were worn for one week, collected, read, and then re- distributed. The experiment was conducted for five weeks, resulting in a total sampling of 400 individuals, 25 in each areal unit (that is, 50 in each of the eight geological regions). Tho details of this study are discussed by Segall, Kstimates of population exposure from the moan values of the Harvard dosimeter data in the various areas are given in the last column of Table 2. These air dose values are derived from Segall’s data?:? by assuming that cach of the dosimeters, worn on the body surface, read 100 per cent of the cosmic-ray ionization and 85 per cent of the y-ray ionization in free air. The latter figure is based primarily on the recent measurements of body attenuation factors by Spiers and Overton’. The Health and Safety Laboratory and Harvard population exposure results are plotted as a function of estimated mean bedrock radioactivity in Fig. 1, with the respective rogression lines indicated. Plotted also in Fig. 1 oo iS] Meanradiation-level (mr./week) 5 0 } j | L L } 10 20 30 40 50 60 70 Mean bedrock radioactivity (p.p.m. eU) Fig. 1. Environmental radiation in the eight selected areas of northern New England as a function of estimated mean bedrock radioactivity. A, Population exposure estimates from Harvard dosimeters; B, population exposure estimates based on Health and Safety Laboratory in situ measurements; C, mean weekly outdoor y doses 6