From a regulatory point of view, nanomaterials are generally considered to be materials with features between 1 and 100 nanometres. These materials have been associated with possible nanometre-scale-specific safety issues1, triggering a global effort to set specific regulatory requirements. Legislators have established nanomaterial definitions that are country- or region-specific so that, potentially, the same material may be classified as a nanomaterial, or not, depending on the definition used. Whether a material is regarded as a nanomaterial or not in a legal context may impact, for example, the data requirements.

Here we examine the (main) differences between the definitions in the various jurisdictions, how to achieve a global definition, and propose a way forward. In this Comment, we provide an overview of whether and how nanomaterials are defined or described in the chemicals legislation or guidance in the Organisation for Economic Co-operation and Development (OECD) member countries, the European Union (EU) and other relevant economies. We highlight how differences between the definitions affect which materials are considered to be nanomaterials. We discuss whether international convergence could be achieved. Finally, we propose a generalized naming convention that can be applied to any specific definition of nanomaterial.

A multitude of regulatory nanomaterial definitions

A definition of a nanomaterial for international technical application is provided by the International Organization for Standardization (ISO) (Table 1). It does not explicitly refer to particles, nor does it include a quantitative threshold of the fraction of material that must be at the nanoscale for the whole material to be considered a nanomaterial. Therefore, the ISO definition is not suitable for regulatory purposes, because most real-life particulate materials are mixtures of nanometre-sized and non-nanometre-sized particles and contain particle agglomerates and/or aggregates.

Table 1 Nanomaterial definitions and comparison of major aspects
Full size table

The OECD agreed on a size-based working definition of a nanomaterial to be used for the sole purpose of its Working Party on Manufactured Nanomaterials (WPMN) in order to progress discussions of regulatory needs and to develop methodological support for nanotechnologies. The OECD published a Council Recommendation2 that concludes that the management of the possible risks of nanomaterials is covered by existing regulatory frameworks for chemicals.

These developments helped legislators globally to define the term ‘nanomaterial’ clearly for each country’s chemicals legislation. However, it is challenging to achieve global convergence of these definitions of nanomaterial3,4. Additionally, there are different approaches to what makes a definition ‘legal’, how something should be ‘regulated’ and whether regulatory needs are sufficiently addressed by guidance, guidelines or standards instead of legislation. Moreover, different jurisdictions apply different underlying conceptual principles for the safety assessment and management of chemicals. This leads to considerable differences regarding the meaning of the term nanomaterial in legal contexts.

Identifying and counting nanoparticles

Most nanomaterial definitions require that particles at the nanometre scale are identified and counted. Regulatory definitions may include a quantifiable threshold of the fraction of particles that must be at the nanometre scale, in which case quantification of particles is essential5,6,7. The particle size measured depends on the method applied, the external dimension chosen to represent its size8, and the way particles are counted9. The OECD Test Guideline 125 (ref. 10) includes electron-microscopy-based methods, which are the only methods that provide the information required by EU legislation to identify nanomaterials. Electron microscopy allows particles to be identified and counted. Particles can be on their own or may be constituents in aggregates (consisting of strongly bound constituent particles) and agglomerates (consisting of weakly bound constituent particles and/or aggregates). It furthermore allows the external dimensions (‘size’) of irregularly shaped particles8 to be measured and the size distribution to be evaluated.

However, even starting from the same electron micrograph, the resulting number of particles and size distribution can change spectacularly depending on how the particles are counted9. Figure 1 illustrates different ways of counting agglomerated and aggregated particles according to different definitions of a nanomaterial. This is a key reason for differences in classification as a nanomaterial in different legal contexts.

Fig. 1: Common ways of counting aggregates, agglomerates and their constituent particles.
figure 1

Agglomerates and aggregates can be excluded from the counting (left), counted as one particle (middle), or all their constituent particles can be counted individually (right). For example, such ways of counting are used in legislation, in various ISO standards and by the automated particle counting of instruments. Figure inspired by ref. 9.

