| ; IN CONFIDENCE (i) The size of the thyroid glands into which the radioactive iodine was concentrated, and the way in which gland size varies with age. Estimates of gland size made in different countries do not vary zreatly, and the sizes assumed for the Marshallese children, in the absence of direct data, are typical average values. (ii) The types of radioiodine taken into the body, and whether by inhalation, drinking water or food (since this affects the time and duration of exposure). The types of radioiodine present at any time since nuclear fission are well established on physical grounds, and the assumed modes and durations of intake seem reasonable. (iii) The amount of these radioiodines incorporated in the cow The estimates of internal exposure in the initial phases depend upon three types of assumption: PN of the relevant period of exposure. TOF ae -~7- The average thyroid dose may thus have been lower than estimated, and it seems unlikely to have been higher. It must be emphasized however that these are estimates of the likely averace dose from internal radiation. Doses received by different children are likely to have differed considerably from the average appropriate for their age, owing to individual variations in size of gland, in amount of contaminated water drunk, or air inhaled, and in the discharge rate of iodine from the thyroid gland. I apologize for the considerable length of this renort and recognize that much of it deals with minor points of technical or medical detail. I felt however that, on questions of the tvpe which your Committee has raised and with which it must be concerned, it ‘vas 228 / continued... 7 Tae diere the estimate nas to be based on measurements of the amounts excreted in pooled urine specimens taken 15 days after exnosure, and on assumptions as to the proportion of the initial uptake that will be excreted during this 15th day. The original assumption was that 9.95 to 0.2% of the initial uptake would be excreted on that day. I have recalculated this figure on the basis of the best later estimates of which I am aware for the speed of discharge of iodine from the normal thyroid and its apyearance in the urine, and obtain a figure of [.097, in good agreement with tne central value for the original assumptions. I have also seen a calculation by Dr. Rall and Dr. Berman based directly on measurements of iodine turnover in five ifarshallese people. This gives a higher percentage, and therefore a lower estimate of radiation exposure as based on the measured urinary excretion. It should also be added that, if the thyroid radiation itself altered any of these (normal) values, it would do so by accelerating the discharge of iodine from the gland, and pernaps also by increasing the proportion excreted in the urine. both these changes would thus lower the estimate of thyroid dose. 7 thyroid and hence the radiation exposure of glands of any given size.