of long lived fission products. The Sr90 air concentration history shows a marked general increase following the Ivy and Castle tests and thus appears to directly relate to the increased stratospheric storage following these tests. The general equivalence of the concentrations observed for the four stations, widely separated in geographical location, is strong indication that relatively old debris rather than fresh fission product activity is involved, Except for a few of the highest concentration values observed, the Sr90 data do not reflect any considerable contribution from individual small weapons tests for which troposvheric washout rates of several weeks or less have been estimated, ' An air filter device, horizontally oriented near the ground surface in general will not collect large particles which fall directlyor particles tranped in raindrops, Instead, it will collect a portion of those particles which mix downward between rains and persist in surface air plus some of the particles carried in the air downdraft associated with rains for which the scavenging efficiency of rains is low. Thus, the Sr70 surface air concentration data do not necessarily relate to the total deposition rate in any direct manner » The Sr? concentrations observed in surface air during the Fall 195) and Spring 1955 are an order of magnitude lower than the limited U, S, and British measurements of upper troposphere concentration during the same period, The numbers are not necessarily inconsistent when consideration is given to dilution by cleaner surfaceair during downward mixing, to reduction resulting from scavenging by rains, and to removal from surface air by the filtration action of vegetaion foliage and the action of fog and dew. The mean troposphere concentration of Sr90 is undoubtedly a factor of 5 to 10 times higher than surface air values and thus corres onds to a total tropospheric air burden of one megaton of fission or so. These considerations provide additional basis