=n ANG Th smal iistribution with a mean of 55 mrad/y and a standard deviation of » 20 mrad/y(12), The contributions of the three sources, K-40 and the decay auughters of U-238 and Th-252 have been measured spectrometrically. The results are indicated in tabie 1, aicng with their inferred soil contents that would ath produce these radiation levels. In general, readings in populated areas outside the range of 5 to 50 mrad/y for any of these sources, and 25 to 100 mrad/y for The EAF ytht of rim ing ypi! the total radiation, would be considered somewhat anomalous. are readily observed near rock outcrops, but these occur relatively rarely in populated areas. This fact and a number of other factors related to urban Living tend to reduce the influence of bedrock geology on the observed radiation levels, and to somewhat homogenize the distribution of radiation sources that influence population exposure. This phenomenon has been clearly observed in perhaps the most detailed study of population exposure in regions of the United States, that in northern New Engiand (13) , dic ati rar. to be th Higher readings