Risks of Plastic Debris: Unravelling Fact, Opinion, Perception, and Belief

Abstract

Researcher and media alarms have caused plastic debris to be perceived as a major threat to humans and animals. However, although the waste of plastic in the environment is clearly undesirable for aesthetic and economic reasons, the actual environmental risks of different plastics and their associated chemicals remain largely unknown. Here we show how a systematic assessment of adverse outcome pathways based on ecologically relevant metrics for exposure and effect can bring risk assessment within reach. Results of such an assessment will help to respond to the current public worry in a balanced way and allow policy makers to take measures for scientifically sound reasons.

Figure 1

Generic ecological risk assessment framework for plastic debris linking protection goals, problem definition, exposure assessment, effect assessment, and risk characterization. “ERM” is the ecologically relevant metric for a specific adverse outcome pathway (AOP) concerning the interaction of a particular plastic particle type (i) with a particular species or species age group of interest (j). For an explanation of the tiers used within the effect assessment, see the text.

Figure 2

Examples of ecologically relevant metrics (ERMs). A risk assessment for plastic debris could systematically distinguish among different species (here, lugworms, sharks, and humans), plastic types (here, nanoplastic, contaminated microplastic, clean microplastic, and two types of macroplastic), and ERMs [here, number concentration (#) or mass concentration (C)]. For the three species, example plastic types and their ERMs are provided with a provisional likeliness of effect indicated. Nanoplastic, which can penetrate cell membranes, could be available for all species with the number concentration as a useful ERM. Chemically contaminated microplastic would be available for all species with a chemical mass concentration as a useful ERM. “Clean” microplastics, that is, with chemical concentrations (far) below effect thresholds, would imply a physical risk for worms and for humans, but probably much less risk for a basking shark, and would cause a chemical risk for none of the species. Large plastic debris would be irrelevant for ingestion risks to species like worms or humans but might be relevant for the shark. Per species, the combined stressor effects need to be assessed, for instance, using approaches as described in the text.

Figure 3

Separating the chemical component (horizontal arrow) from the physical component (vertical arrow) of the risk of plastic debris. If a certain type of plastic particle causes an adverse effect on an organism different from that of another type of particle, then it needs another ERM. Risk assessment of the plastic particle proceeds following adapted versions of existing frameworks for chemicals and/or particles.