Investigation of a potential cotumorigenic effect of the dioxides of nitrogen and sulfur, and of Diesel-engine exhaust, on the respiratory tract of Syrian golden hamsters
Syrian golden hamsters (480 males and 480 females) allocated into 24 groups were exposed 19 hours per day and 5 days per week for 6, 10.5, 15, or 18 months to total diesel exhaust, diesel exhaust without particles, a mixture of nitrogen-dioxide (5 parts per million (ppm) high 2) and sulfur dioxide (10ppm), or clean air. Two exposure groups from each test atmosphere were also treated by a single subcutaneous injection of either 3 mg or 6mg of diethylnitrosamine/kg of body weight to evaluate an enhancing effect of diethylnitrosamine on exposure-related changes. Morphologicel evaluation was done by histopathology. Minor changes of the larynx and trachea were investigated by scanning electron miroscopy, which showed a loss of ciliated cells in all exhaust-exposed groups. After exposure to diesel exhaust with or without particles, focal metaplasia and dysplasia of the respiratory epithelium were seen in the oldest animals by scanning electron microscopy. In the same specimens, attached muco us droplets indicated changes in mucous cells and mucous viscosity. Only the exposure to total diesel exhaust significantly increased the tumor rate in the upper respiratory tract of male hamsters treated with 6mg of diethylnitrosamine per kg of body weight. At the lower diethylnitrosamine dose no exposure-related effects on the tumor rates could be observed. The results appear to be insufficiently conclusive to demonstrate that diesel-engine exhaust should be classified as a cocarcinogen or enhancer for the test system used.