effect of man-made structures on ambient radiation fields, indoors. existed between indoor and outdoor radiation-levels. Several rooms in each dwelling were surveyed, including the living-room and at least one bedroom. Again, a strong uniformity exhibited itself in that the mean indoorlevels were close to 70 per cent of the corresponding outdoor levels in each area‘. This may be related to the fact that the vast majority of the dwellings were of wood-frame construction, with the building materials appearing to act generally as y-ray shields with relatively little activity of their own. With such data at hand, an estimate of mean population exposure to environmental radiation can be obtained by calculating a suitably weighted average of the indoor and outdoor readings of the survey instruments. Taking into consideration the greater occupancy time indoors of the average individual, the mean exposure levels have been estimated to be 80 per cent of the mean outdoorterrestrial y-dose rates given in Table 1, plus the contribution from the ionizing components of the cosmic radiation at the ground altitudes of the various areas. No correction of the cosmic-ray figures for typical structural shielding has been made, since this would be a reduction of the order Natural 1962+ Cosmic Outdoor Total y* Dunham Dolomite Beldens Formation Glacial Drift Monkton Formation Fitchburg Granite Littleton Formation Binary Granite Conway Granite * Includesfall-out, averaged over all locations within an area, + Obtained as indicated in the text; in units of air doserate, Outdoor Natural y of 10 per cent or less, which is comparable to the present 6U (p.p.m.) Geological category MEAN WEEKLY DOSES IN ME./WEEK Table 2. Portabie scintillation detector readings were made in 160 private homes and apartments in the main towns to ascertain whether any consistent relationship | Mean population exposure Dosimeters since most individuals spend a large fraction of their time uncertainty in the absolute cosmic-ray ionization intensity. Table 2 shows the population exposure data arranged by geological region. The mean weekly outdoor doses in air are given for both natural and total (natural plus fallout) y-radiation and for cosmic rays, and the Health and Safety Laboratory population exposure estimates for the time of the survey (August 1962, including fall-out) and for the natural emitters only (that is, the mean:life-time levels neglecting fall-out) are also given. The importance of the spectrometric technique is emphasized by the fact that estimates of the integrated natural y-dose were obtainable even under conditions of near-maximum fall-out contamination. In many population investigations, it is just this quantity that is desired. The Harvard investigation involved the use of a set of 200 Victoreen model 362 condenser ionization-chamber pencils, along with a stable pulse height readout system!51? which is designed to allow readings of 1-0 + 0:2 mr. at the 95 per cent confidence level with a single pencil. Mechanical and thermal stability was tested, and correc- tions made for average leakage rates observed in the laboratory. These dosimeters were distributed in pairs to five individuals in standard occupational categories in each of 16 areal units, half urban and half rural. The dosimeters 5