AN OVERVIEW OF THE PAST, PRESENT, AND FUTURE CONCERNS SURROUNDING THE ASBESTOS CONTENT OF VERMICULITE ORE FROM LIBBY, MONTANA

As we meet this November, we mark five years since a small town in northwestern Montana was placed in the national spotlight as the newest “asbestos problem” in the USA. The problem originates from the asbestos content of the vermiculite ore mined near Libby. Unfortunately, the geological conditions that formed this world-class vermiculite deposit by alteration of existing biotite in an ultramafic intrusion also altered the pyroxenes to amphibole and amphibole-asbestos. The vermiculite mine began production in the early 1920’s and ceased operation in 1990. During that time, it was one of the world’s largest vermiculite mines. After mining, the vermiculite ore was expanded by rapid heating. This expanded product has several industrial uses, most commonly in gardening products; however, the commercial product produced from the Libby ore, termed Zonolite, was also used as home attic insulation. The asbestos content of this product and the vermiculite ore have been measured as high as a few percent, but more often is considerably less. Interestingly, the majority of the amphibole-asbestos at the former mine are the non-regulated amphibole species winchite and richterite. The major health concern was initially focused on the few hundred miners. Studies in the mid-1980’s showed their death rates were approximately 2.5 times higher than expected for asbestosis, mesothleioma, and lung cancer. Five years ago, concern shifted to the few thousand residents of the community and most recently to homeowners across the USA who have Zonolite as an attic insulation. The popular media has estimated as many as 35 million homes in the USA may contain Zonolite; however, common-sense dictates these estimates are far too high because there are fewer than 65 million homes in the USA. Regardless, it is clear that individuals with occupational exposure in the duster operations at the mine suffered increase asbestos-related deaths. The health risk becomes less clear for non-occupational exposure of the Libby residents. Finally, it would seem unlikely that Zonolite attic insulation would pose a significant health risk to a homeowner because exposure levels decrease by orders of magnitude between the miners and the homeowners.