Critical gaps in nanoplastics research and their connection to risk assessment

Abstract

Reports of plastics, at higher levels than previously thought, in the water that we drink and the air that we breathe, are generating considerable interest and concern. Plastics have been recorded in almost every environment in the world with estimates on the order of trillions of microplastic pieces. Yet, this may very well be an underestimate of plastic pollution as a whole. Once microplastics (<5 mm) break down in the environment, they nominally enter the nanoscale (<1,000 nm), where they cannot be seen by the naked eye or even with the use of a typical laboratory microscope. Thus far, research has focused on plastics in the macro- (>25 mm) and micro-size ranges, which are easier to detect and identify, leaving large knowledge gaps in our understanding of nanoplastic debris. Our ability to ask and answer questions relating to the transport, fate, and potential toxicity of these particles is disadvantaged by the detection and identification limits of current technology. Furthermore, laboratory exposures have been substantially constrained to the study of commercially available nanoplastics; i.e., polystyrene spheres, which do not adequately reflect the composition of environmental plastic debris. While a great deal of plastic-focused research has been published in recent years, the pattern of the work does not answer a number of key factors vital to calculating risk that takes into account the smallest plastic particles; namely, sources, fate and transport, exposure measures, toxicity and effects. These data are critical to inform regulatory decision making and to implement adaptive management strategies that mitigate risk to human health and the environment. This paper reviews the current state-of-the-science on nanoplastic research, highlighting areas where data are needed to establish robust risk assessments that take into account plastics pollution. Where nanoplastic-specific data are not available, suggested substitutions are indicated.

Keywords: Bayesian network; microplastic; nanoplastic; risk assesment; toxicity.

FIGURE 1

Numbers of papers on different size categories of plastic pollutants based on searches of three databases (i.e., Google Scholar—hatched grey, Scopus—grey, and Web of Science - black) (A) Macroplastic pollution washed up in the Katmai National Park, Alaska July 2021 (B) Exposed Zebrafish (1–20μm, tire particles) with ingested microplastics (C) Exposed Daphnia (40nm, fluorescent polystyrene). Detailed methods for literature search are described in Supplementary Material.

FIGURE 2

Calculation of equivalent nanoplastic particle count, volume, and surface area using a mass balance approach detailed in Supplemental spread sheet.

FIGURE 3

Diagram of potential sources and flow of micro- and nanoplastics to soil, and uptake by plants and soil-dwelling organisms (Accinelli et al., 2019; Bläsing and Amelung, 2018; Carr et al., 2016; Corradini et al., 2019; Dris et al., 2016; Li et al., 2020a; Murphy et al., 2016; Nizzetto et al., 2016a; O'connor et al., 2019; Qi et al., 2018; Rillig et al., 2017; Steinmetz et al., 2016; Wang et al., 2016; Wright et al., 2020).

FIGURE 4

A conceptual model for an ecological risk assessment framework for microplastics and nanoplastics for the San Francisco Bay estuary. The cause-effect framework is at the top and fits the source-stressor-habitat/location-effect-endpoint/impact structure.