NORA Manufacturing Sector Strategic Goals
927ZJLSa - Respiratory Deposition of Airborne Carbon Nanotubes (NTRC)Start Date: 10/1/2009
End Date: 9/30/2011
Principal Investigator (PI)Name: Pramod Kulkarni
Funded By: NIOSH
Primary Goal Addressed9.0
Secondary Goal Addressed
Attributed to Manufacturing
This project aims to develop an approach to estimate fractional deposition of SWCNT aerosols in human and rodent lungs. The proposed approach combines experimental measurements of equivalent diameters of airborne SWCNTs with the spherical particle dosimetry modeling to estimate their deposited dose. This project directly addresses an important step in the risk assessment of inhalation exposure to airborne SWCNT particles—a key research need specified by NIOSH NTRC. An expected intermediate output of the project will be a developed generic approach to estimate fractional deposition of nonspherical particles that combines measured equivalent diameters of nonspherical aerosol particles with spherical particle dosimetry modeling.
Studies have shown that airborne single-walled carbon nanotubes released during simulated work practices involving handling of large quantities of their dry powders are mainly agglomerates and not single fibers (Maynard et al., 2004). These agglomerates have nanoscale as well as macroscale features; they typically possess low overall aspect ratios, often less than 5, and have porous structures with extremely high surface area-to-mass ratio. The inertial impactor measurements have shown that the overall enveloping physical size of airborne single-walled nanotube agglomerates is much larger than their aerodynamic size, by a factor of up to 10 (Baron et al., 2008). This implies that their diffusion-equivalent size could be much larger than their aerodynamic size. Therefore, lung dosimetry calculations based on aerodynamic size would grossly overestimate total particulate dose below 500 nm. Below this size, Brownian diffusion is an important deposition mechanism in the lung. This means that in an inhalation study, if most inhaled particles have their aerodynamic diameter below 500 nm, an actual particulate does that produces a given biological response is now much less—and the corresponding inferred toxicity could be much more—than originally estimated based on the aerodynamic size. Therefore, in order to perform reliable risk assessment of single-walled carbon nanotubes, it is necessary to develop an approach that can estimate fractional particulate dose accounting for their nonspherical and heterogeneous nature.
This project deals with developing an approach for estimating fractional deposition of particles in the lung that combines the experimental measurements of equivalent diameters of nonspherical single-walled carbon nanotube aerosol particles with the conventional spherical particle dosimetry modeling. Laboratory experiments will be designed and conducted to measure diffusion and aerodynamic diameters of airborne nanotube aerosols generated using different techniques. Tandem mobility-aerodynamic and mobility-mass measurements will be performed on nanotube aerosols generated by ultrasonic nebulization, electrospray, and dry dispersion techniques. Equivalent diameters, effective density, dynamic shape factor, and fractal dimensions will be obtained for the entire submicrometer size range. Correlations from these measurements will then be used as an input to spherical particle dosimetry model. The relative difference in nanotube deposition with respect to spherical particle deposition will be probed. Experiments will also be designed to understand the extent of variability in morphology across nanotube aerosols generated by various techniques, and the resulting impact on their fractional deposition in the lung. The approach developed in this project is generic and could be applied to other nanoaerosols such as multi-walled carbon nanotubes and fibers. This project directly addresses an important step in the risk assessment of inhalation exposure to airborne SWCNT particles—a key research need specified by the NIOSH Nanotechnology Research Center.
The outcomes of research proposed in this project will be presented in technical conferences which will provide a platform to obtain stakeholder feedback. The outcomes will also be published in peer-reviewed journals. The outcomes of this project may also be used by EID/NIOSH to perform risk assessment of nanotubes.
• Describe the problems or needs and why they are important to address. Industrial production of novel nanomaterials such as single- and multi-walled carbon nanotubes (SWCNTs and MWCNTs) is growing rapidly due to their unique physical, chemical, and biological properties. While their unique functionalities make them attractive for technological applications, the very same properties may possibly create adverse health effects after an inhalation exposure. Recent toxicological studies of fibrous and tubular nanostructures have shown that at extremely high doses these materials are associated with fibrotic lung responses, and result in inflammation and an increased risk of carcinogenesis. As a result, there is a growing concern over the potential health risks from inhalation exposure to these materials in industrial environments.
• Identify the surveillance system or information used to define the occupational safety or health need and design for your project. Evidence suggests that most often airborne SWCNT particles are agglomerates of nanotubes and nanoropes. The complex, nonspherical, and heterogeneous nature of these agglomerates means that their transport properties can not be described by one single size parameter. Using the aerodynamic diameter, which is often the diameter of choice in field measurements, to calculate their lung deposition may lead to an error below 500 nm diameter. In order to accurately account for diffusional deposition below 500 nm, one must use diffusion diameter. Our own laboratory measurements suggest that the diffusion diameter could be larger by a factor of 4 or more compared to the aerodynamic diameter. This implies that using aerodynamic diameter to compute lung deposited dose in inhalation studies will lead to a gross overestimation of dose. This also means that a much smaller amount of dose now produces the same biological response, implying that the material could be biologically more active.
• Describe how the project will address the problems or needs stated above. This proposal deals with this important knowledge gap that needs to be addressed in order to conduct reliable risk assessment of airborne SWCNT particles—a key research need specified by the NIOSH NTRC.
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