3 Nanoparticle exposure assessment: methods, sampling techniques, and data analysis I.J. YU, Hoseo University, Korea, G. ICHIHARA, Nagoya University, Japan and K. AHN, Hanyang University, Korea DOI: 10.1533/9780857096678.2.47 Abstract: Nanotechnology is now applied to many industries, resulting in a wide range of nanomaterial-containing products, such as electronic components, cosmetics, medicines, vehicles, and home appliances. Nanoparticles can be released throughout the life cycle of nanoproducts, including manufacture, consumer use, and disposal, thereby involving workers, consumers, and the environment in potential exposure. However, there is no current consensus on the best sampling method for characterizing manufactured nanoparticle exposure. Therefore, this chapter addresses nanoparticle exposure assessment methods, sampling techniques, and data analysis. Key words: nanomaterial, nanoparticle, exposure assessment, sampling techniques, data analysis.
3.1
Introduction
ISO/TS 80004-1 (ISO, 2010) and ISO/TS 27687 (2008a) both offer definitions of terms relevant to nanotechnology (see Table 3.1). Such definitions are subject to change owing to continued developments in the field but provide a useful foundation for this chapter. Nanoparticle exposure assessment presents a unique challenge in the field of occupational and environmental health. Comparing nanoparticles with non-nanoscale particles requires quite distinct nanoparticle sampling and exposure assessment methods from those conventionally used in occupational and environmental health, as shown in Table 3.2. There is no current consensus on the best sampling method for characterizing exposure to manufactured nanoparticles. In the case of manufactured nanomaterials, such as carbon nanotubes (CNTs), carbon nanofibres (NIOSH, 2010), and ultrafine TiO2 (NIOSH, 2011), there are only a few quantitative occupational exposure levels (OELs) in terms of mass concentration, as suggested by the US National Institute for Occupational Safety and Health (NIOSH), although company-developed OELs for workplace management are beginning to emerge, together with benchmark doses based on toxicity data. Qualitative assessments comparing particle concentrations at the emission source with background particle concentrations are frequently used to identify emission sources of nanomaterials and implement measures for exposure mitigation. 47 © 2014 Woodhead Publishing Limited
Table 3.1 Terms and definitions for nanotechnologies: ISO/TS 27687 (2008a) and ISO/TS 80004-1 (ISO, 2010) Term
Definition
Nanoscale Nanotechnology
Size range from approximately 1 nm to 100 nm Application of scientific knowledge to manipulate and control matter on a nanoscale to make use of related size- and structure- dependent properties and phenomena, as distinct from properties and phenomena associated with individual atoms, molecules or bulk materials Material with any nanoscale external dimension, internal structure, or surface structure. This generic term also includes nano- objects and nanostructured materials Material with one, two or three external nanoscale dimensions Composition of interrelated constituent parts that include one or more nanoscale regions Material with internal or surface nanostructure
Nanomaterial
Nano- object Nanostructure Nanostructured material Engineered nanomaterial Manufactured nanomaterial Incidental nanomaterial Nanomanufacturing
Nanomaterial designed for specific purpose or function Nanomaterial intentionally produced for commercial purposes with specific properties or composition Nanomaterial generated as unintentional by- product
Intentional synthesis, generation or control of nanomaterials or fabrication steps on nanoscale for commercial purposes Nanomanufacturing Combination of activities for intentional synthesis, process generation or control of nanomaterials or fabrication steps on nanoscale for commercial purposes Nanoscale Effect attributable to nano- objects or nanoscale regions phenomenon Nanoscale property Characteristic of nano- object or nanoscale region Particle Minute piece of matter with defined physical boundaries Agglomerate Collection of weakly bound particles or aggregates or mixtures of the two, where the resulting external surface area is similar to the sum of the surface areas of the individual components Aggregate Particle comprising strongly bonded or fused particles, where the resulting external surface area may be significantly smaller than the sum of the calculated surface areas of the individual components Nanoparticle Nano- object with all three external dimensions on a nanoscale Nanoplate Nano- object with one nanoscale external dimension and two other significantly larger external dimensions Nanofibre Nano- object with two similar nanoscale external dimensions and one significantly larger external dimension Nanotube Hollow nanofibre Nanorod Solid nanofibre Nanowire Electrically conducting or semi- conducting nanofibre Quantum dot Crystalline nanoparticle exhibiting size- dependent properties due to quantum confinement effect on electronic state
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Table 3.2 Comparison of sampling methods used for non-nanoscale particles and nanoparticles Non-nanoscale particles
Nanoparticles
Size Respirable Sampling Sampling equipment
500 nm to 10 μm Respirable + inhalable Personal > area Cyclone, impactor, filter
