Nanotechnology White Paper - US Environmental Protection Agency

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32 EPA Nanotechnology White Paper 20 nm 20 nm (A) (B) Figure 17. Transmission Electron Microscope (TEM) image of aerosol-generated TiO2 nanoparticles. (A) Un-aggregated and (2-5 nm) (B) and aggregated (80-120 nm), used in exposure studies to determine the health impacts of manufactured nanoparticles. Nanoparticle aggregation may play an important role in health and environmental impacts. (Images courtesy of Vicki Grassian, University of Iowa [Grassian, et al., unpublished results]) A given nanomaterial can be produced in many cases by several different processes yielding several derivatives of the same material. For example, single-walled carbon nanotubes can be produced by several different processes that can generate products with different physical-chemical properties (e.g., size, shape, composition) and potentially different ecological and toxicological properties (Thomas and Sayre, 2005; Oberdörster et al., 2005a). It is not clear whether existing physical-chemical property test methods are adequate for sufficiently characterizing various nanomaterials in order to evaluate their hazard and exposure and assess their risk. It is clear that chemical properties such as boiling point and vapor pressure are insufficient. Alternative methods for measuring properties of nanomaterials may need to be developed both quickly and cost effectively. Because of the current state of development of chemical identification and characterization, current chemical representation and nomenclature conventions may not be adequate for some nanomaterials. Nomenclature conventions are important to eliminate ambiguity when communicating differences between nanomaterials and bulk materials and in reporting for regulatory purposes. EPA’s OPPT is participating in new and ongoing workgroup/panel deliberations with the American National Standards Institute (ANSI), the American Society for Testing and Materials (ASTM), and the International Organization for Standardization (ISO) concerning the development of terminology and chemical nomenclature for nanosized substances, and will also continue with its own nomenclature discussions with the Chemical Abstracts Service (CAS). 3.3 Environmental Fate of Nanomaterials As more products containing nanomaterials are developed, there is greater potential for environmental exposure. Potential nanomaterial release sources include direct and/or indirect releases to the environment from the manufacture and processing of nanomaterials, releases from
EPA Nanotechnology White Paper oil refining processes, chemical and material manufacturing processes, chemical clean up activities including the remediation of contaminated sites, releases of nanomaterials incorporated into materials used to fabricate products for consumer use including pharmaceutical products, and releases resulting from the use and disposal of consumer products containing nanoscale materials (e.g., disposal of screen monitors, computer boards, automobile tires, clothing and cosmetics). The fundamental properties concerning the environmental fate of nanomaterials are not well understood (European Commission, 2004), as there are few available studies on the environmental fate of nanomaterials. The following sections summarize what is known or can be inferred about the fate of nanomaterials in the atmosphere, in soils, and in water. These summaries are followed by sections discussing: 1) biodegradation, bioavailability, and bioaccumulation of nanomaterials, 2) the potential for transformation of nanomaterials to more toxic metabolites, 3) possible interactions between nanomaterials and other environmental contaminants; and 4) the applicability of current environmental fate and transport models to nanomaterials. 3.3.1 Fate of Nanomaterials in Air Several processes and factors influence the fate of airborne particles in addition to their initial dimensional and chemical characteristics: the length of time the particles remain airborne, the nature of their interaction with other airborne particles or molecules, and the distance that they may travel prior to deposition. The processes important to understanding the potential atmospheric transport of particles are diffusion, agglomeration, wet and dry deposition, and gravitational settling. These processes are relatively well understood for ultrafine particles and may be applicable to nanomaterials as well (Wiesner et al., 2006). However, in some cases, intentionally produced nanomaterials may behave quite differently from incidental ultrafine particles, for example, nanoparticles that are surface coated to prevent agglomeration. In addition, there may be differences between freshly generated and aged nanomaterials. With respect to the length of time particles remain airborne, particles with aerodynamic diameters in the nanoscale range (
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