(Reprinted from Nature, Vol. 206, No. 4985, pp. 658-662, May 15, 1965) MEASUREMENT OF THE EXPOSURE OF HUMAN POPULATIONS TO ENVIRONMENTAL RADIATION By WAYNE M. LOWDER and WILLIAM J. CONDON Health and Safety Laboratory, U.S. Atomic Energy Commission, New York HE accurate determination of representative exposure-levels of large human populations to ionizing radiation in the environment has proved to be a problem of considerable interest to the biologist and of comparable difficulty for the physicist. In an attempt to evaluate existing techniques for obtaining such information, the Health and Safety Laboratory and the Harvard School of Public Health in 1962 undertook concurrent investigations of population exposure to environmental radiation in selected areas of the States of Vermont and New Hampshire using two independont methods. These investigations have been discussed by Segall? and by Lowderet al.?, and the extensive results are presented in detail in more recent reports’:+. In this article, we directly compare the two sets of population exposure measurements, discuss briefly some of the alternativo methods available for such surveys, and present some goneral conclusions relating to the state of the art which can be derived from our experience in the New England work. The results given here, which partially supersede the preliminary results reported previously? *, providea useful background for considering the general problems associated with making such measuremonts and interpreting them properly. The areas chosen for investigation (see refs. 1-4) contain a considerable proportion of the population of the two states, including the major urban centres. Interest in these arcas was stimulated initially by the fact that the various underlying bedrock formations appear to differ widely in mean content of naturally occurring radclionuclides, as estimated by either direct field and laboratory sample radiometry or inferences from information on similar formations elsewhere’, It seemed possible that these differences in mean bedrock radioactivity might be reflected in significant differences in moan radiation exposure between the populations of these arcas. Entirely unrelated approaches were utilizod by the Health and Safety Laboratory and Harvard groups in 407830