Lung tumor induction upon long‐term low‐level inhalation of crystalline Silica

F-344 rats were exposed 6 hours/day, 5 days/week for up to 24 months to 1mg/cubicmeter SiO2 or 5 mg/cubicmeter TiO2. A third group of rats inhaled filtered air. No treatment-related effects on life span or causes of deaths were observed. The final sacrifices started 6 weeks after the end of the exposure interval. The lung weight of the silica-exposed animals doubled, while the TiO2-treated animals had lung weights similar to those of the control group. An incidence of 18 % of lung tumors was observed in the silica exposed animals. Tumors were first observed after 21 months in the study. The incidence of lung tumors in the control and TiO2-exposed groups was 2-3%. The significance of the results is that the silica-induced tumors were detected after a relatively low exposure level. The small particle size with corresponding large surface area of the silica used may be one factor for the increased tumor rate observed.