Corporation, in its first SUNSHINE calculation, estimated that 25,000 MI equivalent of fission products could be detonated before contamination from strontium-90 would reach the maximum permissible concentration on a world-wide basis. Their assumption was that this quantity of fission products, spread uniformly, would involve the deposition of 200 micrograms of strontium-90 per acre. In arriving at the value given, which would involve a maximum permissible concentration on a world-wide basis, there was included the calculation for the amount of stable strontium in soil and the stable calcium to strontium ratio in soil and plants. Thus, any amount of strontium-90 in excess of 200 micro- grams per acre would exceed the maximm permissible concentration. They did not attempt to place any evaluation on the biological consequences of exceeding this value. In considering the world-wide contamination problem, particularly as it exists at present, it is pertinent to discuss the natural radioactive material which is airborne. Experimental findings show an intermittent widespread fall-out ranging from a ratio of one part arti- ficial radioactivity (fission product) to six parts natural radioactivity at a station in Alaska, to one part artificial radioactivity to fourteen parts natural radioactivity in Washington, D.C. It has also been found that deposition is lower at night, and that rainfall tends to clear the atmosphere of all radioactivity for a period of time ranging from one hour to almost a day. The natural gamma background due to radioactivity is affected by the airborne particulate matter discussed above. However, the total gemna background is the sum of that due to airborne radioactivity and to radium and wranium in the soil plus that due to cosmic radiation. The latter accounts for a considerable fraction of the total gamma radiation, so that levels of one part artificial activity and fourteen parts natural radioactivity in sir samples result_in an insignificant increase in the gamma background. One particular continuous recording gamme, survey meter has been in operation at the Massachusetts Institute 61