Appendix G External Dose Estimates from Global Fallout

This report provides estimates of the external radiation exposure and whole body effective dose received by residents of the continental U.S. during the period 1953-2000 from nuclear weapons tests. Doses were calculated for tests carried out in the Pacific by the U.S. and U.K. and by the U.S.S.R. at various sites in the former Soviet Union during the years 1952-62. Estimates are given on a county by county basis for each month from 1953-1972. The average population dose from the fallout from all of these tests was about 0.7 mSv, about equivalent to 2-3 years of external radiation exposure from natural background. In contrast to the fallout from tests at the Nevada Test site, the variation in exposure across the country from “global” fallout was relatively small, reflecting primarily variations in annual rainfall. Precipitation was the main mechanism for the deposition of fallout from these mostly high-yield thermonuclear tests that injected most of their debris into the stratosphere. Thus residents of counties in the eastern and Midwestern U.S. that received above average rainfall were impacted more than residents of the more arid Southwestern states. Since the states downwind from the NTS that were impacted most by the NTS fallout are in general more arid than the eastern U.S., the areas most impacted by NTS fallout were in general least impacted by “global” fallout. In contrast to fallout from the NTS where most of the exposure was due to the short-lived radionuclides (primarily I-Te-132 and Ba-La-140), Zr-Nb-95 was the major contributor to external dose from “global” fallout during the years of testing. The total dose through 2000 was dominated by the long-lived Cs-137. Cs-137 present in soil continues to result in a small radiation exposure to the public even at the present time. As was the case for NTS fallout, the most exposed individuals were outdoor workers, the least exposed, persons who spent most of their time indoors in heavily constructed buildings. The deposition of all radionuclides that contribute significantly to external exposure, as well as a few that contributed significantly (Sr-89, Sr-90) to internal radiation exposure via the ingestion pathway, were calculated on a county by county and test by test basis. The general pattern of deposition is discussed. In general the population-weighted total deposition of long-lived radionuclides such as Sr-90 and Cs-137 was about a factor of about 10-15 greater than that from NTS fallout. However, the population-weighted deposition of short-lived isotopes such as I-131 was generally much less than from NTS fallout.