reasonable values for these factors and the various weekly doses into the formula, we get: P = 0-8 [0-60 + (0-2) (0-37) + (0-3) (1-18) + In] + 0-2 [0-60 + 0-37 + 1-18] = 1-25 + 0-8 I, mr./week Since we have determined that the indoor total y-levels average 0-7 of the outdoor levels in these areas, we find that: Th=05J], + 0-42, = 0-66 mr./week os ee en Substituting this in the above expression for P, we get: } | 2 3 This result is not strongly dependent on the particular values assumed for the various factors in the above equation. It is quite consistent with the similarly calculated Health and Safety Laboratory population exposure estimates, and much lower than the dosimeter results. The mean contribution from building materials to population exposure would have to be close to 2 mr./ week to validate the dosimeter data, which is considerably higher than the measured values for the total indoor y-dose rate in most of the 160 residences whereseintillation detector readings were made. Even without such evidence, it seems to be an unreasonably high value to assign to mean regional indoor radiation-levels produced by radio- 4 week (HASL) osure estimates as a function of comtatory (HASL) estimates for the eight d areas examined ittle doubt that the dosimeter ‘an be shown by carrying out a ais of the various contributions -level, P, utilizing the accurate tory measurements of outdoor se rates. If I,, In and Iy are r dose-rate contributions from l-out y-radiation, respectively, -dose rate pro uced by sources we can write the following an + In) + fo (Le + Ly + In) cy time factors for indoor and vely, and sf and sp, are mean » buildings and residences for ral y-radiation. Substituting 8 P = 1-8 mr./week activity in building materials. For it implies total indoor -doses averaging approximately 3 mr./week, wheroas the scattered data given in the 1962 United Nations report?® indicate that readings of 1 mr./week are typical of normal situations in wood or brick houses. The results of both surveys indicate that the range of population exposure to environmental radiation is quite narrow throughout the regions studied. It follows that northern New England does not provide a good ‘laboratory’ for the study of the effect on large human populations of differences in long-term environmental radiation exposure. Of much greater significance is the correlation between the two entirely independent and undoubtedly somewhat imprecise techniques for estimating these exposure-levels, This correlation can be at least partially understood as a consequence of the relatively high degree of uniformity in radiation-levels observed within each area. Under such fortunate conditions, the methodof using a few hundred field measurements to infer the total radiation profile has yielded apparently realistic values for population exposure, for which the Harvard dosimeter data provide strong qualitative support. Spiers et al.’, in their discussion of the extensive population investigation in Scotland, have already indicated someof the difficulties involved in obtaining and interpreting data of this type. Butit can be concluded from the work recorded here that 9