» average).'® ot eave gd a Baty BN Ra ray le be ae vty We In the absence of atmospheric tests these levels are expected to continue to decline generally except for small transitory rises during the next few spring seasons. The annual (1963) national average for those areas of the United States showing the highest values was 26 “strontium units” in milk. This is less than the 32 “strontium units” predicted and should foretell less in the bones than predicted.' Incidentally, the amount of strontium 90 in the milk produced around the Nevada Test Site is among the lowest in the country. In general, past predictions of levels of strontium 90 in bones have been too high. This is due in part to the selection of data in the upper ranges to avoid underestimations of radiation ex- posure. Even so, it is remarkable that the observed amounts of strontium 90 in bones have been within about a factor of two of the predicted amounts considering the fact that such predictions require the application of many scientific dis- ciplines—nuclear physics, meteorology, chemistry, plant and animal physiology, etc.—often to new situations. That segment of the U.S. population whose bones will receive the highest radiation dose are children born in 1963 in regions of heavier rainfall. The total radiation exposure to these children—from internally deposited as well as external radionu- clides—has been predicted to be about 465 milliroentgens (0.465 roentgen) accumulated over a 70-year period. ! Evaluation The predicted average 70-year radiation dose to the bones of the age group receiving the highest exposure from all past tests —about 465 milliroentgens (0.465 roentgen) from all radioactive materials within and outside the body—is about five percent of the bone dose received during the same 70-year period from natural background sources. F. CARBON 14 Background information Carbon 14 is produced naturally by interaction of cosmic rays with the nitrogen inthe atmosphere. Althoughits radio- active half-life is long—5760 years—the process of natural production had been going on for such a great time that the rate of production and rate of decay were in equilibrium, ie., just as much is formed each year as decays away, until nuclear test detonations were initiated. There is a constant exchange of carbon 14 atoms between the atmosphere and the surface of the earth on the one hand, and the deep ocean on the other, 9000941 16