Radiological Research Laboratory, Columbia University, New York City In view of the serious damage which may result from too much exposure to radiation, it becomes extremely important to determine the amount which can be tolerated without producing any undesirable effects. The first impulse might be to say that all irradiation should be avoided, and then no reaction could be produced, no matter how low its threshold. Besides the impracticability of this from the point of view of the construction of protective barriers, it is actually impossible because of the nature of the world around us. The entire earth and all its inhabitants are continually bathed in ionizing radiations of low intensity. Some of these, cosmic rays, come from outside the earth's atmosphere. Some, earth radiation, come from our terrestrial surroundings. The earth's crust and all natural waters contain measurable amounts of radioactive matter; in general the concentration does not vary greatly from one locality to another, except in the...