Maintenace of Stellite and Tungsten Carbide Saw Tips: Determinants of Exposure to Cobalt and Chromium

Abstract

This study measured the exposure of saw filers in lumber mills to cobalt and chromium, which are present in tungsten carbide, saw steel, and stellite used in saw blade tips. It also determined which work locations, activities, and jobs were related to elevated air concentrations. The study was conducted at eight lumber mills, each visited on four occasions at least one month apart. All consenting filers had personal full-shift air samples taken and analyzed. Observations of tasks, locations, and activities were made at 10-minute intervals. Supplementary data was collected about saw tip composition, local exhaust ventilation, and concentrations of metals in grinding coolants. A total of 278 air samples were collected from 112 saw filers. Cobalt concentrations ranged from less than the detection limit (0.64 µg/m3) to 106 µg/m3. Chromium concentrations ranged from less than the detection limit (0.74 µg/m3) to 55 µg/m3. For both metals the majority of measurements were below detection limits. A stepwise multiple linear regression method without log transformation of the metal concentrations, and therefore using robust variance estimates, was chosen to identify the determinants of exposure. Exposures to cobalt were positively associated with wet and dry grinding of tungsten carbide but not stellite saw tips. Bulk samples of grinding coolant used in tungsten carbide grinding machines often had concentrations of cobalt an order of magnitude higher than coolant used in stellite machines. Chromium concentrations were positively associated with wet grinding of chipper knives made of chromium-containing steel, and to a lesser extent with welding of stellite and saw steel. Results show that airborne exposures may not be predicted by the amount of metal in the parent material. Standard hygiene control methods like wet grinding may not reduce exposures. The rationale for and limitations of the measurement and statistical methods are discussed.