[en] The inhalation of small activities of Iodine-131 during radioiodine therapy seems to be unavoidable. According to radiation protection procedures measurement of radioiodine in air and regularly incorporation measurements have to be carried out. By routine incorporation measurements using a shadow shield whole body counter in the Donauspital Vienna, usually only few persons (30 %) with incorporated I-131 activities are detected. The measured activities are low (< 1 kBq). In the third quarter of 1996 a dramatic increase of positive scan results has been found. At this time also higher activities (up to 17 kBq) were reported. The results of external dosimetry showed no increase of radiation dosage. Some amount of activities has been detected in the containment of the therapeutic capsules. Therefore measurements of iodine-131 in the air have been carried out. Activities of more than 10 kBq/m³ have been found. The activity in the atmosphere is the major reason for the positive scan results. After the improvement of the ventilation system the activities detected showed a remarkable reduction of the measured quantities, but the number of persons with positive scan results remained unchanged. Comparison with therapeutic capsules of other manufacturers showed an activity release reduced by a factor of 10 to 100 compared with the capsules used at the Donauspital. Radiation doses caused by inhalation of iodine-131 range from 0,1 mSv to 21 mSv depending on individual differences in handling procedures. According to the results of this work the capsules were withdrawn by the manufacturer. A new product with addition of sodium thiosulfate and a pH adjustment showed a dramatic reduction (from 71 kBq to 800 Bq) of the iodine diffusion through the gelatine shell. (author)