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The concentrations of the various radioactive decay products of radium in the air have been determined by observing the beta activity (1) of pieces of filter paper through which air has been passed, and (2) of chemically separated isotopes obtained from such filter papers. The relative amounts of long‐lived and short‐lived products found indicate that, besides the radioactive decay of the substances, some other process which removes radioactive particles from the air is active. The mean life of the particles with respect to this removing process is found to be about 10 days. The hypothesis that this process is the capture of the radioactive particles by rain droplets (clouds) is consistent with measurements made of the radioactive content of rain‐water. On the other hand, the removal of the ionized radioactive particles by the electric field of the earth leads to a much smaller effect.

The radioactive aerosol content of the surface air in Antarctica has been measured continuously since 1956, first at Little America station and later at the South Pole station. On‐site measurements were made daily of the radioactivity due to bomb‐produced fission products and to naturally occurring radionuclides of the radon and thoron series. Later, composite samples covering 3‐month periods were subjected to radiochemical analysis for such long‐lived radionuclides as Sr90, Cs137, Ce144, Pm147, and Pb210. The natural radioactivity was lower in Antarctica than has been observed at any other geographical location, as might be expected from, the small area of exposed land surface in the vicinity. Fission‐product concentrations, however, often exceeded those found in the southernmost part of South America and exhibited more well‐defined seasonal variations, with maximums in the antarctic summer.

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