LIBBY AMPHIBOLE: UNIQUE GEOLOGY, UNIQUE DISEASE

In rural northwestern Montana, vermiculite was mined from 1919 to 1990. Hundreds of thousands of tons of ore was processed from a mine 6 miles outside of Libby, MT. Vermiculite ore was used for many purposes locally and it was transported by rail and processed throughout the USA and Canada. This vermiculite deposit contained unique asbestiform minerals as well. Originally thought to be tremolite Libby’s asbestiform minerals were later reclassified as a solid solution of primarily winchite (84%) richterite (10%) and tremolite (5%).

Asbestiform fiber contamination of vermiculite was observed to cause asbestos related disease (ARD) in vermiculite workers, but in the 1990s families of workers as well as those exposed environmentally were becoming ill with ARD. This led to a Seattle PI newspaper article in 1999 which publically revealed the probability of a broad public health problem related to mining, processing and transporting the Libby ore.

An initial asbestos screening program was carried out by ATSDR in 2000-2001. This screening revealed that 18-19% of the 7,100 previous or existing Libby residents who participated had radiographic changes consistent with asbestos exposure. This cohort has since been followed for asbestos health impacts by the Center for Asbestos Related Disease. Observed health effects from this exposure shows unique characteristics compared to commercial asbestos exposures.

The unique features associated with exposure to Libby fibers include radiographic findings of progressive pleural disease with little to no lung scarring. Progression of pleural disease can be associated with very rapid progression with patients moving from normal functioning to advanced disease in as little as 3 -5 years. Additionally a severe level of chest pain is sometimes associated with the pleural disease.

These unique health effects have prompted epidemiological cohort comparison studies between those exposed to Libby fibers and those exposed to chrysotile asbestos. Preliminary results suggest a significant prevalence of autoimmune antibodies from Libby exposure that is not seen with exposure to chrysotile. Both ongoing and planned future studies will be extremely valuable for validation of these observations.