "start of the pipe", and their emissions are inherently a result of human material and product use and not secondary defragmentation in nature.

Depending on the definition used, nanoplastics are less than 1 μm (i.e. 1000 nm) or less than 100 nm in size.[25] 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 concentrations. The presence of nanoplastics in the North Atlantic Subtropical Gyre has been confirmed[26] and recent developments in Raman spectroscopy and nano-fourier-transform infrared (nano-FTIR) technology[27] are promising answers in the near future regarding the nanoplastic quantity in the environment.

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.[28] Nanoplastics are also shown to cross the epithelial membrane of fish accumulating in various organs including the gall bladder, pancreas, and the brain.[29][30] Little is known on adverse health effects of nanoplastics in organisms including humans. In zebrafish, polystyrene nanoplastics can induce a stress response pathway altering glucose and cortisol levels, which is potentially tied to behavioral changes in stress phases.[31] 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 [32][33][34]

Sewage treatment plants, also known as wastewater treatment plants (WWTPs), remove contaminants from wastewater, primarily from household sewage, using various physical, chemical, and biological processes.[36] Most plants in developed countries have both primary and secondary treatment stages. In the primary stage of treatment, physical processes are employed to remove oils, sand, and other large solids using conventional filters, clarifiers, and settling tanks.[37] Secondary treatment uses biological processes involving bacteria and protozoa to break down organic matter. Common secondary technologies are activated sludge systems, trickling filters, and constructed wetlands. The optional tertiary treatment stage may include processes for nutrient removal (nitrogen and phosphorus) and disinfection.[37]

Microplastics have been detected in both the primary and secondary treatment stages of the plants. A groundbreaking 1998 study suggested that microplastic fibers would be a persistent indicator of sewage sludges and wastewater treatment plant outfalls.[38] A study estimated that about one particle per liter of microplastics are being released back into the environment, with a removal efficiency of about 99.9%.[36][39][40] A 2016 study showed that most microplastics are actually removed during the primary treatment stage where solid skimming and sludge settling are used.[36] When these treatment facilities are functioning properly, the contribution of microplastics into oceans and surface water environments from WWTPs is not disproportionately large.[36][41]

Sewage sludge is used for soil fertilizer in some countries, which exposes plastics in the sludge to the weather, sunlight, and other biological factors, causing fragmentation. As a result, microplastics from these biosolids often end up in storm drains and eventually into bodies of water.[42] In addition, some studies show that microplastics do pass through filtration processes at some WWTPs.[20] According to a study from the UK, samples taken from sewage sludge disposal sites on the coasts of six continents contained an average one particle of microplastic per liter. A significant amount of these particles was of clothing fibers from washing machine effluent.[43]

Wear and tear from tires significantly contributes to the flow of (micro-)plastics into the environment. Estimates of emissions of microplastics to the environment in Denmark are between 5,500 and 14,000 tonnes (6,100 and 15,400 tons) per year. Secondary microplastics (e.g. from car and truck tires or footwear) are more important than primary microplastics by two orders of magnitude. The formation of microplastics from the degradation of larger plastics in the environment is not accounted for in the study.[44]

The estimated per capita emission ranges from 0.23 to 4.7 kg/year, with a global average of 0.81 kg/year. The emissions from car tires (100%) are substantially higher than those of other sources of microplastics, e.g., airplane tires (2%), artificial turf (12–50%), brake wear (8%), and road markings (5%). Emissions and pathways depend on local factors like road type or sewage systems. The relative contribution of tire wear and tear to the total global amount of plastics ending up in our oceans is estimated to be 5–10%. In air, 3–7% of the particulate matter (PM_2.5) is estimated to consist of tire wear and tear, indicating that it may contribute to the global health burden of air pollution which has been projected by the World Health Organization (WHO) at 3 million deaths in 2012. The wear and tear also enter our food chain, but further research is needed to assess human health risks.[45]

Some companies have replaced natural exfoliating ingredients with microplastics, usually in the form of "microbeads" or "micro-exfoliates". These products are typically composed of polyethylene, a common component of plastics, but they can also be manufactured from polypropylene, polyethylene terephthalate (PET), and nylon.[46] They are often found in face washes, hand soaps, and other personal care products; the beads are usually washed into the sewage system immediately after use. Their small size prevents them from fully being retained by preliminary treatment screens at wastewater plants, thereby allowing some to enter rivers and oceans.[47] In fact, wastewater treatment plants only remove an average of 95–99.9% of microbeads because of their small design . This leaves an average of 0-7 microbeads per litre being discharged.[48] Considering that one treatment plant discharges 160 trillion liters of water per day, around 8 trillion microbeads are released into waterways every day.[48] This number doesn't account for the sewage sludge that is reused as fertilizer after the waste water treatment that has been known to still contain these microbeads.[49]

