Microplastics

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Microplastics are small pieces of plastic that pollute the environment.[1] While there is some contention over their size, the U.S. National Oceanic and Atmospheric Administration (NOAA) classifies microplastics as less than 5 mm in diameter.[2] They come from a variety of sources, including cosmetics, clothing, and industrial processes.

Two classifications of microplastics currently exist: primary microplastics are manufactured and are a direct result of human material and product use, and secondary microplastics are derived from the breakdown of larger plastic debris like the macroscopic parts that make up the bulk of the Great Pacific Garbage Patch.[3] Both types are recognized to persist in the environment at high levels, particularly in aquatic and marine ecosystems. Plastic pellets created for use by manufacturers are sometimes referred to as nurdles.[4]

Because plastics do not break down for many years, they can be ingested and incorporated into, and accumulated in, the bodies and tissues of many organisms.[5] The entire cycle and movement of microplastics in the environment is not yet known, but research is currently underway to investigate this issue.

Polyethylene based microspherules in toothpaste
Microplastic fibers identified in the marine environment
Microplastics in sediments from rivers
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.

Classification[edit]

Primary microplastics[edit]

These 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.[6] 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.[7]

Secondary microplastics[edit]

These 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 chemical processes can reduce the structural integrity of plastic debris, resulting in fragmentation. 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.[7] The prevalence of microplastics with uneven shapes suggests that fragmentation is a key source.[8]

Other sources: as a by-product/dust emission during wear and tear[edit]

Examples of these include dust from car and truck tires,[9]synthetic textiles, ropes, paint and waste treatment. These sources of microplastics are quite recently recognized and are somewhere between primary and secondary microplastics. A Norwegian Environment Agency review report about microplastics published in early 2015[9] 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.

Sources[edit]

Polystyrene foam beads on an Irish beach

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.[10] Through such methods, it has been shown that there are microplastics from multiple sources in the environment.

Microplastics could contribute up to 30% of the "plastic soup" polluting the world’s oceans and, in many developed countries, are a bigger source of marine plastic pollution than the visible larger pieces of marine litter, according to a 2017 IUCN report.[11]

Sewage treatment plants[edit]

Sewage treatment plants remove contaminants from wastewater, primarily from household sewage, using physical, chemical, and biological processes. Microplastics have been detected in both the primary and secondary treatment stages of the plants. 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%.[12][13][14] A 2016 study showed that most microplastics are actually removed during the primary treatment zone where solid skimming and sludge settling are used.[12] The contribution of microplastics into oceans and surface water environments from sewage treatment plants is minimal.[12][15]

Car and truck tires[edit]

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 tyres 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.[16]

Cosmetics industry[edit]

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, and nylon.[17] They are often found in face washes, hand soaps, and other personal care products, so 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.[18]

Clothing[edit]

Studies[19] have shown that many synthetic fibers, such as polyester, nylon and acrylics, can be shed from clothing and persist in the environment.[20] One load of laundry can contain more than 1,900 fibers of microplastics, with fleeces releasing the highest percentage of fibers.[21] 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.[22]

Manufactured goods[edit]

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. 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.[7]

Coastal tourism[edit]

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. Tourism and recreational activities are sources of plastics discarded along beaches and coastal resorts. Marine debris observed on beaches also arises from beaching of materials carried on in-shore 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.

Shipping[edit]

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. However, shipping remains a dominant source of plastic pollution, having contributed around 6.5 million tons of plastic in the early 1990s.[23]

Natural calamities[edit]

Floods or hurricanes can accelerate transportation of waste from land to the marine environment. A California study revealed that after a storm, the transport of plastics increased from 10 to 60 microplastics per m3. The study showed how the waste was transported and deposited at much greater distances from the river mouth than usual. A similar study conducted near the southern coast of California showed an increase of microplastics from 1 to 18 pieces per m3 after a storm.[citation needed] The abundance and global distribution of microplastics in the oceans has steadily increased over the last few decades with rising plastic consumption worldwide.[citation needed]

Potential effects on the environment[edit]

Microplastic particles ingested by a larval perch

Participants at the International Research Workshop on the Occurrence, Effects and Fate of Microplastic Marine Debris at the University of Washington at Tacoma [24] concluded that microplastics are a problem in the marine environment, based on:

  • the documented occurrence of microplastics in the marine environment,
  • the long residence times of these particles (and, therefore, their likely buildup in the future), and
  • their demonstrated ingestion by marine organisms.

