range in the measured dose rates in each case (+10 per cent about the mean for most field readings) meant that relatively few locations were required for this purpose. Most of the measurements were made in the larger towns, as these contained the bulk of the population. The mean y-dose rates over open ground derived from the spectrometer readings are given in Table | for each of the eight regions chosen for examination. The main towns and the corresponding bedrock formations are indicated. The spectrometric procedures allow determinations of the component dose rates from potassium-40, the uranium and thorium series, and the main fall-out y-emitters (8Zr- Nb, Ru, Rh, 47Cs). Estimates of mean soil concentrations of the natural radioisotopes can be calculated from the component dose rates, assuming uniform depth distribution in the ground?*", Such concentration values are representative of the true soil contents of radium, uranium and thorium only when these isotopes aro in radioactive equilibrium with their y-emitting daughters. These results are included in Table 1, and the em for determ eo ination necessar lation tural i and fall-out Yaadion: my, at on detector s for survey s ofthe oor measurem ent location as r measurements otail elsewhere11-1s neSur vey tech- T cent (S.D.) for th 106 an over. ‘approximately + 19easured total mean values for the natural emitters arc combined to give a mean ‘equivalent uranium’ (eU) concentration for the upper layers of the ground, that is, the amount of uranium in equilibriam with its daughtors that would yield the same y-dose rate as the potassium and the uranium and thorium series in the actual situation. Also listed for comparison purposes are the moan equivalent uranium contents of the various bedrocks as estimated by Billings®. It is noteworthy that the rangeof soil activities is much narrower than that inferred for the bedrock formations. The average fall-out levels indicated in Table L apply specifically to July and August 1962, whon almost all readings fell between 2 and 3 ur.jh. When some of these sites were re-checked in 1963, the fall-out dose rates were nearly twice as great, ranging from 3 to 5 ur./h. In all cases, *Zr—*Nb was the dominant contributor. By May 1964, levels of 1 ur./h were typical, the °Zr—*®Nb and other short-lived emitters having nearly disappeared, leaving 137Cs as the most important fall-out y-omitter. These resnlts are quite consistent with the more detailed information available for the New York City arca during this period's, In addition to the combined ionization chamber and spectromoter readings summarized in Table 1 (5-10 in each region, except for 16 at Conway), separate ionizationchamber readings were made at many other sites along with a number of independent portable scintillometer surveys. The uniformity of the outdoor radiation levels in each area was remarkable. 2 In general, streets and side- walks did not significantly alter the obsorved profile. Any attempt to estimate population exposure to environmental radiation must take into account the 3