Measurement and Control of Occupational Exposure to Engineered Nanoparticles

Abstract

This dissertation consists of three studies concerning the measurement and control of inhalation exposure to engineered nanoparticles in the workplace. A key component in evaluating the potential health risks posed by nanomaterials is understanding how a worker may be exposed to airborne engineered nanoparticles. This information is not only needed for establishing safer nanomaterial work practices, toxicology studies also require doses which are relevant to actual workplace exposure. Emission monitoring and exposure assessments were conducted in nanotechnology workplaces in an effort to answer these questions. In Chapter 2, the synthesis of silicon carbide (SiC) nanoparticles in a prototype inductively coupled thermal plasma reactor and other supporting processes, such as the handling of precursor material, the collection of nanoparticles, and the cleaning of equipment, were monitored for particle emissions and potential worker exposure. The purpose of this study was to evaluate the effectiveness of engineering controls and best practice guidelines developed for the production and handling of nanoparticles, identify processes which result in a nanoparticle release, characterize these releases, and suggest possible administrative or engineering controls which may eliminate or control the exposure source. No particle release was detected during the synthesis and collection of SiC nanoparticles and the cleaning of the reactor. This was attributed to most of these processes occurring in closed systems operated at slight negative pressure. Other tasks occurring in more open spaces, such as the disconnection of a filter assembly from the reactor system and the use of compressed air for the cleaning of filters which collected synthesized SiC nanoparticles, resulted in releases of submicrometer particles with a mode size of ~ 170 – 180 nm. Observation of filter samples under scanning electron microscope confirmed that the particles were agglomerates of SiC nanoparticles. In Chapter 3, results are presented from an assessment of potential exposure to multi-walled carbon nanotubes (MWCNTs) conducted at an industrial facility where polymer nanocomposites were manufactured by an extrusion process. Recent animal studies have shown that carbon nanotubes (CNTs) may pose a significant health risk to those exposed in the workplace. To further understand this potential risk, effort must be taken to measure the occupational exposure to CNTs. Exposure to MWCNTs was quantified by the thermal-optical analysis for elemental carbon (EC) of respirable dust collected by personal sampling. All personal respirable samples collected (n = 8) had estimated 8-hour time weighted average (TWA) EC concentrations below the limit of detection for the analysis, and about one half the recommended exposure limit for CNTs of 1 µg EC/m3 as an 8-hour TWA respirable mass concentration. Potential exposure sources were identified and characterized by direct-reading instruments and area sampling. Area samples analyzed for EC yielded quantifiable mass concentrations inside an enclosure where unbound MWCNTs were handled and near a pelletizer where nanocomposite was cut, while those analyzed by electron microscopy detected the presence of MWCNTs at six locations throughout the facility. Through size selective area sampling, it was found that the airborne MWCNTs present in the workplace were in the form of large agglomerates. This was confirmed by electron microscopy, where most of the MWCNT structures observed were in the form of micrometer-sized, ropey agglomerates. However, a small fraction of single, free MWCNTs was also observed. It was found that the high number concentrations of nanoparticles, approximately 200,000 particles/cm3, present in the manufacturing facility were likely attributable to polymer fumes produced in the extrusion process. Chapter 4 details the development of a theoretical model to predict the filtration efficiency of electret filter media. Electret filters are filters whose fibers are semi-permanently charged. This charge results in electrostatic interactions between particles and fibers which can increase filtration efficiency without increasing the pressure drop across the filter, thus making electret media well-suited for use in local exhaust ventilation and respiratory protection. The numerical results were used to develop surrogate models which considered a random fiber orientation, filter solidity, particle interception, and dielectrophoretic, image, and Coulomb forces. The mean error of predicted single fiber efficiencies for the surrogate models of charged and uncharged particles were 6% and 0.8%, respectively. Aerosol penetration estimated by the surrogate models for a best fit effective surface charge density were compared to experimentally evaluated electret filters. Coefficients of determination for the fitted effective surface charge densities ranged from 0.82 to 0.98 and the effective surface charge densities were comparable to values reported in the literature.