In order to quantify the comparative clearance of chrysotile and the amphibole asbestos tremolite, both fibers were evaluated in an inhalation biopersistence study that followed the European Commission recommended guidelines. In addition, the histopathological response in the lung was evaluated following the short-term exposure.
In order to quantify the dynamics and rate by which these fibers are removed from the lung, the biopersistence of a sample of commercial grade chrysotile from the Coalinga mine in New Idria, CA, of the type Calidria RG144 and of a long-fiber tremolite were studied. For synthetic vitreous fibers, the biopersistence of the fibers longer than 20 µm has been found to be directly related to their potential to cause disease. This study was designed to determine lung clearance (biopersistence) and the histopathological response. As the long fibers have been shown to have the greatest potential for pathogenicity, the aerosol generation technique was designed to maximize the number of long respirable fibers. The chrysotile samples were specifically chosen to have 200 fibers/cm3 longer than 20 µm in length present in the exposure aerosol. These longer fibers were found to be largely composed of multiple shorter fibrils. The tremolite samples were chosen to have 100 fibers/cm3 longer than 20 µm in length present in the exposure aerosol. Calidria chrysotile fibers clear from the lung more rapidly (T1/2, fibers L>20 µm = 7 h) than any other commercial fiber tested including synthetic vitreous fibers. With such rapidly clearing fibers, the 5-day exposure would not be expected to result in any pathological change in the lung, and the lungs of animals that inhaled Calidria chrysotile showed no sign of inflammation or pathology and were no different than the lungs of those animals that breathed filtered air.