alistic estimates of population al radiation. The Health and sonducted in situ measurements ,0th out of doors and within eneral radiation profile over the chted these results by ostimates n the various types of locations. ilized on a large scale by Spiors ind another investigation ofthis witzorland in 1961 (ref. 8). The k a more direct approach and dosimeters to representative on to be worn on the person. nparing the two sets of results, on exposure’ as the free air dose ndividuals in their usual rounds only the air dose from the more of the environmental radiation ial y- and ionizing cosmic radiabe justified by the fact that the 1 and at various depths in the own fractions of the free air nore penetrating components of d significantly contribute to the test interest, that is, those of the yuld be noted that the possibly contribution from cosmic ray ned by the techniques used in Laboratory radiation survey was August 1962, and subsequent e made in May and September » instrumentation included higha chambers for total dose-rate ometer system for determination (particularly necessary for disnatural and fall-out y-radiation), n detectors for surveys of the utdoor measurement location as easurements. The survey techl elsewhere!-!5, provide an overmt (S.D.) for the measured total proximately +10 per cent for 1ponents of the total radiation gs were taken in large, flat open ated areas (for example, parks, 3) with the instruments placed The number of measurements in 1 by its population and size, the es, and the observed range and readings. A sufficient number t @ reasonable radiation profile each area. The quite limited 2 range in the measured dose rates in each caso (+ 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, us these contained the bulk of the population. Tho mean y-dose rates over open ground derived from the spectrometer readings are given in Table 1 for each of the eight regions chosen for examination. The main towns wid the corresponding bedrock formations are indicated. The spectrometric procedures allow determinations of the component dose ratos from potassium-40, the uranium and thorium series, and the main fall-out y-emitters (PZr-Nb, 1 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!®1, 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. Theso results are included in Table 1, and the mean valucs for tho natural emitters are combined to givo a mean ‘equivalent uranium’ (eU) concontration for the upper layers of the ground, that is, the amount of uranium in equilibrium with its daughters that would yield the same y-dose rate as the potassium and the uraninin and thorium series in the actual situation. Also listed for comparison purposes are the mean equivalent uranium contents of the various bedrocks as estimated by Billings’. It is noteworthy that the range of soil activities is much narrower than that inferred for the bedrock formations. Tho average fall-out levels indicated in Table 1 apply specifically to July and August 1962, when almost ail readings fell between 2 and 3 yr.jh. When some of these sites wore re-checked in 1963, the fall-out dose rates wero 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 87Cy3 as the most important fall-out y-emitter. Theso results are quite consistent with the more detailed information available for the Now York City aroa during this period. 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. In general, streets and sidewalks did not significantly alter the observed profilo. Any attempt to estimate population exposure to onvirommnental radiation must take into account the 3