EU scientific committee publishes opinion on definition of nanomaterials

(Nanowerk News) The EU Commission's Scientific Committee for Emerging and Newly Identified Health Risks (SCENIHR) has published a 46-page paper – Scientific Basis for the Definition of the Term "nanomaterial" (pdf) – where it basically concludes that size should be the basis for the scientific definition of the term nanomaterials.
Executive Summary
With the expected increase in the applications of nanotechnology, there is an urgent need to identify by clear unequivocal descriptions what can be considered as a nanomaterial and what should not be. This need to identify a nanomaterial comes from the uncertainty regarding the safety evaluation and risk assessment of nanomaterials. The SCENIHR was invited to provide advice on the essential elements of a science-based overarching working definition of the term "nanomaterial". More specifically they were asked to identify:
  • The types of physical and chemical properties particular to nanomaterials,
  • The threshold(s) at which nanomaterial-specific properties could be expected to occur and
  • Potential methodology for nanomaterial characterisation
  • The scientific opinion concluded that:
  • Whereas physical and chemical properties of materials may change with size, there is no scientific justification for a single upper and lower size limit associated with these changes that can be applied to adequately define all nanomaterials.
  • There is scientific evidence that no single methodology (or group of tests) can be applied to all nanomaterials.
  • Size is universally applicable to define all nanomaterials and is the most suitable measurand. Moreover, an understanding of the size distribution of a nanomaterial is essential and the number size distribution is the most relevant consideration.
  • The scientific opinion recognised however that specific requirements regarding risk assessment for regulatory purposes for certain areas and applications may require modifications of any overarching definition.
    The major question for both regulators and manufacturers is to identify when a material or product can be considered a nanomaterial. This opinion has tried to address the complexity and the uncertainties and provide advice on the essential scientific elements for a working definition of the term "nanomaterial" for regulatory purposes using specific examples that may be applicable to certain classes of nanomaterials.
    It should be stressed that "nanomaterial" is a categorisation of a material by the size of its constituent parts. It neither implies a specific risk nor does it necessarily mean that this material actually has new hazard properties compared to its constituent parts. However, size influences bio-distribution (and distribution kinetics) in an organism or in an ecosystem which should be taken into consideration in the risk assessment of nanomaterials.
    There is sufficient evidence to indicate that there can be a change in some properties of the material when it or its constituent parts are at the nanoscale which is, for instance, due to the increased surface-to-volume ratio. These nano-specific properties raise concerns over their potential to cause harm to humans and the environment. The reaction rate of nanoparticles often relates to the available surface area. Consequently, chemical reactivity per mass dose increases for smaller particles of the same type; this effect may or may not be associated with an increase in biological activity or toxicity. It is this possible change in reaction rate that warrants the careful evaluation of possible risks associated with nanotechnology products. However, it is not currently possible to identify a specific size at which a specific property would change or appear, or a specific property that is introduced or changed with size.
    Several international and national organisations have proposed definitions for the nanoscale and for nanomaterials (summarised in Annex I). In most of the proposed definitions, the size refers to one or more external dimensions or an internal structure within a specified size range. An upper limit of 100 nm or approximately 100 nm is commonly used. However, there is no scientific evidence to qualify the appropriateness of this value or any other single upper limit. Some definitions also include a reference to specific properties or nano-specific properties.
    It should be noted that, in the metric system, the "nanoscale" is the range below 1 micrometre (?m) and above 999 picometre (pm). Criteria relevant for the discrimination between nano and non-nano are discussed using a working definition for the nanoscale being approximately 1 to 100 nm. Any material with one or more internal or external dimensions in the nanoscale is then considered a nanomaterial. The feasibility of including specific properties as elements of a definition was assessed.
    When considering any definition for nanoscale and nanomaterial, size is the predominant feature. This requires that adequate validated methodologies are available for carrying out measurements at the nanoscale (i.e. below 1 ?m). Any nanomaterial should be described by its size, and by its number size distribution, including the methodologies used for the measurement.
    Solely referring to size as "one or more external dimensions" will NOT capture aggregates and agglomerates of primary particles or, importantly, more complex multi-component nanomaterials that are used in medical and cosmetic applications as their external dimension is likely to be larger than a specified upper size limit. The inclusion of a reference to "internal structure" with the same specified range as the external dimensions will include materials that consist of aggregates, agglomerates and multicomponent assemblies within the scope of the definition. This would also include nanoporous and nanocomposite materials. If possible, also for the constituents of multicomponent nanomaterials the size and number size distribution should be provided.
    To distinguish a nanostructured material from a non-nanostructured material, the volume specific surface area (VSSA) can be a complementary criterion, based on its integral material surface area per unit volume. For powders and/or dry solid materials, the VSSA can be determined using a nitrogen absorption methodology called the BET method after Brunauer, Emmett and Teller (Brunauer et al. 1938). A limitation of the BET-method is that it is only applicable to powders and/or dry solid materials and not to nanomaterials embedded in solids and suspensions. Other methods exist that can be used for the analysis of both powders and particles dispersed in liquids but most still remain difficult to use on a routine basis. More developments are needed in the area of analysing powders and particles in liquids. Expressing the surface area related to the volume instead of mass allows for an additional criterion independent of the density and size or size distribution of the nanomaterial. A VSSA above 60m2/cm3 would indicate an average size below 100 nm, thus indicating a high nanomaterial or nanostructure content.
