Comparing the Definitions of Microplastics based on Size Range: Scientific and Policy Implications

Introduction

The term ‘Microplastic’ was coined about 20 years ago by Thompson et al. (2004). Defining the term microplastics (MP) can stir strong sentiments in some microplastic research and management circles. Some venture that if these particles had not been called ‘microplastics’, but instead a more generic term such as ‘broken plastics’ or ‘small plastics’ that the definition would not be as contentious. Some believe in a broad definition that includes all small secondary and primary plastic particles, while others state that the ‘micro-’ and ‘nano-’ prefixes should only be used in the classic terminology of the metric system (e.g., 1 to1000 μm for micro and 1 to 1000 nm for nano). Still others propose classifying these plastic particles by function. It is a classic exchange of ideas between “lumpers” and “splitters”, and the struggle between “purist” and “unitarian” thought.

While this Perspective would not venture to suggest it will immediately bring consensus, we believe we can offer a brief comparison of some of the definitions that exist for MP globally and, possibly, some of the reasoning behind them. To this end, we have assembled a table of current definitions (sometimes draft definitions) that federal and state agencies in the United States, international governments and organizations, and research groups have applied (Table 1). A motivation for preparing this Perspective is that for research scientists, practitioners, managers, and regulators, a consistently applied definition would facilitate a more coherent discussions around the fate and transport, and adverse effects to human health and the environment of MP. In addition, when joint or overlapping management jurisdictions occur, it is useful to have a consensus definition or knowledge of how definitions differ.

Table 1).

Summary of definitions of microplastics (MPs) and, in some cases, nanoplastics (NPs) from global organizations including federal agencies in the United States (US), US states, other countries, international organizations, and professional societies.

Organization	Type	Definition (by size)	Comments
National Oceanic and Atmospheric Administration (NOAA)	Federal	< 5 mm	NOAA (2015)
Environmental Protection Agency (EPA)	Federal	5 mm to 1 nm	https://www.epa.gov/water-research/microplastics-research
United States Geological Survey (USGS)	Federal	5 mm to 1 μm	Operationally dependent on investigation; Personal communication
Food and Drug Administration (FDA)	Federal	5 mm to 1 μm for MP &1000 nm to 1 nm NP	Duncan et al. (2024)
Agency for Toxic Substances and Disease Registry (ATSDR) and National Center for Environmental Health (NCEH)/Centers for Disease Control (CDC)	Federal	Inclusive definition of a MP is useful with further categories defined depending upon the objective of the study	Operational dependant on investigation; personal communication
National Institute for Standards and Technology (NIST)	Federal	No definition	Operational dependant on investigation; personal communication
Consumer Product Safety Commission (CPSC)	Federal	No definition	Under consideration; personal communication
Department of Defense (DOD)/Army Corps of Engineers (ACE)	Federal	No definition	Personal communication
State of California	State	5 mm to 1 nm	Regulatory; https://www.waterboards.ca.gov/board_decisions/adopted_orders/resolutions/2020/rs2020_0021.pdf
Interstate Technology and Regulatory Council (ITRC)	Interstate Organization	5 mm to 1 nm	https://mp-1.itrcweb.org/introduction/#1_1
ASTM International	International Organization	< 5 mm	ASTM International (2023)
European Chemical Agency (ECHA)	International Organization	≤ 5 mm	https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=OJ%3AJOL_2023_238_R_0003&qid=1695804976302
European Food Safety Authority (EFSA)	International Organization	5mm to 100 nm MP & 100 nm to 1 nm NP	Allan et al. (2021)
International Organization for Standardization (ISO)	International Organization	5 mm to 1 μm for MP & < 1 μm for NP	https://www.iso.org/standard/78033.html
Organisation for Economic Co-operation and Development (OECD)	International Organization	< 5 mm	https://www.oecd.org/environment/waste/policy-highlights-policies-to-reduce-microplastics-pollution-in-water-focus-textiles-and-tyres.pdf
United Nations	International Organization	5 mm to 1nm	Allan et al. (2021)
Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection (GESAMP)	International Organization	< 5 mm	http://www.gesamp.org/publications/guidelines-for-the-monitoring-and-assessment-of-plastic-litter-in-the-ocean

We understand a truly wholistic discussion of the definition of MP could include the types of polymers, kinds of additives, range of densities, and even shape, color, and other morphological properties. For this Perspective, we will limit the discussion to the simple starting point of the size range of MP. In addition, this discussion of MP materials (i.e., repeating polymeric subunits) excludes carbon-based and metallic engineered nanomaterials (ENM) which have existing definitions based largely on size (Hansen 2010). In this Perspective, we made every effort to locate readily available definitions from the scientific literature, governmental and organizational reports and websites, and knowledgeable colleagues. However, the comparison below is very likely not complete and is missing some definitions. In addition, any errors in Table 1 are entirely the authors.

