If we ask a random person what a microplastic is, they might respond that it’s something that can only be seen with a microscope. The answer is both yes and no, and this may be surprising, because in this neologism, science and common sense don’t entirely align. Let’s start with the definition. The word ‘microplastic’ was set to refer to the small plastic fragments that have become one of the main threats to the environment and human health. The Merriam Webster dictionary lists the word in an imprecise manner: ‘a very small piece of plastic especially when occurring as an environmental pollutant’. Despite the vagueness of the definition, it includes two important elements that define microplastics: size and their nature as anthropogenic pollutants. In what follows, I will explain how size defines microplastics in a precise way.
The observations of Dr. Kara Lavender Law’s group precisely documented the accumulation of small plastic debris in the North Atlantic Gyre. The results of 22 years of studies (from 1986 to 2008) on samples collected using trawl nets were published in Science in 2010, enabling the quantification of the number of particles and the mass of floating plastic in one of its main accumulation zones. In this work, nets with a mesh size of 335 microns were used for practical reasons, as finer nets clogged up quickly. The published data indicated that 12% of the plastic particles were bigger than 10 mm in their larger dimension. The term ‘microplastic’ existed at the time this article was published (in fact, it’s used three times), although it did not yet have a precise definition.
The term ‘microplastic’ was coined by Richard Thompson and collaborators in another celebrated (and brief) article that appeared in Science in 2004 as an acronym for ‘MICROscopic PLASTIC.’ With this neologism, the authors referred to small granular or fibrous fragments with an approximate dimension of around 20 microns that were being found in sampling campaigns of beaches and seas in the United Kingdom. Much smaller, therefore, than those that were being detected floating is floating debris in the oceans.
The first precise definition of microplastics came from the Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection (GESAMP). In their Report Number 90, issued in 2015, they defined microplastics as ‘particles in the size range of 1 nm to < 5 mm.’ The reason for the upper limit of 5 mm, which is somewhat inconsistent with the use of the prefix ‘micro,’ is due to the need to ensure continuity with the scientific data collected up to that point, mostly using plankton nets with mesh sizes in the hundreds of microns range. The fact that 5 mm refers to the ‘major dimension,’ which is a non-trivial detail, was not explicitly stated until 2019 in GESAMP’s Report Number 99: ‘Guidelines for the monitoring and assessment of plastic litter and microplastics in the ocean’.
The image shows several plastic fragments with dimensions larger than a few millimetres and a filament that, despite its reduced diameter, does not fit the definition of microplastic due to its length being over 5 mm. The issue posed by objects with high aspect ratio (length divided by width) is complex and will be addressed in another post. For comparison, a human hair is shown, with an average thickness around 70-80 microns. Plastics equal or larger than 5 mm are referred to as mesoplastics (5-25 mm), macroplastics (25 mm-1 m), or megaplastics (> 1 m). This classification comes from the aforementioned GESAMP Report Number 90 and should be understood in reference to the major dimension, although it wasn’t specified as such at that time. Plastics smaller than 1 micron, approximately the size of a cell of the bacterium Escherichia coli, are termed nanoplastics. The definition of nanoplastic warrants a more detailed treatment.
At the origin of plastic waste research, there was no lower limit for the size of microplastics. In GESAMP’s Report Number 90, it was stated that ‘microplastics extend to nanometric scales.’ Recently, the RAC (Committee for Risk Assessment) and SEAC (Committee for Socio-Economic Analysis) committees of the European Chemicals Agency (ECHA) recommended a lower size limit of 100 nm for the purpose of implementing restrictions on intentionally produced microplastics, which are the so-called ‘primary microplastics’. After considerable discussion, the Committees did not establish a lower limit due to practical reasons, although they acknowledged that there are commercial products with plastic additives < 100 nm. Therefore, the 100 nm limit only applies to the restrictions imposed on manufacturers by the REACH Regulation, specifically targeting substances placed on the EU market, and is not generally suitable for environmental studies.
Secondary microplastics are those produced by the degradation of larger plastics and not intentionally manufactured at that size. A lower size limit for secondary microplastics, and in general for microplastics in environmental studies is generally established as 1 µm. Below this limit, plastics are commonly referred to as ‘nanoplastics’. As in the case of microplastics, the nomenclature doesn’t align well with the use of the prefix ‘nano’, which in the case of synthetic nanoparticles are those with at least one dimension < 100 nm. However, the 1 µm boundary suits the characteristics and behaviour of these small plastic particles in the environment. In the first of their two articles on the subject, Julian Gigault and his team proposed that unintentionally produced particles (secondary plastics) with colloidal behaviour and a size range between 1 nm and 1 µm should be termed nanoplastics. (Once again, the lower limit is relevant: 1 nm is the size of a sucrose molecule, and in that range, the concept of ‘particle’ ceases to make sense.) Precisely among the distinctive characteristics of nanoplastics, the most relevant is their ability to remain in colloidal suspension in water and to undergo Brownian motion in the air. The micrometre roughly marks the difference between colloidal/Brownian behaviour and that of particles that settle or deposit due to gravity.
In summary, microplastics encompass a vast range of sizes, spanning from 1 µm, equivalent to the size of a bacterium, to < 5 mm, comparable to that of a garden ant. This range is as large as the difference between an ant and a blue whale. It is important to note that the limits of 1 µm and 5 mm are operational and do not denote absolute distinctions.
Science, Plastic and Environment 