[en] Radioactive nuclei that emit most of their energy via low energy electrons, like tritium and iodine-125, are very radiotoxic when they are built in into the DNA of cells. The problems discussed are the local distribution of absorbed dose and the radiation quality. The absorbed dose in the cell nucleus depends on the distribution of the radioactive nuclei and on the range of the emitted particles. For the radioactive nuclei H-3 and I-125 which are specifically incorporated into the DNA, hence into the cell nucleus, the absorbed dose in the nucleus is considerably larger than the mean absorbed dose valid for homogeneous distribution of the radioactive atoms. Therefore the Dose Distribution Factor, DDF, was defined which is the absorbed dose in the cell nucleus per unit radioactivity administered for a heterogeneous distribution of activity, devided by the corresponding dose from a uniform distribution of the same amount of activity. The DDF is evaluated for H-3, C-14, P-32, I-125 and I-131. The radiation quality of H-3 and I-125 is compared to the radiation quality of external radiations