FIBER BURDEN AND PATTERNS OF ASBESTOS-RELATED DISEASE IN WORKERS WITH HEAVY MIXED AMOSITE AND CHRYSOTILE EXPOSURE

To attempt to determine the mineralogic factors that relate to the appearance of specific types of asbestos-related disease in workers with heavy mixed exposure to amphiboles and chrysotile, we analyzed the pulmonary asbestos fiber burden in a series of 144 shipyard workers and insulators from the Pacific Northwest. Amosite was found in all lungs, and tremolite and chrysotile in most lungs, but the vast majority of fibers were amosite. Tremolite and chrysotile concentrations were significantly correlated, indicating that the tremolite originated from chrysotile products, but no correlation was found between tremolite or chrysotile concentration and amosite concentration. Time since last exposure was correlated with decreasing amosite concentration and the calculated clearance half time was about 20 yr. In a multiple regression analysis that accounted for the presence of more than one disease in many subjects, a high concentration of amosite fibers was correlated with the presence of airway fibrosis and asbestosis, whereas subjects with mesothelioma, lung cancer, pleural plaques, or no asbestos-related disease had about the same, much lower, amosite concentration. No relationship was found between the concentration of chrysotile or tremolite and any disease. Analysis of fiber size measures (length, width, aspect ratio, surface, mass) showed that pleural plaques were strongly associated with high aspect ratio amosite fibers and suggested that mesotheliomas were associated with low aspect ratio amosite fibers. We conclude that, in this population, the major residual fiber is amosite, and only amosite concentrations correlate with the presence of specific diseases, raising questions about the role of chrysotile in disease induction. There are distinct differences in the relationship of fiber burden and disease comparing workers with heavy amosite exposure to chrysotile miners and millers; in particular, mesothelioma appears at much lower amosite burdens than does asbestosis, in contrast to the situation previously reported for chrysotile-induced mesothelioma. Amosite clearance from the lung is extremely slow, requiring decades. Except for pleural plaques, the association of fiber size and disease remains uncertain.