This is because the low penetrating power of the beta radiation results in the absorption of almost all. of the ionizing radiation in tissue within a few millimeters of the particle. It is this low level chronic radiation that is considered as a possible carcinogen. Part of the bomb debris within the mushroom cloud of large yield detonations goes to a very high altitude and is considered to be deposited in the stratosphere. Another portion of the debris from a large yield detonation, and essentially all of the debris from many low yield weapons, is deposited in the troposphere. Particles initially deposited in the stratosphere and large enough to be affected by gravitational attraction to the earth go from the stratosphere to the troposphere through the atmospheric layer known as the tropopause. However, there is no known mechanism for moving particles of molecular dimensions and only slightly larger from the stratosphere to the troposphere. Several dust samples at altitudes up to 90,000 feet have been collected by the Health and Safety Laboratory of the New York Operations Office of the Atomic Energy Commission following test operations in the Pacific Proving Ground, the collections being made during the sumer of 1954, The data indicate inconsistent variation, or non-uniform mixing, at altitudes from 80,000 to 90,000 feet. Values of 0.025 to 0.56 disinte- grations per minute per cubic meter of air were obtained of which 0.7% to 4.7% was due to strontium-90. Samples collected at 40,000 feet by jet aircraft showed considerable variation, with only samplers on the right and left wings of a particular aircraft showing consistent results. There was a change from flight to flight on the same day in the same area, but the consistent results from right to left wing of the jet indicated a real variation in total radioactivity through which the aircraft flew. These values ranged from 0.021 to 3.2 disintegrations per minute per cubic meter of air. Although the methods of sampling at 90,000 feet differed from those at 40,000 feet, the results indicate that there is about the same order of magnitude of radioactivity at the two levels, with the radiation levels being slightly higher at 40,000 feet. 56