Calibration study on subchronic inhalation toxicity of man-made vitreous fibers in rats.

This 3-mo inhalation study investigated the biological effects of a special-purpose glass microfiber (E-glass microfiber), the stone wool fiber MMVF21, and a new high-temperature application fiber (calcium-magnesium-silicate fiber, CMS) in Wistar rats. Rats were exposed 6 h/day, 5 days/wk for 3 mo to fiber aerosol concentrations of approximately 15, 50, and 150 fibers/ml (fiber length >20 microm) for E-glass microfiber and MMVF21. For the CMS fiber only the highest exposure concentration was used. During a 3-mo postexposure period, recovery effects were studied. In the highest exposure concentration groups, gravimetric concentrations were 17.2 mg/m3 for E-glass microfiber, 37 mg/m3 for MMVF21, and 49.5 mg/m3 for the CMS fiber. After 3 mo of exposure, lung retention of fibers longer than 20 micro m per lung was 17 x 10(6) for E-glass microfiber, 5.7 x 10(6) for MMVF21, and 0.88 x 10(6) for CMS. After 3 mo of recovery the concentration of the long fiber fraction was decreased to 38.4%, 63.9%, and 3.0% compared to original lung burden for the E-glass microfiber, MMVF21, and CMS, respectively. Biological effects measured included inflammatory and proliferative potential, histopathology lesions, and the persistence of these effects over a recovery period of 3 mo. Generally, observed effects were higher for E-glass microfiber when compared to MMVF21. The following clear dose-dependent effects on E-glass microfiber and MMVF21 exposure were observed as main findings of the study: increase in lung weight, in measured biochemical parameters and polymorphonuclear leukocytes (PMN) in the bronchoalveolar lavage fluid (BALF), in cell proliferation (BrdU-response) of terminal bronchiolar epithelium, and in interstitial fibrosis. The values observed in the proliferation assay on the carcinogenic E-glass microfiber indicate that this assay has an important predictive value with regards to potential carcinogenicity. Surprisingly, for the biosoluble CMS fiber, fibrogenic potential was detected in this study. The results of the CMS exposure group indicate that effects may be dominated by the presence of nonfibrous particles and that fibrosis may not be a predictor of carcinogenic activity of fiber samples, if the fiber preparation contains a significant fraction of nonfibrous particles. In summary, this study demonstrates the importance of fiber dust contamination by granular components. For future subchronic studies a longer posttreatment observation period would be advisable.