Currently, microplastics are predominantly defined as plastic particles below 5 mm in size (Arthur et al., 2009) . This includes most pellets, which are macroscopic and easily distinguished from actual pieces resulting from plastic fragmentation, and with a different chemical composition as they often lack the chemical additives added in the compounding phase of plastic manufacturing.
Moreover, there is still no agreement regarding the upper and lower size limits for microplastics and no clear scientific justification for the currently applied size boundaries. The 5 mm upper limit for microplastics is not based on actual evidence but rather on pragmatism (Hartmann et al, 2019). For both, practical (technical) and theoretical reasons, the LabPlas project consortium supports the definition of microplastics as plastic particles below 1 mm. When reported in 2004, the term microplastics was used to describe fragments of plastic around 20 μm in diameter (Thompson et al. 2015). We endorse R. Thompson’s perspective on the upper limit “perhaps a more intuitive boundary following the SI classification of <1 mm” (Thompson et al. 2015).
This definition offers several advantages:
- it is in line with the international system (SI) of units
- it is more harmonic with the limit of size easily identifiable macroscopically
- it is compatible with most common standard ISO mesh sizes
- it excludes industrial pellets, which have a different chemical composition from manufactured plastics
Additionally, the LabPlas project consortium is among the group of scientists that suggested the now generally accepted term, “small micro- and nano-plastics (SMNP)”, needed to differentiate between plastic particles with different routes of exposure. Particles larger than 10 µm are unlikely to get across biological membranes. Also, from a practical standpoint, recent advances in MP sampling moved the lower threshold of catchable particles from the 100 µm of mesh techniques (manta trawl, plankton nets) to the 10 µm of filtering techniques and micro-FTIR characterization. This aligns with the 10 to 20 µm upper threshold of particle size ingested by most zooplanktonic organisms and efficiently retained by bivalve filter feeders. Furthermore, 10 µm is also a benchmark in atmospheric particle monitoring (PM10 and PM2.5), and below 10 µm, electrostatic surface charges become significant, causing aggregation with organic matter in cohesive sediment flocs, which represents a potential pathway of exposure via ingestion to aquatic organisms.
Based on these considerations, we propose defining SMNP, as plastic particles below 10 µm.
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