fallout by many pathways including external radiation as a plume or cloud passes over the region, external exposure to radioactivity deposited on the ground and remaining there for extended periods, and internal exposure to radioactivity that accumulates in the body from inhalation or ingestion of plants, meat, and milk.
A collection of data is available to map exposure rates across the United States from radioactivity deposited on the ground as a result of fallout (NCI, 1997). Analysis of the data has revealed that doses from external radiation to radiosensitive tissues are small and, in all but a few cases, not significantly greater than those from natural background radiation (CDC-NCI, 2001).
The most important pathway is the ingestion of iodine through the consumption of milk (NCI, 1997). Iodine is absorbed rapidly in the gastrointestinal tract and almost completely transferred to the blood through the small intestine. Of the iodine in the blood, about 30% is transferred to the thyroid, and the remainder is eliminated by excretion. Stable iodine in the thyroid is reduced by 50% in approximately 120 days. The isotope iodine-131 (131I) has a radioactive half-life of 8 days. Thus, more than 90% of the atoms of 131I taken up in the thyroid will decay there. Beta particles from that decay deliver a substantial dose to the thyroid.
There has therefore been an extensive effort to determine the dose to populations from 131I originating in the atmospheric weapons-testing program at the NTS. The decade-long effort has produced a dose calculator that can conveniently be used to estimate the dose to a person on the basis of age, location, and milk consumption (NCI, 1997) (http://ntsi131.nci.nih.gov/).
We have used the calculator to provide information that might be useful in identifying geographic areas in the continental United States that might be eligible for compensation through RECA. To begin the process, we created an imaginary person. The person was a male born on January 1, 1948, who remained in a single county throughout the testing period. He consistently drank one to three glasses of processed milk from local dairies. We computed the total dose to the person’s thyroid. We repeated that process for every county in Utah. Several counties with sufficient fallout data were subdivided into regions, and we performed the dose calculation for each region separately. Figure 5.1 shows the results of our calculations. Each circle represents the computed dose for a specific county or sub-county region in Utah. The counties on the X axis are arranged from the highest thyroid dose to the lowest.
The variation in dose is large, ranging from 30 mGy for the lowest value in Iron County to 210 mGy in Washington County.
We assumed that the person was diagnosed with thyroid cancer some time after the testing at the NTS stopped. We then determined whether he was eligible for compensation through RECA. The solid circles in Figure 5.1 represent counties where he would be eligible for compensation, and the open circles represent counties in which he would not. Some counties with relatively low doses are