This is an issue at the household level because it has been estimated that around 808 trillion beads per household are discharged in a single day whether due to cosmetic exfoliates, face wash, toothpaste, or other sources. Although many companies have committed to phasing out the use of microbeads in their products, according to research, there are at least 80 different facial scrub products that are still being sold with microbeads as a main component.[48] This contributes to the 80 metric tons of microbead discharge per year by the United Kingdom alone, which not only has a negative impact upon the wildlife and food chain, but also upon levels of toxicity, as microbeads have been proven to absorb dangerous chemicals such as pesticides and polycyclic aromatic hydrocarbons.[48] The restriction proposal by the European Chemical Agency (ECHA) and the reports by UNEP and Tauw suggest that there are more than 500 microplastic ingredients that are widely used in cosmetics & personal care products.[50]

Studies have shown that many synthetic fibers, such as polyester, nylon, acrylics, and spandex, can be shed from clothing and persist in the environment.[51][52] Each garment in a load of laundry can shed more than 1,900 fibers of microplastics, with fleeces releasing the highest percentage of fibers, over 170% more than other garments.[53][43] For an average wash load of 6 kg, over 700,000 fibres could be released per wash.[54]

Washing machine manufacturers have also reviewed research into whether washing machine filters can reduce the amount of microfiber fibers that need to be treated by water treatment facilities.[55]

These microfibers have been found to persist throughout the food chain from zooplankton to larger animals such as whales.[7] The primary fiber that persist throughout the textile industry is polyester which is a cheap cotton alternative that can be easily manufactured. However, these types of fibers contribute greatly to the persistence to microplastics in terrestrial, aerial, and marine ecosystems. The process of washing clothes causes garments to lose an average of over 100 fibers per liter of water.[43] This has been linked with health effects possibly caused by the release of monomers, dispersive dyes, mordants, and plasticisers from manufacturing. The occurrence of these types of fibers in households has been shown to represent 33% of all fibers in indoor environments.[43]

Textile fibers have been studied in both indoor and outdoor environments to determine the average human exposure. The indoor concentration was found to be 1.0–60.0 fibers/m^3, whereas the outdoor concentration was much lower at 0.3-1.5 fibers/m^3.[56] The deposition rate indoors was 1586–11,130 fibers per day/m^3 which accumulates to around 190-670 fibers/mg of dust.[56] The largest concern with these concentrations is that it increases exposure to children and the elderly, which can cause adverse health effects.

The manufacture of plastic products uses granules and small resin pellets as their raw material. In the United States, production increased from 2.9 million pellets in 1960 to 21.7 million pellets in 1987.[57] In 2019, the United States produced 121.46 billion pounds of plastic resin.[58] Through accidental spillage during land or sea transport, inappropriate use as packing materials, and direct outflow from processing plants, these raw materials can enter aquatic ecosystems. In an assessment of Swedish waters using an 80 µm mesh, KIMO Sweden found typical microplastic concentrations of 150–2,400 microplastics per m³; in a harbor adjacent to a plastic production facility, the concentration was 102,000 per m³.[20]

Many industrial sites in which convenient raw plastics are frequently used are located near bodies of water. If spilled during production, these materials may enter the surrounding environment, polluting waterways.[24] “More recently, Operation Cleansweep, a joint initiative of the American Chemistry Council and Society of the Plastics Industry, is aiming for industries to commit to zero pellet loss during their operations”.[20] Overall, there is a significant lack of research aimed at specific industries and companies that contribute to microplastics pollution.

Recreational and commercial fishing, marine vessels, and marine industries are all sources of plastic that can directly enter the marine environment, posing a risk to biota both as macroplastics, and as secondary microplastics following long-term degradation. Marine debris observed on beaches also arises from beaching of materials carried on inshore and ocean currents. Fishing gear is a form of plastic debris with a marine source. Discarded or lost fishing gear, including plastic monofilament line and nylon netting, is typically neutrally buoyant and can, therefore, drift at variable depths within the oceans. Various countries have reported that microplastics from the industry and other sources have been accumulating in different types of seafood. In Indonesia, 55% of all fish species had evidence of manufactured debris similar to America which reported 67%.[59] However, the majority of debris in Indonesia was plastic, while in North America the majority was synthetic fibers found in clothing and some types of nets. The implication from the fact that fish are being contaminated with microplastic is that those plastics and their chemicals will bioaccumulate in the food chain.

