CHAPTER 4 THE RADIOIODINE PROBLEM -- INHALATION The discovery a decade later of severely damaged thyroids in those Marshallese who were exposed as children to the fallout from the March 1, 1954, BRAVO shot, in two ins tances amoun ting to complete ablation, and almost surely due to radioiodine, > raised the question of the pathway by which that exposure occurred. In earlier analyses, it was generally assumed that ingestion through food and drinking water was the principal pathway, and not inhalation. Direct data on the thyroid exposure were not available, partly because the problem was not appreciated then, and partly because gamma spectral analysis was in its infancy. Direct measurement of thyroid burden was not possible as it is today. On the other hand , it is readily demonstrable that there were massive external and internal exposures to a wide mix of fission products, including the radioiodines. The severity of the thyroid damage suffered by the Marshallese has raised the specter of a possible neglected but important danger from radioiodine in fallout particles. It also became important to investigate the possible routes of_entry--ingegtion or inhalation. This has led to recent studies by Cole”? and Norman’ on the threat of inhalation of radioiodine. In the fission process, the iodine radionuclides (I-131, 132, 133 and 135) are mainly produced as decay products of the precursor nuclides of Sb and Te, Although these precursors are less volatile than iodine itself, almost all the iodine radionuclides would be expected to condense late in the temperature history of the nuclear cloud and thus on the surface of the fallout particles, This tendency for surface condensation would make the radioiodines liable to leaching and later assimilation by plants and animals. In addition, significant volatilization of iodine takes place in the evaporation of water solutions of iodide, and when moist warm air is passed over iodine-coated, pseudo-fallout particles. This effect can be orders of magnitude greater on coral (carbonate) than on siliceous particles. Cole found one set of circumstances in which he concluded that inhalation of radioiodine would be a real and significant hazard following nuclear attack: where people are in a fallout the most intense part of a fallout field,* and there is standing water near the shelter ventilator intakes, and thermal inversion. Fallout in rain he excluded because occurs in coincidence with a strong inversion. shelter near appreciable an extended rain seldom Examination of data from atmospheric tests does not yield a basis for clear-cut conclusions about the hazard of iodine inhalation. The *Because iodine is usually fractionated out of the larger particles that fall in the intense part of the fallout field, these circumstances are generally limited to overlapping fallout fields. 16