The United States National Oceanic and Atmospheric Administration or NOAA defines microplastics as plastic fragments that are less than five millimeters in length. These materials are not a category of synthetic polymers. They are specifically a direct product of various industries involved in petrochemical processing and manufacturing and degraded offshoots of materials made from polymers that enter and contaminate the environment.
Marine biologist Richard Thompson was one of the first scientists who raises concerns over microplastics. Researcher A. Katsnelson explained that these minuscule and invisible plastic fragments are considered environmental pollutants and a public health hazard because they can enter the food chain. Various species ranging from zooplankton and marine creatures to land animals end up ingesting them inadvertently. Studies have also noted the presence of microplastics in tissues and even cells of wild marine and land animals.
Understanding the sources of microplastics also involves understanding their two general classifications. These are primary microplastics and secondary microplastics. Primary microplastics are direct industrial products that are already five millimeters in size or less before entering the environment. Secondary microplastics come from the degradation of larger plastics or other polymer-based materials that have entered the environment.
The Major Sources of Microplastics
1. Microfibers from Clothing and Fabric
Several modern fabrics used in different clothing items and related products are made of synthetic fibers or a mixture of natural and synthetic fibers. Examples of these products include sportswear and swimwear that are made of nylon or polyester, undergarments or innerwear marketed as microfiber clothing, the heat-resistant synthetic fiber Kevlar used to make bulletproof gears, and leather substitutes such as Ultrasuede.
Other manufacturers have also used microfibers in place of cotton and leather to produce a variety of products. These include microfiber towels or cloths used for cleaning glass and painted metal surfaces; thermal insulation materials used in sleeping bags and vehicle covers, bedroom items such as fleeces and bed covers, and water-resistant furniture coating.
The improper disposal of these products and their wear and tear contribute to the environmental accumulation of microplastics. M. A. Browne et al. discussed several experiments that sampled wastewater from domestic washing machines. These experiments showed that a single garment could produce greater than 1900 fibers per wash. This suggests household wastewater is one of the sources of microplastics and causes of microplastic accumulation.
2. Microbeads from Cosmetics and Medical Research
Microbeads are solid plastic particles that are less than one millimeter in size and are made of polyethylene or manufactured from other petrochemical plastics such as polypropylene and polystyrene. These materials were once popular in the cosmetic industry in which they were included in the formulation of personal care products. Examples of these products include exfoliating facial washes, soap scrubs, and toothpaste.
The field of medical research and health sciences also uses microbeads in various research undertakings and lab processes. These include microscopy techniques, fluid visualization, fluid flow analysis, cell isolation and culture, genetic analysis techniques, drug studies and drug delivery, medical diagnostics, and process troubleshooting.
Microbeads end up in bodies of water because their small size prevents wastewater treatment facilities from filtering them. Several countries have banned these materials from personal care products. These include the United States, the United Kingdom, selected countries in the European Union, India, South Africa, Taiwan, Australia, and New Zealand. Some companies in developing countries still use these materials in product formulation.
3. Sludge from Sewage Treatment Plants
It is important to reiterate the fact that the microfibers from clothing items and microbeads from personal care products end up in wastewater and natural bodies of water. Some sewage treatment facilities do not have the capacity to filter these minuscule particles completely. This is especially true for facilities in developing and underdeveloped countries in which processes are not as sophisticated as those found in developed countries.
There are facilities in developed countries with solid skimming and sludge-settling processes that can remove a large portion of microplastics from wastewater. Browne et al. took samples of sewage sludge disposal sites on the coasts of six continents. These samples revealed that there was an average of one particle of microplastic per liter of sludge
It is also worth mentioning that sludge collected from wastewater treatment facilities is also used as a critical component of soil fertilizers in several countries. These fertilizers can release microplastics into the environment because they are exposed to weather, sunlight, and other natural factors. The minuscule plastic fragments from these sludge-based soil fertilizers eventually and specifically end up in storm drains and directly in bodies of water.
