Clearance of deposited silicon carbide whisker from rat lungs inhaled during a 4-week exposure

Abstract

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 μm 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 μm, respectively. The daily average exposure concentrations were 10.4 ± 0.5 mg/m3 (214 ± 31 fibers/ m/) 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 onecompartment 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.