Abstract
Abstract
Nanotechnology enables manufacturers to enhance the performance of products by incorporating materials that are in the scale of 1–100 nm. There are environmental and health questions about engineered nanomaterials that may be entering the environment from degradation of nano-enabled products, from degradation of waste from these products at the end of the life-cycle, and from waste materials generated during manufacturing. Measurement methods and instrumentation are nonspecific, making it difficult to differentiate between naturally occurring and engineered particles; furthermore, the background levels of naturally occurring materials in the nanoscale are unknown. Because the uncertainties in this area are high, the Industry Consortium of Environmental Measurement of Nanomaterials (ICEMN)† was formed to provide regulators ideas on how to develop methods and support these measurements. The reviews of the science that follow, initiated by the ICEMN organization and led by specific ICEMN members, are an approach to use. It is presented as a guide to regulatory scientists and others who are not engaged in direct research, but have a need to understand the complexities of developing data on environmental concentrations of engineered nanomaterials.
Introduction
Although scientific and policy bodies have stated that nanomaterials are not intrinsically toxic (SCENIHR, 2009; Holdren et al., 2011), there is interest in evaluating if and how many engineered nanomaterials may be entering the environment from degradation of nano-enabled products, from degradation of waste from these products at the end of the life-cycle, and from waste materials generated during manufacturing. Identification of waste streams has relied on use patterns, estimates of manufacturing volumes, and laboratory models (Gottschalk et al., 2009). Measurement methods and instrumentation are nonspecific, making it difficult to differentiate between naturally occurring and engineered particles (Handy et al., 2008); furthermore, the background levels of naturally occurring materials in the nanoscale are unknown (Wiesner et al., 2009).
Because the uncertainties in this area are high, the Industry Consortium of Environmental Measurement of Nanomaterials (ICEMN) was formed to work with regulators to develop methods and support these measurements. The consortium consists of industry and academic scientists who expressed an interest in addressing concerns originally voiced by the State of California in its data gathering activity on measuring environmental concentrations of nanomaterials—not to respond to that data call, in as such, but to provide strategies for California or other states that had similar questions. Certainly, the National Science Foundation has already established centers for environmental implications of nanotechnology, notably the University of California (UC-CEIN) and Duke University (CEINT). These academic groups have provided significant advances in testing strategies for environmental effects and for measurement approaches. The literature contains numerous reports describing measurement methods for specific nanomaterials in the environment (Heymann et al., 1996; Isaacson et al., 2009), and there are many reviews of the effects of engineered nanomaterials on environmental biota. However, there are few that discuss general measurement techniques and strategies, per se. A recent review (von der Kammer et al., 2012) summarizes how environmental samples can be analyzed for carbon nanomaterials (nanotubes and fibers), quantum dots, silver, and metal oxides of titanium and cerium. The authors identify sample preparation, multiple instrumentation for analysis, and the sensitivity limitations of instruments to determine the low levels likely to be present as important considerations. The reviews in this issue attempt to go beyond that by discussing the need for information on background and extending the discussion to airborne particles. These reviews are presented as a guide to regulatory scientists and others who are not engaged in direct research, but have a need to understand the complexities of developing data on environmental concentrations of engineered nanomaterials. This work is not meant to be exhaustive, but is meant to lead individuals to areas where a deeper understanding can be gained. In that way, the reviews contained in this issue can build on and complement the work done by UC-CEIN, CEINT, and other academic programs by adding an industrial perspective; the authors represent industry and academia, including UC-CEIN. These reviews represent the contribution of the ICEMN.
Discussion
Defining nanomaterials
As stated above, a nanomaterial is considered to be a particle in the size range of 1–100 nm. The NNI defines it as a particle with one dimension between 1–100 nm that has unique properties (NNI, 2012). While this definition is correct for many new materials, it is limiting because not all materials that could be considered to be nanomaterials are between 1–100 nm, and that all nanomaterials have unique properties (European Commission, 2012). Some particles considered to be nanomaterials have diameters above 100 nm, and some particles with diameters less than 100 nm do not have unique properties relative to the same substance, but a larger diameter. The International Council of Chemical Associations (ICCA) has suggested that regulatory nanomaterial definitions include several elements to help clarify what materials are included and what materials are not of concern. Those elements state that nanomaterials are solid, particulate substances that are intentionally manufactured at the nanoscale. Nanomaterials consist of nano-objects as defined by the ISO (2011), but without the word approximately to describe the size range because the ambiguity introduced by approximately will be problematic.
The scientific and regulatory communities recognizes that the upper limit of 100 nm is a common and internationally accepted limit for the definition of a nanomaterial [e.g., ISO, European Commission, Australian National Industrial Chemicals Notification and Assessment Scheme (NICNAS), Organization for Economic Cooperation and Development, Health Canada, U.S. Environmental Protection Agency, U.S. Food & Drug Administration (Handy et al., 2008)], but a key review revealed that most novel size-dependent environment, health, and safety properties of an important class of nanomaterials have been shown to occur below 30 nm (Auffan et al., 2009). Thus, 100 nm represents a conservative limit that, when combined with the other elements, allows the definition to include materials of interest. Furthermore, it is important to recognize that individual nanoparticles or nano-objects rarely exist as discrete particles. Because of the surface forces, nano-objects cluster into aggregates (strongly fused or bonded) and agglomerates (loosely bound), which can be far larger than 100 nm. Furthermore, populations of nanomaterials can be mixed with nano-objects interspersed with aggregates and agglomerates.
