Study Subjects

Study subjects were vermiculite miners, millers, and processors. Workers also may have been assigned jobs in the screening plant, railroad loading dock, expansion plants, or an office located in the town of Libby (several miles from the mine).

The cohort was enumerated in May 1982, and study subjects were followed through December 2001. The design allowed a minimum 20 years of followup since first exposure, with 65 years of follow-up for the earliest hired workers. Demographic and work history data were abstracted from company personnel and pay records.

A database created by the National Institute for Occupational Safety and Health (NIOSH) in the 1980s contained demographic data, work history, and vital status at the end of 1981 for 1,881 workers. The data were validated against company records on microfilm at NIOSH, and work history data were reabstracted.

One person was removed from the cohort because company records stated that he was hired but never worked. Nine workers with Social Security numbers listed in company records were excluded because demographic and work history data were not available, leaving 1,871 potential study subjects.