Plastic pollution: Where are we regarding research and risk assessment in support of management and regulation?

More research on plastic particles and using quantitative exposure-response designs, material, size and shape-specific metrics, and harmonized analytical methods are needed to provide suitable data for ecological risk assessments of plastic pollution and for management and policy. Humanity is facing an unprecedented global plastic pollution crisis because of the dramatic increase in plastic use in recent years and the mismanagement of plastic waste around the world. It has been estimated that annual global plastic production in the 1950s was around 1.7 million tons (Geyer et al., 2017). This increased to 299 million tons in 2013 (Geyer et al., 2017) and to over 367 million tons in 2020 (PlasticsEurope, 2021). In total, 8.1 billion tons of plastics have been produced since 1950 (Geyer et al., 2017). In addition, the COVID-19 pandemic has required the production of billions of facial masks and a substantial amount of other plastic medical materials in the past two years. As a result, the pandemic has added a large additional burden of plastic waste to the environment. If the current rates of plastic production and use continue, by 2050 annual global plastic production will increase to 590 million tons. This will bring the total global plastic production to about 34 billion tons by 2050 (Statista, 2022). Only one-third of plastics are recycled and reused, and two-thirds are discarded into the inland environment; many will later find their way to aquatic ecosystems (Geyer et al., 2017). Plastics tend to accumulate in the oceans due to their longevity and capabilities for long-range transport. Moreover, the persistence of plastics in the environment, especially the petroleum-based plastics, and their tendency to fragment into micrometer- and nanometer-sized particles (called microplastics and nanoplastics, respectively; hereafter called plastic particles), create unique challenges for plastics pollution when compared to other anthropogenic chemical pollution, such as dichloro-diphenyl-trichloroethane (DDT) and polychlorinated biphenyls (PCBs). The lifetimes of chemical pollutants in the environment are often shorter than the lifetime of petroleum-based plastics, even for many persistent compounds. This poses potential exposures and risks to living organisms including humans and presents a warning to society. A question that can be asked is, “Where are we at the present time regarding research in support of ecological risk assessment, management, and regulation of plastic pollution?” A search on Google Scholar with the keywords “microplastics” and “nanoplastics” from the 1900s, when plastic production began (Baekeland, 1909), to the present time (April 21, 2022), yielded a total of 25 300 publications on microplastics and 14 100 on nanoplastics (Figure 1). Various aspects of plastic particles have been investigated, including environmental monitoring, fate, transport, effects, and risk assessments. Most of this work (98%) was conducted over the past 10 years. This rapid increase in research interest can be explained by the recent growing awareness and concern about the plastic pollution problem. Among the publications on plastic particles, a small number (~20) were found to be related to ecological risk assessments for plastic particles. However, suitable literature for a quantitative risk assessment of plastic particles (i.e., using probabilistic approaches) is scarce, especially for effect concentration data. Most ecological risk assessments for plastic particles involved additional extrapolation steps using mathematical models to estimate and harmonize exposure and effect data to characterize risks. As a result, the estimated risks are associated with high uncertainties that limit their applicability in risk management. The concept of microplastics and nanoplastics, defined based on particle size, is generic. It excludes consideration of the physical characteristics of the particles, which can lead to misinterpretation of effects (Covernton et al., 2019). Plastic particles within the size range of microplastics (1 µm-5 mm by earlier researchers or 1 µm-1 mm proposed by recent researchers), can have different shapes and types and behave differently in the natural environment. These different behaviors result in different environmental fates, bioaccumulation, and effects on living organisms. For example, plastic particles with a sharp and pointy shape could be entangled in the digestive systems of living organisms and could induce more physical effects than soft and smooth particles, which can be more easily excreted (Canniff & Hoang, 2018; Hoang & Felix-Kim, 2020). Perhaps the definition of plastic particles should take a classification of particle type into consideration to differentiate plastics not only by size but also by shape and type. Among the common types of plastic particles (fiber, fragment, bead) found in the environment, fibers have been reported to be the most dominant. Defining and classifying plastic particles into fiber and nonfiber (fragment and/or film, bead) particles, such as fiber micro- and nanoplastics and fragmented micro- and nanoplastics, would be helpful for the interpretation of effects. In addition, limitations on analytical techniques and the current lack of standardized methods for sample preparation and analysis have resulted in inconsistent data for risk assessment (Hung et al., 2020). Environmental exposure and effect concentrations of plastic particles have been reported in various measurement units, such as in mass units (e.g., mg/L, mg/km2, mg/kg, etc.) or in count units (i.e., # particles/L, # particles/kg, # particles/km2, # particles/organisms, etc.). These mixed units have limited comparison of environmental concentrations with effect concentrations to characterize risks. Further, plastic particles might not produce adverse effects if they are not ingested and bioaccumulated in living organisms. Therefore, bridging relationships between exposure, particle ingestion and accumulation, and effect concentrations of plastic particles is important for risk characterization. To conduct a quantitative risk assessment, data from research that examines the relationships between a stressor and a response are usually needed. However, this research design has not often been used in published plastic particle research. Although significant research efforts on plastic pollution have been made over the past 10 years, the literature reflects a picture of “a lot, but inadequate” information for quantitative risk assessments in support of management. More research on plastic particles and using quantitative exposure-response designs, material, size and shape-specific metrics, and harmonized analytical methods are needed to provide suitable data for ecological risk assessments of plastic pollution and for management and policy.