    Data on the size distribution should be taken into account when describing a nanomaterial. When only a part of the material has a size within the size range of the definition or description, it should be clear whether and when such a material will be considered a nanomaterial. This may be by allowing a part (a certain percentage) of the number size distribution to be below a certain threshold or by using the information on the size distribution itself. Based on its geometric mean and geometric standard deviation, a material might be considered a nanomaterial when >0.15% of the material, as indicated by the number size distribution, has a size below the designated upper size limit.
    As size is a key element in any definition of the term "nanomaterial", there seems to be a need for the development of validated standardised methods to determine size (including that of its constituent parts where relevant) and its corresponding distribution to ensure comparability of results.
    There is a multitude of possibilities for the application of coatings and surface modifications to nanomaterials. Purposely applied and environmentally acquired coatings can have a major impact on nanomaterial interaction with biological systems. The coating and core together control the properties of a given nanomaterial. As a result, it is not useful to look at either the properties of the core or of the coating in isolation as they may not be representative of how the nanomaterial will behave in a given environment. The diversity in coatings of nanomaterials makes it impossible to include criteria based on surface properties within a definition as these properties may vary with coatings applied.
    Several physico-chemical properties from the OECD Working Party on Manufactured Nanomaterials (WPMN) list of characteristics were evaluated as possible discriminators for the identification of a nanomaterial. They were crystalline phase, photocatalytic activity, zeta potential, redox potential, radical formation potential, water solubility and the octanol-water partition coefficient. It was concluded that while all of these properties are very useful for risk assessment, none of them appear to be universally applicable as a criterion within a definition for all nanomaterials.
    Like any other material, nanomaterials can be degraded either mechanically, chemically or by dissolution; in fluids, they can form agglomerates or stable dispersions depending on solvent chemistry and surface coating. Features associated with solubility (and degradability) of nanomaterials are very important for risk assessment in view of the possibility for persistence and accumulation both in man and the environment. These features include size and shape, water solubility, surface charge and surface reactivity. However, these features cannot be translated into a definition as they are part of the characterisation of a nanomaterial and can change for each individual nanomaterial depending on chemical composition, surface modification and the immediate environment of the nanomaterial.
    Certain nanomaterials and composite materials may contain internal or external structures at the nanoscale that were incorporated to confer nanospecific characteristics. As the external dimensions of nanocomposites would be typically larger than 100 nm, a definition based solely on external size would not consider most nanocomposites to be nanomaterials. The internal structure with a size at the nanoscale would be an element to include in a definition, as then nanocomposites will be included in the definition of a nanomaterial. There are also nanocomposites where one phase is the solid bulk. Exclusion criteria would have to be developed to avoid considering macroscopic composite objects as nanomaterials.
    In order to specifically designate purposely made nanomaterials within regulations, the term "engineered" or "manufactured" may be used. When considering purposely made nanomaterials, the meaning of "engineered" or "manufactured" needs to include the processing (e.g. grinding or milling resulting in size reduction, or chemical processing) of materials to obtain materials at the nanoscale.
    In conclusion, size (and its distribution) is universally applicable to all nanomaterials and is the most suitable measurand. A defined size range would facilitate a uniform interpretation. For regulatory purposes the number size distribution should also be considered using both the geometric mean size and its geometric standard deviation for further refinement of the definition. Alternatively a specific fraction of the number size distribution might be allowed to be within the specified size ranges of the definition. For dry powders, the volume specific surface area (VSSA) may be added to the size as a complementary discriminator to identify nanomaterials. In addition, the definition should include both external and internal nanostructures.
    A size of 1 nm is proposed as the lower limit for the definition of nanomaterials. However, around 1 nm the distinction between molecules, nanoclusters and nanoparticles is unclear. In general molecules should be excluded. However, exceptions can be made to allow for inclusion of certain specific entities.
    At the moment, no scientific data are available to indicate that a specific size associated with special properties due to the nanoscale can be identified for nanomaterials in general. There is no scientific evidence in favour of a single upper limit. Although an upper limit of 100 nm is commonly used, there is no scientific evidence to qualify the appropriateness of this value. Notably, the use of a single upper threshold value might be too limiting for the classification of nanomaterials and a differentiated approach might be more appropriate. This approach could be based on a relatively high upper threshold for which it is assumed that the size distribution at the lower end will most likely be above the lower, more critical threshold. The lower threshold would be the critical one for which extensive nano-specific information would be needed in order to perform case-by-case risk assessment.
    For specific areas and applications, modifications of any overarching definition may be needed due to specific requirements regarding risk assessment for regulatory purposes.
    Source: European Commission