TS.1. Introduction
One part of Section 7(a) of Public Law 97-414 directs the Secretary of Health and Human Services to "conduct scientific research and prepare analyses necessary to develop valid and credible methods to estimate the thyroid doses of Iodine 131 (131I) that are received by individuals from nuclear bomb fallout (and) to develop valid and credible assessments of the exposure to Iodine 131 that the American people received from the Nevada atmospheric nuclear bomb tests."
The National Cancer Institute (NCI) was requested to respond to this mandate. In so doing, a task group, established to assistthe NCI in this effort, suggested that it might be possible to estimate, for each of the most important tests, the 131I exposures from fallout for representative individuals and for the populations of each county of the contiguous U.S. during the time of the tests. About 100 of the tests carried out at the Nevada Test Site (NTS), with yields ranging from less than one kiloton to 74 kilotons of TNT, resulted in off-site detection of radioactive materials. The radiation exposures from 90 tests, representing almost 99% of the total activity of 131I that had been released into the atmosphere, have been estimated in this report. They include 3 tests of the Ranger series (January-February 1951), 6 tests of the Buster-Jangle series (October-November 1951), 8 tests of the Tumbler-Snapper series (April-June 1952), 11 tests of the Upshot-Knothole series (March-June 1953), 13 tests of the Teapot series (February-May 1955), 23 tests of the Plumbbob series (May-October 1957), 9 tests of the Hardtack-Phase II series (September-October 1958), and 17 tests conducted between September 1961 and December 1970.
The most significant atmospheric weapons tests with respect to fallout occurred in the 1950s, during which time most of the monitoring of environmental radioactivity consisted of gross ß or g measurements. Therefore, the estimation of 131I exposures dating back to the 1950s must essentially be derived from the original measurements of gross ß or g activity, or from mathematical models.
Exposures to 131I in fallout resulted mainly from the pasture-cow-milk food chain. In the assessment of the 131I exposures and thyroid doses from that food chain on a continental scale, estimates need to be made, for each of the approximately 3,100 counties then existent in the contiguous United States, of:
- the activities of 131I deposited on soil and vegetation,
- the amounts of 131I consumed by dairy cows and the resulting 131I concentrations in cows' milk,
- the 131I ingested by people, and
- the radiation absorbed doses to the thyroid from 131I in the thyroids of people.
In addition, exposures to 131I in fallout resulted from other, usually less important exposure routes (e.g., inhalation of 131I-contaminated air, and ingestion of foodstuffs other than fresh cows' milk). These exposure routes also are analyzed in this report, but in a much less detailed manner.