Presence of Airborne Fibers in Tungsten Refining and Manufacturing Processes: Preliminary Characterization

Abstract

In tungsten refining and manufacturing processes, a series of tungsten oxides (WO _X ) 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 WO_X fibers. The purpose of the present study was to identify and provide a preliminary characterization of airborne tungsten-containing fiber dimensions, elemental composition, and concentrations in the U.S. tungsten refining and manufacturing industry. To provide the preliminary characterization, 10 static air samples were collected during the course of normal employee work activities and analyzed using standard fiber sampling and counting methods. Results from transmission electron microscopy analyses conducted indicate that airborne fibers with length > 0.5 μ m, diameter > 0.01 μ m, and aspect ratio ≥ 3:1, with a geometric mean (GM) length of ∼ 2.0 μ m and GM diameter of ∼ 0.25 μ m, were present on 9 of the 10 air samples collected. 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. Results from an air sample collected at the carburization process indicated the presence of fibers composed primarily of tungsten with oxygen and carbon, and traces of other elements. Based on National Institute for Occupational Safety and Health standard fiber counting rules, airborne fiber concentrations ranged from below the limit of detection to 0.14 f/cm ³ . The calcining process was associated with the highest airborne fiber concentrations. More than 99% (574/578) of the airborne fibers identified had an aerodynamic diameter ≤10 μ m, indicating that they were capable of reaching the thoracic regions. 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.

Keywords:

• electron microscopy
• hard-metal manufacturing
• occupational exposure
• thoracic
• tungsten blue oxide
• tungsten oxide bronze

ACKNOWLEDGMENTS

This research was supported in part by the National Toxicology Program through Interagency Agreement number Y1-ES-9045-10 with the National Institute for Occupational Safety and Health.

The authors gratefully acknowledge the support and contributions of employees and management at the facility surveyed. The authors also recognize Paul Baron, David Dankovic, Margaret Quinn, Dan Crane, Cheryl Estill, and Lauralynn Taylor McKernan for their constructive manuscript review, guidance, and comments. Thanks also are extended to Martin Petersen, Misty Hein, and James Deddens for providing statistical consultation.

The findings and conclusions in this report are those of the authors and do not necessarily represent the views of the National Institute for Occupational Safety and Health.

Notes

^A = The recommended quantitative working range of NMAM 7400 is 0.04 to 0.5 f/cm³ for a 1000-L air sample with a preferable fiber concentration range of 100 to 1300 f/mm² on the media.^{( 28 )}

^B = Limit of detection; calculated based on NMAM 7402, 1 confirmed f/mm² above 95% of expected mean blank value.^{( 29 )}

^C = NMAM 7400 counting rule “A.”^{( 28 )}

^D = NMAM 7400 counting rule “B.”^{( 28 )}

^E = Using NMAM 7400.^{( 28 )}

^F = Elemental content of fibers with L ≤ 5 μ m.

^G = Estimated airborne concentrations using MLE method.^{( 30 )} When less than half of the sample results were above the LOD, only the range and number of detectable samples were reported.

^H = Airborne concentration for up to a 10-hr work shift during a 40-hr workweek.

^I = Airborne concentration for a conventional 8-hr workday and a 40-hr workweek.

^A Orientation averaged.

^C AM = Arithmetic mean.

^D SD = Standard deviation.

^E GM = Geometric mean.

^F GSD = Geometric standard deviation.