cloud drop pick up, the radioactive fallout may enter the biosphere by normalbiological proc- esses. Radioactive Sr*° and radioactive Cs!* are the two principal isotopes which have this facility and are produced in high yield by the fission reaction and are of long enoughlifetimes to be disseminated world-wide particularly by the stratospheric mechanism, about 28 years half life for each. Strontium-90 is produced at a level equivalent to about 1 mc of Sr°*® per square mile of the earth’s surface for every two megatons of fission energy, and radiocesium is produced at about 50 per cent higher yield. Of the two isotopes, Sr*°, because of its chemical similarity to calcium, collects in human bone, where it is held for years and where its radiations might then cause deleterious effects to the health of the individual, such as leukemia or bone cancer. It is interesting that Sr®’ constitutes a relatively less important genetic hazard because of the short range of its radioactive radiation and the fact that it is not held in the reproductive organs. Radiocesium stays in the human body only six or eight months on the aver- age, because it has no permanent structure like the bone for which it has a natural affinity. As a result, the amount of radiation occurring from internally ingested radiocesium is much less and mostlikely is subject to palliative measures calculated to reduce its time in the body. Strontium-90 taken into the bone, however, appears to be stored for many years, the exact time not being known very well.?4 Radiostrontium is taken into the body becauseof its similarity to calcium, butthere is a definite difference in chemical behavior which causes animal organismsto prefer calcium. Thus the radiostrontium content of newly deposited bone calcium is less than that for food calcium, In many countries, the principal source of calcium is milk products, so the fact that cow’s milk has only one-seventh the strontium in it per gram of calcium that the cow’s food has, and that milk taken into the human body similarly deposits calcium in the bones with only half the Sr®° content of the milk itself means that human beings naturally have a lower Sr*°-tocalcium ratio for new bone than for the food source by something like a factor of 15 for dairy products. On the other hand, vegetation containing Sr*® also deposits its strontium relatively inefficiently with a factor of something like 4 less strontium in the bone from these sources than is carried in the vegetable food itself, all relative to calcium. In some countries where calcium in the human diet comes principally from vegetables other sources of calcium contribute, some of which contain essentially no Sr*’, namely sea food. Because fallout is diluted so quickly by the action of the waves in the ocean, the concentration of the radioactive strontium in the sea calcium is very much lower thanit is in the soil of the land in which the grass and vegetable crops grow. This difference becomes even larger when theeffects of direct leaf and stem base pick up are considered. This perhaps accounts for the high values reported by Ogawa” for rice in Japan. So, fish from the sea are naturally at the lowest level in radio- strontium and sea food should be the lowest source of calcium among ordinary human foods. With all of these factors taken together, the world populations assimilate calcium at a much lower radiostrontium content than is exhibited by land plants to a very considerable degree. Eckelmann, Kulp and Schulert!®> have given a detailed sample calculation recently, based on their extensive measurements on human bone. 5. The biological hazard from the radioactive fallout from weapons testing is not well known, and like many biological problems the determination of the hazard in any exact way seems to be almost impossibly difficult. Fortunately, however, it is possible to compare the radiation from radioactive fallout with the intensities of natural radiation to which weare always exposed. For example,it is clear that the present level of the radiostrontium in the bones of young children which are, of course, closest to being in equilibrium with the fallout, since adults have had their bones some time even before there was any radioactive fallout, is about 2 mr/year as compared to an average natural dosage of 150 to 200 mr/year, about 1 to 2 per cent of the dosage from natural sources to the bones depending upon location. Natural radioactivity present in the ground, building materials and even in our own bodies gives us an average total dose at sea level of about 150 mr/year, and medical X-rays add something like another 150 mr. The radiocesium taken into the body and the penetrating radiations from non-assimi- lable radioactive fallout contribute perhaps another three or four per cent to the whole body dosage. Thus the total dosage to freshly formed human boneis at most five per cent of the natural dosage. Furthermore, we do know that the variations in natural background dosages from place to place are enormous in magnitude as compared to the average value, and of course as comparedto the fallout dosage. For example, it has been found”* that exposure rates from 256