AbstractThe vermiculite ore and concentrate of a mine and mill near Libby, Montana, was found to be contaminated with fibrous tremolite‐actinolite. Of 599 fibers (length greater than 5 μm and width greater than 0.45 μm) counted in eight airborne membrane filter samples, 96% had an aspect ratio greater than 10 and 16% had an aspect ratio greater than 50. Additionally, 73% of the fibers were longer than 10 μm, 36% were longer than 20 μm, and 10% were longer than 40 μm. Estimates of exposure before 1964 in the dry mill were 168 fibers/cc for working areas, 182 fibers/cc for sweepers, 88 fibers/cc for skipping, and 13 fibers/cc for the quality control laboratory. In 1964–1971, exposure estimates for these areas were 33, 36, 17, and 3 fibers/cc, respectively. Estimates of exposures in the mine before 1971 ranged from 9–23 fibers/cc for drillers and were less than 2 fibers/cc for nondrilling jobs. All 8‐hr TWA job exposure estimates decreased from 1972‐1976, and from 1977‐1982 were less