In the early 1970s, it became a concern that exposure to the
mineral fibers associated
taconite ore processed in
Silver Bay, Minnesota would cause
asbestos-related disease including
gastrointestinal cancer. At that time data gaps existed which have now been significantly reduced by further research. To further our understanding of the types of airborne fibers in
Silver Bay we undertook a geological survey of their source the Peter Mitchell Pit, and found that there are no primary
asbestos minerals at a detectable level. However we identified two non-
asbestos types of fibrous minerals in very limited geological locales. Air sampling useful for risk assessment was done to determine the type, concentrations and size distribution of the population of airborne fibers around
Silver Bay. Approximately 80% of the airborne fibers have elemental compositions consistent with
cummingtonite-grunerite and the remaining 20% have elemental compositions in the
tremolite-
actinolite series. The mean airborne concentration of both fiber types is less than 0.00014 fibers per milliliter that is within the background level reported by the World Health Organization. We calculate the risk of
asbestos-related
mesothelioma and
lung cancer using a variety of different pessimistic assumptions. (i) that all the non-
asbestos fibers are as potent as
asbestos fibers used in the EPA-IRIS listing for
asbestos; with a calculated risk of
asbestos-related
cancer for environmental exposure at
Silver Bay of 1 excess
cancer in 28,500 lifetimes (or 35 excess
cancers per 1,000,000 lifetimes) and secondly that
taconite associated fibers are as potent as
chrysotile the least potent form of
asbestos. The calculated risk is less than 0.77 excess
cancer case in 1,000,000 lifetimes. Finally, we briefly review the epidemiology studies of grunerite
asbestos (amosite) focusing on the exposure conditions associated with increased risk of human
mesothelioma.