- 19 repair could be complete. For genetic tissue it is gener- ally believed that there is no repair of damage once incurred, but in a recent experiment by Russell with mice, "...low dose rates of radiation turned out to be only onefourth as effective in producing mutations as the same dose given at high dose rates." (Joint Committee on Atomic Energy 1959). At the present time experimental data on the biological effects from very low radiation doses are not available; therefore, it cannot be said positively that there is or is not a threshold dose. Before considering the hazard from radioisotopes that have been added to our environment by man, consider- ation will be given to the jonizing radiations to which we are exposed in nature. Irradiation by natural sources is relatively constant in any one area but varies from area to area with local geological conditions. External emitters from natural sources include cosmic rays and the radioactive isotopes present in the crust of the earth and in the air. Cosmic rays account for about one quarter of the natural background radiations. isotopes x40 and ol4, Internal emitters include the radiowhich exist as a small percentage of these elements and are normal constituents of the body, and