a. gastrointestinal tract, from the gross fission product b. thyroid, from isotopes of iodine, and ce. bone, principally from isotopes of strontium and barium- activity, 4 % . lanthanum. a . 4 ' The solubility of the fallout material is a major factor in determining the resultant fate, and thus radiation doses, within the body. The solubility varies, depending among other factors upon the surface over which the detonation occurred. The fallout material collected in soil samples at the Nevada Test Site has been quite insoluble, i.e., only a few percent in distilled water and roughly 20-30 percent in 0.1 N HCI. However, it would be expected that the activity actually present in drinking water supplies would be principally in soluble form. The water collected from a well and a cistern on the Island of Rongelap about 21 months after the March 1, 1954 fallout, was found to have about 80 percent of the activity in the filtrate, but there was an undetermined amount that settled to the bottom. Other data suggest the material to have been about 10-20 percent soluble in water. Figure 7 shows relative doses to the body organs, based on the assumptions that (a) 90% of the material is insoluble (when calculating doses to the gastrointestinal tract), (b) all of the isotopes of iodine are soluble (when estimating doses to the thyroid), and (c) 25% of the ingested strontium isotopes and 7% of the barium—lanthanum reached the bones. It may be seen that ingestion of a given amount of fission product activity on the fourth and fifth days may result in nearly two and one-half times the dose to the thyroid as to the lower large intestine. For a con- timuous consumption of fallout material from the first hour to the 30th