The term “nanotechnology” was coined in the 1970s to describe the field in which molecules could be manipulated to form unique materials.* It is generally recognized, however, that small particles, even ones with unique properties, have existed for centuries (NNI, 2012). Further, some of these small particles occur naturally. While it may be possible to distinguish naturally occurring metal oxides from engineered ones using sophisticated techniques, it is not practical to do so when measuring environmental concentrations. This fact makes measurement of concentrations of engineered materials difficult because background levels of naturally occurring materials of the same substance are not known.
It is important to recognize that particles, even intentionally manufactured particles, are not all of one size; a population of particles generally has a distribution of sizes. Thus, while the average (mean or median) diameter of a particle may be above 100 nm, there may be a number of particles that fall below 100 nm. To ensure that such a population correctly gets identified as nanomaterial, criteria for how many of those particles need to be <100 nm and how are they measured need to be established. Recently, the European Commission (2011) and Australian NICNAS (2011) established criteria based on the number of particles (50% and 10%, respectively) to identify how many particles <100 nm define a nanomaterial, whereas the U.S. Environmental Protection Agency uses mass (10% [w/w]). Regardless of which criterion is used, the measurement method must be realistic and practical, that is, spending hours looking at electron micrograph images to count the distribution of sizes is not practical (Carr et al., 2012).
Use of nanomaterials
Nanomaterials can enhance the performance of products; that is a statement on which many agree (e.g., NNI, European Commission). How nanomaterials are used depends on the application. For personal care products (e.g., cosmetics, sunscreens), nanomaterials such as titanium dioxide or zinc oxide are coated to allow them to stay suspended as individual particles in the medium (usually an oil-in-water emulsion). For most other consumer product applications, the nanomaterials are part of a more stable matrix, for example, catalytic converters for automotive vehicles, tires, concrete, and coatings. For some selected consumer products, nanomaterials are part of the clothing matrix. During the lifecycle of these products, nanomaterials may be released into the environment. If they enter the environment, how they enter the environment, and how much enters the environment is currently under investigation by several groups. The assumption is that nanomaterials enter the environment, and their concentration is of interest, and it is the purpose of the subsequent articles in this issue to guide regulators in asking the questions from an enlightened perspective of how to answer the question of how to measure those concentrations.
Scope of ICEMN
A review of the literature suggests that there are many gaps in our understanding and in the methodology to conduct measurements to determine the concentration of engineered nanomaterials in the environment (von der Kammer et al., 2012). The ICEMN agreed to address at least some of these gaps to aid regulators in posing questions from an understanding of how to measure the concentration of engineered nanomaterials in the environment. These reviews provide a list of primary and secondary references on releases of nanomaterials into the environment; this includes release from finished products during processing, normal use, and end of life release. Entry into the environment may be via release to air (from the stack during processing to make the finished product or during normal wear), release to water (from the wastewater or normal wear), or release to soil (from normal wear or via another environmental source like air). These reports will summarize the extant data, identifying any data gaps in understanding how nanomaterials may be released from products.
The reviews also provide an understanding of the efficiency of wastewater treatment to remove nanomaterials and the subsequent medium where nanomaterials might deposit (surface water, sediment, biota, sludge). Based on information for a limited set of inorganic nanomaterials, wastewater treatment may be quite efficient in extracting particles from the wastewater (Rezic, 2011; Wang et al., 2012). However, there are data gaps and opportunities for more research.
Conclusions
The reviews outline measurement strategies, including issues regarding determining background levels and instrumentation requirements. Common conclusions throughout the subsequent review articles are:
• lack of information on background levels of naturally occurring or incidental small particles that can contribute to the overall measurement; • strategies for determination of background levels need to account for spatial and/or temporal variations; • the inability to rely on a single instrument because of biases and limitations; • the need to include complete characterization of the analyte to understand the physical characteristics in the environment; • the need for proper sample preparation before analysis; and • the need to laboratory scale methods because real-time measurements are inadequate.
In the end, regulatory scientists will need to collaborate with environmental scientists, metrologists (analytical chemists), and geochemists to answer the questions. It is the hope of the ICEMN that the information presented in the subsequent reviews will help foster that dialogue. The information presented supplements the report previously presented by von der Kammer and coworkers (2012) and adds the need to develop a sound strategy to determine background levels of nanomaterials of interest.
Footnotes
Author Disclosure Statement
No competing financial interests exist.
*
“1974: Tokyo Science University Professor Norio Taniguchi coined the term nanotechnology to describe precision machining of materials to within atomic-scale dimensional tolerances” (NNI, 2012).