average).'§ 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 exposure. 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 disciplines—nuclear physics, meteorology, chemistry, plant and animal physiology, etec.—often to new situations. That segmentof the U.S. population whose boneswill 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-yearperiod. ! 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 radio- active 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. with the latter constituting a reservoir holding about 96 percent of the atoms. Nuclear detonations can also produce carbon 14 by inter- action of the neutrons, produced at the time of the explosion, with nitrogen of the atmosphere. Approximately 400 mega- tons of total yield fired in the air (surface bursts “lose” about one-half of the neutrons into the ground) will produce a sufficient amountof carbon 14 to equal the amount normally pres- ent in that part of the earth’s biosphere that determines radiation exposure to man. However, half of this newly-added carbon 14 “disappears” into the deep ocean within about 33 years.'® One-half of that remaining in the atmospherelikewise “disappears” in the following 33 years, until only a few percent remains. Radioactive isotopes act chemically similar to their stable counterparts so that not only is stable carbon but also carbon 14 found in all living cells. Thus, although carbon 14 emits a beta particle of very low energy that travels a very short distance it nevertheless irradiates essentially the whole body at a rate of approximately one milliroentgen (0.001 roentgen) per year. This is the natural background rate for carbon 14. The Data Since nuclear weapons testing started 511 million tons total energy yield have been released. Considering the conditions of firing (surface versus air bursts) about the same amountof carbon 14 was produced from all past tests as is normally pres- ent in that part of the earth’s biosphere that determinesradia- tion exposure to man. Assuming that most of the carbon 14 produced by the detonation will “disappear” into the deep ocean with a half-time of 33 years, the estimated whole body exposure for 70 years is 37 milliroentgens (0.037 roentgen).! After this 70-year period the dose rate from bomb produced earbon 14 will be about one-quarter of that at the start,i.e., F. CARBON 14 Background Information Carbon 14 is produced naturally by interaction of cosmic rays with the nitrogen in the atmosphere. Although its radioactive 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, i.e., just as muchis 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, 16 about one-quarter of one milliroentgen (0.00025 roentgen) per year. Thereafter, the activity will persist for thousands of years but at ever decreasing levels. Evaluation The radiation exposure from carbon 14 may account for roughly one-third of the total radiation dose from fallout over the next 70 years. Because of its long radiological half-life, it will persist at low levels of activity for thousands of years. However, even before the 70-year period is completed the dose rate from carbon 14 will be so low as to be non-measurable. This does not mean that the radiation is not “there”butit will aed