Initiating the risk assessment process for inhaled particulate materials: development of short term inhalation bioassays.

This study describes a short term inhalation bioassay in rats to predict the potential for inhaled particles to produce chronic lung disease in humans (e.g., pulmonary fibrosis). To validate the method, rats were exposed for 6 h or 3 days to various concentrations of two reference materials: (1) a known fibrogenic material (i.e., aerosolized alpha-quartz silica particles in the form of Berkeley Min-U-Sil (Pennsylvania Glass and Sand Company, Pittsburgh, PA), or (2) carbonyl iron (CI) particles, as a negative control. Cells and fluids from groups of sham and dust exposed animals were recovered by bronchoalveolar lavage (BAL). Alkaline phosphatase, lactate dehydrogenase and protein values were measured in BAL fluids at several times postexposure. Cells were identified, counted, and evaluated for viability. The lungs of additional exposed animals were processed for histopathology. Although particle deposition patterns for the two dusts were similar, brief exposures to silica particles produced a persistent pulmonary inflammatory response characterized by neutrophil recruitment at sites of particle deposition and consistently elevated biomarkers of cytotoxicity in BAL fluids. In addition, alveolar macrophage clearance functions were impaired. Progressive histopathologic lesions were observed within 1 mo after a 3-day exposure. Light and electron microscopy of silica exposed lung tissue revealed a chronically active pulmonary inflammatory response characterized by hyperplasia of Type II alveolar epithelial cells and the infiltration of macrophages and neutrophils into alveoli and interstitial compartments. The lesions were progressive, leading to the development of a multifocal, granulomatous-type pneumonitis within 2 mo postexposure. In contrast to the observed effects of silica, 3-day exposures to CI particles produced no significant adverse biochemical or histopathological effects on pulmonary tissues. These results demonstrate that short term, high dose inhalation exposures of silica produce effects similar to those previously observed using intratracheal instillation or chronic inhalation models and lend support to this method as a reliable short term bioassay for evaluating the pulmonary toxicity and mechanisms associated with exposure to new and untested respirable materials.