The response of a pressurized argon‐filled ionization chamber to charged particles in the cosmic radiation near 50°N geomagnetic latitude has been determined as a function of atmospheric depth from mountain altitudes to sea level. The γ contributions to the various ionization chamber readings were subtracted out by utilizing concurrent determinations of the γ ionization from γ‐ray energy spectra obtained with a scintillation spectrometer. Conversion of relative response to absolute free air ionization is accomplished by normalizing the mean chamber reading at sea level to a calculated value of 2.1±0.1 ion pairs cm−3/sec−1 atm−1 (STP), derived from existing experimental data on muon, electron, and proton fluxes and energy distributions. The curve of absolute ionization versus atmospheric depth has the same shape as that obtained by Millikan in 1932, but with significantly lower absolute values. It is inferred that many present determinations of free‐air ionization, primarily conducted at higher altitudes, may be as much as 50% too high.

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Critical evaluations of existing data on cosmic radiation in the atmosphere and in interplanetary space have been carried out in order to estimate the exposure of the world's population to this important component of natural background radiation. Data on population distribution and mean terrain heights on a 1 x 1 degree grid have been folded in to estimate regional and global dose distributions. The per caput annual dose equivalent at ground altitudes is estimated to be 270 µSv from charged particles and 50 µSv from neutrons. More than 100 million people receive more than 1 mSv in a year, and two million in excess of 5 mSv. Aircraft flight crews and frequent flyers receive an additional annual dose equivalent on the order of 1 mSv, though the global per caput annual dose equivalent from airplane flights is only about 1 µSv. Future space travellers on extended missions are likely to receive dose equivalents in the range 0.1-1 Sv, with the possibility of higher doses at relatively high dose rates from unusually large solar flares. These results indicate a critical need for a better understanding of the biological significance of chronic neutron and heavy charged particle exposure.

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