Existing Classification Schemes for Nanomaterials
There are many different ways to classify nanomaterials including chemical composition, similarities in shapes, location within the final end-use product and risk-based analysis. Classification schemes that are based on similarities in chemical composition are discussed further in this section. For clarity, this document does not reference all existing classification schemes for nanomaterials,.
Classification of Nanomaterials by Chemical Composition
A nanomaterial classification scheme based on similarities in chemical composition could work well for regulatory programs which are based on traditional chemical frameworks. International bodies such as the Organization for Economic Cooperation and Development (OECD), as well as leading scientists in the nanomaterial field, are currently working on chemical-based classification of nanomaterials in order to utilize analogue/read-across information to determine potential safety implications.
The OECD’s Working Party on Manufactured Nanomaterials (WPMN) was formed in 2006 to spearhead international co-operation in understanding the health-related and environmental safety-related aspects of manufactured nanomaterials among member countries. One of the organization’s key projects is the testing of a representative set of 13 nanomaterials for human health and environmental safety. Nanomaterials which are already in use or will be soon, including fullerenes (also known as "bucky balls"), single-wall (SWCNTs) and multi-wall (MWCNTs) carbon nanotubes are being tested for their physiochemical parameters, environmental degradation and accumulation, environmental toxicology, and mammalian toxicology. Within each 'class' of the 13 nanomaterials, different forms of chemically similar nanomaterials have been identified for testing; for example, different sizes and surface coatings of the same core nanomaterial - such as titanium dioxide - will be included in the research. One of the intents of this testing is to be able to utilize read-across information on these chemically similar nanomaterials. To build on this theme, the OECD WPMN is also organizing an expert workshop in 2014 to establish categories of nanomaterials with the expected goal of feeding into testing, read across/structure-activity relationships, risk assessment and risk management.
Stone et al. (2010) have developed a classification scheme for nanomaterials based on similarities in chemical composition from an environmental perspective. In their work, the proposed classes are carbon, metals or metal oxides, and organic (see Figure 1 below).
Figure 1: Nanomaterial classification scheme suggested by Stone et al. (2010)
Classification for Regulatory Purposes
Several suggestions for nanomaterial classification schemes for RCC regulatory purposes emerged during discussions with stakeholders and other experts at the March 20, 2013 RCC Nanotechnology Initiative Workshop in addition to the examples discussed above and in Appendix I. There was consensus that many of the proposed classification schemes do not meet Canada/US regulatory requirements, while others still require extensive research in order to be considered. Stakeholders suggested that nanomaterial classifications schemes could be based on the following, (if/when sufficient science is available):
- Exposures (powdery/aerosolized, liquid exposure, life cycle analysis, and consumer exposure);
- Use profiles (industrial use only, consumer, commercial);
- Toxicological mode of action of nanomaterials (e.g., structure activity relationships); and,
- Physicochemical properties (e.g., surface activity, catalytic activity, electronic activity).
One of the goals of the classification scheme developed through the RCC Nanotechnology Initiative is to use it as a framework to support the selection of appropriate analogue/read-across information to be used in substance-specific risk assessments for nanomaterials when possible. At present, there are no nanomaterial-specific regulatory frameworks in either Canada or the US.
The Canada/US Programs acknowledge that sufficient comprehensive scientific knowledge does not yet exist to develop a validated classification scheme for nanomaterials, as was done when classification schemes were created for traditional chemicals. However, a classification scheme for nanomaterials based on similarities in chemical composition which allows analogue/read-across information to be utilized will provide the Canada/US Programs with a good starting point for nanomaterial classification, and is considered suitable given the existing regulatory frameworks. This proposed nanomaterial classification scheme will be refined as scientific knowledge of nanomaterials continues to increase, eventually focusing on specific characteristics such as mode of action.
The following section introduces the nanomaterial classification scheme developed as part of the RCC Nanotechnology Initiative. It also highlights certain physicochemical parameters that may be important in identifying whether two nanomaterials share sufficient similarities to utilize analogue/read-across information.
 Nel, A.; Xia, T.; Mädler, L.; Li, N.Science, 2006, 311, 622-627.
 Olson, M.; Gurian, P. J. Nanopar. Res., 2012, 14, 786.
Available online at: http://www.oecd.org/env/ehs/nanosafety/sponsorshipprogrammeforthetestingofmanufacturednanomaterials.htm
 Stone, V.; Nowack, B.; Baun, A.; Brink, N.; Kammer, F.; Dusinska, M.; Handy, R.; Hankin, S.; Hassellov, M.; Joner, E.; Fernandes, T. Sci. Total Env., 2010, 408, 1745-1754.
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