One study analyzed the plastic-derived chemical called polybrominated diphenyl ethers (PBDEs) in the stomachs of short-tailed shearwaters. It found that one-fourth of the birds had higher-brominated congeners that are not naturally found in their prey. However, the PBDE got into the birds' systems through plastic that was found in the stomachs of the birds. It is therefore not just the berska plastics that are being transferred through the food chain but the chemicals from the plastics as well.[60]

Shipping has significantly contributed to marine pollution. Some statistics indicate that in 1970, commercial shipping fleets around the world dumped over 23,000 tons of plastic waste into the marine environment. In 1988, an international agreement (MARPOL 73/78, Annex V) prohibited the dumping of waste from ships into the marine environment. In the United States, the Marine Plastic Pollution Research and Control Act of 1987 prohibits discharge of plastics in the sea, including from naval vessels.[61][62] However, shipping remains a dominant source of plastic pollution, having contributed around 6.5 million tons of plastic in the early 1990s.[63][64] Research has shown that approximately 10% of the plastic found on the beaches in Hawaii are nurdles.[65] In one incident on July 24, 2012, 150 tonnes of nurdles and other raw plastic material spilled from a shipping vessel off the coast near Hong Kong after a major storm. This waste from the Chinese company Sinopec was reported to have piled up in large quantities on beaches.[24] While this is a large incident of spillage, researchers speculate that smaller accidents also occur and further contribute to marine microplastic pollution.[24]

In one study, 93% of the bottled water from 11 different brands showed microplastic contamination. Per liter, researchers found an average of 325 microplastic particles.[66] Of the tested brands, Nestlé Pure Life and Gerolsteiner bottles contained the most microplastic with 930 and 807 microplastic particles per liter (MPP/L), respectively.[66] San Pellegrino products showed the least quantity of microplastic densities. Compared to water from taps, water from plastic bottles contained twice as much microplastic. Some of the contamination likely comes from the process of bottling and packaging the water.[66]

Newborn drinks milk from a baby bottle

In 2020 researchers reported that polypropylene infant feeding bottles with contemporary preparation procedures were found to cause microplastics exposure to infants ranging from 14,600 to 4,550,000 particles per capita per day in 48 regions. Microplastics release is higher with warmer liquids and similar with other polypropylene products such as lunchboxes.[67][68][69]

Since the emergence of the COVID-19 pandemic, the usage of medical face masks has sharply increased to reach approximately 89 million masks each month. Single use face masks are made from polymers, such as polypropylene, polyurethane, polyacrylonitrile, polystyrene, polycarbonate, polyethylene, or polyester. The increase in production, consumption, and littering of face masks was added to the list of environmental challenges, due to the addition of plastic particles waste in the environment. After degrading, disposable face masks could break down into smaller size particles (under 5mm) emerging a new source of microplastic.[70]

A report made in February 2020 by Oceans Asia, an organization committed to advocacy and research on marine pollution, confirms "the presence of face masks of different types and colors in an ocean in Hong Kong".[70]

Microplastics can become embedded in animals' tissue through ingestion or respiration. Various annelid species, such as deposit-feeding lugworms (Arenicola marina), have been shown to have microplastics embedded in their gastrointestinal tracts. Many crustaceans, like the shore crab Carcinus maenas, have been seen to integrate microplastics into both their respiratory and digestive tracts.[52][76][77] Plastic particles are often mistaken by fish for food which can block their digestive tracts sending incorrect feeding signals to the brains of the animals.[9]

It can take up to 14 days for microplastics to pass through an animal (as compared to a normal digestion period of 2 days), but enmeshment of the particles in animals' gills can prevent elimination entirely.[76] When microplastic-laden animals are consumed by predators, the microplastics are then incorporated into the bodies of higher trophic-level feeders. For example, scientists have reported plastic accumulation in the stomachs of lantern fish which are small filter feeders and are the main prey for commercial fish like tuna and swordfish.[78] Microplastics also absorb chemical pollutants that can be transferred into the organism's tissues.[79] Small animals are at risk of reduced food intake due to false satiation and resulting starvation or other physical harm from the microplastics.

