Affiliations of authors: E. S. Gilbert, R. Tarone, E. Ron (Division of Cancer Epidemiology and Genetics), A. Bouville (Division of Cancer Biology), National Cancer Institute, Bethesda, MD.
Correspondence to: Ethel S. Gilbert, Ph.D., National Institutes of Health, Executive Plaza North, Rm. 408, Bethesda, MD 20892-7368.
We thank Dr. Archer for his comments. We note, however, that higher doses are not restricted to counties in western states with small populations (1). For example, we calculate an average dose of 3.2 cGy for the north central region (Iowa, Illinois, and Michigan) in Dr. Archer's Table 1, which is not substantially lower than the average dose of 3.8 cGy for the mountain region (Colorado, Montana, and New Mexico). We agree that risk estimates from our study may be biased for several reasons, including especially the errors in doses when studying a mobile population, and we state in the abstract that various problems ". . . preclude making a quantitative estimate of risk due to exposure" (2). We question whether further analyses can overcome the basic limitations of this ecologic study.
With regard to the thyroid dose estimation, the small doses estimated for the 1951 tests are due to the fact that all 1951 tests with relatively large yields (>10 kilotons) were detonated at heights above ground of more than 300 meters. As a consequence, most of the radioactive materials stayed aloft and little was deposited on the ground over the territory of the continental United States. In addition, the 1951 tests were clustered around February 1 and November 1, at times when cows are off pasture in many regions of the United States and, therefore, the 131I transfer from ground contamination to milk is minimal. Finally, the average doses for 1952 and 1958 were 0.7 and 0.0 cGy, not 0.7 and 0.8 Gy, respectively, as stated by Dr. Archer.
REFERENCES
1 National Cancer Institute. Estimated exposures and thyroid doses received by the American people from iodine-131 in fallout following Nevada atmospheric nuclear bomb tests, a report from the National Cancer Institute. Washington (DC): U.S. Department of Health and Human Services; 1997.
2
Gilbert ES, Tarone R, Bouville A, Ron E. Thyroid cancer rates
and 131I doses from Nevada atmospheric nuclear bomb tests. J Natl Cancer
Inst 1998;90;1654-60.
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