Quality Criteria for Microplastic Effect Studies in the Context of Risk Assessment: A Critical Review

Abstract

In the literature, there is widespread consensus that methods in plastic research need improvement. Current limitations in quality assurance and harmonization prevent progress in our understanding of the true effects of microplastic in the environment. Following the recent development of quality assessment methods for studies reporting concentrations in biota and water samples, we propose a method to assess the quality of microplastic effect studies. We reviewed 105 microplastic effect studies with aquatic biota, provided a systematic overview of their characteristics, developed 20 quality criteria in four main criteria categories (particle characterization, experimental design, applicability in risk assessment, and ecological relevance), propose a protocol for future effect studies with particles, and, finally, used all the information to define the weight of evidence with respect to demonstrated effect mechanisms. On average, studies scored 44.6% (range 20-77.5%) of the maximum score. No study scored positively on all criteria, reconfirming the urgent need for better quality assurance. Most urgent recommendations for improvement relate to avoiding and verifying background contamination, and to improving the environmental relevance of exposure conditions. The majority of the studies (86.7%) evaluated on particle characteristics properly, nonetheless it should be underlined that by failing to provide characteristics of the particles, an entire experiment can become irreproducible. Studies addressed environmentally realistic polymer types fairly well; however, there was a mismatch between sizes tested and those targeted when analyzing microplastic in environmental samples. In far too many instances, studies suggest and speculate mechanisms that are poorly supported by the design and reporting of data in the study. This represents a problem for decision-makers and needs to be minimized in future research. In their papers, authors frame 10 effects mechanisms as "suggested", whereas 7 of them are framed as "demonstrated". When accounting for the quality of the studies according to our assessment, three of these mechanisms remained. These are inhibition of food assimilation and/or decreased nutritional value of food, internal physical damage, and external physical damage. We recommend that risk assessment addresses these mechanisms with higher priority.

Figure 1

Cumulative frequency distributions for MP particle sizes used in effect tests for aquatic biota. The majority of studies tested a size range, which implies that separate cumulative distributions can be plotted for the minimum (Min), the maximum (Max) and the average size tested across studies.

Figure 2

Number of studies reporting a particular shape (A) or polymer type (B) for the microplastics used in the exposure tests (from a total of 124 records for shapes and 145 records for polymer types). PS = polystyrene, PE = polyethylene, PVC = polyvinyl chloride, PP = polypropylene, PET = terephthalate, PA = polyamide, N/A = not analyzed, PLA = polylactic acid, PMMA = poly(methyl methacrylate), PC = polycarbonate, PE-Acrylate = polyethylene-Acrylate, EVA = ethylene-vinyl acetate, PHB = polyhydroxybutyrate, ABS = acrylonitrile butadiene styrene, SAN = styrene acrylonitrile resin, and POMH = polyoxymethylene-homopolymer.

Figure 3

Number of studies evaluating the effects of MP on organisms of a certain taxonomic group (A) and on a particular end point (B) (from a total of 134 records for organisms and 252 records for end points).

Figure 4

QA/QC quantitative system scores from n = 105 studies. Average scores per criterion with categories “particle characterization”, “experimental design”, “applicable for RA”, and “ecological relevance”. Each study is assigned a criterion value of either 2 (adequate), 1 (adequate with restrictions), or 0 (inadequate) points, for each of the 20 criteria.

Figure 5

QA/QC quantitative system scores from n = 105 studies. Scores per study with categories “particle characterization”, “experimental design”, “applicable for RA”, and “ecological relevance”. *Studies with involvement of 1 or more of the authors of the present paper. Detailed scores and full references are provided in Table S3 and the SI reference list, respectively.