234 LOWDER, BECK, AND CONDON fall. By the end of 1963, dose rates in the eastern United States had declined to approximately 1 ur/hr, mainly from "Cs, and have since remained close to that value. No increased gamma radiation has been noted in New-York from fallout from the 1964 Chinese test. INTRODUCTION Estimates of long-term human exposure to environmental radiation require detailed information on the properties of the various natural and man-made components of the environmental radiation field in areas of human habitation and activity. Since 1955 the Health and Safety Laboratory (HASL) has been conducting a study of the properties of the more penetrating components of the natural environmental radiation field, which include the cosmic radiation and gamma rays from radioisotopes in the ground and the lower atmosphere. Sensitive ionization chambers have been developed which provide accurate measurements of the total dose rate. The significant contribution in recent years from gamma-emitting fission products to the total dose rate has made necessary the development of supplementary techniques for differentiating between the natural and fallout components. Gamma-ray spectra obtained directly in the field with a large Nal(T1) detector have been found to provide sufficient detail to allow reasonably precise and accurate estimates of the dose-rate contributions from the gamma rays of ‘°K and the daughters of 7**U and ?*Th and also from those of the significant fallout-gammaemitters, in particular, *°Zr—®Nb, !Rh, and "Cs, The success of these techniques has made possible the con- tinuation of our natural-radiation studies through periods of substantial fallout deposition besides providing information relevant to considerations of human exposure to fallout-gammaradiation. Several hundred gamma spectra at over 200 different locations in the United States have been obtained over the past several years. These measurements were taken to obtain information on gamma-radiation fields in particular areas of interest, to study the applicability and validity of our spectrometric methods over a wide range of environ- mental radiation fields, and to evaluate the performance of our instrumentation under differing field conditions. We shall briefly outline here the methods used to infer dose rates from these spectra, present some evidence to indicate the validity of these dose-rate estimates, and discuss some of the results of our various measurements. The complete data for all locations are being presented in a series of HASL reports.’In addition, we have published several review articles,“~* and a detailed report on our spectrometric techniques has recently been published.’ RL