4. Rubbers from Vehicle Tires and Footwear
Another source of microplastics or a cause of their environmental accumulation is the wear and tear of vehicles and rubber-based footwear. Findings from a study spearheaded by the Environmental Protection Agency of Denmark suggest that secondary microplastics from products or items such as tires and footwear have a more critical negative environmental impact than primary plastics by two orders of magnitude.
Researchers P. J. Kole et al. mentioned that the estimated per capita emission of microplastics from tires ranged from 0.23 to 4.7 kilograms per year with a global average of 0.81 kilograms per year. Road vehicles produce more of these minuscule plastic fragments than sources such as airplane tires, artificial turf, brake wear, and road markings.
The aforementioned researchers concluded further that the relative contribution of wear and tear of tires in the plastic pollution in oceans was between 5 and 10 percent. In addition, aside from polluting the oceans, P. J. Kole et al. mentioned further that the wear and tear of tires contribute around 3 percent and up to 7 percent of the particulate matter in the air. This additional finding suggests another possible source of air pollution.
5. Industrial and Manufacturing Activities
End-use plastics are produced using granules and small resin pellets as raw materials. Plastic production facilities can contribute to the release of microplastics into the environment through accidental spillage, inappropriate packaging, and direct plant outflow. Mathew Cole et al. mentioned a research revealing microplastic concentrations of 150 to 2400 particles per cubic meter in a harbor adjacent to a plastic production facility.
Commercial and recreational fishing are also a source of microplastics. Fishing activities contribute to the unwarranted disposal of plastic-based materials such as fishing gear, plastic monofilament lines, nylon netting, and packaging materials. These plastics would become secondary microplastics through long-term degradation.
It is also important to underscore the fact that the shipping industry has been considered a primary contributor to marine plastic waste. J. G. B. Derraik reviewed studies conducted during the early 1990s. The review concluded that shipping had contributed to 6.5 million tons of discarded plastics in the ocean. Remember that these materials would eventually degrade into smaller fragments to become secondary microplastics.
FURTHER READINGS AND REFERENCES
- Arthur, C., Baker, J., and Bamford, H. eds. 2008. Proceedings of the International Research Workshop on the Occurrence, Effects, and Fate of Microplastic Marine Debris. MD: National Oceanic and Atmospheric Administration.
- Browne, M. A., Crump, P., Niven, S. J., Teuten, E., Tonkin, A., Galloway, T., and Thompson, R. 2011. “Accumulation of Microplastics on Shorelines Worldwide: Sources and Sinks.” Environmental Science & Technology. 45(2): 9175-9179. DOI: 1021/es201811s
- Cole, M., Lindeque, P., Halsband, C., and Galloway, T. S. 2011. “Microplastics as Contaminants in the Marine Environment: A Review.” Marine Pollution Journal. 62(12): 2588-2597. DOI: 1016/j.marpolbul.2011.09.025
- Derraik, J. G. B. 2002. “The Pollution of the Marine Environment by Plastic Debris: A Review.” Marine Pollution Bulletin. 44(9): 842-852. DOI: 1016/S0025-326X(02)00220-5
- Katsnelson, A. 2015. “News Feature: Microplastics Present Pollution Puzzle.” Proceedings of the National Academy of Sciences. 112(18): 5547-5549. DOI: 1073/pnas.1504135112
- Kole, P. J., Löhr, A. J., Van Belleghem, F., Ragas, A. M. J. 2017. “Wear and Tear of Tyres: A Stealthy Source of Microplastics in the Environment.” International Journal of Environmental Research and Public Health. 14(10): 1265. DOI: 3390/ijerph14101265
- Lassen, C., Hansen, S. F., Magnusson, K., Norén, F., Hartmann, N. I. B., Jensen, P. R., Nielsen, T. G., and Brinch, A. 2015. Microplastics: Occurrence, Effects, and Sources of Releases to the Environment in Denmark. Copenhagen: The Danish Environmental Protection Agency. ISBN: 978-87-93352-80-3