A study done at the Argentinean coastline of the Rio de la Plata estuary, found the presence of microplastics in the guts of 11 species of coastal freshwater fish. These 11 species of fish represented four different feeding habits: detritivore, planktivore, omnivore and ichthyophagous.[80] This study is one of the few so far to show the ingestion of microplastics by freshwater organisms.

Bottom feeders, such as benthic sea cucumbers, who are non-selective scavengers that feed on debris on the ocean floor, ingest large amounts of sediment. It has been shown that four species of sea cucumber (Thyonella gemmate, Holothuria floridana, H. grisea and Cucumaria frondosa) ingested between 2- and 20-fold more PVC fragments and between 2- and 138-fold more nylon line fragments (as much as 517 fibers per organism) based on plastic-to-sand grain ratios from each sediment treatment. These results suggest that individuals may be selectively ingesting plastic particles. This contradicts the accepted indiscriminate feeding strategy of sea cucumbers, and may occur in all presumed non-selective feeders when presented with microplastics.[81]

Bivalves, important aquatic filter feeders, have also been shown to ingest microplastics and nanoplastics.[82] Upon exposure to microplastics, bivalve filtration ability decreases.[83] Multiple cascading effects occur as a result, such as immunotoxicity and neurotoxicity.[84][85][86] Decreased immune function occurs due to reduced phagocytosis and NF-κB gene activity.[84][86] Impaired neurological function is a result of the inhibition of ChE and suppression of neurotransmitter regulatory enzymes.[86] When exposed to microplastics, bivalves also experience oxidative stress, indicating an impaired ability to detoxify compounds within the body, which can ultimately damage DNA.[85] Bivalve gametes and larvae are also impaired when exposed to microplastics. Rates of developmental arrest, and developmental malformities increase, while rates of fertilization decrease.[82][87] When bivalves have been exposed to microplastics as well as other pollutants such as POPs, mercury or hydrocarbons in lab settings, toxic effects were shown to be aggravated.[83][84][85]

Not only fish and free-living organisms can ingest microplastics. Scleractinian corals, which are primary reef-builders, have been shown to ingest microplastics under laboratory conditions.[88] While the effects of ingestion on these corals has not been studied, corals can easily become stressed and bleach. Microplastics have been shown to stick to the exterior of the corals after exposure in the laboratory.[88] The adherence to the outside of corals can potentially be harmful, because corals cannot handle sediment or any particulate matter on their exterior and slough it off by secreting mucus, expending energy in the process, increasing the likelihood of mortality.[89]

Marine biologists in 2017 discovered that three-quarters of the underwater seagrass in the Turneffe Atoll off the coast of Belize had microplastic fibers, shards, and beads stuck to it. The plastic pieces had been overgrown by epibionts (organisms that naturally stick themselves to seagrass). Seagrass is part of the barrier reef ecosystem and is fed on by parrotfish, which in turn are eaten by humans. These findings, published in Marine Pollution Bulletin, may be "the first discovery of microplastics on aquatic vascular plants...[and] only the second discovery of microplastics on marine plant life anywhere in the world."[90]

It is not just aquatic animals which may be harmed. Microplastics can stunt the growth of terrestrial plants and earthworms.[91]

In 2019, the first European records of microplastic items in amphibians’ stomach content was reported in specimens of the common European newt (Triturus carnifex). This also represented the first evidence for Caudata worldwide, highlighting that the emerging issue of plastics is a threat even in remote high-altitude environments.[92]

Zooplankton ingest microplastics beads (1.7–30.6 μm) and excrete fecal matter contaminated with microplastics. Along with ingestion, the microplastics stick to the appendages and exoskeleton of the zooplankton.[93] Zooplankton, among other marine organisms, consume microplastics because they emit similar infochemicals, notably dimethyl sulfide, just as phytoplankton do.[94][95] Plastics such as high-density polyethylene (HDPE), low-density polyethylene (LDPE), and polypropylene (PP) produce dimethyl sulfide odors.[94] These types of plastics are commonly found in plastic bags, food storage containers, and bottle caps.[96] Green and red filaments of plastics are found in the planktonic organisms and in seaweeds. [97]

Not only do animals and plants ingest microplastics, some microbes also live on the surface of microplastics. This community of microbes form a slimy biofilm which, according to a 2019 study,[98] has a unique structure and possesses a special risk, because microplastic biofilms have been proven to provide a novel habitat for colonization that increases overlap between different species, thus spreading pathogens and antibiotic resistant genes through horizontal gene transfer. Then, due to rapid movement through waterways, these pathogens can be moved very quickly from their origin to another location where a specific pathogen may not be naturally present, spreading the potential disease.[98]

