LATE EFFECTS OF RADIOACTIVE IODINE IN FALLOUT there are 10 units, and at 7 times 7 (49 hr), 1 unit. A word has to be said about local factors involved, because these have played a major role in the Marshallese explosion. In general, one can differentiate underwater explosions, underground explosions, surface explosions, low-altitude, and high-altitude explosions. One critical factor is whether the fireball created by the explosion touches the earth or the ocean. If it does, it can volatilize as much as 100,000 tons of earth, water, and debris; and this large mass of material produces two effects on fallout. First, the volatilized material is subjected to neutron radiation so that large amounts of induced isotopes are produced, and secondly, as the mass of volatilized material expands and cools, it conde .es and produces particles large enoughtosettle to the ground. In the Marshall Islands after the explosion, there was so much of this material that large white particles drifting down from the sky gave the appearance of a snowfall. This is close-in fallout, and it occurs from 10 to 100 miles from the point of detonation. It results from an injection of large amounts of material into the tro- phosphere (the atmosphere from ground level to 10 miles). Stratospheric injection, which is into the earth’s atmosphere more than 10 miles from the ground, results in a much more general fallout because of the much slower settling of debris that has a half-time of some years. In general, this type of injection occurs with high-level ex- plosions, and the fallout is worldwide in distribution. The biological consequences of radioactive fallout are complicated by several factors. If radioiodine falls out on pasture, cows grazing there can consumerelatively large amounts of anything deposited on the grasses. In addition, the iodine is concen- trated in the milk 10 to 40 times the level in blood; hence, the radioiodine in milk has been concentrated considerably over that deposited on the ground. Finally, the 1217 humans who consume milk concentrate about one third of the radioiodine that they ingest in their own thyroids. Iodine in the human thyroid remains in the gland a long time; in a normal individual iodine in the thyroid has a half-life of 60 days. Oneadditional factor is the problem of the child versus the adult. A 1-year-old child, for example, has a 1-g thyroid; an adult has a 20-g thyroid. The proportional uptake in both is the same, so that the child has ‘foughly 20 times as much radioiodine deposited per gram of thyroid tissue as an adult. Wecan ratherbriefly conclude and point out that fallout depends on a variety of circumstances. It depends on the nuclear device exploded. It depends on whereit is exploded so that it can give close-in fallout, medium trophospheric fallout, or longrange stratospheric fallout. The problems associated with fallout depend on the kind of isotopes involved. In the case of iodine it is complicated by biological concentration through cattle and through humans, and there is every reason to expect that it could be a serious consequenceof fallout. Dr. Rossins: Ordinarily we leave discussion until the end, but we are going to shift emphasis somewhathere, so if anyone in the audience has any questions on the technical matters that Dr. Rall has brought up, we could have some questions now. Dr. JAN WotFF: Is there a chemical selection at the moment of fission favoring one element of a given mass over another of the same mass? Dr. Ratt: In general, there is no chemical fractionation or preference in the production of fission products. Thefinal result, however, depends on whether isotopes of a given mass are stable and how long- or short-lived they may be if they are radioactive. Dr. Puitippe V. Carpon: You said that there was a yield of 2.5% for 1*'I in a thermonuclear explosion. If there is a ground burst does that go up substantially? petits seer Volume 66, No. 6 June 1967