Computed Tomography in Industrial Hygiene

Abstract

A variety of air monitoring methods are used by industrial hygienists to evaluate human exposures to contaminants, monitor process emissions and leaks, and determine the effectiveness of ventilation systems. Although methods may vary in the length of time over which the samples are obtained, essentially all of the methods use point samplers. Therefore, the results are spatially limited to the discrete locations of the sampling devices. In addition, when the concentrations are integrated over time, they are temporally limited to the length of the sample time. Limited spatial and temporal resolution is important because it reduces an industrial hygienist's ability to evaluate and control exposures to chemicals effectively. When sampling devices are placed in the breathing zones of workers, the results relate only to the physical location of the workers or to the paths that they travel during the sampling period. Industrial hygienists take these spatially limited results and assume they are representative of the larger unsampled workforce. This assumption may not always be valid; in practice, it is difficult to select a representative subset of individuals to sample because the concentration distributions in a room are unknown. Before choosing a subset of workers to sample, a larger homogeneous group is usually created based upon similarities in the tasks they perform and in the local environments in the rooms where they work. The local environment is important because contaminant flow patterns are strong determinants of exposure; however, environmental similarity is difficult to predict. Data on ventilation systems and airflow patterns are usually lacking and individuals selected for sampling may not be truly representative. An entirely new air monitoring technique, for both the occupational and environmental field, may provide spatially and temporally resolved estimates of contaminant concentrations noninvasively (does not pump air out of a space through collection media), and in real-time, over large areas. This technique combines the real-time chemical detection methods of optical remote sensing, such as an open-path Fourier transform infrared (OP-FTIR) spectrometer, with the mapping capabilities of computed tomography (CT). This environmental CT system generates near real-time spatially and temporally resolved two-dimensional concentration maps of multiple chemicals at low limits of detection (ppb–low ppm) for an entire area. Not just another nifty tool, this technology represents a major departure from conventional industrial hygiene air sampling methods and could allow researchers to understand and evaluate human exposures, source emissions, and chemical transport in ways that are unavailable using conventional methods. This technique provides a powerful tool for visualizing air contaminant species, concentrations, and flows in industry and outdoors in the community. Each tomographic concentration map provides a snapshot, which represents a short time period (minutes), of the concentration and location of contaminant plumes in a slice or plane through the air. As measurements are obtained over the day, the reconstructed concentration maps are linked together to provide a powerful tool for visualizing the flow of air contaminants over space and time. Keywords: Design; CT aligorithms; ORS geometrics; Field studies; Background optical remote sensing instruments; Challenges