HORI Hajime Univ. of Occup. & Environ. Health, School of Medicine, Associate Professor, 医学部, 助教授
HIGASHI Toshiaki Univ. of Occup. & Environ. Health, School of Medicine, Associate Professor, 医学部, 助教授
Pulmonary deposition and the clearance of deposited particles from lungs are very important factors to induce pneumoconioses and tumors.
In this experiment, 42 Wistar male and 45 Wistar female rats were exposed to glass fiber (GB-100R, Advantec Toyo, Co., Japan) (content : SiO_2 63 %) for 6 hrs/day, 5 days/week for 4 weeks, and for chronic inhalation toxicity test, 27 Wistar male rats were also exposed to glass fiber for 1 year. The mass median aerodynamic diameter was 2.7 mum by an Andersen sampler, and the count median diameter and length determined by a scanning electron microscope were 0.51 mum and 5.5 mum, respectively. The daily average exposure concentration was controlled by a continuous fuluidized bed with a screw feeder and an overflow pipe at 10.8 <plus-minus> 3.3 and 9.8 <plus-minus> 3.0 mg/m^3 during the exposure period in the male group and female group for 4-week exposure, respectively and 2.2 <plus-minus> 0.6 mg/m^3 for 1-year exposure. The fibrous particle concentration
s in the exposure chamber were monitored by a light scattering method and were constant during the exposure.
The rats were sacrificed at 24 hours after 1, 2, 3 and 4 weeks exposure and after 3.5, 5.5, 6 and 12 months clearance period following a 4-week exposure. For chronic effect of glass fiber on lung deposition, they were killed just after the 1-year exposure and after 1 year clearance period following a 1-year exposure. The wet lung weight and the silica concentration in thelungs were measured and the lungs were treated for low temperature ashing (ca. 150 ﾟC) by using a plasma asher.
After ashing, these samples were melted with sodium carbonate in a platinum pot to measure the silicon content by the absorption spectrophotometry.
The amount of deposited silicon in rat lungs by exposure to glass fiber was calculated from the difference value of measured silicon content between the exposed rats and controls. The deposited amount of glass fiber which was observed up to 300 ug in the 4-week exposed male rats, was proportional to the exposure period. For chronic exposure, the deposited amount of glass fiber was observed 1.5 mg and 0.6 mg after the 1-year clearance just after 1-year exposure. The apparent deposition fraction which is defined as the deposited amount in the lungs to the amount of the inhaled glass fiber during the exposure, was 4.5 % and 4.7 % in male and female, respectively. There was no significant difference in the apparent deposition fraction between male and female rats.
During the clearance period, the deposited silicon content decreased with the increasing of the clearance period. The biological half time in the one-compartment model was determined to be 1.5 months in male and female rats ; for 4-week exposure, but 9.2 months for 1-year exposure. The biological half time increased with the increasing of the exposure period. The cause of this may be decrease of clearance activity by fiber deposition, however, there were no malignant tumor in this experiment. 10. KEY WORDS Less