~16.it is known that patclicuia> ~adioisotopes, such as iodine 131, represent a relatively corstent active debris. proportion of the total radioc- Hence, if a measurezent of the total auount of fallout is obtained it 15 ;ossible te calculate the amount of Lodine 13] produced. Fron this value ore can estimate the amount of iodine i3i present "age" of the fallout, i.e. in ltailcut voy determining the the tine between its production in the nuclear explosion anc its measurement, for like all radio-~ isotopes i tine 13. decays with time. Thus. if one makes a measurement of the tctal gamma or beta radioactivity emitted by a sample of fallout, and can alco determine its “age,” it is possible to estimate the amount of iodine 131 present. The relevant calculations are presented in detail in the Appendix. During this interval some "fractionation" may possibly occur, i.e., as the fallout drifts along, some isotopes may become deposited out sooner than others. Not much is known about this process. In keeping with general practice in thic field (see for exazple, Dunning, Hearings,Radiation Subcommittee, 1959. Biologicai and Environmental Effects of Nuclear ar, p. 443), the possible effects of fractionation are not considered in our caiculations. Once an estimate of iodine 131 on the ground is available, it ‘s possibile quite readily to calculate how much of it will go intc milk, and how much of the radioiodine in the milk will become concentrated in the thyroid of a child drinking cne quvrt of milk per day (the standard usually used in fsllout calcuicticn).