Comparison of Definitions

Thompson’s original use of the term ‘Microplastic’ (Thompson et al. 2004) was soon followed by a 2008 expert workshop and the first size-based definition of “5 mm and less” without a specific lower size limit expressed (NOAA 2009). Over time, federal agencies in United States started adopting their own definitions. For example, the National Oceanic and Atmospheric Administration (NOAA), which has the oldest definition of MP in the federal government with < 5 mm with no lower limit (NOAA 2015) and the U.S. Environmental Protection Agency’s broadly inclusive definition of 5 mm to 1 nm (U.S. EPA 2024) (Table 1). Other agencies have more operational or narrative definitions like the United States Geologic Survey (USGS) which states for the purposes of ecological risk assessments, MP are 5 mm to 1 μm with the understanding that this range can change depending on the project goals. The Food and Drug Administration (FDA) uses 5 mm to 1μm for MP and 1000 nm to 1 nm for nanoplastic (NP). Other federal agencies including National Institute for Standards and Technology (NIST), Consumer Product Safety Commission (CPSC), Centers for Disease Control (CDC), Agency for Toxic Substances and Disease Registry (ATSDR), and Department of Defense (DOD)/Army Corp. of Engineers (ACE) have no official definition or apply definitions that are project specific (Table 1).

It is important to note that the State of California has the only legally binding definition of MP established by statute in the United States. It is broadly inclusive as 5 mm to 1 nm (Table 1). The Interstate Technology Regulatory Council (ITRC), an advisory group to many States in the United States, adopted the same definition as California.

On the European and other international fronts, the European Union (EU), ASTM International, the European Chemicals Agency (ECHA), the United Nations (UN), and the Organization for Economic Cooperation and Development (OECD) have broadly inclusive definitions generally 5 mm to 1 nm (Table 1). The Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection (GESAMP) defines an upper limit of less than 5 mm. In some cases, there is a bit of wiggle room around the lower limit; for example, the EU’s European Food Safety Authority (EFSA) uses 100 nm and he International Standards Organization (ISO) has a lower cutoff of 1 μm for MP and classifies anything less than 1 μm as NP. Finally, it is worth acknowledging that many countries and international organizations have not yet officially defined MP.

Implications of Definitions

There are many reasons to have differing definitions; for example, the U.S. EPA is a regulatory agency, and therefore has a broad, inclusive definition of MP because if, and when, a regulation or guidance for MP is codified into federal law, the definition should be as broad as possible. This would encourage covering the most inclusive grouping of similarly featured materials and avoid the need to promulgate a separate regulation for another category of MP based on size, such as for NP. In addition, from a regulatory perspective, potential loopholes in a MP definition should be avoided. For example, if the lower limit of a MP definition was 1 μm, it would preclude a very similar sized 995 nm particle from regulation. This loophole would undermine the human and ecological health protections of a regulation seeking to reduce exposures to the smaller, potentially more harmful MP.

Ideally, a definition would be agnostic to the analytical instrumentation, as we understand that these tools are changing rapidly and becoming increasingly sensitive, faster, and cost-effective; therefore, the definition should not be limited by instrumentation capabilities at the current moment. As research scientists, we recognize that even though specific agencies and organizations may apply a range of definitions for MP to fit organizational goals, on a scientific basis, for each research investigation, there may be the need to operationally-define specific characteristics of those MP being studied. For example, if environmental samples (e.g., seawater or lake water) have been passed through a 1 mm filter and then a 20 μm filter to isolate MPs, the size of the MPs in this study are considered: < 1 mm and > 20 μm.

Finally, the lower size limit definitions presented in this Perspective were selected by national and international organizations and research groups for many reasons including a desire to capture the smaller sizes that are likely to be bioavailable to aquatic organisms and represent the greatest ecological risk, and potentially, human health threats. In addition, as previously stated, these lower size limits reflect the rapidly changing analytical landscape of MP and the ability to measure ever smaller particles.

Pathways Forward

A clear and inclusive definition allows us to effectively frame research concerning MP, while providing an umbrella for consistency across a range of policy goals and regulatory actions. A common definition of MP would help to rationally streamline interaction and collaboration, nationally and internationally, among the different sectors and members of the MP community. Possible pathways forward include adopting consensus through organizations that develop scientific standards such as ISO or ASTM International. A possible example comparable to adopting a definition for MP based on size range is the UN’s Environmental Programme 2004 Stockholm Convention that established a definition for persistent organic pollutants (POPs) based on specific chemical properties (UN Environmental Programme 2024). Currently, 34 chemicals are defined as POPs based on the Convention. Similarly, adoption of a common definition for MP would not preclude organizations from having more limited or broader definitions for specific investigations or regulatory purposes, while providing a consensus understanding among the MP community.