Carcinogenicity of Sublimed Urethane in Mice through the Respiratory Tract

The carcinogenicity of sublimed urethane (ethyl carbamate) in air was examined with mice. JCL:ICR mice were nursed in a plastic cage inside a vinyl chamber which was ventilated 4 times per hour. The mice were exposed to urethane gas for various periods by passing air which contained a high concentration of sublimed urethane (1.29 μg/ml) into the vinyl chamber, or by placing a vessel containing crystalline urethane inside the vinyl chamber so that it was filled with spontaneously‐sublimed urethane gas at a low concentration (0.25 μg/ml). When female mice were killed 5 months after exposure, lung tumor frequency increased almost linearly with the number of days of exposure in the low concentration experiment, but increased in a non‐linear manner in the high concentration experiment. In terms of nearly the same total dose, i.e., (concentration of urethane gas in air) × (days of inhalation), one day of exposure to urethane gas at the low concentration induced lung tumors at a significantly higher frequency than 1/4 day of exposure to urethane gas at the high concentration. When male mice were killed at 12 months after exposure to examine the progressive change of induced tumors, malignant, invasive and metastatic tumors were found to have been induced more frequently in the lung after exposure to urethane gas at the low concentration (0.25 μg/ml for 10 days) than at the high concentration (1.29 μg/ml for 4 days), although the total dose in the former group was about half of that in the latter. Continuous exposure to urethane gas for a longer period at the low concentration seems to be more efficient for the induction, promotion and/or progression of lung tumors than the exposure for a shorter period at the high concentration.