Human contamination and accumulation of MPs can occur via food (due to packaging, sea food), air (inhaling contaminated air) and drinking water causing cytotoxicity, hypersensitivity, unwanted immune response, and acute response like hemolysis.[99] Fish is a significant source of protein for the human population, accounting for 6.1% of all protein consumed globally in 2007.[100] The microplastics ingested by fish and crustaceans can be subsequently consumed by humans as the end of the food chain.[101] Many additional researchers have found evidence that these fibers had become chemically associated with metals, polychlorinated biphenyls, and other toxic contaminants while in water. The microplastic-metal complex can then enter humans via consumption.[52]

The primary concern with human health in regards to microplastics is more directed towards the different toxic and carcinogenic chemicals used to make these plastics and what they carry. It has also been thought that microplastics can act as a vector for pathogens as well as heavy metals.[102] Pollutants may be absorbed into the surface of MPs with enough surface area.[103] Furthermore, induced electrostatic charge on the highly resistive plastic caused by high-speed manufacturing equipment during plastic production can increase the pickup of pollutants while in the environment. Additionally, low polarity at the surface of the MPs make them more hydrophobic. This hydrophobic nature allows to adsorption of hydrophobic chemicals into surface of MP. Lipid-loving chemicals like POPs (including polychlorinated biphenyls PCBs, polycyclic aromatic hydrocarbons (PAHs) and organochlorine pesticides such as DDT and DDE) have the right characteristics to adsorp and concentrate on the plastics hydrophobic surface forming a micelle shape-like structure[103] More specifically, pregnant women in particular are in danger of causing birth defects to male infants such as anogenital distance, penile width, and testicular descent.[65] This comes from phthalate exposure and DEHP metabolites that interfere with the development of the male reproductive tract.

PET, a common durable plastic used in bottled water, juice and soda leaches antimony in amounts that exceed U.S. safety guidelines under high temperature. HDPE commonly used in food packaging can percolate estrogenic chemicals when exposed to heat, boiling water and sunlight causes breast cancer, endometriosis, altered sex ratios, testicular cancer, poor semen quality, early puberty and malformations of the reproductive tract. Polyvinyl chloride (PVC) is used to wrap meat and sandwiches, floats in the tub in the form of bath toys, makes for stylish jackets and household plumbing, leaches toxic chemicals when in contact with water. Four chemical softeners used with PVC interfere with the body's production of hormones. Polystyrene, a common type of plastic used in packaging for takeout food and the fishing industry, can release carcinogen when in contact with hot beverages.

Bisphenol A (BPA) is a substance that is an ingredient used to harden plastic that can also cause a wide range of disorders. Cardiovascular disease, type 2 diabetes, and abnormalities in liver enzymes are a few disorders that can arise from even small exposure to this chemical.[65] Although these effects have been more widely studied than other types of plastics, it is still used in the production of much clothing (polyester).

A flame retardant called Tetrabromobisphenol A (TBBPA) is used in many different types of plastics such as those found in microcircuits. This chemical has been linked to disruptions in thyroid hormones balance, pituitary function, and infertility in lab rats.[104] The endocrine system is affected by TBBPA through disruption of the natural T3 functions with the nuclear suspension in pituitary and thyroid.

Many people can expect to come in contact with various types of microplastics on a daily basis in the aforementioned sources (see sources). However, the average citizen is exposed to microplastics through their various types of food included in a normal diet. The report "Human Consumption of Microplastics" mentions that the average person eats at least 50 000 microplastic particles a year and breathes in a similar quantity.[105][106]

Researchers in China, for instance, tested three types of table salt samples available in supermarkets and found the presence of microplastics in all of them. Sea salt has the highest amounts of microplastics compared to lake salt and rock/well salt.[107] Sea salt and rock salt which are commonly used table salts in Spain have also been found to contain microplastics.[108] The most common type of microplastic found in both these studies was polyethylene terephthalate (PET).

