In March 2004, the National Institute for Occupational Safety and Health (NIOSH) received a joint request for a Health Hazard Evaluation (HHE) from the management of U.S. Magnesium LLC, the United Steel Workers of America, and the Environmental Protection Agency (EPA). The requesters asked NIOSH to assess employee exposures to chlorinated hydrocarbons (CHCs), including chlorinated dibenzo-p-dioxins (PCDDs), chlorinated dibenzofurans (PCDFs), and polychlorinated biphenyls (PCBs), certain pesticides (e.g., hexachlorobenzene [HCB]), and chlorine, all generated as byproducts during magnesium production at the U.S. Magnesium plant in Rowley, Utah. NIOSH investigators conducted a walk-through survey of the facility in May 2004, and made return visits in August and November 2004. The HHE involved quantitative exposure assessments, employee medical and occupational history questionnaires, and biological monitoring. NIOSH investigators determined blood levels of CHCs in a sample of longest-tenured workers in specific areas where CHCs were likely generated. Several measured chlorine exposures exceeded the NIOSH ceiling Recommended Exposure Level (REL) of 0.5 part per million (ppm) with peak exposures between 10 and 50 ppm. Carbon tetrachloride (CCl4), known to be generated in the production process, was measured by personal breathing zone (PBZ) samples, and exposures were mostly very low. Hand wipes and surface samples found evidence of HCB (from 0.14 to 3.5 micrograms [µg]) and indicated the potential for dermal exposure. Of forty-two PBZ and two area air samples collected for HCB and PCBs, five exceeded the exposure criterion (REL). Full-shift air sampling results for HCB ranged from 0.096 micrograms per cubic meter (µg/m3) for a sample collected on a foundry operator to 5.3 µg/m3 for a maintenance helper working on the sixth floor reactor. In total, 5 of 42 PBZ samples (or approximately 12% of the total sample set) exceeded or were very close to an adjusted Threshold Limit Value for HCB. Air sample results indicate that certain congeners of PCB were present in workplace air above the minimum detection concentration (MDC). Bulk sample results indicated the presence of HCB at 250 micrograms per gram (µg/gr) and three congeners of PCB in collected dust. Nine of the twelve dioxin-like PCB congeners with assigned TEFs (dioxin-like PCB congeners have been assigned 2,3,7,8-TCDD Toxicity Equivalency Factors [TEFs], indicating their toxicity relative to 2,3,7,8-TCDD, which itself has been assigned a TEF of 1.0) were detected, including congener numbers 77, 105, 114, 123, 126, 156, 167, 169, and 180. Congener 20, which does not have an assigned TEF, was also detected. The 10 congeners that were detected ranged from trace (between the limit of detection [LOD] and limit of quantification [LOQ]) to 2 µg/sample for congener #209. Since only the Arochlor series of PCB have occupational exposure criteria, these results indicate that certain congeners of PCB are present in workplace air above the MDC; however, the meaning in terms of health risks is unclear. Of the 30 workers interviewed, 60% reported headaches and 80% reported having had acute upper respiratory symptoms from exposure to chlorine gas at some time during their employment. The blood sample results of the workers revealed that levels of PCDDs and PCDFs were well below levels reported in association with observable health effects. We compared the 30 workers' average blood levels for CHC using the World Health Organization-toxic equivalency quotient (WHO-TEQ 98) and found the average level in the 30 workers was higher than the level found in the general population. The workers' mean blood level of 2,3,7,8-TCDD was well below the level found to be associated with observable health effects in all published studies. However, 2,3,7,8-TCDD and PCBs are considered to be potential human carcinogens, and a no-risk threshold for human exposures does not exist. The blood HCB levels were higher than the general population, but studies have not found observable clinical health problems at these levels. NIOSH investigators conclude that biological and environmental monitoring results show evidence of work-related exposure to dioxins, dibenzofurans, PCBs, HCB, and chlorine. Because health-based biological exposure indices are not available for all of these compounds (even less information is available for the combination of exposures), health risk consequences are unclear. Industrial hygiene monitoring results found the areas of the plant with the greatest risks for HCB and PCB exposures were the reactor building and the electrolytics area. Reactor maintenance workers, particularly less experienced workers, had the highest exposures. Respirator use throughout the areas evaluated was observed to be sporadic at best. Sampling results found peak chlorine exposures at greater than 10 and up to 50 ppm, which represents a serious risk for unprotected workers. NIOSH investigators determined that an occupational health hazard due to exposures to chlorinated hydrocarbons including chlorinated dibenzo-p-dioxins (PCDDs), chlorinated dibenzofurans (PCDFs), and polychlorinated biphenyls (PCBs); certain pesticides (e.g., hexachlorobenzene [HCB]); and chlorine existed for workers at U.S. Magnesium. Recommendations for controlling workplace exposures include identifying fugitive emissions and inadequately controlled processes, controlling emissions by engineering controls, enhancing local exhaust ventilation, improving housekeeping, and enforcing the use of personal protective equipment. Additional recommendations are included at the end of this report.