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.