In a 1300-MW(electric) pressurized water reactor power plant, the sources of the airborne 131I species were determined over a period of five months. During power operation, the main source of the radio-logically decisive elemental 131I was the exhaust from the hoods in which samples from the primary coolant are taken and processed. During refueling outage, elemental 131I was mainly contributed by the containment purge air. By efficient filtration of these exhausts, a reduction of the ingestion dose, caused by the total 131I stack release, by a factor of nearly 4 during power operation and of possibly 10 during refueling outage can be accomplished.