An example of bioaccumulation in the food chain that leads to human exposure was a study of the tissue samples of mussels to approximate concentration of microplastics. The study extrapolated that an average citizen might be exposed to 123 particles/year/capita of microplastics through mussel consumption in the UK.[109] Considering different diets, it was also estimated that microplastic exposure could rise to 4,620 particles/y/capita in countries with higher shellfish consumption.[109] Humans, on average, are predicted to be exposed to microplastics more in household dust than by consuming mussels.[110]

A 2018 study conducted on eight individuals from Europe and Japan found microplastics in human feces for the first time. All participants were found positive for at least one type of microplastic after all of them had consumed plastic-wrapped food and drunk water from plastic bottles while six had also eaten seafood. It was noted though that the study was small-sized, preliminary and unable to reveal the true origin of the plastic particles.[111][112]

According to a comprehensive review of scientific evidence published by the European Union's Scientific Advice Mechanism in 2019, "little is known with respect to the human health risks of nano- and microplastics, and what is known is surrounded by considerable uncertainty". The authors of the review identify the main limitations as the quality or methodology of the research to date. Since "the poison is in the dose", the review concludes that "there is a need to understand the potential modes of toxicity for different size-shape-type NMP combinations in carefully selected human models, before robust conclusions about ‘real’ human risks can be made".[71]

Approximately half of the plastic material introduced to the marine environment is buoyant, but fouling by organisms can cause plastic debris to sink to the sea floor, where it may interfere with sediment-dwelling species and sedimental gas exchange processes. Several factors contribute to microplastic's buoyancy, including the density of the plastic it is composed as well as the size and shape of the microplastic fragments themselves.[113] Microplastics can also form a buoyant biofilm layer on the ocean's surface.[114] Buoyancy changes in relation to ingestion of microplastics have been clearly observed in autotrophs because the absorption can interfere with photosynthesis and subsequent gas levels.[115] However, this issue is of more importance for larger plastic debris.

[116]

Plastic particles may highly concentrate and transport synthetic organic compounds (e.g. persistent organic pollutants, POPs), commonly present in the environment and ambient seawater, on their surface through adsorption.[117] Microplastics can act as carriers for the transfer of POPs from the environment to organisms.[63][64]

Additives added to plastics during manufacture may leach out upon ingestion, potentially causing serious harm to the organism. Endocrine disruption by plastic additives may affect the reproductive health of humans and wildlife alike.[64]

Plastics, polymers derived from mineral oils, are virtually non-biodegradable. However, renewable natural polymers are now in development which can be used for the production of biodegradable materials similar to those derived from oil-based polymers.

Polystyrene foam beads on an Irish beach

Great Pacific garbage patch — Pacific Ocean currents have created 3 "islands" of debris.[118]

Microplastics enter waterways through many avenues including deterioration of road paint, tyre wear and city dust entering the waterways, plastic pellets spilled from shipping containers, ghost nets and other synthetic textiles dumped into the ocean, cosmetics discharged and laundry products entering sewage water and marine coatings on ships degrading.[9]

Some microplastics leave the sea and enter the air, as researchers from the University of Strathclyde discovered in 2020.[119] Some remain on the ocean's surface; microplastics account for 92% of plastic debris on the ocean's surface, according to a 2018 study.[8] And some sink to the ocean floor. Australia's national science agency CSIRO estimated that 14 million metric tons of microplastics are already on the ocean floor in 2020.[120] This represents an increase from a 2015 estimate that the world's oceans contain 93–236 thousand metric tons of microplastics[121][122] and a 2018 estimate of 270 thousand tons.[123]

The Ocean Conservancy has reported that China, Indonesia, Philippines, Thailand, and Vietnam dump more plastic in the sea than all other countries combined.[124]

A study of the distribution of Eastern Pacific Ocean surface plastic debris (not specifically microplastic, although, as previously mentioned, most is likely microplastic) helps to illustrate the rising concentration of plastics in the ocean. Though admitting further research is needed to predict trends in ocean plastic concentration, by using data on surface plastic concentration (pieces of plastic km⁻²) from 1972 to 1985 n=60 and 2002–2012 n=457 within the same plastic accumulation zone, the study found the mean plastic concentration increase between the two sets of data, including a 10-fold increase of 18,160 to 189,800 pieces of plastic km⁻².[125]

Arctic ocean microplastics come mainly from Atlantic sources, especially Europe and North America.[126]

In 2020 scientists created what may be the first scientific estimate of how much microplastic currently resides in Earth's seafloor, after investigating six areas of ~3 km depth ~300 km off the Australian coast. They found the highly variable microplastic counts to be proportionate to plastic on the surface and the angle of the seafloor slope. By averaging the microplastic mass per cm³, they estimated that Earth's seafloor contains ~14 million tons of microplastic – about double the amount they estimated based on data from earlier studies – despite calling both estimates "conservative" as coastal areas are known to contain much more microplastic. These estimates are about one to two times the amount of plastic thought – per Jambeck et al., 2015 – to currently enter the oceans annually.[127][128][129]