Sampling metric
Mass, number (fibre)
Background concentration Particle size distribution measurement TEM/SEM
Not usually measured
1000 nm) indicates the likely presence of larger particles and/or nanoscale particle agglomerates. Then, the presence of nanoscale particles, larger particles or nanoscale particle agglomerates can be verified by a TEM or SEM analysis. Selectivity is a critical issue when characterizing exposure using an airborne particle number concentration. Airborne nanoscale particles are present in many workplaces and often originate from multiple sources, such as combustion, vehicle emissions, and infiltration of outside air. However, particle counters are
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generally not selective as regards the particle source or composition, making it difficult to differentiate between incidental and process-related nanoscale particles when using just the number concentration. While CPCs and OPCs can be used to identify the sources of nanoscale particles, filter-based samples can be used to verify the chemical composition and shape of the nanoscale particles in order to differentiate between incidental and manufactured nanomaterials. Although this issue is not unique to particle number concentration measurements, differences on the scale of orders of magnitude can exist in aerosol number concentrations, depending on the number and type of source of the particle emissions. Therefore, monitoring over several days and during different seasons can provide a better understanding of the variability. For example, rainy days tend to have more incidental particles, whereas the operation of a vacuum pump at the start of the working day has been found to increase the particle number, together with welding operations in other locations in the laboratory, indicating that other operations besides nanomaterial manufacturing can be involved in increasing the particle number concentration (Lee et al., 2011). For CPC, a particle number concentration of upper particles over 100 000 particles per cm3 may indicate a data interpretation error, and placing a diluter consisting of a modified HEPA filter cartridge upstream of the inlet should be considered (Peters et al., 2006; Heitbrink et al., 2007; Evans et al., 2008). The analysis of air samples by TEM and SEM using energy dispersive X-ray (EDX) spectrometry can provide information on the elemental composition of the nanomaterials. However, TEM and SEM analyses can be compromised if there is a particle overload on the filter. Alternatively, if the loading is too sparse, an accurate assessment of the particle characteristics may not be possible. It should be noted that, while sampling near the emission source raises the efficiency of the sampling and represents the worstcase scenario, it does not represent the worker exposure. However, this kind of sampling is necessary to identify nanoparticle emissions in the workplace and can be used to mitigate workplace exposure.
3.9
Conclusion and future trends
Several exposure and emission assessment guidelines for nanomaterials recommend taking background measurements of nanoparticle concentrations when measuring the particle number concentration (ISO, 2007; OECD, 2009c; BSI, 2010; Brouwer et al., 2012). Various background measurement strategies for assessing workplace emissions of or exposure to nanomaterials have been suggested (Kuhlbusch et al., 2011; Ramachandran et al., 2011; Brouwer et al., 2012), including measuring the nanomaterial concentrations before and after the process (time-series approach), measuring the outdoor ambient concentrations, and sampling at the intake of certain processes that may or may not be from the outside (spatial approach). Measurements can also be made simultaneously using process-related monitoring or pre- and post-process monitoring (Kuhlbusch et al.,
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2011; Ramachandran et al., 2011). Thus, an industrial hygienist needs to determine the average airborne particle number concentration for various processes and adjacent work areas using a CPC and OPC before or after the processing or handling of nanomaterials. The exposure assessment of carbon nanotubes, such as SWCNTs and MWCNTs, remains a real challenge in the field of occupational hygiene, as there have been relatively few studies on CNT sampling, and the best sampling filters and methods have yet to be established. While most number counting devices, such as a CPC and OPC, do not work for CNTs, size measurements using a SMPS also do not work due to the arc charge caused by the charged CNTs in the differential mobility analyser. Thus, a fibre paradigm has been suggested for CNTs, where the toxicity of CNTs is related to the fibre dimensions. Plus, various counting methods for CNT tubes or fibres have also been developed. Determination of the mass concentration of airborne CNTs by measuring the elemental carbon remains, however, a challenge due to the detection limits and complicated nature of current analytical methods.
3.10
Acknowledgement
This research was supported by the Nano R&D program through the Korean National Research Foundation funded by the Korean Ministry of Education, Science, and Technology (2011-0019171).
3.11
References
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