The wide range of sizes, shapes and applications of microplastics, as well as their diverse chemistries, make characterizing their environmental fate and effects difficult.
In spite of its vast commercial production since the 1950s, environmental contamination by plastic has only recently garnered widespread attention, with the initial focus driven by images of large floating islands of debris in the world’s oceans. However, as the amount of plastic production outpaces carbon emissions (Hale et al., 2020) and global use has thrust plastics into a seemingly permanent part of daily life, focus has shifted to the much smaller sized, yet more widespread, microplastics. The term microplastics, first coined by Thompson et al. in 2004 (Thompson et al., 2004), refers to plastic particles smaller than 5 millimeters (mm) in diameter, which is slightly smaller than the width of a pencil (see Figure). Microplastics are ubiquitous in the environment across the globe, including some of Earth’s most remote locales (e.g., in Arctic Ocean ice and the Pacific Ocean’s Mariana Trench).
So, what exactly are microplastics? At a fundamental level, plastics are materials that consist of long chains of organic polymers, or strings of small molecules (monomers) that are bound together in a repeating pattern, similar to a bicycle chain. These polymer chains are synthetic, often derived from petroleum-based feed stocks, and include such materials as polyethylene, polypropylene, polystyrene, polyester, and polyvinyl chloride (aka PVC). While the scientific names of these polymers may be unfamiliar to many people, they form the basis of many products we use in our everyday lives. For example, polyethylene is considered to be the most widely used plastic and has many applications, depending on its formulation density, that range from malleable materials such as grocery bags and bubble wrap to rigid structures like bottles, drain pipes, and medical devices. Some plastics are intentionally manufactured at microplastic sizes to be used in personal care products, cleaning agents, coatings, paints, oil and gas drilling fluids, resins, or as abrasives. Nevertheless, these intentionally manufactured microplastics, known as “primary microplastics,” are thought to represent only a small fraction of the microplastics found globally in the environment.
The ability to observe and measure microplastics consistently in different environmental media may be one of the most formidable scientific challenges that lies ahead.”
Although designed to resist environmental degradation, plastics are vulnerable to various types of weathering, including photo-oxidation, or polymer breakdown from sunlight exposure, and in some cases, biodegradation (e.g., bacterial consumption). Plastics are also susceptible to physical weathering processes like chipping and cracking of brittle structures, or flaking and abrasion from softer products such as tires or synthetic clothing. These processes produce what are known as “secondary microplastics.” Regardless of the mechanism, as large plastic materials break down, they shed smaller pieces, characterized as microplastics, and may further fragment into nanoplastics with a size range of less than 1 micrometer (µm), approximately 20 times smaller than the diameter of a human hair. Researchers have posited that plastic debris exists in a continually transitional state between macroplastic (larger plastic pieces) and nanoplastic (Hartmann et al., 2019).
Microplastics have diverse chemistries, shapes, and textures, which affect how they move through the environment, as discussed in the second article of this Trends issue. These characteristics also affect how microplastics can be observed or sampled in any particular media. For example, seawater samples collected from the ocean surface that are filtered for plastic debris above a 200 µm cut-off will likely contain a radically different plastic particle size distribution from those collected from the seawater column below the surface. This is because a greater fraction of smaller, more dense plastic materials are likely to sink in the seawater column compared to larger, more buoyant pieces that remain at the surface (see Hale et al., 2020, Figure 3).
The ability to observe and measure microplastics consistently in different environmental media may be one of the most formidable scientific challenges that lies ahead. Apart from their small size, microplastics occur in multiple media, including water, sediment, soil, biosolids, air, and biological tissue (both human and animal). Understanding the extent of microplastic contamination across these media depends on the development of methods for standardizing material extraction from bulk sample matrices, and for characterizing properties such as polymer chemistry, shape, and potential additives. Under legislative mandate, the California State Water Resources Control Board (a branch of CalEPA) recently released standardized testing methodologies to detect and quantify microplastics in drinking water, which is considered a first step toward regulating these materials in water supplies.
The science on microplastics continues to grow exponentially, as demonstrated by the number of peer reviewed studies published each year on this topic over the past decade. However, despite these rapid advancements and our vast knowledge about the production of plastics since their introduction in the 1950s, questions about the ultimate fate of microplastics in terrestrial and marine systems and their potential biological effects are expected to remain at the cutting edge of scientific discovery over the next decade.
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Hale, RC; Seeley, ME; La Guardia, MJ; Mai, L; Zeng, EY. 2020. “A global perspective on microplastics.” J. Geophys. Res. Oceans 125(1):e2018JC014719. doi: 10.1029/2018JC014719.
Hartmann, NB; Hüffer, T; Thompson, RC; Hassellöv, M; Verschoor, A; Daugaard, AE; Rist, S; Karlsson, T; Brennholt, N; Cole, M; Herrling, MP; Hess, MC; Ivleva, NP; Lusher, AL; Wagner, M. 2019. “Are we speaking the same language? Recommendations for a definition and categorization framework for plastic debris.” Environ. Sci. Technol. 53(3):1039-1047.
Lim, X. 2021. “Microplastics are everywhere – but are they harmful.” Nature 593(7857):22-25. doi: 10.1038/d41586-021-01143-3.
Thompson, RC; Olsen, Y; Mitchell, RP; Davis, A; Rowland, SJ; John, AW; McGonigle, D; Russell, AE. 2004. “Lost at sea: Where is all the plastic?” Science 304(5672):838. doi: 10.1126/science.1094559.