Kelly et al. found 96 microplastic particles from 14 different types of polymers in an ice core sampled in 2009 from east Antarctica.[130] Plastic pollution has previously been recorded in Antarctic surface waters and sediments as well as in Arctic sea ice, but this is thought to be the first time plastic has been found in Antarctic sea ice. Relatively large particle sizes suggest local pollution sources.[130]

Microplastics have been widely detected in the world's aquatic environments.[73][131] The first study on microplastics in freshwater ecosystems was published in 2011 that found an average of 37.8 fragments per square meter of Lake Huron sediment samples. Additionally, studies have found MP (microplastic) to be present in all of the Great Lakes with an average concentration of 43,000 MP particle km⁻².[132] Microplastics have also been detected in freshwater ecosystems outside of the United States. In Canada, a three-year study found a mean microplastic concentration of 193,420 particles km⁻² in Lake Winnipeg. None of the microplastics detected were micro-pellets or beads and most were fibres resulting from the breakdown of larger particles, synthetic textiles, or atmospheric fallout.[133] The highest concentration of microplastic ever discovered in a studied freshwater ecosystem was recorded in the Rhine river at 4000 MP particles kg⁻¹.[134]

Due to their ubiquity in the environment, microplastics are widespread among the different matrices. In marine environments, microplastics have been evidenced in sandy beaches,[135] surface waters,[136] the water column, and deep sea sediment. Upon reaching marine environments, the fate of microplastics is subject to naturally occurring drivers, such as winds and surface oceanic currents. Numerical models are able to trace small plastic debris (micro- and mesoplastics) drifting in the ocean,[137] thus predicting their fate.

A substantial portion of microplastics are expected to end up in the world's soil, yet very little research has been conducted on microplastics in soil outside of aquatic environments.[138] In wetland environments microplastic concentrations have been found to exhibit a negative correlation with vegetation cover and stem density.[73] There exists some speculation that fibrous secondary microplastics from washing machines could end up in soil through the failure of water treatment plants to completely filter out all of the microplastic fibers. Furthermore, geophagous soil fauna, such as earthworms, mites, and collembolans could contribute to the amount of secondary microplastic present in soil by converting consumed plastic debris into microplastic via digestive processes. Further research, however, is needed. There is concrete data linking the use of organic waste materials to synthetic fibers being found in the soil; but most studies on plastics in soil merely report its presence and do not mention origin or quantity.[7][139] Controlled studies on fiber-containing land-applied wastewater sludges (biosolids) applied to soil reported semiquantitative recoveries of the fibers a number of years after application.[140]

Microplastics were found in every human tissue studied by graduate students at Arizona State University.[141] In December 2020 microplastic particles were found in the placentas of unborn babies for the first time.[142][143]

Airborne microplastics have been detected in the atmosphere, as well as indoors and outdoors. In 2019 a study found microplastic to be atmospherically transported to remote areas on the wind.[144] A 2017 study found indoor airborne microfiber concentrations between 1.0 and 60.0 microfibers per cubic meter (33% of which were found to be microplastics).[145] Another study looked at microplastic in the street dust of Tehran and found 2,649 particles of microplastic within 10 samples of street dust, with ranging samples concentrations from 83 particle – 605 particles (±10) per 30.0 g of street dust.[146] Microplastics and microfibers were also found in snow samples.[147] However, much like freshwater ecosystems and soil, more studies are needed to understand the full impact and significance of airborne microplastics.[71]

China banned in 2018 the import of recyclables from other countries, forcing those other countries to re-examine their recycling schemes.[lower-alpha 1] The Yangtze River in China contributes 55% of all plastic waste going to the seas.[lower-alpha 2] Including microplastics, the Yangtze bears an average of 500,000 pieces of plastic per square kilometer.[156] Scientific American reported that China dumps 30% of all plastics in the ocean.[157]

