Standard Practice for Collection of Non-Fibrous Nanoparticles Using a Nanoparticle Respiratory Deposition (NRD) Sampler
5.1 Exposures to high concentrations of aerosolized fine and ultrafine non-fibrous metal particles, including manganese (Mn), chromium (Cr), and nickel (Ni) generated during processes that involve high energy such as welding or smelting, may elicit deleterious health effects. Animal and epidemiological studies have associated welding and related work processes with a wide range of adverse health effects, including upper respiratory effects (rhinitis and laryngitis), pulmonary effects (pneumonitis, chronic bronchitis, decreased pulmonary function), potential neurological disorders (manganese-induced Parkinsonism), and high lung cancer and pneumoconiosis death rates. Manganese has been associated with neurological diseases.
5.2 Nanoparticles produced from metals, or their oxides and chalcogenides, have found many industrial uses. Examples of nanometals include silver (Ag), gold (Au), iron (Fe), copper (Cu), cadmium (Cd), zinc (Zn), platinum (Pt), and lead (Pd); examples of nanometal oxides include aluminium oxide (Al2O3), magnesium oxide (MgO), zirconium dioxide (ZrO2), cerium(IV) oxide (CeO2), titanium dioxide (TiO2), zinc oxide (ZnO), iron(III) oxide (Fe2O3), and tin(II) oxide (SnO); examples of nanometal sulfides include copper monosulfide (CuS), cadmium sulfide (CdS), zinc sulfide (ZnS), silver sulfide (AgS), tin sulfide (SnS), and many sulfides of Ni and cobalt (Co); examples of nanometal selenides include zinc selenide (ZnSe), cadmium selenide (CdSe), and mercury selenide (HgSe). Both the manufacture and use of these nanoparticles can result in particle inhalation, and consequent ill-effects. A stronger association has often been found between adverse health and cellular effects and inhalation of nanoparticles compared to larger particles of the same composition.
5.3 Aerosol sampling methods generally specify the collection of workplace air samples using inhalable and related samplers. These exposure assessment methods, as well as the use of respirable and thoracic samplers (ISO 7708), are inadequate for measurements of nanoparticle exposure when paired with gravimetric analysis. Large particles (>1 μm) weigh substantially more than nanoparticles typical of fumes and, consequently, obscure the ability to detect nanoparticles through gravimetric filter sampling. Additionally, most size-selective samplers collect all particles in the fraction of aerosol that can penetrate into the respiratory tract. Particle deposition, which is governed by the principles of impaction, interception, and diffusion (ISO 13138), is typically overestimated by these samplers.
5.4 There is a need to measure nanoparticle airborne concentrations apart from larger particles. An NRD sampler selectively collects nanoparticles in a manner similar to their typical deposition in the human respiratory tract. The constant motion of nanoparticles causes them to diffuse and potentially deposit in all regions of the respiratory tract, from the head airways to the deep alveolar region, as described by the ICRP (2). NRD samplers are designed to follow a nanoparticulate matter (NPM) deposition curve based on the ICRP model for deposition of particles smaller than 300 nm (the minimum in deposition for submicrometre particles) while removing the larger particles (1). Size-selective samplers (respirable, thoracic, and inhalable) mimic particle penetration rather than particle deposition. Many studies of welding fume have noted that size distribution of welding fume particles brackets the airways deposition minimum so that a substantial proportion of the fume is not deposited in the airways following inhalation (3-7). The use of an NRD sampler, however, approaches exposure assessment from a deposition estimation perspective (8) and provides a more relevant and physiological procedure for measuring actual hazards to workers (such as welders) posed by nanoparticle exposure. This knowledge is critical to the development of toxicological studies aimed at finding links between deposition of metal-containing nanoparticles and adverse health effects.
5.5 Welding fumes are dominated by incidental nanoparticles (particles with any external dimension in the nanoscale), but also include larger particles generated by splatter. Current animal and epidemiological studies investigate exposure to welding fumes without differentiating between nanoparticles and larger particles. Welding fume nanoparticles have been found to induce more toxic effects at the cellular level and to generate more reactive oxygen species (ROS) activity when compared to larger particles.
