The differences between
chrysotile asbestos, a
serpentine mineral, and
amphibole asbestos have been debated extensively. Many studies have shown that
chrysotile is cleared from the lung more rapidly than
amphibole. 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. This article presents the results of this study through 90 days after cessation of exposure. Following the termination of the study, a subsequent article will provide the complete results through 12 mo after cessation of 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 microm 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 microm 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 microm in length present in the exposure
aerosol. Calidria
chrysotile fibers clear from the lung more rapidly (T1/2, fibers L > 20 microm = 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. Following this 5-day exposure to
tremolite, the
tremolite fibers once deposited in the lung parenchyma do not clear and almost immediately result in
inflammation and a pathological response in the lung. At the first time point examined, 1 day after cessation of exposure,
inflammation was observed and
granulomas were already formed. By 14 days postexposure these microgranulomas had turned fibrotic, and by 90 days postexposure the severity of the
collagen deposits had increased and interstitial
fibrosis was observed in one of the rats. These findings provide an important basis for substantiating both kinetically and pathologically the differences between
chrysotile and the
amphibole tremolite. As Calidria
chrysotile has been certified to have no
tremolite fiber, the results of the current study together with the results from toxicological and epidemiological studies indicate that this fiber is not associated with
lung disease.