reduction in dose rate caused by radon-222 emanation from the soil.* The reproducability of individual measurements has also been investigated at several standard locations in the New York City area. In addition, our ionization chamber measurements have been directly compared with those of other investigators,® Based on these findings we conservatively estimate stand- ard deviations of an individual determination of + 10% for th232 and K40, + 20% for u238, and + 0.5 ur/hr for total fallout for our individual component dose rate values, while + 7% and + 0.5 uxr/hr would be reasonable uncertainties to assign to the total natural and "best value" respectively. terrestrial gamma dose rate values, These error estimates are of course only educated guesses and include both systematic uncertainties in the various calibration factors and experimental and statistical uncertain- ties involved in individual determinations. Further comparison of our data with soil sample measurements, as well as the expected decrease in fallout contributions as a result of the test ban treaty, should enable us to evaluate more accurately the validity of our dose rate estimates, especially the contribution from the u*38 series. RESULTS AND DISCUSSION Table IlIcontains a detailed presentation of the data for the surveys covered by this report. Any interpretation of these data in terms of radiation exposure to the population should consider carefully the significance of open field dose rate measurements in terms of their relation to dose rates in homes, on concrete roadways, in buildings, and at other locations. We have found that the total dose rate levels inside typical New England homes are about 70% of the outdoor natural levels.” This factor does of course depend strongly on the type of building material, and in these areas wood frame houses were quite prevalent. Furthermore, natural radiation levels may vary considerably with time due to such factors as snow cover, soil moisture, and natural fallout.” Thus, individual measurements are not necessarily representative of the average gamma radiation field. This is particularly true of the isolated spot measurements which constitute a large proportion of the data reported. These measurements are of significance only so far as they fit into a meaningful pattern.