5.6 An NRD sampler was initially designed with nylon screens as the diffusion stage for the collection of nanoparticles (1), including welding fume (8, 9), although it was noted at the time that laboratory tests of this embodiment had not also included agglomerated particles, such as those which characterize welding fume. An additional collection mechanism, interception, was later found to play an important role as the sample collection of agglomerated nanoparticles progressed to higher loadings. Performance of the nylon screens for agglomerated particles was found to be affected by accumulated nanoparticle fraction loadings greater than 1 mg. The change in performance was accompanied by an increase in pressure drop across the screens to 14.3 kPa (57 in. of water) (5), which would cause many sampling pumps to fault. At the American Conference of Governmental Hygienists (ACGIH) Threshold Limit Value (TLV)5 for welding fume of 5 mg/m3, a one-hour sample at 2.5 L/min will collect 0.75 mg. Since the nanoparticle fraction of welding fume is typically less than half the total mass in air (3), the nylon screens are effective in sampling welding fume for one-hour or less as was borne out in field studies (9).
5.7 A new diffusion stage substrate, polyurethane foam, has characteristics more closely resembling human airways (example, Ref (10)) and may be preferable for collecting agglomerated materials in higher loading scenarios (11). In addition, polyurethane foam does not contain titanium dioxide allowing this sampler to be used to assess nanoparticle titanium dioxide.
5.8 The sampler with polyurethane foam has been shown to mimic the ICRP deposition curve closely when sampling spherical nanoparticles up to 100 nm diameter. Agglomerated particles collected in foam begin to show significant deviations from the simple curve as their size and shape factor increase (11). In Figure 3 of Ref (11), the curve modeling behavior of particles through foam is adjusted according to the dynamic shape factor of the aerosol and the sampler collection is shown to continue to match the modified curve at larger particle sizes. Since foam has proven to be a useful surrogate for lung deposition at larger particle sizes, it can be hypothesized that the adjusted foam model also will mimic the behavior of nanoparticle agglomerates in the lung. Enhanced deposition of larger agglomerates has been observed for agglomerated silica particles in human lung-casts (12) demonstrating that it may be necessary for an adjustment to the ICRP curve for agglomerates in this size range. However, until future research has identified a more precise adjustment to the ICRP deposition curve for agglomerated particles in the human airways the relationship of foam collection to human airways deposition remains a hypothesis.
5.9 An accurate measurement of flow rate through an NRD sampler is required for experiments where sampling devices and filter materials are to be compared as to the size distribution aerosol they capture. Air flow rate affects the efficiency with which a sampler will capture a particular aerodynamic size of particles. Furthermore, air flow rate through a sampler may affect the distribution of aerosol particles captured on the filters and deposited on the sampler collection substrates and walls. To determine aerosol concentration from a mass of captured particles it is necessary to set and measure flow rates accurately.
Note 2: Refer to Guide E1370 for guidance on the development of appropriate exposure assessment and measurement strategies.
1.1 This practice describes specified apparatus and procedures for collection of non-fibrous airborne metal nanoparticles generated during work activities.
1.2 Nanoparticle respiratory deposition (NRD) samplers are designed to follow a nanoparticulate matter (NPM) deposition curve based on the International Commission on Radiological Protection (ICRP) model for deposition of particles smaller than 300 nm (the minimum deposition for submicrometre particles) while removing the larger particles (1).2
1.3 This practice is applicable to personal and area sampling during work processes and situations where metal nanoparticles may be generated (for example, welding, smelting, shooting ranges).
1.4 This practice is intended for use by professionals experienced in the use of devices for occupational air sampling (such as cyclone samplers).
1.5 This practice is not applicable to the sampling of fibrous nanoparticles such as carbon nanotubes.
1.6 Detailed operating instructions are not provided owing to differences among various makes and models of suitable devices and instruments. The user is expected to follow specific instructions provided by the manufacturers of particular items of equipment. This practice does not address comparative accuracy of different devices nor the precision between instruments of the same make and model.
1.7 This practice contains notes that are explanatory and are not part of the mandatory requirements of the method.
1.8 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.9 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.10 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
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