Date of Degree
PhD (Doctor of Philosophy)
Occupational and Environmental Health
First Committee Member
Peters, Thomas M.
Second Committee Member
Anthony, Theresa R.
Third Committee Member
Fethke, Nathan B.
Fourth Committee Member
Nonnenmann, Matthew W.
The research presented in this doctoral dissertation aimed to improve knowledge on methods to evaluate exposures to carbon-containing nanomaterials and to develop optimized respiratory filters properties to protect workers from these exposures while minimizing discomfort due to breathing resistance.
In the initial study, a novel laboratory-based system generated aerosols of four carbon-containing powders (carbon black, a small-diameter (< 8 nm) multi-walled carbon nanotube (MWCNT), a large-diameter (50-80 nm) MWCNT, and a nickel-coated MWCNT) to evaluate the effectiveness of NIOSH Method 5040 for measuring masses as low as 1 μg. A targeted mass of a powder ranging from 1 to 30 μg was deposited on filters for gravimetric and elemental carbon (EC) analysis. The gravimetric mass was compared to the EC mass, and a regression model developed for each powder type. Additionally, the limit of detection (LOD) of the NIOSH Method 5040 for each powder type was determined. The regression models had significant slopes relative to zero for all powder types with all but carbon black demonstrating a statistical difference between the two methods. The LOD of NIOSH Method 5040 ranged from 4.5 for small-diameter MWCNTs to 31.8 μg for nickel-coated MWCNTs. Assuming a sample flow rate of 4.2 L/min and an 8-hour sample duration, the concentration-based LOD for NIOSH Method 5040 ranged from 2.2 μg/m3 for small-diameter MWCNTs to 15.8 μg/m3 for nickel-coated MWCNTs. These results indicate the analysis of EC is affected by the structure and elemental content of the CNTs. Additionally, based on the LOD determined for each powder type, the method may not be sufficient to assess exposures at and below the recommended exposure limit accurately without sampling durations longer than 8 hours.
A second study used a laboratory-based system to evaluate an aethalometer response to carbon-containing nanomaterials including carbon black and MWCNTs. Concentrations ranging from 1 to 20 μg/m3 were generated to evaluate the device at concentrations expected in occupational settings. The concentration of the aerosol was measured by an aethalometer alongside a sample collected for EC analysis using NIOSH Method 5040. Additionally, NIOSH Method 7300 was used to determine the concentration of nickel during trials with a nickel-coated MWCNT to determine if the method along with the aethalometer can be used to assess metal-coated MWCNTs. A regression model was developed for each powder type, and the slopes for each were significant relative to zero. The LOD of the aethalometer ranged from 0.56 μg/m3 for nickel-coated MWCNTs to 7.2 μg/m3 for small-diameter MWCNTs. These results indicate the response of the aethalometer may be affected by particle structure and elemental content. NIOSH Method 5040 performed better than the aethalometer for all powder types except the nickel-coated MWCNT. Additionally, based on the LOD determined for each powder, an aethalometer may not be able to assess low-level exposures.
In the third study, a mathematical model was used to predict the particle penetration and pressure drop of respirator filters with varying filter thickness, fiber diameter, solidity, and electrostatic charge. Particle penetration was determined experimentally for two different commercially available respirator filters against a sodium chloride aerosol using a scanning mobility particle sizer (SMPS). Optimized filter designs were developed using the model to minimize the pressure drop by adjusting the filter depth, fiber diameter, and solidity of the filter. The model and experimental data were used to maintain a 5% maximum penetration against nanoparticle exposures while minimizing breathing resistance. Model results indicated electrostatic charging played a significant role in improving collection efficiency of respirator filters while not increasing the breathing resistance of the filter. Filter thickness and solidity also played a key role in minimizing breathing resistance. Pressure drop decreased with decreasing solidity, however, the filter depth increased to maintain the collection efficiency of the filter. This increase in filter depth introduced a decision point of determining the practical implications of increased filter thickness on the end user. Filter depth increases dramatically as the solidity decreases below 0.20. The breathing resistance that corresponds to this design is heavily dependent on the face velocity and electrostatic charge of the filter. The electrostatic charge should be maximized during filter production as this was the dominant collection method for nanoparticle aerosols.
This study evaluated two methods to measure exposures to carbon-containing nanoparticles to include carbon nanotubes in the workplace. Additionally, with the use of mathematical models, a design for a respiratory filter was developed to control against these exposures while minimizing the breathing resistance of the filter.
A traditional method used to measure elemental carbon exposures in the workplace was initially evaluated. A novel experimental setup deposited a specific mass of carbon nanoparticles onto a filter, and this mass was compared to the mass of elemental carbon using the traditional method. The study found that the traditional analytical method underreported the mass of carbon nanotubes present, and a correction factor would be needed to correct the measurements to avoid underestimating the risk of exposure to workers.
This study also used an instrument to measure concentrations to carbon-containing nanoparticles and compared these concentrations to the traditional method mentioned previously. An instrument determines the concentration of carbon in the air by passing the air through a filter and measuring how much light is absorbed by a filter as it darkens from the carbon. The instrument measured concentrations of carbon nanotubes below 10 μg/m3.
However, the measurement of the instrument varied significantly from the traditional analytical method depending the type of carbon-containing material measured.
The design of the filter focused on determining how the properties of a filter can influence the collection of nanoparticles and the breathing resistance. The study found that the thickness, solidity, and the electrostatic charge of the filter play a significant role in optimizing the filter.
aethalometer, black carbon monitor, carbon nanotubes, elemental carbon, filters, nanoparticles
xii, 89 pages
Includes bibliographical references (pages 72-81).
Copyright © 2017 Craig Alan Holder
Holder, Craig Alan. "Investigation of exposure assessment methods and filtration of carbon nanotubes." PhD (Doctor of Philosophy) thesis, University of Iowa, 2017.