Full size image

Table 1 reviews the legislative texts by region and the major aspects of each nanomaterial definition. The following countries use the ISO definition and are not listed in Table 1: South Africa, Thailand and China. Other countries, such as Argentina, Brazil, Chile and Colombia, do not currently define nanomaterials in either their chemicals legislation or guidance.

Outlook

The understanding of what constitutes a nanomaterial is converging towards a global agreement that nanomaterials have at least one dimension at the nanometre scale (between 1 and 100 nanometres). On the other hand, several aspects of the many definitions continue to differ substantially (Table 1), especially regarding how they consider agglomerates and aggregates, which may each be counted as one particle or their constituent particles may be counted. Other variations between the criteria in the definitions include the origin of the material (manufactured, natural or incidental), the metrics used (number or mass of particles), the threshold (the fraction of nanometre-scale particles that is required for the material to be considered a nanomaterial), as well as whether the evaluated material must exhibit nanometre-scale-related properties. Other issues are the chosen measurand and applied measurement methods, both of which influence the measured particle size and thereby the particle size distribution.

Convergence of the regulatory definitions of a nanomaterial could be achieved by agreeing internationally on which particles to count, and how—we note that the World Health Organization did this for counting fibres11—as well as agreeing on the other criteria listed above. However, it seems unlikely that regulatory definitions of a nanomaterial will converge into one globally accepted definition soon.

Regulatory considerations for the same material that result in contradicting outcomes between countries create ambiguity for stakeholders. At the least, clarity on the detailed data and metadata to be assessed should be facilitated. Importantly, agreed methods for measuring and counting particles must allow for flexibility in the data analysis, such as in deciding which particles are counted in electron microscope micrographs. The reporting of the measurement outcomes should include such detailed metadata.

To increase the global transparency of the legal identification and assessment of nanomaterials, we propose a naming convention that indicates, at a glance, which definition criteria, in addition to the size range, were applied in assessing the material. Any additional criteria included in the definition can also be listed.

Our proposed naming convention for nanomaterials is based on four parameters, combined as follows to give a nanomaterial name of the form: nanomaterial(origin/C1-C2-C3/metrics/threshold). In this name, ‘origin’ can take the values ‘manufactured’ (m), ‘incidental’ (i), ‘natural’ (n) or a combination thereof. The counting C1-C2-C3 indicates how the particles are counted: C1 refers to the individual constituent particles, and can have the value c (they are counted) or 0 (not counted); C2 refers to the agglomerates, and can have the value c (constituent particles in agglomerates are counted), aggl (agglomerates are counted as one particle) or 0 (agglomerates are not counted); and C3 refers to the aggregates, and can have the value c (constituent particles in aggregates are counted), aggr (aggregates are counted as one particle) or 0 (aggregates are not counted). The metrics used in the definition refers to the basis of the size distribution, which can have the values (for example) number, mass, surface or volume. The threshold is the percentage: any number larger than 0 up to 100, described with mathematical symbols, such as > (greater than), and no specified threshold would be indicated as ns.

Examples of how the naming convention applies to existing definitions are: nanomaterial(m,i,n/c-c-c/number/>50) (the European Commission’s recommended definition), or nanomaterial(m/c-aggl-aggr/mass/>1) (a nanomaterial according to the US Environmental Protection Agency).

We believe that using this general universal nanomaterial naming convention would be a valuable step towards more transparency. It could, for example, be associated with the output of the measurement methods described in the OECD Test Guideline 125 (ref. 10) and standards that report particle numbers, which would allow the user immediately to identify the most relevant methods for measuring the size (distribution) for the purpose as well as irrelevant ones.

Moreover, the proposed nomenclature would help users to understand the content of nanomaterial information databases, thus facilitating comparison of data. It would enable a clearer communication of the basis used for analysing experimental measurements, which would be a major achievement. In addition, we could also refine the description of the subclass ‘nanomaterial’ in ontologies for mapping terms related to nanosafety, such as the eNanoMapper ontology11, and use it to interlink these databases. This would contribute to the creation of a federated database system.