Vermiculite, Respiratory Disease, and Asbestos Exposure in Libby, Montana: Update of a Cohort Mortality Study

Vermiculite is a naturally occurring mineral mined in the United States, Brazil, Argentina, Mexico, South Africa, Zimbabwe, Kenya, Uganda, Egypt, India, Russia, China, Japan, and Australia. Mined vermiculite ore is milled to produce vermiculite concentrate of various sizes and grades. When rapidly heated, vermiculite concentrate expands to form small, light-weight, accordion-shaped granules. Vermiculite is used in construction products (loose-fill attic insulation, acoustic finishes, spray-on fireproofing, gypsum plaster, concrete mixes for swimming pools), consumer products (packing materials, adsorbent in laboratories), agricultural and horticultural products (animal feed, bulking agent, fertilizers, pesticides, seed encapsulant, hydroponics, potting mixes, soil conditioners), and in industrial products (brake shoes and pads, drilling muds, furnaces, filters, insulator blocks, paints, and sealants) [U.S. Environmental Protection Agency (EPA) 2006]. Vermiculite from the mine that operated near Libby, Montana, from the early 1920s until 1990 was contaminated with asbestos and other fibrous amphibole minerals, crystalline silica, and talc. The U.S. Geological Survey has characterized the respirable fraction of asbestiform amphiboles contaminating the Libby vermiculite as approximately 84% winchite, 11% richterite, and 6% tremolite (Meeker et al. 2003). The raw Libby ore was estimated to be 21–26% asbestos by weight; the mill feed was 3.5–6.4% asbestos; airborne dust in the dry mill was 40% asbestos (Wake 1962); and the vermiculite concentrate shipped to numerous processing plants in the United States and other countries was 0.3–7.0% asbestos before expansion (Amandus et al. 1987; Atkinson et al. 1981). Previous studies of Libby workers documented increased risk of lung cancer and nonmalignant respiratory disease among highly exposed workers with at least 1 year tenure (Amandus and Wheeler 1987; McDonald et al. 1986, 2004). Reports of respiratory disease mortality among community residents and household contacts of Libby vermiculite workers suggested increased risk from transient exposure or ambient community exposure [Agency for Toxic Substances and Disease Registry (ATSDR) 2002; Schneider 1999]. Cross-sectional radiographic screening conducted in Libby for the ATSDR revealed that 6.7% of community residents with no occupational or familial exposure have radiographic evidence of asbestos-related disease (Peipins et al. 2003). These findings suggest that risk from asbestos-contaminated vermiculite may not be limited to those with high-intensity occupational exposure. This report expands the previously studied occupational cohort to include all white men hired at Libby from September 1935 through December 1981. The intent here is to describe the mortality experience of workers exposed to Libby amphibole fibers (tremolite asbestos, winchite, richterite) over the full range of exposure and employment duration. Occupational respiratory disease mortality among Libby workers is compared to that expected based on the mortality experience of the U.S. population, and standardized mortality ratios (SMRs) and standardized rate ratios (SRRs) for asbestosis, lung cancer, and all nonmalignant respiratory diseases are presented.