The measurements performed in the exhaust air streams of two FRG boiling water reactors (BWRs) during normal operation show that the radioecologically decisive elemental 131 I is largely released to the environment by the unfiltered reactor and turbine building exhausts (order of magnitude of throughput: 100 000 m3/h each). It is concluded that, with respect to the release of elemental 131I, a reasonable improvement of the iodine filtration concept implemented in the two BWRs is not possible.