[en] In production of 20 Ci ⁹⁹Mo from ²³⁵U fission, about 120 Ci of radioiodine (¹³¹I, ¹³²I, and ¹³³I) is involved. To remove airborne radioiodine from the exhaust air from production apparatus and minimize radioiodine release to the atmosphere, the ventilation system is equipped with 2 units of Model-FD charcoal filter (KI₃-Impregnated charcoal 2 inch thick of Barnebey-Cheney Co.). From September 1976 to December 1977, 21 runs of ⁹⁹Mo production involving airborne radioiodine were carried out. The ventilation system was operated continuously for the whole 15 months period; variation in removal efficiency of airborne radioiodine from the exhaust air stream was observed. In the runs valuable experiences were gained in operation and maintenance of the ventilation system including activated charcoal filter and health-physics management of such facility. Following are the results: (1) Airborne radioiodine from ⁹⁹Mo production apparatus is reduced to 10^-3% of the original quantity. (2) When the ventilation system is operated at a maximum air flow rate through the filter, the average efficiency during 15 months is over 98%. (3) Airborne radioiodine released from ⁹⁹Mo production apparatus to the ventilation system is less than 5% particulate iodine and alkyl iodines and more than 95% inorganic iodine. (4) Airborne radioiodine released from the stack is less than 28 μCi/run, which is below the limit in regulations on Radioisotope Production Laboratory. (auth.)