Background and aims: In tungsten refining and manufacturing processes, a series of tungsten oxides (WOx) are typically formed as intermediates in the production of tungsten powder. Studies in the Swedish tungsten refining and manufacturing industry have shown that intermediate tungsten refining processes can create WOx fibers. The present study was conducted to characterize airborne tungsten containing fiber dimensions, elemental composition, and concentrations in the U.S. tungsten refining and manufacturing industry. Methods: During the course of normal employee work activities, seven personal breathing zone and 62 area air samples were collected and analyzed using standard fiber sampling and counting methods to determine dimensions, composition, and airborne concentrations of fibers. Mixed models were used to identify relationships between potential determinants and airborne fiber concentrations. Results: Results from transmission electron microscopy analyses conducted indicate that airborne fibers with length > 0.5 um, diameter > 0.01 um, and aspect ratios > 3:1 were present on 35 of the 69 air samples collected. Overall, the airborne fibers detected had a geometric mean (GM) length of approximately 3 um, and GM diameter of approximately 0.3 um. Ninety-seven percent of the airborne fibers identified had an aerodynamic diameter < 10 um, indicating that they were capable of reaching the thoracic regions. Energy dispersive x-ray spectrometry results indicate that airborne fibers prior to the carburization process consisted primarily of tungsten and oxygen, with other elements being detected in trace quantities. Based on NIOSH fiber counting "B" rules (length > 5 um, diameter < 3 um, aspect ratio > 5:1), airborne fiber concentrations ranged from below the limit of detection to 0.085 f/cc, with calcining being associated with the highest airborne concentrations. The mixed model procedure indicated that process temperature had a marginally significant relationship to airborne fiber concentration. Discussion and conclusions: The finding that temperature was marginally significant was unexpected, since heated processes such as calcining created the highest airborne fiber concentrations. Until more is known about the durability and potential health effects associated with airborne tungsten-containing fibers, it would be prudent to take steps to limit or eliminate occupational exposures.