Subchronic inhalation toxicity of 3.5-microm diameter carbon fibers in rats.

No effects were seen when rats were exposed for 6 h day(-1), 5 days per week, for 16 weeks to an atmosphere of 20 mg m(-3) of carbon fibers (25 x 10(6) fibers m(-3)) and small amounts of fiber particulate resulting from preparation of the ultimate test material. The carbon fibers were made from polyacrylonitrile fiber. They were 3.5 microm in diameter and 72% were 10-60 microm long. Histopathological evaluations were made after each of 4, 8, 12 and 16 weeks of exposure and after 36 and 80 weeks of recovery. No effects due to the exposure were seen, as judged by clinical signs, body weight, organ weight, organ-body weight ratios, organ-brain weight ratios, gross and microscopic examination of internal organs, special stains of lung tissue for fibrous tissue, reticulin and fat and pulmonary function measurements. Non-fibrous particles were seen in the pulmonary lymphoid clearance system and in alveolar histiocytes (macrophages) at each histopathological evaluation period. Non-fibrous particles were seen in histiocytes in the mucociliary clearance system after 12 and 16 weeks of exposure. Fibers were seen in the nasal cavity at each histopathological examination. Fibers also occasionally were seen in tissue sections from the lower respiratory tract, but their exact location in life could not be determined. There was no indication of fibrosis.