42 BIOLOGICAL EFFECTS OF ATOMIC RADIATION 



general latitude of the test, since east-west motions in the atmosphere predominate over north- 

 south motions and the tropospheric residence time is short. 



Delayed fallout, the primary concern of this report, originates from the portions of nu- 

 clear clouds which penetrate into the stratosphere. This is a region above the troposphere, 

 separated from it by a discontinuous interface called the tropopause. The emphasis on delayed 

 fallout arises because it constitutes the bulk of the world-wide fallout, and at this time is the 

 major source of artificial radioactivity left in the atmosphere. This will continue to be the 

 case unless large-scale testing is resumed. As a result of recent findings indicating the short 

 storage time of some stratospheric debris, the distinction between intermediate and delayed 

 fallout is not as clear-cut as was formerly considered. 



The fission products comprising delayed fallout and probably much of the intermediate 

 fallout are in very small particles. Recent evidence tends to confirm the fact that most strato- 

 spheric debris is on particles smaller than 0.1 micron (0.000039 inches). It has also been 

 observed that most of the Sr^" and Cs"" is in soluble form. 



Carbon- 14 results from the interaction of neutrons produced in a nuclear explosion with 

 atmospheric nitrogen. It normally combines into carbon dioxide, or possibly carbon monox- 

 ide, natural gaseous components of the atmosphere. Undoubtedly some C* falls out locally 

 as calcium carbonate and some remains in the troposphere, but measurements indicate that 

 the excess over the natural background cannot be associated with tropospheric fallout. 



III. Recent Observations 



The number and variety of measurements of long-lived fission products and of C" made 

 in the past few years are sufficient to provide a description of the important features of world- 

 wide fallout. These measurements include extensive Sr^" soil analyses over all parts of the 

 world, utilizing improved techniques and quality controls which permit increased confidence 

 in the data. Networks for collecting and analyzing precipitation for Sr'" have become much 

 more widespread and are using ion-exchange techniques for extracting the strontium from 

 the precipitation. Systematic measurements of the air concentration of fission products have 

 also increased and monthly profiles of the concentration of several specific isotopes along 

 the 80°W meridian have proved valuable in studying large-scale phenomena. Some progress 

 has been made in studying fallout over the 70% of the globe covered by water. Measure- 

 ments of Sr''" in surface and deep waters indicate that much of the strontium remains in the 

 upper mixed layers. Because of the movement of ocean waters, and of some mixing into deep 

 waters, interpretation of these data in terms of world-wide fallout is difficult. Collections of 

 precipitation aboard U. S. ocean weather ships and of precipitation and soil samples on islands 

 have also contributed to our understanding of fallout over the oceans. It is assumed that 

 fallout over the oceans tends to parallel that over land at similar latitudes. An unsolved 

 problem remains in the sampling of snow. This is particularly important in polar regions 

 where it is difficult to distinguish between fresh and old blowing snow. Impetus for expanding 

 radioactivity measurements and for collecting and collating data has resulted from the Inter- 

 national Geophysical Year program. Various measurements of C" in air, water, and vegeta- 

 tion show the growth of this isotope in our environment as the result of nuclear testing. 



A very useful series of measurements for studying large-scale circulation features in the 

 atmosphere resulted from the unique production of tungsten- 185 (W'"') in the Pacific test 

 series held by the U. S. in the summer of 1958. Measurements made more than six months 



