. natAV be fen ver purpose of making conservative dose estimates they may be considered to be in radioactive equilibrium with radon-222. The long-lived daughters of radon-222 have separate existences in the environment with polonium-2Z10 being much less than the lead-210 activity which in turn et hs is a small fraction of the radon-222 activity. ai t in man, however, are usually close to equilibrium. The CEN ott > of “igh ark GoM The levels of lead and polonium Thorium Series (Tables 11-14) The thorium series decays through a chain which is very similar to that of uranium. The short lives of Ra-228 and Rn-222 modify the behaviour considerably and exposures from airborne daughter products of thoron are relatively small. The major contribution of the thorium series to human exposure is in external gamma radiation. External Radiation The external radiation exposure of man comes from three sources, cosmic ra radiation, terrestrial gamma, and gamma radiation from airborne radionuclides. The latter is calculated as an exposure of about 5 mrad/y, but it is always ineluded when measuring the terrestrial gamma dose rates. Cosmic ray dose estimates have been summarized by unscEar‘3) , The ionizing component amounts to 28 mrad/y at sea level in the middle latitudes and is about 10% less near the equator. This dose rate is approximately doubled for each 2000 meter increase in altitude for the first few kilometers. The cosmic ray neutron dose rate is of the order of 1 mrad/y. Measurements of gamma dose rates with scintillation spectrometers in many populated areas of the United States indicate total natural terrestrial radiation fields that fall within rather narrow limits at most locations. Neariy 90% of the several hundred available readings can be represented by an approximate: 7