The composition of the radioiodine species that is released in a hospital by nuclear medical therapy is investigated. More than 90% are found to be organic iodine. This leads to a reduction of the calculated radiation exposure in the environment by more than a factor of ten with regard to pure elemental iodine releases. The organic iodine is not formed by chemical reactions in the air of the rooms butoriginates in patients, who are treated with 131I. Effective half-lives and release factors for exhalation were determined by monitoring the exhaled air of the patients.