Clearance of Deposited Silicon Carbide Whisker from Rat Lungs Inhaled during a 4-Week Exposure

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Pulmonary deposition and the clearance of deposited fiber particles from lungs are key determinant factors in assessing potential carcinogenicity and fibrogenicity. Forty-two Wistar male rats (9 wk old) were exposed to silicon carbide whisker (SiCW) for 6 h/d, 5 d/wk for 4 wk. The mass median aerodynamic diameter was 2.5 pm as detected by an Andersen sampler, and the geometric mean fiber diameter and length determined by a scanning electron microscope were 0.4 and 2.2 pm, respectively. The daily average exposure concentrations were 10.4 ± 0.5 mg/m^3 (214 ± 31 fibers/ml) during the exposure period. The rats were sacrificed after 3 d, 2 wk, 1, 2, 3, 6 and 12 months after 4-wk exposure. At the time of sacrifice, the wet lung weights and the deposited SiCW amounts in the lungs were measured by an X ray reference method. The amount of SiCW deposited was 0.60 ± 0.09 mg 3 d after a 4-wk exposure. The apparent deposition fraction was 4.8 ± 0.7 (%). During the clearance period, the amount of SiCW deposited in the rat lungs decreased exponentially with the increasing duration of the clearance period. The biological half time in the one-compartment model was determined to be 4.0 months which is similar to the values for glass fiber, potassium titanate whisker and aluminium-silicate ceramic fiber under similar exposure conditions.
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