Abstract
A survey of occupational fiber exposures during vermiculite (1318009) mining and milling operations was conducted. The survey was intended to estimate exposures as part of a mortality and morbidity study of miners exposed to tremolite (14567738) and to estimate a ratio for converting respirable airborne dust exposure to fiber exposure. Air and bulk samples were analyzed for tremolite or actinolite (1332214) at a vermiculite mine (SIC-1499) and mill near Libby, Montana. Samples were collected over the period 1956 to 1982 by various groups including the state of Montana, NIOSH, and the Mine Safety and Health Administration, as well as by the company. Time weighted average exposures were computed for various job categories. Tremolite/actinolite exposures before 1964 averaged 182 fibers per cubic centimeter (f/cc) for sweepers, 88f/cc for skipping, and 13f/cc for the quality control laboratory. From 1964 to 1971, the corresponding exposures were 36, 17, and 3f/cc, respectively. Exposures during 1972 to 1976 decreased annually. Exposures during 1977 to 1982 in most mining areas were less than 1.0f/cc and in the mill ranged from 0.6 to 1.0f/cc. Ninety-six percent of 599 sized fiber samples had an aspect ratio greater than 10, 67 percent greater than 20, and 16 percent greater than 50. Seventy three percent of the fibers were longer than 10 microns, 36 percent longer than 20 microns, and 11 percent longer than 40 microns. All fibers counted had a width less than 2.49 microns. The authors conclude that fiber exposures were extremely high in the mine and mill before 1974.