In the US, some states have taken action to mitigate the negative environmental effects of microplastics.[158] Illinois was the first US state to ban cosmetics containing microplastics.[65] On the national level, the Microbead-Free Waters Act 2015 was enacted after being signed by President Barack Obama on December 28, 2015. The law bans "rinse-off" cosmetic products that perform an exfoliating function, such as toothpaste or face wash. It does not apply to other products such as household cleaners. The act took effect on July 1, 2017, with respect to manufacturing, and July 1, 2018, with respect to introduction or delivery for introduction into interstate commerce.[159] On June 16, 2020, California adopted a definition of 'microplastics in drinking water', setting the foundation for a long-term approach to studying their contamination and human health effects.[160]

On July 25, 2018, a microplastic reduction amendment was passed by the U.S. House of Representatives.[161] The legislation, as part of the Save Our Seas Act designed to combat marine pollution, aims to support the NOAA's Marine Debris Program. In particular, the amendment is geared towards promoting NOAA's Great Lakes Land-Based Marine Debris Action Plan to increase testing, cleanup, and education around plastic pollution in the Great Lakes.[161] President Donald Trump signed the re-authorization and amendment bill into effect on October 11, 2018.

On June 15, 2018, the Japanese government passed a bill with the goal of reducing microplastic production and pollution, especially in aquatic environments.[162] Proposed by the Environment Ministry and passed unanimously by the Upper House, this is also the first bill to pass in Japan that is specifically targeted at reducing microplastic production, specifically in the personal care industry with products such as face wash and toothpaste.[162] This law is revised from previous legislation, which focused on removing plastic marine debris. It also focuses on increasing education and public awareness surrounding recycling and plastic waste.[162] The Environment Ministry has also proposed a number of recommendations for methods to monitor microplastic quantities in the ocean (Recommendations, 2018).[163] However, the legislation does not specify any penalties for those who continue manufacturing products with microplastics.[162]

The European Commission has noted the increased concern about the impact of microplastics on the environment.[164] In April 2018, the European Commission's Group of Chief Scientific Advisors commissioned a comprehensive review of the scientific evidence on microplastic pollution through the EU's Scientific Advice Mechanism.[164] The evidence review was conducted by a working group nominated by European academies and delivered in January 2019.[165] A Scientific Opinion based on the SAPEA report was presented to the Commission in 2019, on the basis of which the commission will consider whether policy changes should be proposed at a European level to curb microplastic pollution.[166]

In January 2019, the European Chemicals Agency (ECHA) proposed to restrict intentionally added microplastics.[167]

The European Commission's Circular Economy Action Plan sets out mandatory requirements for the recycling and waste reduction of key products e.g. plastic packaging. The plan starts the process to restrict addition of microplastics in products. It mandates measures for capturing more microplastics at all stages of the lifecycle of a product. E.g. the plan would examine different policies which aim to reduce release of secondary microplastics from tyres and textiles.[168] The European Commission plans to update the Urban Waste Water Treatment Directive to further address microplastic waste and other pollution. They aim to protect the environment from industrial and urban waste water discharge. A revision to the EU Drinking Water Directive was provisionally approved to ensure microplastics are regularly monitored in drinking water. It would require countries must propose solutions if a problem is found.[9]

The Environmental Protection (Microbeads) (England) Regulations 2017 ban the production of any rinse-off personal care products (such as exfoliants) containing microbeads.[169] This particular law denotes specific penalties when it is not obeyed. Those who do not comply are required to pay a fine. In the event that a fine is not paid, product manufacturers may receive a stop notice, which prevents the manufacturer from continuing production until they have followed regulation preventing the use of microbeads. Criminal proceedings may occur if the stop notice is ignored.[169]

The Clean Oceans Initiative is a project launched in 2018 by the public institutions European Investment Bank, Agence Française de Développement and KfW Entwicklungsbank. The organisations will be providing up to €2 billion in lending, grants and technical assistance until 2023 to develop projects that remove pollution from waterways (with a focus on macroplastics and microplastics) before it reaches the oceans.[9]

• "The plastic in our bodies," Politico Europe, 5 May 2019
• "Microplastics can also travel by wind, new study shows," ZME Science, 15 April 2019
• "Microplastic pollution revealed 'absolutely everywhere' by new research," The Guardian, 7 March 2019
• "Plastic straws are little, but they are part of a huge problem," Washington Post, 9 September 2018
• "Microplastic pollution in oceans is far worse than feared, say scientists," The Guardian, March 2018

• Plastic Planet, Werner Boote and Gerhard Pretting
• Addicted To Plastic on YouTube
• Addicted to Plastic at Bullfrog Films and at Cryptic Moth
• Plasticized on YouTube
• The problem with microplastics on YouTube