33 00189 ly in the stratosphere and the particle size, although not known, must be very small, probably less than 0.1 micron, It is distributed by the stratospheric winds in the east-west or west-east direction, and there is also thought to be a slow circulation toward the poles, Movement into the troposphere can take place by slow settling or by seasonal changes in the altitude of the tropopause, The exchange may be most prevalent at the break in the tropopause near the middle latitudes, Once transfer from the stratosphere is completed, the material will be deposited rela- tively quickly in the same manner as intermediate fallout (Reference 23}, 1,2,4 Procedures for the Determination of Fallout Partition, The hazards of nuclear weapons testing are associated primarily with worldwide fallout, since local fallout can be controlled by selection of the test site and the proper winds aloft so that its area of deposition will be of minor consequence to the population of the world, Introduction of radio- nuclides, such as sr?°, into the human environment via world-wide fallout has a potential effect on the whole population and their significance has been studied in great detail (Reference 24), As a result of these studies, it has been concluded that certain radionuclide levels at the earth's surface canbe tolerated and that these levels can be maintained within acceptable limits by restrictions on the rate of nuclear testing. This is reached in the of "equilibrium" is n io it nd co a at th t based on the concep 23 ERC SAN BRUNO A>