of the ionization chamber should show up in the data analysis. None were evident. Denver and Colorado Springs, Colo. The Denver area was of special interest. Higher than average terrestrial radiation levels had been measured with air ionization chambers during earlier HASL surveys.* These high levels were confirmed by the October 1962 data and were shown to be due to higher than average soil con- centrations of K49 and Th232, particularly the latter. These measurements were rechecked in 1963 and a number of new measurements were taken. Natural terrestrial levels ranging from 8-15 ur/hr were found. Since cosmic ray dose rate contributions at these altitudes are 6-7 ur/hr,°® total natural dose rates from penetrating radiations are in the range 14-22 ur/hr. This may be compared to total natural dose rate values of about 10-12 ur/hr in the New York City area, In addition, fallout further increased these levels by 2-3 ur/hr in October 1962 and 1-2 wr/hr in October 1963. This area is the only one yet found in the continental United States where a reasonably large population is exposed to ambient radiation levels 50-100% higher than is usual. SUMMARY The wide diversity of sites and conditions encountered on these surveys in the southeast, central, and western United States were extremely helpful in evaluating the validity of our methods of spectrum analysis as well as for studying the performance of our instrumentation in a variety of field situations. The instruments generally performed well and the dose rates inferred from our spectrometric methods and ionization chamber readings were in close agreement over a wide range of terrestrial and cosmic environmental radiation fields. The data obtained on these surveys indicated that fallout usually contributed a significant amount to the total terrestrial dose rate. Fallout levels encountered in the Spring of 1963 during the survey through the southeast were relatively high (4-5 ur/hr) compared to the levels encountered on the western surveys in October of 1962 and 1963 (2-4 ur/hr). The effect of precipitation on fallout dose rate was indicated by the - 10 -