Exposure Assessment

The mining, milling and processing operations at Libby; conditions of exposure; and job-specific estimates of asbestos exposure intensity have been thoroughly described previously (Amandus and Wheeler 1987; Amandus et al. 1987, 1988; McDonald et al. 1986).

Briefly, miners extracted vermiculite ore from an open-pit mine. The ore was processed in a dry mill (1935–1976) and/or two wet mills (1955–1990) that operated on Vermiculite Mountain. The resulting concentrate was shipped by railroad to processing plants where the vermiculite was expanded for use in loose-fill attic insulation.

Before 1975, exposures in the mine ranged from 9–23 fibers per cubic centimeter (fibers/cc) of air for drillers; exposures in other mining jobs were estimated to be < 2 fibers/cc.

Early fiber exposures in the dry mill were as high as 182 fibers/cc during sweeping operations; by 1964, exposures in the mill had been reduced by 80%. Amandus et al. (1987) estimated that by 1972 exposures in all work areas were < 1 fiber/cc as an 8-hr time-weighted average, compared with today’s Occupational Safety and Health Administration (OSHA) asbestos standard of 0.1 fiber/cc.