So far, research has mainly focused on larger plastic items. Widely-recognized problems facing marine life are entanglement, ingestion, suffocation and general debilitation often leading to death and/or strandings. This causes serious public concern. In contrast, microplastics are not as conspicuous, being less than 5 mm, and are usually invisible to the naked eye. Particles of this size are available to a much broader range of species, enter the food chain at the bottom, become embedded in animal tissue, and are then undetectable by unaided visual inspection.

Microplastics have been detected not just in marine but also in freshwater systems in three continents (Europe, North America and Asia)[25] Samples collected across 29 Great Lakes tributaries from six states in the United States were found to contain plastic particles, 98% of which were microplastics ranging in size from 0.355mm to 4.75mm.[26]

Biological integration into organisms[edit]

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.[20][27][28]

Additionally, 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.[29]

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.[30] 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.[30] 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, and they expend a large amount of energy in the process, increasing the chances of mortality.[31]

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.[32] Zooplankton, among other marine organisms, consume microplastics because they emit similar infochemicals, notably dimethyl sulfide, just as phytoplankton do.[33][verification needed][34] Plastics such as high-density polyethylene (HDPE), low-density polyethylene (LDPE), and polypropylene (PP) produce dimethyl sulfide odors.[33] These types of plastics are commonly found in plastic bags, bleach, food storage containers, and bottle caps.[35]

It can take at least 14 days for microplastics to pass through an animal (as compared to a normal digestion periods of 2 days), but enmeshment of the particles in animals' gills can prevent elimination entirely.[27] 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.[36] Microplastics also absorb chemical pollutants that can be transferred into the organism's tissues.[37] 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.[38]. This study is one of the few so far to show the ingestion of microplastics by freshwater organisms.

Humans[edit]

Fish is the primary source of protein for nearly one-fifth of the human population.[39] The microplastics ingested by fish and crustaceans can be subsequently consumed by humans as the end of the food chain. In a study done at the State University of New York, 18 fish species were sampled and all species showed some level of plastics in their systems.[20][27] 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.[20]

Microplastics find their way to human consumption not only from sea food but also from table salt. Researchers in China 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. [40]. Sea salt and rock salt which are commonly used table salts in Spain have also been found to contain microplastics.[41].The most common type of microplastic found in both these studies was polyethylene terephthalate (PET).

Buoyancy[edit]

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. 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.[42] However, this issue is of more importance for larger plastic debris.

Persistent organic pollutants[edit]

Plastic particles may highly concentrate and transport synthetic organic compounds (e.g. persistent organic pollutants, POPs), commonly present in the environment and ambient sea water, on their surface through adsorption.[43] Microplastics can act as carriers for the transfer of POPs from the environment to organisms.[23]

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.[44]

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 that of oil-based polymers.[citation needed]

Policy and legislation[edit]

With increasing awareness of the detrimental effects of microplastics on the environment, groups are now advocating for the removal and ban of microplastics from various products. One such campaign is "Beat the Microbead", which focuses on removing plastics from personal care products.[17] The Adventurers and Scientists for Conservation run the Global Microplastics Initiative, a project to collect water samples to provide scientists with better data about microplastic dispersion in the environment.[45] UNESCO has sponsored research and global assessment programs due to the trans-boundary issue that microplastic pollution constitutes.[46] These environmental groups will seemingly keep pressuring companies to remove plastics from their products in order to maintain healthy ecosystems.[47]

United States

In the US, some states have taken action to mitigate the negative environmental effects of microplastics. Illinois was the first US state to ban cosmetics containing microplastics.[citation needed] 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.[48]

Action for creating awareness[edit]

On April 11, 2013 in order to create awareness, artist Maria Cristina Finucci founded The Garbage patch state[49] under the patronage of UNESCO and the Italian Ministry of the Environment.[50]

The U.S. Environmental Protection Agency (EPA) launched its "Trash-Free Waters" initiative in 2013 to prevent single-use plastic wastes from ending up in waterways and ultimately the ocean.[51] EPA collaborates with the United Nations Environment Programme–Caribbean Environment Programme (UNEP-CEP) and the Peace Corps to reduce and also remove trash in the Caribbean Sea.[52] EPA has also funded various projects in the San Francisco Bay Area including one that is aimed at reducing the use of single-use plastics such as disposable cups, spoons and straws, from three University of California campuses.[53]

Cleanup[edit]

Computer modelling done by The Ocean Cleanup, a Netherlands foundation, has suggested that collecting devices placed nearer to the coasts could remove about 31% of the microplastics in the area.[54] In addition, some bacteria have evolved to eat plastic, and some bacteria species have been genetically modified to eat (certain types of) plastics.[55]

See also[edit]

References[edit]

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